ME-166-Lecture-01-Internal-Combustion-Engine-Fundamentals.pdf

ME 166 Thermal and Fluids Engineering Design Lecture 01: Internal Combustion Engine (ICE) Fundamentals Main Reference ❖ Carlo N. Grimaldi and Federico Millo, Internal Combustion Engine (ICE) Fundamentals, Handbook of Clean Energy Systems, 2015 John Wiley & Sons, Ltd. YouTube Videos o https...

ME 166 Thermal and Fluids Engineering Design Lecture 01: Internal Combustion Engine (ICE) Fundamentals Main Reference ❖ Carlo N. Grimaldi and Federico Millo, Internal Combustion Engine (ICE) Fundamentals, Handbook of Clean Energy Systems, 2015 John Wiley & Sons, Ltd. YouTube Videos o https://www.youtube.com/watch?v=ID1FEE2dpL8 o https://www.youtube.com/watch?v=x4LIO_xs1es o https://www.youtube.com/watch?v=hbfkbcdw_OM o https://www.youtube.com/watch?v=-3gzQVGEqF4 o https://www.youtube.com/watch?v=4maq0qUb0JA o https://www.youtube.com/watch?v=__eoXGWUxew o https://www.autodeal.com.ph/articles/car-features/3-cylinder-vs-4-cylinder-engines-are-three-pistons-enough o https://www.goodwood.com/grr/road/news/the-best-three-cylinder-engines/ o https://www.youtube.com/watch?app=desktop&v=fSeGGusaekM o https://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/diesel-fuel-better-environment.htm o https://www.eia.gov/energyexplained/diesel-fuel/diesel-and-the-environment.php o https://www.carparts.com/blog/a-brief-comparison-of-diesel-vs-gasoline- emissions/#:~:text=A%202017%20study%20indicates%20that,t%20produce%20the%20toxic%20pollutant. o https://www.jdpower.com/cars/shopping-guides/what-do-dohc-sohc-and-ohv-stand-for o https://www.team-bhp.com/forum/technical-stuff/7633-engine-better-sohc-vs-dohc.html o https://www.tvsmotor.com/media/blog/sohc-vs-dohc-differences- explained/#:~:text=When%20a%20single%20camshaft%20governs,the%20setup%20is%20called%20DOHC. o https://www.carparts.com/blog/sohc-vs-dohc-whats-the-difference-and-which-is-better/ o https://www.youtube.com/watch?v=lNnwn8OlPHM o https://cartreatments.com/sohc-vs-dohc/ o https://www.youtube.com/watch?v=OtbwfGMrlNU o https://www.youtube.com/watch?v=bE_1JYrlYYU o https://www.youtube.com/watch?v=d4ZX665LPFw Internal Combustion Engines: Important Characteristics ❖ Among the factors important to evaluate include: 1) Engine performance over operating range as quantified by a) maximum power (or maximum torque) at each speed b) power and speed range of satisfactory operation 2) Fuel consumption and fuel cost 3) Engine air pollutant and noise emissions 4) Initial cost of engine installation 5) Reliability, durability, maintenance requirements, and their effects on operating costs Lecture Outline I. INTRODUCTION: ENGINE TYPES AND CATEGORIES o Internal vs External Combustion Engines o Rotary vs Reciprocating Engines o Main Classification Criteria of Reciprocating Internal Combustion Engines o Additional Classifications Criteria o Gasoline vs Diesel Fuel Comparison II. MAIN GEOMETRICAL AND OPERATING CHARACTERISTICS o Displacement, stroke/bore ratio, and compression ratio o Power, torque, work, and other performance parameters o Energy conversion efficiency, fuel and air consumption III. EFFICIENCY ANALYSIS o Real operating cycles and ideal thermodynamic reference cycles o Breakdown of energy losses o Introduction to technologies for increasing efficiency at part load IV. ENGINE CONTROL V. ENGINE EMISSIONS o Introduction o Legislation framework for pollutant emissions o Legislation framework for CO2 emissions o Hybrid powertrains Introduction: Engine Classifications Internal & External Combustion Engines ❖ Engines are machines designed to provide mechanical energy to a wide variety of systems by converting the chemical energy of a fuel. On the other hand, devices that are capable of producing a mechanical output starting from different types of energy, such as electric or hydraulic energy, are usually called motors. ❖ Internal combustion engines (ICEs) exploit the conversion of the chemical energy contained in suitable fuels—typically hydrocarbons (HCs)—into mechanical energy, owing to a combustion process. The heat generated by oxidation reactions of elements such as carbon or hydrogen produces a temperature increase in the fluid that acts as the working fluid in the power plant. The enthalpy increase is then exploited to generate mechanical work in suitable machines, thanks to the fluid expansion. ❖ If the combustion takes place within the working fluid itself, the machine is then called an ICE; on the contrary, if the working fluid receives the heat from the combustion products remaining separated from them by a solid surface (for instance, in a heat exchanger), the machine is referred to as external combustion engine. Internal & External Combustion Engines ❖ Typical examples of external combustion engines are steam turbine plants or Stirling engines, whereas the ICEs category includes gas turbines and reciprocating and rotary ICEs. ❖ External combustion engines operate according to a thermodynamic closed cycle, as the working fluid undergoes thermodynamic transformations in a closed loop without any need of being replaced. ❖ On the contrary, the fluid operating in an ICE undergoes chemical transformations that require its periodical replacement with fresh fluid, thus performing a thermodynamic open cycle. ❖ Even though gas turbine plants can also be designed to operate through a closed cycle, they are typically operating in open cycle: consequently, strictly speaking, they should be included in the ICEs category. Nevertheless, only reciprocating and rotary engines are commonly designated as ICEs and will therefore be discussed in this lecture. Classification: Rotary vs Reciprocating Rotary Engines: Operating Principles ❖ Although reciprocating ICEs represent the majority of the ICEs that are currently being produced in large quantities around the world, different rotary ICEs have also been developed over the years, and the one proposed by F. Wankel (1965) has been produced in small series for automotive, motorcycle, and aircraft applications. ❖ In this engine, variable volumes are obtained by the rotation of a rotor with an approximately triangular section, with slightly curved sides, inside a trochoidal casing: ❖ The three edges of the rotor, in conjunction with the housing walls, generate three cells with variable volumes, in which a combustible mixture is firstly inducted, then compressed, burned, and finally expanded and exhausted. Parts of Typical Rotary Engine Vehicles Utilizing Rotary Engines: Past & Present 1967 NSU Ro80 1973 Chevrolet Corvette GT Birotor 1985 AutoVAZ 2108 Lada Samara 1973 Citroen GS Birotor 2012 Mazda RX-8 1978-2002 Mazda RX-7 Rotary Engine Advantages & Disadvantages (vs Reciprocating) ❖ Advantages: ✓ Good Balancing of Masses ✓ Smooth and Uniform Instantaneous Torque Pattern ✓ Compact Design ✓ Provides High Specific Power per Unit Volume and per Unit Weight ❖ Typical Problems & Drawbacks  Difficult Sealing  High Heat Losses  High Fuel Consumption  HC Emissions  High Maintenance Costs ❖ Difficulty in meeting stricter emission standards, reliability and durability problems has significantly limited the diffusion of this engine. Rotary Rebirth: 2024 MX-30 EV Range Extender (Generator) o https://www.foxnews.com/auto/revolutionary-mazda-rotary-engine-electric-car Reciprocating Engines ❖ As a matter of fact, the totality of the ICEs worldwide is currently of the reciprocating type, which was invented more than 150 years ago (Cummins, 1976), and it is characterized by a piston in reciprocating motion into a cylinder, which is closed at the opposite end by a cylinder head, thus producing a cyclic variation of the cylinder volume. ❖ Reciprocating ICEs, hereinafter simply designated as ICEs, are widely diffused, particularly in the transportation (mainly for ground and marine transport) and in the power generation sector. ❖ ICEs are known for their favorable power density figures (including, for transport applications, the onboard energy source storage, i.e., the fuel tank); simplicity, and relatively low manufacturing and service costs, as well as to the wide variety of power ranges (from kW to MW) and usable fuels [liquid, such as gasoline, diesel fuel, and ethanol, and gaseous, such as liquefied petroleum gas (LPG) and compressed natural gas (CNG)]. Reciprocating Engines ❖ The piston moves between two extreme positions, the top dead center (TDC), closest to the cylinder head, and the bottom dead center (BDC), at the largest distance from the cylinder head. These two positions correspond respectively to the minimum cylinder volume Vmin and to the maximum cylinder volume Vmax. ❖ The piston is linked by the connecting rod to the crank that transmits the motion to the engine shaft. The entire mechanism so converts the reciprocating linear motion of the piston into the rotational motion of the crankshaft. Reciprocating Engine: Commonly Encountered Terms ❖ Piston: Force generated by combustion is transferred to the piston. The piston is split into many parts, this includes a piston skirt, piston crown, piston rod and piston pin. ❖ Cylinder Wall: Also referred to as the ‘cylinder liner’ and forms the combustion chamber. ❖ Rotary Motion: Force created by the combustion process is transferred to the piston and then the crankshaft. The process causes the crankshaft to rotate and the piston to reciprocate linearly. ❖ Cylinder Bore: Internal diameter of the cylinder liner. Calculate the cylinder displacement by calculating the cylinder bore and piston stroke. Reciprocating Engine: Commonly Encountered Terms ❖ Top Dead Centre (TDC): Represents the maximum transit of the piston in the direction of the cylinder valves. ❖ Bottom Dead Centre (BDC): Represents the piston’s point of maximum transit in the direction of the cylinder base. In other words, the piston will not travel further towards the cylinder base than the BDC reference point. ❖ Clearance Volume: Distance from the TDC of the Stroke and the top of the cylinder liner. ❖ Stroke: Measurement of the total distance travelled by the piston (TDC to BDC). The reference point is measured from the top of the piston crown. Typical Engine Valve & Crankshaft/Piston Operation Typical Engine Valve & Crankshaft/Piston Operation Camshaft Arrangement: OHV vs SOHC vs DOHC o https://www.pakwheels.com/blog/dohc-vs-sohc-engines/ Valve & Piston Timing: Belt vs Chain Sample Cylinder Arrangements ICE Classification: Cylinder Arrangement ❖ In multicylinder engines, operating cycles in different cylinders are generally shifted in order to uniformly distribute the torque pulses over the engine cycle, and obtain a smoother torque pattern: equally spaced ignition intervals between cylinders are typically used (e.g., 180° intervals for a 4S, four-cylinder engine). ❖ Equally spaced ignition intervals also lead to crankshaft architectures with a good balance of the inertia forces that are due to the rotation of the crank throws and reciprocating motion of the pistons. ❖ However, for the engines with more than five or six cylinders, the inline arrangement leads to a quite long engine, raising severe torsional vibration issues; therefore, for engines with more than six cylinders, generally a V arrangement is used, with two banks of cylinders set at a certain “V angle” to each other. ❖ As the cylinders are aligned in two separate planes or “banks,” they appear to be in a “V” when viewed along the axis of the crankshaft. ❖ Typically used “V” angles are 60° (V6, V12) and 90° (V8). ICE Classification: Cylinder Arrangement ❖ If the angle between the two banks is equal to 180°, then the engine is usually referred to as boxer or opposite cylinders. ❖ Subaru and Porsche are two manufacturers known for their 180° engine configurations. ❖ Other architectures, such as U or radial or opposed pistons, which were used in the past for motorcycle and aviation engines, have been nowadays abandoned. o https://engineerine.com/the-12-types-of-cylinder-engines-layouts/ ICE Classification: Various Cylinder Arrangements ICE Classification: Various Cylinder Arrangements ICE Classification: Various Cylinder Arrangements ICE Classification: Various Cylinder Arrangements Reciprocating Engine Configurations: Common I-4 & I-6 Reciprocating Engine Configurations: Inline 3 Cylinder Engines Reciprocating Engine Configurations: 5 Cylinder Engines Massive Wärtsila V-Diesel Engines For Power Generation ICE Classification Criteria Classification of Internal Combustion Engines Classification: Ignition Source ICE Classification: Ignition Source ❖ The most important criterion is undoubtedly related to the combustion process and, strictly linked to this, to the type of fuel that they can burn. ❖ The two main classes of engines are then the Otto or spark ignition (SI) engines, which are mainly operated with fuels as gasoline, on the one side, and the Diesel or compression ignition (CI) engines, which mainly use diesel fuel, on the other side. Spark Ignition: Otto Cycle ICE Classification: Ignition Source ❖ SI engines burn fuels with relatively low reactivity such as gasoline, CNG, or LPG, that can be mixed with air to form a combustible, homogeneous air/fuel mixture, and then compressed into the engine cylinder to reach temperatures of about 700K and pressures of about 20 bar: in this condition, the ignition of the mixture is not possible without an external energy input, which is typically provided by a spark generated by a suitable plug. ❖ The spark energy is quite limited in comparison with the energy that will be released by the combustion process: approximately a few joules vs several hundreds of joules for an automotive engine. ❖ The spark must be sufficient to locally ignite the mixture and crucial to trigger the combustion process and allow a precise timing of its start. Spark Ignition Engines: Flame Front Propagation ❖ The lack of mixture ignition may lead to a combustion misfire, which represents one of the main problems that can affect a proper SI engine operation. From the first kernel ignited by the spark, the combustion then spreads through the fresh mixture: layer after layer, the flame front travels through the chamber, thanks to the heat transferred from the burned gases zone to the surrounding layer of unburned mixture, which is in this way ignited, until the last zones (also called end gas) father from the spark are reached. ❖ The flame front speed is typically about 20–40m/s, thanks to the turbulence inside the mixture (the turbulence corrugates the flame front, thus increasing the heat exchange area, and so the flame propagation speed). ❖ As the flame front speed is proportional to turbulence intensity, and turbulence intensity increases with the engine revolution speed, the flame front will increase with engine speed, thus compensating the reduction of time available for the combustion. Spark Ignition Engines: Flame Front Propagation & Knock ❖ Thanks to this mechanism, there is virtually no limit to the maximum engine speed for SI engines, at least as far as the combustion process is concerned (for instance, F1 engines can run up to 20 krpm). ❖ On the other hand, high values of flame propagation speed can be achieved only if the air/fuel mixture is close to the stoichiometric ratio. When an SI engine has to be operated at part load, it is impossible to reduce only the amount of fuel burned per cycle without reducing the amount of air too, thus requiring the use of a throttling device at the intake for load control, thus resulting in additional efficiency penalties at part load. ❖ In addition, particular operational conditions of the engine (leading to high temperatures and pressures of some portions of the charge), abnormal combustion phenomena can occur, such as preignition and knock. Preignition may occur when a hot spot such as valve edge or a glowing deposit on a combustion chamber surface ignites the unburned mixture before the regular spark ignition. Spark Ignition Engines: Engine Knock ❖ Preignition can result in high temperature and pressure levels that can lead to severe engine damages. ❖ It is an extremely dangerous phenomenon, as it can be self exciting (the higher temperatures reached because of the more advanced combustion can further increase the hot spot temperature, thus making preignition occurring earlier and earlier). It can also be triggered by knock, the other main type of abnormal combustion. ❖ Knock occurs when the “end gas”, that is, the charge fraction that is located farther from the spark, autoignites spontaneously and simultaneously before the arrival of the flame front. ❖ This abnormal combustion causes a sudden rise of the cylinder pressure, followed by the propagation of high frequency (5–15 kHz) pressure waves inside the combustion chamber, which are transmitted through the engine structure to the surrounding environment, producing a characteristic metallic noise called knock and causing severe engine damage because of high temperature fatigue stresses. Spark Ignition Engines: Elimination of Engine Knock ❖ To avoid the risk of knock occurrence in an SI engine, a careful design must be undertaken by complying with limitations concerning the maximum flame path length, which limits the combustion chamber dimensions and primarily the cylinder bore, and the maximum admissible temperatures and pressures of the end gas, which limit the maximum compression ratio of the engine, as well as the maximum boost level for turbocharged engines. Spark Ignition Engines: Thermodynamic Cycle Analysis Spark Ignition Engines: Thermodynamic Cycle Analysis ❖ Finally, it should be pointed out that, even if SI engines are often referred to as Otto engines, this does not mean that they feature an instantaneous, isochoric combustion process, because a certain amount of time is needed for the flame front to travel from the spark plug to the end gas. ❖ Nevertheless, they are often referred to as Otto engines because the Otto thermodynamic cycle represents the reference, ideal cycle for this category of engines. Spark Ignition Engines: Multiple Spark Plugs / Cylinder Spark Ignition: Atkinson Cycle ATKINSON CYCLE (SVPSVP): Typical For Gas Hybrid Vehicles ❖ Developed by British Engineer James Atkinson (1882). Ideal Atkinson Cycle consists of: 1→2 Isentropic Compression 2→3 Isochoric Heat Addition 3→4 Isobaric Heat Addition 4→5 Isentropic Expansion 5→6 Isochoric Heat Rejection 6→1 Isobaric Heat Rejection ❖ The goal of the modern Atkinson cycle is to make the pressure in the combustion chamber at the end of the power stroke equal to atmospheric pressure. When this occurs, all available energy has been obtained from the combustion process. ❖ For any given portion of air, the greater expansion ratio converts more energy from heat to useful mechanical energy—meaning the engine is more efficient. ATKINSON CYCLE (SVPSVP): Typical For Gas Hybrid Vehicles ❖ The Atkinson cycle delays the intake valve’s closing until the piston has completed 20 to 30 percent of its upward travel on the compression stroke. As a result, some of the fresh charge is driven back into the intake manifold by the rising piston so the cylinder is never completely filled (hence the low-speed power reduction). ❖ The payoff comes after ignition when the piston begins descending on the expansion (also called power) stroke. ATKINSON CYCLE (SVPSVP): Typical For Gas Hybrid Vehicles ❖ Consistent with Atkinson’s original thinking, the shortened intake stroke combined with a full-length expansion stroke squeezes more work out of every increment of fuel. ❖ In most engines, the compression ratio is set as high as the engine can stand short of detonation in pursuit of power and efficiency. Compression and expansion ratios are the same in an Otto engine. Atkinson wins on efficiency because its expansion ratio is significantly larger than its compression ratio. Supplementary YouTube Videos: Atkinson Cycle Operation Supplementary YouTube Videos: Atkinson Cycle Operation Supplementary YouTube Videos: Atkinson Cycle Engines ❖ CNET Car Tech 101: Atkinson Cycle Overview o https://www.youtube.com/watch?v=gD2AQuhbHdk ❖ Toyota Canada: Atkinson Cycle Engine o https://www.youtube.com/watch?v=WKKILW3Zj_Y ❖ Engineering Explained: Atkinson Cycle Operation o https://www.youtube.com/watch?v=z45fM2N-4C4 ❖ Driving 4 Answers – Cycles: Atkinson vs Miller vs Otto o https://www.youtube.com/watch?app=desktop&v=NvtcTXey0U8 Compression Ignition: Diesel Cycle Compression Ignition Engines: Diesel Cycle ❖ CI engines burn fuels with high reactivity, such as diesel fuel, typically featuring long, straight chain HC molecules (such as cetane, C16H34), in which the preliminary reactions of the oxidation process proceed quite quickly at high temperatures and pressures, so that the autoignition process is favored and an SI system is no longer necessary. ❖ However, such fuels cannot be mixed with air and then compressed into the cylinder, because otherwise the combustion process will start spontaneously during the compression stroke. ❖ Therefore, these fuels are injected as a high pressure liquid spray into the compressed air, near to the end of the compression stroke: the fuel liquid jet atomizes into small fuel droplets, which, surrounded by the hot compressed air, quickly evaporate, and after the fuel vapor is mixed with air, the combustion process starts spontaneously after a short ignition delay. Compression Ignition Engines: Compression Ratio ❖ To obtain this, compression ratios higher than those typical of SI engines are used. The compression ratio in a gasoline-powered engine will usually not be much higher than 10:1 due to potential engine knocking (autoignition) and not lower than 6:1. A stock Honda S2000 engine (F22C1) has a compression ratio of 11.1:1. ❖ The Diesel engines have the compression ratio that normally exceed 14:1 and ratios over 22:1 are also common. The Honda i-DTEC engine operates at 16.0:1. Compression Ignition Engines: RPM Limitations ❖ In the CI engines, the start of combustion is then indirectly controlled by the injection timing, and the features of the combustion process are quite different from the SI ones, because two different types of flames are always present: premixed flames in the first combustion phase, followed by diffusion flames. ❖ Mostly caused by the presence of the diffusion flame, whose speed is lower than that of the premixed flame and governed by the processes of vaporization of the spray liquid droplets and of the fuel vapors diffusion, it is impossible for the combustion process in diesel engine to self-adjust its characteristics to the time reductions connected with increasing engine revolution speeds. ❖ Consequently, CI engine maximum speed is generally limited below 5000 rpm. ❖ The autoignition process of CI engines, result in multiple ignition points that allow a robust combustion process spreading throughout the combustion chamber. Thus, unlike SI engines, there are no strict requirements to keep the air/fuel ratio close to the stoichiometric value to assure a proper flame propagation speed. Compression Ignition Engines: Thermodynamic Cycle ❖ At part load, the injected fuel quantity can then be reduced while maintaining the same quantity of inducted air into the cylinder, without any need of throttling and therefore reducing the pumping losses during the gas exchange phase. ❖ Finally, it is worth mentioning that, although these engines are often referred to as diesel engines, this does not mean that combustion will take place at constant pressure as in the Diesel thermodynamic cycle, but only that this is the reference thermodynamic cycle for this kind of engines. ❖ It should be mentioned that new types of engines are currently being developed, characterized by innovative combustion process features, such as HCCI (homogeneous charge compression ignition) engines, which present significant differences from the previously described conventional combustion processes and will be discussed later. Compression Ignition Engines: Diesel Cycle Compression Ignition: Dual Cycle Compression Ignition Engines: Dual Cycle ❖ The dual combustion cycle (also known as the mixed cycle, Trinkler cycle, Seiliger cycle or Sabathe cycle) is a thermal cycle that is a combination of the Otto cycle and the Diesel cycle, first introduced by Russian-German engineer Gustav Trinkler. ❖ Heat addition at constant volume is followed by heat addition at constant pressure: o UWMC YouTube Video: https://www.youtube.com/watch?v=l20_5-kwu-E Compression Ignition Engines: Dual Cycle ❖ The dual cycle engine is a technology that has been developed to help reduce these harmful effects on air quality. It reduces fuel consumption by up to 40% while providing more power than an equivalent diesel engine. Initially used in marine diesel engines. ❖ The Dual Cycle is an ideal cycle that better approximates the actual performance and operation of a most modern compression-ignition engines. ❖ One of the main advantages of dual cycle engines is that they can achieve higher thermal efficiency and lower specific fuel consumption than Otto or Diesel engines, especially at high compression ratios and high loads. o UWMC YouTube Video: https://www.youtube.com/watch?v=l20_5-kwu-E Classification: Cycle Duration ICE Classification: 4-Stroke vs 2-Stroke ❖ Another very important criterion for the classification of the ICEs refers to the duration of the working cycle, depending on the number of strokes that are necessary to complete the whole operating cycle, including both the combustion and the charge replacement processes: the two corresponding engine categories are then defined as four stroke (4S) engines and two-stroke (2S) engines. Cycle Duration: 4-Stroke Engines ICE Classification: 4-Stroke Engine Operation ❖ In 4S engines, four piston movements between BDC and TDC are required to complete a working cycle, during which six different phases are taking place: intake, compression, combustion, expansion, exhaust blowdown, and exhaust displacement. ❖ These phases occur during the following four piston strokes, according to the subsequent cyclic sequence, conventionally starting from the intake: ICE Classification: 4-Stroke Engine Operation 1) Intake Stroke (From TDC to BDC). Fresh mixture in SI or fresh air in CI engines is aspirated into the cylinder through the intake valve, which typically opens with a slight advance before TDC and closes with a certain delay after BDC, in order to increase the mass of the inducted charge. Spark Ignition Engine Compression Ignition Engine ICE Classification: 4-Stroke Engine Operation 2) Compression Stroke (From BDC to TDC). In this phase, the charge is compressed while both intake and exhaust valves are closed. Towards the end of the compression stroke, the combustion is initiated via SI or fuel injection (CI). Spark Ignition Engine Compression Ignition Engine ICE Classification: 4-Stroke Engine Operation 3) Power Stroke (From TDC to BDC). The hot burned gases expand, pushing down the piston and making on it a work that is several times higher than the work done by the piston during the compression stroke. Toward the end of this stroke, the exhaust valves open and part of the burned gases are discharged from the cylinder, thanks to the pressure differential (blowdown exhaust). Spark Ignition Engine Compression Ignition Engine ICE Classification: 4-Stroke Engine Operation 4) Exhaust Stroke (From BDC to TDC). The piston pushes out the burned gases that still remain in the combustion chamber (displacement exhaust). Near to the end of this stroke, the intake valves open, while shortly after the TDC, the exhaust valves close and a new cycle begins. Spark Ignition Engine Compression Ignition Engine Cycle Duration: 2-Stroke Engines Internal Combustion Engines: 2-Stroke Operation ❖ In 2S engines, the complete operating cycle just requires two piston strokes (i.e., one crankshaft revolution). In order to obtain a higher power output, here the two strokes used for the gas exchange are suppressed, and substituted by a scavenging process, that is, by the displacement of the burned gas when piston is approaching the end of the power stroke by means of a fresh charge that has been pressurized in an external compressor or blower. ❖ In the simplest design, the blower can be obtained by means of the crankcase itself, the volume of which varies in opposition with the cylinder volume, so that the minimum crankcase volume (and thus the maximum crankcase pressure) is reached when the piston is at BDC in the main cylinder. ❖ Moreover, a more compact design in comparison with 4S is possible, because the intake and exhaust valves can be replaced by ports in the cylinder liner, the opening and closure of which can be directly controlled by the piston motion. Internal Combustion Engines: 2-Stroke Operation ❖ Two-stroke engine with crankcase scavenging: compression, combustion, and scavenging phases: ❖ The two strokes are as follows: o Compression Stroke. After closing the inlet and exhaust ports, the piston compresses the cylinder charge (in the meantime, the volume in the crankcase increases, drawing fresh charge into the crankcase). Toward the end of the compression stroke, the combustion is triggered via SI or fuel injection (CI). Internal Combustion Engines: 2-Stroke Operation o Power Stroke. The hot burned gases expand, pushing the piston down and making mechanical work on it. Toward the end of this stroke, the exhaust port opens and part of the burned gases are expelled from the cylinder, thanks to the pressure differential. Later, the scavenging ports are opened, and the pressurized charge displaces the burned gases, so that a new cycle can then start after the piston has reached the BDC. ❖ Again, as for the 4-Stroke Operation, six different phases take place during the two strokes: scavenging, intake, compression, combustion, expansion, and blowdown. ❖ If simple ports in the cylinder walls are used, the intake port edge must be lower than the exhaust port, in order to allow the blowdown phase; this will cause a short circuit of a part of the inducted fresh charge at the beginning of the compression stroke, as the exhaust port will remain open for a while after the intake port closure. Internal Combustion Engines: 2-Stroke Operation ❖ The scavenging process represents the Achilles’ heel of the 2S engine, because in its simplest layout with simple ports in the cylinder walls, a part of the fresh charge will flow directly to the exhaust port causing high fuel consumption and HC emissions in SI engines. ❖ For these reasons, the use of 2S SI engines has been traditionally limited to small power utility engines (such as lawn mowers, saw chains, outboard engines for boat propulsion, and small scooters), where these cons were thought to be acceptable owing to the high simplicity, low cost, and high power density of these engines. ❖ 2S is also used in large CI engines for marine and stationary applications (about 1m bore) for which the short circuit of a part of the scavenging flow (made by compressed air only) can be accepted, and where they are usually preferred to 4S owing to the excessively high thermomechanical stresses that poppet valves used in 4S should withstand (stresses increase with valve diameter, which is proportional to cylinder bore). Internal Combustion Engines: 2-Stroke Operation ❖ Currently there are no examples of 2S engines for mass production automotive applications, while also the adoption of 2S engines for two- and three-wheel vehicles application and utility engines has been constantly decreasing, owing to the more and more severe emissions regulations. ❖ Pros & Cons - Additional Discussion: ❖ Driving 4 Answers o https://www.youtube.com/watch?v=eKUEZY3R3cI ❖ The Engineers Post o https://www.youtube.com/watch?v=8dAbcbAJRw8 ❖ Concerning Reality o https://www.youtube.com/watch?v=0rza6B5mDQ4 ❖ Automotive Explained o https://www.youtube.com/watch?v=PT515pCskS4 Two-Stroke Engines vs Four-Stroke Engines ❖ Note the power stroke for every revolution, compared to two revolutions for the four- stroke. High power to weight ratio. ❖ Two-stroke engines use a crankcase to pressurize the air-fuel mixture before transfer to the cylinder. ❖ Unlike four-stroke engines, they cannot be lubricated by oil contained in the crankcase and sump. ❖ Fuels supplied to two-stroke engines are mixed with oil so that it can coat the cylinders and bearing surfaces along its path. The ratio of petrol to oil ranges from 10:1 to 50:1 by volume. ❖ Disadvantages: Incomplete combustion, highly polluting. Unstable idling, high vibration and noise emission. More wear, shorter life span. Narrow power band. Two-Stroke Engines vs Four-Stroke Engines ❖ Motorcycles and Outboard Marine Engines were traditionally dominated by two-stroke engines. These are compact, simple and lightweight; with fewer moving parts (i.e. crank, rod and piston). Low maintenance. ❖ Two Stroke Engines have found less market share due to tightening Emission Standards and Fuel Efficiency. ❖ Two-stroke engines are still found in a variety of small propulsion applications, such as outboard motors, small on- and off-road motorcycles, mopeds, scooters, tuk-tuks, snowmobiles, go-karts, ultralight and model airplanes. Classification: Air Supply ICE Classification: Air Supply ❖ Additional criteria of classification of the ICEs may concern a variety of other aspects, such as the methods adopted to supply air and fuel, the cooling systems, the cylinder arrangement, and the most common applications, and they will be briefly described in the following sections. ❖ In naturally aspirated (NA) engines, fresh air is drawn into the cylinder directly from the surrounding environment, at atmospheric pressure, by the piston motion. ❖ However, the engine power density can be remarkably increased by means of supercharging or turbocharging, that is, by creating a pressurized environment at the engine intake: in this way, a higher mass of air will be inducted per each engine cycle, allowing a higher quantity of fuel to be burned, and thus achieving a higher work per cycle. ❖ The compressor that is needed to pressurize the intake air can be driven by the engine shaft (supercharging) or by means of a turbine using the residual energy of the exhaust gases (turbocharging). These methods are known as Forced Induction (FI). Air Supply: Forced Induction – Turbocharger w/ Intercooling ❖ As the compressor also increases the air temperature, an intercooler is usually interposed between the compressor outlet and the engine inlet to cool down the air and increase its density. ❖ The primary purpose of an intercooler is to cool the air that the turbocharger or supercharger has compressed before it enters the engine. By cooling the air, the intercooler reduces the chances of knocking and allows for more air to be forced into the engine, which can increase power output. ❖ Naturally Aspirated Engine vs Cold Air Intake (CAI): Air Supply: Forced Induction – Turbocharger w/ Intercooling Air Supply: Forced Induction - Superchargers o https://www.youtube.com/watch?v=F-Iugo2lqvk Air Supply: Forced Induction SI & CI Engines ❖ The turbocharger blows extra air into the combustion chamber to boost the engine’s compression. The higher air mass enables more fuel to be burned. That provides multiple benefits for the engine, including increased fuel efficiency, performance, and torque output. ❖ Today, Original Equipment Manufacturers (OEMs) can choose between turbochargers or mechanical superchargers. The latter solution provides greater low-end performance and exceptional longevity. However, it is more expensive and often poses a true struggle when it comes to its compliance with tightly packed engine parts. Classification: Air-Fuel Mixture ICE Classification: Air & Fuel Mixture Formation ❖ As far as SI engines are concerned, a homogeneous, combustible air/fuel mixture must be formed before the start of combustion. ❖ The simplest way to achieve such a result is by means of a carburetor, where the air passes through a Venturi throat, which is connected to a fuel reservoir: the highest the air mass flow is, the highest will be the pressure drop in the Venturi throat, and thus the fuel mass flow drawn. ICE Classification: Air & Fuel Mixture Formation ❖ However, such a system does not allow a precise control of the air/fuel ratio, as it is mandatory to obtain a high efficiency of the exhaust gas aftertreatment system, and thus it has been replaced by electronically controlled fuel injection systems. ❖ Electronic Fuel Injection (EFI) more precisely controls the amount of fuel injected into the air stream by means of electronically controlled injectors. ICE Classification: Air & Fuel Mixture Formation ❖ Sample Gasoline and Diesel Fuel Injectors: ICE Classification: Fuel Injection Point – Spark Ignition ❖ Fuel can be injected into the intake port (port fuel injection, PFI) or directly into the cylinder (direct injection, DI), although in this case the injection must take place well in advance before the start of combustion in order to allow the formation of a proper air/fuel mixture. ICE Classification: Fuel Injection Point – Spark Ignition ICE Classification: Fuel Injection Point – Spark Ignition ❖ Cylinder DI has the drawback of carbon deposits on the valve seats since the cleaning effect of fuel flowing over the valve is no longer present. ❖ Toyota’s Dynamic Force D4-S is a current demonstration of port and direct injection implementation. Should minimize carbon deposits with regular port injection. ICE Classification: Fuel Injection Point – Spark Ignition ❖ DI can also enable stratified charge operation, in which a stoichiometric air/fuel ratio is obtained around the spark plug, surrounded by air, in order to avoid the need of throttling the engine at part load, and to obtain a better efficiency. ❖ One famous application is Honda’s 1970s CVCC, or Compound Vortex Controlled Combustion Engine. ICE Classification: Fuel Injection Point – Compression Ignition ❖ In CI engines, the fuel is injected into the cylinder toward the end of the compression stroke. In the most common diesel engines, the DI ones, the injector is directly protruding in the combustion chamber: the mixture is not homogeneous and is formed through the interaction between the fuel spray and the air flow motion patterns. ❖ In the past, for small displacement engines (0.5 cm3 per cylinder or below), the difficulty to obtain a proper fuel/air mixing was overcome by the design of indirect injection (IDI engines) systems; in these engines, a prechamber connected to the main cylinder combustion chamber by a narrow passage was used, allowing a better and faster fuel/air mixing, but causing remarkable efficiency losses. ICE Classification: Fuel Injection Point – Compression Ignition ❖ The development of electronically controlled common rail injection (CRDI) systems, allowing an extremely precise control of small injected fuel quantities and the split of the injection event into two or more injections per cycle, along with a more flexible control of injection pressure has solved these issues, thus remarkably reducing the diffusion of IDI engines, which almost completely disappeared from the automotive scenario. Classification: Engine Cooling ICE Classification: Engine Cooling ❖ Differently from other machines that are crossed by continuous streams of high temperature gases as gas turbines, the periodic operation of the ICEs makes their components to be exposed to gas temperatures that show fast and wide amplitude variations during the engine cycle, ranging from ambient temperature during the intake phase to combustion temperatures higher than 2000 K; owing to the thermal inertia of the engine components, the related thermal stresses are limited to average temperature levels. ❖ Nevertheless, the engine components facing the combustion chamber must be intensively cooled by means of direct air-cooling systems, where an intensive air flow is forced by a fan to pass through finned surfaces on cylinder block and head or by means of liquid cooling systems, where a closed circuit filled with a water/glycol mixture with cooling passages inside the cylinder block and the cylinder head is used. ❖ The cooling liquid temperature is then controlled by means of an air/liquid heat exchanger, usually a radiator, through which an air stream is forced by a fan. ICE Classification: Motorcycle Engine Cooling: Air vs Water ❖ While the former cooling systems are usually preferred for their simplicity, reliability, and absence of maintenance requirements especially for motorcycle and small utility applications, the latter cooling systems are generally adopted in automotive applications, as they allow the achievement of lower thermal stresses in engine components, and a more accurate control of components and lubricant temperatures. Air Cooling: Legacy Air-Cooled Vehicles Water Cooling: Predominant Thermal Management Approach o https://www.facebook.com/watch/?v=196485028894006 Water Cooling: Predominant Thermal Management Approach Classification: Application ICE Classification: Application (listed ❖ Finally,in order ICEs ofclassified can be their power size)ofas on the basis follows. their applications (listed in order of their power size) as follows: o Small Utilities: Lawnmowers, snowblowers, chainsaws, and portable gensets o Road Vehicles: Motorcycles, passenger cars, light commercial vehicles, buses, and long-haul trucks o Off-Road Vehicles: Forklift trucks, agricultural machines, construction and earthmoving machines, and military vehicles o Railways: Locomotives o Aircraft: Airplanes and general aviation o Marine: Boats and inland, coastal, and ocean-going ships, including ships’ auxiliaries o Stationary Engine Installations: Emergency-power gensets, generating plants, and combined heat and power cogeneration plants Comparisons: Gasoline vs Diesel Fuel Comparison: Petrol (C8H18) vs Diesel (C12H23) ❖ Diesel and Gasoline are both petroleum products. Despite the same origin, they have significant differences in their use. Diesel is the fuel typically for big machinery that demands more torque over horsepower. In comparison, gasoline is a lighter fuel that is often preferred for vehicles needing speed over brute strength. ❖ Diesel and gasoline are both refined from crude oil. However, gasoline is more refined than Diesel. This makes gasoline thinner in density and more volatile. So, in practical use, gasoline burns faster, which allows it to produce more power or horsepower. ❖ Despite being a denser fuel, Diesel has the consistency of light oil. This means that it continuously lubricates the cylinder over and over again as it moves around. Also, due to the bigger engine size, the oil can move more freely inside, allowing it to lubricate components more easily. ❖ Diesel fuel is thicker and, as such, has a higher energy density. The viscosity of Diesel also decreases with temperature and burns slower than gasoline. This is why diesel engines run at lower revolutions per minute (RPM), reducing engine wear and tear. Fuel Comparison: Petrol (C8H18) vs Diesel (C12H23) ❖ Whereas Diesel fuel is thicker in density, it evaporates more slowly. It has a higher energy density, which means that 20% more energy is produced from Diesel than the same amount of gasoline. ❖ Caloric Value: Diesel > Gasoline (15 – 20% More Energy/Volume) ❖ On the other hand, gasoline is more of a solvent; it’s more acidic when compared to Diesel. So as gasoline burns inside, it causes corrosion making the engine surfaces rougher. This increases the wear and tear inside the gasoline engine. Fuel Comparison: Petrol (C8H18) vs Diesel (C12H23) ❖ Previously, Diesel emits fewer amounts of CO2 and methane but produces higher levels of harmful nitrogen compounds. Diesel contains more pollutants that need to be extracted before it can meet emission standards for road use. ❖ These are the three most common gasoline types at the pump: regular (87), mid-grade (89-93), and premium (95-100). The only thing that differentiates these grades of gasoline is their octane levels. The higher the octane level, the higher the possible compression ratio. ❖ There are two main types of diesel to be aware of: clear diesel and dyed diesel. Clear diesel is intended for on-road use and is sometimes referred to as “auto diesel”. This fuel is clear with a light-green hue, which becomes darker as the diesel degrades. ❖ Dyed diesel is, as its name suggests, dyed. It’s often dyed with a solvent and can be red, blue, or purple. Dyed diesel has a higher sulfur content than clear diesel and is illegal to use on public roads. The most common uses for dyed diesel are in construction, farming, and aviation vehicles. Fuel Comparison: Petrol (C8H18) vs Diesel (C12H23) ❖ Diesel engines can emit a fair amount of nitrogen compounds and particulate matter as they burn diesel fuel. These facts combined to give diesel fuel a bad environmental name, even though it in fact emits lower amounts of carbon monoxide, hydrocarbons and carbon dioxide than does gasoline. ❖ A 2017 study indicates that diesel cars produce less carbonaceous particulate matter pollution than gasoline cars. Diesel fuel produces more carbon dioxide (CO2) per gallon, but fuel efficiency means it emits less CO2 per mile. Only gasoline cars produce carbon monoxide–diesel cars don't produce the toxic pollutant. ❖ While older diesel fuel produces far more sulfur dioxide, modern low-sulfur and ultra- low-sulfur diesel fuels have roughly the same sulfur content as gasoline. Octane boosters in high-octane gasoline produce ultrafine particulate matter pollutants, while diesel doesn’t need the polluting fuel additives. Fuel Comparison: Petrol (C8H18) vs Diesel (C12H23) ❖ Two of diesel fuel's environmental advantages have always been that diesel engines get better mileage than traditional gasoline engines and diesel fuel requires less refining. ❖ The energy costs required for refining crude oil into gasoline and the limitations placed on gasoline production by limited refineries in the United States both contribute to increased gasoline needs. ❖ To the degree that diesel fuel can offset the need for more gasoline, the environmental toll of refining gas is also reduced. Engine Cycle Comparison: Otto vs Diesel ❖ Almost the same cycle but the most important differences are: o No fuel in the cylinder at the beginning of the compression stroke (no autoignition can occur) in Diesel engines. o Diesel engine uses compression ignition instead of spark ignition. o Because of the high temperature developed during the adiabatic compression, the fuel ignites spontaneously as it is injected. Therefore, no spark plugs are needed. o Before the beginning of the power stroke, the injectors start to inject fuel directly into the combustion chamber and therefore first part of power stroke occurs approximately at the constant pressure. o To maintain constant pressure during expansion, heat addition occurs (i.e. combustion). o Higher compression ratios can be achieved in Diesel engines. o Higher Temperature & Pressure compared to Gas engines. Cycle Thermal Efficiency Comparison: Otto vs Diesel Gasoline Diesel 1 1 𝛽 𝑘 −1 𝜂𝑡ℎ = 1 − 𝜂𝑡ℎ = 1 − [ ] 𝑟𝑣𝑘−1 𝑘−1 𝑟𝑣 𝑘(𝛽−1) ❖ Analytically, it is important to note that the term preceding the parentheses corresponds to the final term in the Otto cycle thermal efficiency equation and that the term in the parentheses is always greater than 1. ❖ Consequently, for any given compression ratio, except in the limit when the cutoff ratio is zero (i.e., no heat addition to the Diesel cycle), the Otto cycle is theoretically more efficient than the Diesel cycle! ❖ For Cutoff Ratio =1 , the two engines have the same thermal efficiency. ❖ The cutoff ratio severely reduces the thermal efficiency of the Diesel cycle as its value increases. Cycle Thermal Efficiency Comparison: Otto vs Diesel ❖ CUTOFF RATIO  – Ratio of cylinder volume after and before combustion process; where V3 - volume at which fuel addition is “cut off” ❖ The cutoff ratio severely reduces the thermal efficiency of the Diesel cycle as its value increases. Cutoff Ratio  𝑉3 𝑣3 𝑇3 𝛽= = = 𝑉2 𝑣2 𝑇2 Engine Cycle Comparison: Otto vs Diesel ❖ For the same compression ratio, Gasoline Engines are more efficient than Diesel Engines…IN THEORY, COLD AIR STANDARD ANALYSIS. ❖ The actual efficiency advantage of the Diesel engine is due to the higher compression ratio, compared to the Gasoline Engine: ❖ Typical Thermal Efficiencies: Gas: Comp Ratio 6:1 – 12:1 Ideal 51-63% Real 20-30% Diesel: Comp Ratio 11.5:1 – 22:1 Real 30-40% Variable Compression Ratio: Nissan’s Problematic VC Turbo ❖ NISSAN VC-Turbo Engines: o https://www.youtube.com/watch?v=j0An3RbXcPg o https://www.youtube.com/watch?v=d3md1EqL1h4 Otto Cycle: Mazda High Compression Engines ❖ Objective: Increasing Compression Ratio for Thermal Efficiency. ❖ Typical SI Engine Range: 6:1 < Compression Ratio < 10:1 ❖ High Compression Ratio SI Engines w/o Auto-Ignition Risk: Mazda’s Innovative Solution: Skyactiv Concept o https://www.youtube.com/watch?v=CoXs1W_iNak Skyactiv-G Concept o https://www.youtube.com/watch?v=HNFeUlnBypk Skyactiv-G + Cylinder Deactivation o https://www.youtube.com/watch?v=iJIVsXPrENc Skyactiv-X SPCCI o https://www.youtube.com/watch?v=8xUdast_RrQ Engine Comparison: Gasoline vs Diesel ❖ Gasoline Engine Advantages: ✓ Generally, more affordable to purchase initially compared with diesel engines. ✓ Gasoline itself tends to be less expensive than diesel fuel in many regions. ✓ Quieter and smoother than their diesel counterparts. ✓ Fewer nitrogen oxides and particulate matter emissions, making them relatively better for the environment. ✓ Fewer maintenance requirements for components like fuel filters and emissions control systems compared with diesel engines. ❖ Gasoline Engine Disadvantages:  Typically, lower fuel efficiency compared with diesel engines, offering fewer miles per gallon.  Generally, less torque production, which might make them less suitable for heavy towing or hauling.  Usually, shorter lifespan compared with diesel engines due to their higher RPM operation and lighter construction. Engine Comparison: Gasoline vs Diesel ❖ Diesel Engine Advantages: ✓ Higher fuel efficiency , providing more miles per gallon compared with gasoline engines. ✓ Higher torque generation, making them well-suited for towing and hauling heavy loads (e.g. construction, transportation, power generation, agriculture, etc). ✓ Fuel generally has a higher energy density, offering more power and better mileage. ✓ Longer lifespans due to their sturdier construction and lower RPM operation. ❖ Diesel Engine Disadvantages:  Often more expensive to purchase initially, the cost of diesel fuel varying and becoming more expensive than gasoline.  Higher nitrogen oxides and particulate matter emissions, contributing to environmental concerns (although advancements in diesel technology are addressing these issues).  Higher maintenance requirements for components like fuel filters and emissions control systems.  Noisier and produce more vibration compared to gasoline engines. Efficiency Comparison: Gasoline vs Diesel Engines ❖ In general, Diesel is 20% more efficient than gasoline. Its thicker density allows it to produce more energy with less amount of fuel. Also, a Diesel engine can have the same amount of power as gas but at a lower RPM. In other words, a slower rate of burning fuel for the same output. ❖ Then there is the higher compression ratio which is directly related to thermal efficiency. The higher it will be, the better the fuel will burn. This means more rich explosions will happen, and more energy is produced. ❖ However, a gasoline engine can never reach as high a compression ratio. If this did occur, excess heat would ignite the fuel resulting in engine-damaging uncontrolled explosions. So, gas engines require a low compression ratio which results in less efficient combustion. Durability Comparison: Gasoline vs Diesel Engines ❖ In general, Diesel engines are considered more durable than gasoline power plants. That’s one reason why it’s common to see so many Diesel work trucks. Comparatively, a Diesel can operate for many more miles than a gas engine before needing maintenance and repair. In addition, a Diesel engine’s life expectancy is around 250,000-500,000 miles. ❖ Diesels are big engines with big cylinders, large crankshafts, strong pistons, and multiple gears. The castings are built thicker, the cylinder walls are built thicker and the oiling system has a higher volume. These power plants are simple and work off of basic gear operations. Further, Diesel engines are made with heavy-duty materials to support continuous operation. Since engine gears are mostly fixed at a set place, there is limited movement and reduced wear and tear. ❖ Gasoline engines, however, are complicated pieces of engineering. Working mostly on chains and belts, they use hundreds of small parts that require more precision. Regular stop-and-go operations add engine stress which increases wear and tear. Cost of Ownership: Gasoline vs Diesel Engines ❖ The cost of ownership of a Diesel vehicle initially seems lower. Diesel cars are more fuel- efficient, are more durable, and need less maintenance. Diesel engines are hard to break. In fact, a diesel engine can easily last 20-30 years (200,000 to 300,000 miles), before needing major repairs. ❖ On the other hand, the Gasoline engine consumes more fuel, needs more frequent maintenance, and may need more attention after 150,000 to 200,000 miles. ❖ This might make the gasoline engine look like an unwise choice. But over the long run, it is seen that gas engines can get more value than Diesel depending on their use. Because despite Diesel being more robust, it’s also costlier. ❖ Comparatively, a vehicle with a Diesel will cost more at purchase time than the exact vehicle with a gasoline engine. Further, maintenance and repair costs for a Diesel engine go above that of a gas engine. ❖ For example, the oil change in a gas engine would be around $20-$40, whereas Diesel oil change costs $50- $70. Diesel engines are larger, so they need more engine oil. Cost of Ownership: Gasoline vs Diesel Engines ❖ In addition, Diesel engines generally have higher fuel prices. At present, Diesel fuel averages $3.27 a gallon. A Chevrolet Silverado has a city mileage of 22 mpg for Diesel and 19 mpg for gas. Over 10,000 miles, the average fuel cost would be around $1,500 for Diesel and $1,400 for gas (at $3.00 per gallon). ❖ So, considering the additional price for a Diesel vehicle, higher fuel costs, and more expensive servicing, the cost of ownership for Diesel is higher than that of gas over ten years at 10,000 miles/year. ❖ Diesel and gasoline engines have their unique advantages. In general, if you drive less than 12,000 miles a year with regular use, then a gas engine will meet your needs. Also, if you prefer driving a car with some “get-up and go,” then you’ll likely want to skip a Diesel. ❖ However, if you drive extensively (15,000+ miles per year), carry heavier cargo, or tow big loads, then a Diesel will make sense. Yes, a Diesel will have higher upfront and fuel costs, but its overall efficiency offsets this. Vehicle Comparison: Gasoline vs Diesel Variants Exhaust Emissions Treatment: Gasoline vs Diesel ❖ Despite their lower carbon dioxide emissions, diesel vehicles are much greater pollutants overall. Namely, heated air in the engine creates nitrogen oxides NOx. Diesel engines without the aftertreatment system emit more of this harmful gas. ❖ An aftertreatment system is a device that reduces harmful exhaust emissions from internal combustion engines. It exists in both diesel and petrol devices. This system helps eliminate the excess combustion gas from the engine via the tailpipe. ❖ A typical diesel exhaust system consists of a particulate filter, a catalytic converter, and a muffler. ❖ Diesel engines use diesel particulate filters (DPF), while gasoline engines use gasoline particulate filters (GPF). Their key role is to capture and eliminate exhaust soot. ❖ A three-way catalyst uses chemical reactions to transform carbon monoxide into carbon dioxide. ❖ Finally, a muffler provides noise reduction. Gradual Phaseout: Gasoline & Diesel ❖ In 2015 48% of new cars sold in Europe were diesel. But Euro 6 emissions have been strangling diesel engines since 2015, making them increasingly more expensive to produce. And, since the new emission standard was introduced, diesel sales in Europe have been steadily declining. ❖ Britain will ban the sale of new petrol and diesel cars and vans from 2035. The scheme only affects the sale of new vehicles so used petrol and diesel cars will be unaffected and will still be available to buy and sell. ❖ California will ban the sale of new gasoline-powered passenger cars and trucks starting in 2035. The Canadian province of Quebec said this week it would ban the sale of new gasoline-powered passenger cars from 2035. ❖ German cities started to introduce bans on older diesel vehicles that emit higher amounts of pollutants than from late 2018. Gradual Phaseout: Gasoline & Diesel ❖ Norway, which relies heavily on oil and gas revenues, aims to become the world's first country to end the sale of fossil fuel-powered cars, setting a 2025 deadline. Fully electric vehicles now make up about 60% of monthly sales in Norway. ❖ The love affair with diesels came to a crashing end in late 2015, though, as the Volkswagen scandal that came to be known as Dieselgate brought new air quality concerns to the fore. The Dieselgate scandal raised the question whether diesel cars were as ‘clean’ as assumed. ❖ The Volkswagen Group were issued with a notice of violation of the Clean Air Act, after finding that VW had programmed TDI diesel engines to activate emissions controls only during lab tests. This allowed the cars to meet the standards, but they actually emitted up to 40 times more nitrogen oxides (NOx for short). YouTube Video: Gasoline vs Diesel Engine Comparison ❖ Useful Comparison Video: ❖ Driving 4 Answers o https://www.youtube.com/watch?v=aWeqyAxlM2M Main Geometrical & Operating Characteristics ICE Typical Values: Main Geometrical & Operating Parameters ❖ The main geometrical characteristics and operating parameters of reciprocating ICEs will be introduced in this section. Typical values of main geometrical and operating parameters for different internal combustion engine categories: Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ The rotation of the crank with radius r is converted through the crank mechanism and the connecting rod with length l into the reciprocating motion of the piston, which moves back and forth in a cylinder with bore b, which is sealed on top by the cylinder head. ❖ The minimum in-cylinder volume Vmin, or clearance volume Vc, is reached when the crank is forming with the cylinder axis an angle  equal to zero, corresponding to the so-called TDC position, Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ whereas the maximum in-cylinder volume Vmax is reached when the crank is forming with the cylinder axis an angle  equal to 180°, corresponding to the so- called BDC position. ❖ The distance covered by the piston between these two extreme positions is the stroke s, which is related to the crank radius r by Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ The volume swept by the piston when it moves from the TDC to the BDC defines the displacement V, which corresponds to the difference between the maximum volume Vmax and the clearance volume Vc and is related to bore b and stroke s by ❖ The ratio between the maximum volume Vmax and the minimum volume Vc is the compression ratio rc: Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ An important operating parameter is the mean piston speed: where N is the crankshaft rotational speed (revolutions per unit of time). ❖ All the abovementioned geometrical and operating parameters are typically falling within well-defined ranges, depending on the engine type and application, as shown in the Table. ❖ Cylinder bore for SI engines is typically limited to values lower than 100 mm, owing to the necessity to limit the flame path and to avoid abnormal combustion phenomena as knock. ❖ In CI engines, where, thanks to the different combustion process, the only limitations are set by thermomechanical stresses on valves (which are increasing with the valves— and cylinder—diameters), bore values can reach 1m in large marine or stationary 2S engines. Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ The stroke to bore ratio (s/b) is typically close to unity, with minimum values close to 0.5 in racing, high speed SI engines, and close to 4 in low speed marine or stationary 2S engines. ❖ As a matter of fact, the higher the s/b ratio is, the lower will be the surface to volume ratio of the combustion chamber (for a given displacement) and the higher will be the thermodynamic efficiency, thanks to the lower heat losses through the combustion chamber walls. ❖ On the other hand, the shorter the stroke is, the lower is the mean piston speed Sp, which in turn has to be limited as both resistance to gas flow into the cylinder and mechanical stresses on the crack mechanism due to the reciprocating parts increase proportionally to Sp. ❖ For these reasons, high speed SI engines, for which Sp is typically limited because of gas flow resistance to a maximum value of 25m/s, may adopt s/b ratios of 0.5, or even lower, so to increase the revolution speed to which engine power is proportional. Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ On the other hand, large marine or stationary 2S engines, where Sp is limited because of mechanical stresses to a maximum value of 10 m/s, may feature s/b ratios of 4 or even larger in order to lower the surface to volume ratio of the combustion chamber and increase the thermodynamic efficiency. ❖ The engine displacement typical values are obviously a consequence of the bore and stroke typical values and limitations. ❖ The number of cylinders is typically chosen on the basis of crankshaft balancing and instantaneous torque smoothness requirements; the higher the number of cylinders is, the better will be the balancing of the crankshaft in terms of centrifugal and reciprocating inertia forces, and the smoother and more uniform the instantaneous torque will be. ❖ Typical numbers of cylinders are therefore ranging from 2 to 12 for automotive applications; reaching up to 18 for stationary engines in order to increase the maximum engine power output, once the cylinder dimensions have reached upper limits. Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ Compression ratio values for SI engines are typically in the range between 8.5 and 12.5, the upper boundary being set by the need to limit end-of-compression charge temperature and pressure in order to avoid knock, although the increase of fuel conversion efficiency with compression ratio (about 3% per unit of compression ratio increase) motivates the choice of high rc values. ❖ As far as CI engines are concerned, rc values may range from 14 to 22, being the lower boundary set by the need to guarantee end-of-compression air temperature and pressure values suitable for fuel ignition, while the upper may generally be set by the need to limit peak pressure values and mechanical stresses on engine components. Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ Define Lambda Λ as the ratio between crank radius r and connecting rod length l: ❖ The typical design value adopted in reciprocating engines is 0.25, which means having a connecting rod length double than the stroke. ❖ High speed racing engines generally adopt shorter rod length, corresponding to Λ ratio of 1/3, whereas low speed large marine and stationary engines feature longer rod designs, with Λ ratios of 1/9. Displacement, Stroke/Bore Ratio, & Compression Ratio ❖ Longer rods lead to smaller angles 𝛽 between the rod and the cylinder axis, thus reducing the force exchanged between piston and liner, and therefore friction losses, with obvious benefits in terms of mechanical efficiency. ❖ On the other hand, longer rods increase the reciprocating and rotating masses, thus increases inertial loads on the crank mechanisms, and are therefore not suitable for high speed operation. Power, Torque, & Mean Effective Pressure ❖ The fundamental performance parameter of an ICE is the power P that the engine can deliver. The power is generally measured by means of a dynamometer (or brake), which is capable to absorb and dissipate the power delivered by the engine while allowing the measurement of the engine torque T and of the rotational speed 𝜔: ❖ The power measured in this way is generally called brake power Pb being the power that the engine is capable to deliver to the load, which can be, depending on the application, a driveline for vehicle propulsion, an electrical generator or, for the laboratory test described earlier, a dynamometer or a “brake.” ❖ The engine brake power Pb can also be expressed as a function of the brake work Wb per engine cycle, that is: Power, Torque, & Mean Effective Pressure ❖ Here z is the total number of cylinders, N the crankshaft rotational speed (revolutions per unit of time), and nR the number of engine revolutions per working cycle (nR =1 for 2S engines and nR =2 for 4S engines). ❖ The brake power Pb and the brake work Wb per cycle are lower than the power and work that are transferred from the gas to the piston, because of mechanical losses such as frictions in the crank mechanism and power necessary to drive ancillaries such as water pump, oil pump, fuel pump, camshaft, and alternator. ❖ The work done by the in-cylinder gas on the piston top surface Ap is called indicated work Wi and can be obtained from the measurement of the in-cylinder pressure versus piston displacement, by integrating the area enclosed on the p, V diagram as shown Power, Torque, & Mean Effective Pressure ❖ Dynamometers: Engine vs Chassis Power, Torque, & Mean Effective Pressure ❖ The corresponding indicated power Pi of the engine can hence be expressed as ❖ The difference between the indicated power Pi and the brake power Pb is the friction power Pf, which is the power necessary to drive the engine accessories and overcome mechanical friction in the crank mechanism: ❖ Correspondingly, for the work per cycle, the following relationship applies: Power, Torque, & Mean Effective Pressure ❖ However, it should be noticed that in a 4S naturally aspirated engine, the pressure during the intake stroke is lower than the pressure during the exhaust stroke, because of pressure losses in the intake and exhaust systems, respectively. ❖ Therefore, the work delivered to the piston during the intake and exhaust strokes (sum of areas B+C on the p−V diagram) is negative and is usually called pumping work. ❖ Consequently, two different definitions of the indicated work Wi can be given as follows: Power, Torque, & Mean Effective Pressure Indicated Work per Cycle ❖ This is the work transfer from the gas to the piston over the operating cycle of the engine. ❖ Indicated quantities are used primarily to identify the impact of compression, combustion, and expansion processes on engine performance. ❖ The indicated work is equal to the area enclosed by cycle on the p-V diagram. Power, Torque, & Mean Effective Pressure ❖ Net Wi, or simply Wi, which is the work delivered to the piston over the whole engine cycle, that is, over a 720 crank angle CA interval, corresponding to areas A−B. Power, Torque, & Mean Effective Pressure ❖ Gross Wi, which is the work delivered to the piston over compression and expansion strokes only, that is, over 360 CA, corresponding to areas A+C. ❖ It represents the sum of the useful work available at the shaft and the work required to overcome all the engine losses. Thus it can be estimated by: where Pb = brake power and Pf = friction power Power, Torque, & Mean Effective Pressure ❖ Recall from thermodynamics that on P-V diagrams, loops that go clockwise (area A) represent net (+) work; and those going counterclockwise (area B) represent (-) work. ❖ Note that in supercharged engines (and sometimes also in turbocharged engines), the pumping work can be positive because the boost pressure can be higher than the exhaust pressure. Power, Torque, & Mean Effective Pressure ❖ Torque and power are values useful to evaluate the performance of an engine in absolute terms, and are of paramount importance in matching the engine to its application, such as in matching an engine to a certain vehicle in automotive applications. ❖ On the contrary, torque and power are unfit to compare the performance of different engines, as the torque depends on the engine displacement and the power on the engine displacement and on the engine revolution speed. ❖ A more meaningful performance parameter is the so called mean effective pressure (MEP), which is given by the ratio between the work per engine cycle W and the engine displacement V. As it has units of force per unit area, it is therefore commonly referred to as a pressure. ❖ From ME 62: Power, Torque, & Mean Effective Pressure 𝑊𝑛𝑒𝑡 × 𝐸𝑛𝑔𝑖𝑛𝑒 𝑆𝑝𝑒𝑒𝑑 𝑊𝑛𝑒𝑡 𝑃𝑜𝑤𝑒𝑟 = × 𝑛𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟𝑠 𝑀𝐸𝑃 = 𝑅𝑒𝑣𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑠 /𝐶𝑦𝑐𝑙𝑒 (𝑛𝑟 ) 𝑉𝑑 ❖ All the indicated, pumping, friction, and brake cycle W described earlier can be related to the cylinder volume, thus obtaining, respectively: Power, Torque, & Mean Effective Pressure ❖ As with a previous derivation, the relationship follows: ❖ The maximum bmep level that can be attained in an ICE depends on the work per displacement, which in turn depends on fuel conversion efficiency and the quantity of fuel that can be burned at each engine cycle, and therefore on the amount of air that can be inducted. ❖ As a result, maximum bmep levels are well established for each engine category (see Summary Table) and can represent a reference when establishing the design target for a new project. ❖ Therefore, the engine displacement, which is necessary to obtain a certain power at a given rotational speed, can be estimated by means of a proper assumption of the bmep level that can be expected for that particular engine category. Power, Torque, & Mean Effective Pressure ❖ Mean Effective Pressure (MEP) – Used in conjunction with reciprocating engines; The AVERAGE PRESSURE that would act on the piston during the entire Power Stroke and produce the same amount of work output as the net work output for the actual cyclic process. Note V1 and V2 are held same. ❖ Theoretical Value: Measure of engine’s capacity to do work that is independent of its displacement. For the same displacement, engine with larger MEP indicates better performance (more torque) at the same speed. ❖ Shows how well an engine is using its displacement to produce work. See video below: o https://www.youtube.com/watch?v=bE_1JYrlYYU ❖ Example of MEP Range: NA Gas Engine: 800 - 1100 kPa FI (Turbo) Gas Engine: 1200 – 1700 kPa NA Diesel Engine: 700 – 900 kPa FI (Turbo) Diesel Engine: 1400 – 1800 kPa Power, Torque, & Mean Effective Pressure ❖ Finally, specific power figures can also be used to compare engines with different sizes; the engine-specific power can be related to either engine displacement or the piston unit area, the latter being proportional to the product between bmep and mean piston speed: ❖ Sample Computations: https://www.youtube.com/watch?v=wYWMi0O0UB4 ❖ 2021 Power List: https://www.roadandtrack.com/new-cars/g6482/10-cars-with-the-highest-specific-outputs/?slide=10 Road-Load Power ❖ This is the power required to drive a vehicle on a level road at steady speed. ❖ This power is used to overcome both Rolling Resistance and Aerodynamic Drag. ❖ An approximate formula for road load power is: where: CR = coefficient of rolling resistance ( 0.012  CR  0.015 ) Mv = mass of vehicle (for passenger cars: curb mass + passenger load of 68 kg) g = acceleration due to gravity a = ambient air density CD = drag coefficient (for cars: 0.3  CD  0.5 ) Av = frontal area of vehicle Sv = vehicle speed Engine Performance Metrics Materials from Dr. E. Quiros Engine Design & Performance Data ❖ Engine ratings usually indicate maximum power, maximum torque, and the speed at which they are achieved. ❖ Torque and speed depend on engine displacement making it difficult to compare engines of different displacements. ❖ For comparing engines of different displacements in a given engine category normalized performance parameters at specified engine operating conditions are more useful. Engine Design & Performance Data 1) At Maximum or Normal Rated Point: a) Mean Piston Speed (Sp) - Measures comparative success in handling loads due to inertia of parts and/or engine friction b) Brake Mean Effective Pressure (bmep) o For naturally aspirated engines, it reflects the product of volumetric efficiency (ability to induct air), fuel/air ratio (effectiveness of air utilization in combustion), and fuel conversion efficiency. o For supercharged engines, it indicates the degree of success in handling higher gas pressures and thermal loading. c) Power per Unit Piston Area - Measures effectiveness with which piston area is used, regardless of cylinder size. d) Specific Weight - Indicates relative economy with which materials are used. e) Specific Volume - Indicates relative effectiveness with which engine space has been utilized. Engine Design & Performance Data 2) At All Engine Speeds running at full throttle/maximum fuel pump setting; o Brake Mean Effective Pressure (bmep) Measures ability to obtain/provide high air flow and use it effectively over the full range. 3) At All Useful Regimes of Operation and particularly in those regimes where the engine is run for long periods of time; a) Brake Specific Fuel Consumption (bsfc), or Fuel Conversion Efficiency (ηf) b) Brake Specific Emissions Specific Fuel Consumption (sfc) ❖ The amount of fuel consumed by the engine per unit power output. A measure of how efficiently the engine is using the fuel supplied to produce work: ❖ Conversion: ❖ Low values of specific fuel consumption are desirable. ❖ Typical Values: SI: bsfc ≈ 75 μg/J = 270 g/kW-hr (0.47 lbm/hp-hr) CI: bsfc ≈ ≤ 55 μg/J = 200 g/kW-hr (0.32 lbm/hp-hr, large engines) Fuel Conversion Efficiency ❖ The ratio of the work produced per cycle to the amount of fuel energy supplied per cycle: ❖ Units & Conversion: ❖ Typical Heating Values (hydrocarbon fuels): 42 - 44 MJ/kg (18,000 -19,000 Btu/lbm) ❖ Note that in an actual operating engine, the fuel energy supplied by the fuel QHV may not be fully released as thermal energy due to incomplete combustion. Air/Fuel & Fuel/Air Ratios ❖ The air/fuel ratio is the ratio of the mass of air to the mass of fuel supplied to the engine. ❖ Typical values for normal operating ranges: o SI: 12 ≤ A/F ≤ 18 ( 0.056 ≤ F/A ≤ 0.083 ) o CI: 18 ≤ A/F ≤ 70 ( 0.014 ≤ F/A ≤ 0.056 ) Volumetric Efficiency ❖ The volume flow rate of air into the intake system divided by the cylinder volume displacement rate. Measures the effectiveness of the engine’s induction process. Only used with 4-stroke cycle engines which have a distinct induction process. where ρa,i = inlet air density ** 𝑚ሶ 𝑎 = mass flow rate of air inducted into the cylinder ma = mass of air inducted into the cylinder per cycle ** If equal to atmosphere air density, then ηv measures pumping performance of entire intake system. If equal to air density in the inlet manifold, then ηv measures pumping performance of the inlet port and valves only. ❖ Typical values: 80 to 90% for Normally Aspirated (NA) engines. CI engines have higher volumetric efficiencies than SI engines. Engine Specific Weight & Specific Volume ❖ These parameters indicate the effectiveness with which the engine designer has used the engine materials and packaged the engine components. Specific Emissions & Emissions Index ❖ Levels of emissions of NOx, CO, unburned hydrocarbons HC, and particulates are important engine operating characteristics. ❖ Concentrations of gaseous emissions in the exhaust gases are usually measured in o parts per million (ppm), equivalent to the mole fraction x 106 o percent by volume (% vol), equivalent to the mole fraction x 102 ❖ Two commonly used indicators of engine emissions levels are: 1) Specific Emissions: mass flow rate of pollutant per unit power; typical units are μg/J, g/kW-hr, and g/hp-hr 2) Emission Index: mass flow rate of pollutant divided by the fuel flow rate; typical units are μg/J, g/kW-hr, and g/hp-hr) Specific Emissions vs Emissions Index mNOx mNOx ( g / s) sNOx = EI NOx = P mf ( kg / s ) mCO mCO ( g / s ) sCO = EI CO = P m f ( kg / s ) mHC mHC ( g / s ) sHC = EI HC = P m f ( kg / s ) m part m part ( g / s) sPart = EI part = P mf ( kg / s ) Relationships of Performance Parameters Materials from Dr. E. Quiros Relationships Between Performance Parameters ❖ The importance of the various engine design and operating parameters on engine performance is shown by expressing power, torque, and mean effective pressure in terms of these parameters. o Power  f ma N QHV ( F / A) P = nR ❖ For 4-Stroke cycle engines, ηv can be included so that:  f v N Vd QHV a ,i ( F / A) P = 2 o Torque  f v Vd QHV a ,i ( F / A) T = 4 Relationships Between Performance Parameters o Mean Effective Pressure mep =  f v QHV a ,i ( F / A) o Specific Power Defined as power per unit piston area, is a measure of the engine designer’s success in using the available piston area regardless of cylinder size. P  f v N L QHV  a ,i ( F / A) = Ap 2 If the mean piston speed is included P  f v S p QHV a ,i ( F / A) = Ap 4 Relationships Between Performance Parameters ❖ These relationships illustrate the direct importance to engine performance of the following: 1) High fuel conversion efficiency, ηf 2) High volumetric efficiency, ηv 3) Increasing the output of a given displacement engine by increasing the inlet air density (e.g., by turbo- or supercharging) 4) Maximum fuel/air (F/A) ratio that can be usefully burned in the engine 5) High mean piston speed Engine Efficiency Analysis Thermodynamic Analysis ❖ The fuel conversion efficiency defined earlier is a measure of the global effectiveness of an engine to transform the chemical energy of the fuel in mechanical energy to the shaft. As this comprehensive term includes the effects of different causes of losses, in order to more clearly identify actions that can be taken to minimize these losses, it may be convenient to split the fuel conversion efficiency into different factors, in order to group losses in homogeneous clusters. ❖ The fuel conversion efficiency 𝜂f can then be split into the product of a mechanical efficiency 𝜂m (that considers the mechanical losses and equals the ratio between the brake work Wb and the indicated work Wi), and of an indicated efficiency 𝜂i (that comprises all the other losses in the conversion process from the fuel energy Ef to the indicated work Wi made by the gases on the piston). Thermodynamic Analysis: Carnot Cycle vs Actual ICE Conditions ❖ Considering a thermodynamic cycle in which an ideal fluid (such as air with constant thermodynamic properties) undergoes state changes by transferring heat and performing work according to ideal thermodynamic transformations, the effective maximum value of the energy conversion efficiency that could be reached in an ideal case is determined. ❖ Although the Carnot cycle offers the maximum possible energy conversion efficiency for any thermodynamic cycle taking place between a lower (Tmin) and an upper (Tmax) temperature limit, it is not suitable as a reference ideal cycle for ICEs processes. ❖ In order to identify a suitable ideal reference cycle, the Carnot cycle should be modified by considering a different heat supply phase, according to the characteristics of the combustion process that is occurring in the ICE. ❖ Consequently, the ideal reference cycles will be the Otto cycle for SI engines, and the Diesel cycle for CI engines, as shown in the following figure. Thermodynamic Analysis: ICE Cycle Representation ❖ The sequence of the thermodynamic transformations for the Otto ideal cycle will therefore be 1–2 isentropic compression, 2–3 constant volume heat supply, 3-4 isentropic expansion, and 4–1 constant volume heat removal. ❖ The sequence of the thermodynamic transformations for the Diesel ideal cycle will therefore be 1–2 isentropic compression, 2–3 constant pressure heat supply, 3-4 isentropic expansion, and 4–1 constant volume heat removal. Thermodynamic Analysis: Cycle Efficiencies ❖ From ME 62, the following efficiencies for the Otto and Diesel cycles were derived as: where 𝜏 is the ratio T3/T2. ❖ The efficiencies of the Otto and Diesel ideal cycles both increase with the compression ratio rc as shown in the figure; although for a given rc ratio Diesel cycle efficiency is lower than Otto cycle efficiency, the higher compression ratio that is commonly used in CI engines may reverse the result of the comparison. Thermodynamic Analysis: Otto vs Diesel Cycle Efficiencies ❖ As an example, for the Otto cycle with rc = 10, 𝛾 = 1.4, the ideal efficiency 𝜂id is about 60%, whereas for the Diesel cycle with rc = 18 and 𝜏 =2, the efficiency is about 63%. ICE Efficiency: Breakdown of Energy Losses ❖ The other losses that are introduced in the real operation of an ICE will now be discussed: Losses Due To Real Fluid Properties ❖ The real working fluid (air+fuel) has a different behavior in comparison with the ideal gas. In particular, specific heat values (cp and cv) increase as temperature increases, leading to lower values of their ratio 𝛾. ❖ Moreover, CO2 dissociation may occur during the combustion reactions, thus reducing the amount of chemical energy of the reactants that can be converted into heat. ❖ Note that this cycle is performed with air and fuel, and is no longer a closed thermodynamic cycle, because modifications in the chemical structure of the working fluid happen, due to the combustion process. The working fluid therefore has to be replaced periodically before the beginning of a new cycle. ICE Efficiency: Breakdown of Energy Losses Thermo-Fluid-Dynamic Losses ❖ When trying to operate an ICE according to its corresponding “reference” ideal cycle, several additional thermofluid-dynamic losses will occur, making the indicated work Wi remarkably lower (usually by about 15–20%). ❖ The thermo-fluid-dynamic losses are primarily due to four types of processes: heat transfer from the cylinder charge to the walls, combustion losses due to its development in a finite time and to its incompleteness, pumping losses during the gas exchange process, and flow leakages (blow-by flow through piston rings). ❖ Heat Transfer to Walls - Although the thermal power that is lost into the engine cooling system is a remarkable fraction of the fuel power input and is comparable with the fraction converted into brake power, it should be pointed out that the complete elimination of heat losses—although impossible to be achieved in practice—could allow only a fraction of the heat transferred to the walls to be converted in useful work, while producing a substantial increase in the exhaust gas enthalpy. ICE Efficiency: Breakdown of Energy Losses ❖ The heat transfer losses are higher at low engine speed because of the longer amount of time available for heat exchange. ❖ Combustion Losses - Even if a stoichiometric or larger than stoichiometric amount of air is used, combustion may still be incomplete, and exhaust gases may contain partially unburned species, such as CO (carbon monoxide). However, combustion inefficiency is generally small (1–2%) and its impact is to be considered mainly for pollutant emissions [such as CO, HC, and PM (particulate matter)] rather than for engine efficiency concerns. ❖ Differently from reference cycles (where heat addition to the operating fluid may be instantaneous, as for the Otto cycle), the combustion process requires a remarkable amount of time to be completed (generally about 60 CA degrees). This causes significant losses, because the fuel energy cannot be converted instantaneously into thermal energy at TDC, thus leading to a lower pressure peak and to a lower expansion work. ICE Efficiency: Breakdown of Energy Losses ❖ Spark advance in SI engines and fuel injection in CI engines are therefore usually advanced before TDC, in order to reduce these losses as much as possible, by placing the combustion heat release around TDC. ❖ Finally, it should be pointed out that in SI engines, the flame speed is extremely sensitive to the air/fuel ratio: for 𝜆 larger than 1.3, the combustion losses will grow so rapidly to make engine operation with leaner mixture impossible. For this reason, the use of a throttling device at the intake of SI engines is necessary for load control, thus causing additional pumping penalties at part load. ❖ Gas Exchange or Pumping Losses - In 4S naturally aspirated engines, the pressure during the intake stroke is lower than the environmental pressure, while the opposite happens during the exhaust stroke, because of pressure losses in the intake and exhaust systems, respectively; therefore, the total work delivered to the piston during the intake and exhaust strokes is negative and is usually called pumping work. ICE Efficiency: Breakdown of Energy Losses ❖ At part load operation, pumping losses may grow remarkably in throttled SI engines. Innovative approaches such as charge stratification in DI engines or variable valve actuation (VVA) try to solve this issue, which represents the Achilles’ heel of the SI engine. ❖ Leakages and Crevice Effects - As cylinder pressure increases, part of the operating fluid flows into crevices (such as the ring pack region), and may eventually reach the crankcase. Although in a well-maintained engine this leakage flow (also known as blow- by) is quite small (usually about 1% of in-cylinder mass), it causes a decrease of the in- cylinder pressure primarily during combustion and expansion, thus reducing the indicated work. Mechanical Losses ❖ Besides the losses due to the real properties of the working fluid and to the thermo- fluid-dynamic effects, a last class of losses has finally to be considered, the mechanical losses, which affect the mechanical efficiency 𝜂m. ICE Efficiency: Breakdown of Energy Losses ❖ Mechanical losses are due to friction between piston and liner and between other components of the crank mechanism and to the power that is requested to drive the engine accessories (oil and water pump, valvetrain, fuel pump, and alternator). ❖ As mechanical losses are often measured through motoring tests (i.e., by driving the engine by means of a dynamometer without fueling it), pumping losses can be included within mechanical losses instead of being considered among fluid-dynamic losses. Therefore, when fmep is measured through a motoring test including pumping losses, the term tfmep (total friction mean effective pressure) is often preferred to avoid ambiguity. (Motoring Setup https://www.youtube.com/watch?v=yessM4UfaqM). Both fmep and tfmep typically increase with engine revolution speed, thus leading to a significant mechanical efficiency decrease in the high speed range. ❖ At constant engine speed, the mechanical efficiency decreases as the engine load decreases, because the power requested to drive the accessories remains constant, while the indicated power decreases: the mechanical efficiency eventually reaches a zero value when the engine is running at idle. ICE Efficiency : Improvement Technologies @ Part Load ❖ As for automotive engines part load operation represents the most frequent operating condition, innovative technologies are currently being developed to mitigate the efficiency decrease at part load, especially for SI engines, which suffer from increased pumping losses at part load owing to throttling. Some of these design improvements include: o Charge Stratification o Variable Valve Actuation o Cylinder Deactivation o Downsizing ❖ A brief introduction to these technologies will be presented in the following section. ICE Efficiency Improvements: Charge Stratification ❖ Spark Ignition Direct Injection engines may operate at part load with a stratified, globally lean charge, thus allowing unthrottled operation and pumping losses decrease. ❖ The fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. ❖ A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines. ❖ However, specific aftertreatment devices are needed to control NOx emissions, as conventional three-way catalysts cannot operate with lean mixtures, and also PM emissions can represent an issue for these engines. ICE Efficiency Improvements: Charge Stratification ❖ In a normal homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. Any attempt to improve fuel economy by running a much leaner mixture (less fuel or more air) with a homogeneous charge results in slower combustion and a higher engine temperature; this impacts on power and emissions, notably increasing nitrogen oxides or NOx. ❖ A stratified charge engine creates a richer mixture of fuel near the spark and a leaner mixture throughout the rest of the combustion chamber. The rich mixture ignites easily and in turn ignites the lean mixture throughout the rest of the chamber; ultimately allowing the engine to use a leaner mixture thus improving efficiency while ensuring complete combustion. ICE Efficiency Improvements: Cha