Internal Combustion Engines 2025-2024 PDF

Introduction Heat Engines Any type of engine or machine which derives heat energy from the combustion of fuel and converts this energy into mechanical work is termed as a heat engine.  Heat engines may be conveniently grouped into two principal classes as follows: 1. External Combustion...

Introduction Heat Engines Any type of engine or machine which derives heat energy from the combustion of fuel and converts this energy into mechanical work is termed as a heat engine.  Heat engines may be conveniently grouped into two principal classes as follows: 1. External Combustion Engines. In this case, combustion of fuel takes place outside the cylinder, as in case of steam engines where the heat of combustion is employed to generate steam which is used to actuate a piston in a cylinder. Other examples of external combustion engines are steam turbine and gas turbine.  These are generally used for driving locomotives, ships, generation of electric power etc. Steam engine An open cycle gas-turbine 2. Internal Combustion Engines. In this case combustion of the fuel with oxygen of the air occurs within the cylinder of the engine. The internal combustion engine group includes:  engines employing mixtures of combustible gases and air, known as gas engines. those using lighter liquid fuels, known as petrol engines  and those using heavier liquid fuels, known as oil, compression ignition engines or diesel engines. These are generally used for road vehicles, aircraft, locomotives and several industrial applications. Petrol engine   External combustion engines offer following advantages over internal combustion engines: (1) Cheaper fuels can be used due to external combustion of fuel. Even solid fuels can be used. (2) High starting torque. (3) Self-starting with the working fluid whereas in case of internal combustion engines, some additional equipment or device is used for starting the engine. (4) Flexibility in arrangement is possible due to external combustion of fuel.  Reciprocating internal combustion engines offer certain advantages over external combustion engines: (1) Higher overall efficiency. (2) Greater mechanical simplicity. (3) Easy starting from cold conditions. (4) Lower weight to power ratio. (5) Lower initial cost. (6) Compact units, therefore require less space. Principles of Internal Combustion Engines Conventional internal combustion engines have one or more cylinders in which combustion of the fuel takes place. A cross-section of an engine cylinder with the principal parts labeled is shown in the Fig. Engine components Video: https://www.youtube.com/watch?v=Y32gDgLq6hE&ab_channel=H ydraulicandpneumaticsystems https://www.youtube.com/watch?v=JhEX94yP04k&ab_channel=TH ORSeLearningSolutions Cylinder: It is a cylindrical shaped container within which a piston travels in a reciprocating linear motion.  It should have high strength to withstand high pressure above 50 bar and temperature above 2000oC.  The ordinary engine is made of cast iron.  heavy duty engines are made of steel alloys or aluminum alloys.  It is in direct contact with the products of combustion so it must be cooled. For cooling of cylinder a water jacket (for liquid cooling used in most of cars) or fin (for air cooling) are situated at the outer side of cylinder. Cylinder block: The cylinder is supported in position in the cylinder block, attached to, or an integral part of the crankcase. Cylinder head: Top end of the cylinder is enclosed by the cylinder head. The cylinder head generally bolted to the cylinder block. A copper or asbestos gasket is provided between the engine cylinder and cylinder head to make an airtight joint. Inlet and exhaust valves: These valves are incorporated in the cylinder head. The number of valves in an engine depends on the number of cylinders as two valves are used for each cylinder.   Inlet valve is meant for admitting air or mixture of fuel and air to the engine cylinder.  Exhaust valve is meant for discharging the products of combustion at appropriate time. The valves are normally kept closed by means of cams geared to the engine shaft. The valves are fitted in the port at the cylinder head by use of strong spring. This spring keep them closed. Both valves usually open inwards. Piston: Transmit the force exerted by the burning of charge to the connecting rod.  It is usually made of aluminum alloy which has good heat conducting property and greater strength at higher temperatures. Ports and manifolds: The passages in the cylinder head leading to the valves are called ports.  The system of pipes which connects the inlet ports of the various cylinders to a common, air or air-fuel intake for the engine is called the inlet manifold.  Thus, a system connecting exhaust ports to a common exhaust pipe is known as exhaust manifold. Connecting rod: Connecting rod connects the piston to crankshaft and transmits the motion and thrust of piston to crankshaft.   It converts reciprocating motion of the piston into circular motion of the crank shaft in the working stroke.  There are two end of connecting rods, one is known as big end and other as small end.  The smaller end of the connecting rod relates to the piston by gudgeon pin or piston pin and bigger end of the connecting rod is connected with the crank with crank pin.  Special steel alloys or aluminum alloys are used for the manufacture of connecting rod. Crankshaft: The crankshaft is the principal rotating member of the engine.  It converts the reciprocating motion of the piston into the rotary motion with the help of connecting rod. The crankshaft mounts in bearing so it can rotate freely.  The shape and size of crankshaft depends on the number and arrangement of cylinders.  It is usually made by steel forging. An extension of this shaft is usually the part through which the external work of the engine is done. Crankcase: The main body of the engine to which the cylinders are attached, and which contains the crankshaft and crankshaft bearings is called the crankcase.   This member also holds other parts in alignment and resists the explosion and inertia forces.  It also protects the parts from dirt etc.  serves as a part of lubricating system and sometime it is called oil sump. All the oil for lubrication is placed in it. Flywheel: It is a big wheel mounted on the crankshaft, whose function is to maintain its speed constant.  It is done by storing excess energy during the power stroke, which is returned during another stroke.  A certain standard terminology concerning volumes and measurements in the cylinder region are presented and shown in the Fig. Basic geometry of reciprocating I.C.E Cylinder bore (D): The inside diameter of the cylinder is called bore and is measured in centimeters or millimeters. Piston area (A): The area of circle of diameter equal to the cylinder bore. Stroke (L): As piston reciprocates inside the engine cylinder, it has got limiting upper and lower positions beyond which it cannot move, and reversal of motion takes place at these limiting positions. The linear distance along the cylinder axis between these two limiting positions is called stroke. Top dead centre (TDC): The position of the cylinder, when it is closest to the top of the cylinder, is called top dead centre. In the case of horizontal engines this is known as inner dead centre. Bottom dead centre (BDC): The position of the piston, when it is farthest from the top of the cylinder, is called bottom dead centre. In case of horizontal engines this is called outer dead centre. Clearance volume (Vc): The volume confined in the cylinder above the top of the piston, when the piston is at top dead centre, is called the clearance volume and is usually measured in cubic centimeters or liters. Displacement volume or Swept volume (Vd): The volume swept through by the piston in moving between top dead centre and bottom dead centre, is called the piston displacement. Cylinder volume (Vt): Total volume of the cylinder when piston is at the bottom dead centre. Total cylinder volume = Swept Volume + Clearance volume Compression ratio (r): The ratio of volume when the piston is at the bottom dead centre (i.e. cylinder volume) to the volume when the piston is at top lead centre (i.e. clearance volume) is called the compression ratio. r = Vt / V c r = (Vd + Vc) / Vc Square engine: An engine that has equal, or nearly equal, bore and stroke dimensions.   Square engines are commonly used in spark-ignition (SI) passenger cars. Under-square engine: An engine with a stroke dimension that is longer than the bore dimension, also known as a long-stroke engine.  These engines are used to perform heavy hauling and vocational work, such as diesel or SI engines used in pick-up trucks. Over-square engine: An engine with a stroke that is shorter than the bore diameter.  Over-square engines are best suited to high-speed operation. Stroke ratios in over-square engines are better adapted to use in high-speed applications where engines need to develop the highest torque at high speed.  Over-square engines, also called short-stroke engines, are commonly used by cars with SI systems. Engine Classifications There are many different types of internal combustion engines. They can be classified by: 1. Application. Internal combustion engines (ICE) are the most common form of heat engines, as they are used in vehicles, boats, ships, airplanes, and trains. 2. Basic Engine Design. Engine has one or more cylinders in which pistons reciprocate back and forth. The combustion chamber is located in the closed end of each cylinder shown in the Fig. Various piston cylinder geometries (a) Single Cylinder. Engine has one cylinder and piston connected to the crankshaft. (b) In-Line. Cylinders are positioned in a straight line, one behind the other along the length of the crankshaft. They can consist of 2 to 11 cylinders or possibly more. In-line four-cylinder engines are very common for automobile and other applications. (c) V Engine. Two banks of cylinders at an angle with each other along a single crankshaft. The angle between the banks of cylinders can be anywhere from 15° to 120°, with 60°-90° being common. V engines have even numbers of cylinders from 2 to 20 or more. V6s and V8s are common automobile engines. (d) Horizontally Opposed Cylinder Engine. Two banks of cylinders opposite each other on a single crankshaft (a V engine with a 180°). These are common on small aircraft and some automobiles with an even number of cylinders from two to eight or more. These engines are often called flat engines (e) W Engine. Same as a V engine except with three banks of cylinders on the same crankshaft. Not common, but some have been developed for racing automobiles. Usually, 12 cylinders with about a 60° angle between each bank. (f) Opposed Piston Engine. Two pistons in each cylinder with the combustion chamber in the center between the pistons. A single- combustion process causes two power strokes at the same time, with each piston being pushed away from the center and delivering power to a separate crankshaft at each end of the cylinder. (g) Radial Engine. Engine with pistons positioned in a circular plane around the central crankshaft. The connecting rods of the pistons are connected to a master rod which, in turn, is connected to the crankshaft. A bank of cylinders on a radial engine always has an odd number of cylinders ranging from 3 to 13 or more. Operating on a four-stroke cycle, every other cylinder fires and has a power stroke as the crankshaft rotates, giving a smooth operation. 3. Working Cycle. Four-stroke cycle or two-stroke cycle: (a) Naturally aspirated (admitting atmospheric air). No intake air pressure boost system. (b) Supercharged (admitting recompressed fresh mixture). Intake air pressure increased with the compressor driven off of the engine crankshaft (c) Turbocharged (admitting recompressed fresh mixture). Intake air pressure increased with the turbine-compressor driven by the engine exhaust gases.  Turbine-compressor driven by the engine exhaust gases 4. Valve location. (a) Valve in block, L head. Older automobiles and some small engines. (b) Valve in head, I head. Standard on modern automobiles. (c) One valve in head and one valve in block, F head. Less common automobiles. (d) Valves in block on opposite sides of cylinder, T head.  1.7 valve locations. 5. Fuel Used. (a) Gasoline (or petrol), (b) Fuel oil (or diesel fuel), (c) Natural gas, (d) Liquid petroleum gas, (e) Alcohols (methanol, ethanol), (f) Hydrogen 6. Method of Mixture Preparation. (a) Carburetor. (b) Fuel injection into the intake ports or intake manifold. (c) Fuel injection into the engine cylinder. Gasoline Fuel Injection Diesel Injection Systems 7. Method of Ignition. (a) Spark Ignition (SI). An SI engine starts the combustion process in each cycle by use of a spark plug. The spark plug gives a high-voltage electrical discharge between two electrodes which ignites the air-fuel mixture in the combustion chamber surrounding the plug. (b) Compression Ignition (CI). The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high temperature in the combustion chamber caused by high compression. 8. Combustion Chamber Design. The design of combustion chamber has an important influence upon the engine performance and its knock properties. The design of combustion chamber involves the shape of the combustion chamber, the location of the sparking plug and the position of inlet and exhaust valves. (a) Combustion chambers for spark ignition engines.  Combustion chambers for spark ignition engines (b) Combustion chambers for diesel engines. In S.I engine mixing takes place in carburetor; however in C.I engines this has to be done in the combustion chamber. To achieve this requirement in a short period is an extremely difficult job.  Combustion chambers for diesel engines 9. Method of Load Control. (a) Throttling of fuel and air flow together so mixture composition is essentially unchanged. Since the fuel flow is metered in proportion to the air flow, the throttle valve in an Otto cycle, in essence, controls the power. (b) Control of fuel flow alone. The inlet air in the diesel engine is unthrottled. The power is controlled by the amount of fuel injected into the cylinder. 10. Method of Cooling. (a) Water cooled. (b) Air cooled. Video: How a Car Engine : https://www.youtube.com/watch?v=ZQvfHyfgBtA&ab_channel= Animagraffs Engine Cycles There are two major cycles used in internal combustion engines: Otto and Diesel. The Otto cycle is named after Nikolaus Otto (1832- 1891) who developed a four-stroke engine in 1876. It is also called a spark ignition (SI) engine, since a spark is needed to ignite the fuel-air mixture. Otto is considered the inventor of the modern internal combustion engine, and the founder of the internal combustion engine industry. Otto Cycle A four-stroke spark ignition engine  As shown in the Fig., the four-stroke Otto cycle has the following sequence of operations: 1. An intake stroke that draws a combustible mixture of fuel and air past the throttle and the intake valve into the cylinder. 2. A compression stroke with the valves closed which raises the temperature of the mixture. A spark ignites the mixture toward the end of the compression stroke. 3. A power stroke resulting from combustion of the fuel-air mixture. 4. An exhaust stroke that pushes out the burned gases past the exhaust valve.  Air enters the engine through the intake manifold, a bundle of passages to distribute the air mixture to individual cylinders. The fuel, typically gasoline, is mixed using a fuel injector or carburetor with the inlet air in the intake manifold, intake port, or directly injected into the cylinder, resulting in the cylinder filling with a homogeneous mixture.  When the mixture is ignited by a spark, a turbulent flame develops and propagates through the mixture, raising the cylinder temperature and pressure. The flame is extinguished when it reaches the cylinder walls.  If the initial pressure is too high, the compressed gases ahead of the flame will auto-ignite, causing a problem called knock. Diesel Cycle The Diesel cycle is named after Rudolph Diesel (1858-1913) who in 1897 developed an engine designed for the direct injection of liquid fuel into the combustion chamber. The Diesel cycle engine is also called a compression ignition (CI) engine, since the fuel will auto-ignite when injected into the combustion chamber. The four-stroke Diesel cycle (see the Fig.) has the following sequence: 1. An intake stroke that draws inlet air past the intake valve into the cylinder. 2. A compression stroke that raises the air temperature above the auto- ignition temperature of the fuel. Diesel fuel is sprayed into the cylinder near the end of the compression stroke. 3. Evaporation, mixing, ignition, and combustion of the diesel fuel during the later stages of the compression stroke and the first part of the expansion stroke. 4. An exhaust stroke that pushes out the burned gases past the exhaust valve. Note:  The inlet air in the diesel engine is unthrottled.  The power is controlled by the amount of fuel injected into the cylinder.  In order to ignite the fuel-air mixture, diesel engines are required to operate at a higher compression ratio, resulting in a higher theoretical efficiency compared to spark ignition (SI) engines.  Diesel engine performance is limited by the formation of smoke, which forms if there is inadequate mixing of the fuel and air. Basic Differences Between Spark Ignition and Compression Ignition Engines While spark ignition and compression ignition engines have much in common, there are certain basic differences that causes their operation to vary considerably, (i) Compression ratio. Spark ignition engines usually operate at lower compression ratios as compared to those for compression ignition engines, (ii) Ignition of fuel. In case of spark ignition engines fuel is ignited by means of electric spark generated inside the engine cylinder by a spark plug. In case of compression ignition engines, fuel is ignited due to high temperature of air at the end of compression stroke. (iii) Method of introduction of fuel. In case of spark ignition engines, fuel is evaporated, mixed with air and then supplied to engine cylinder. On the other hand, in case of compression ignition engines, fuel is injected inside the engine cylinder in the form of a fine spray. Compression ratio (r) Video, The Differences Between Petrol and Diesel Engines: https://www.youtube.com/watch?v=rlK7JIAz9WY&ab_channel=Ca rThrottle Two-Stroke Cycle As the name implies, two-stroke engines need only two strokes of the piston or one revolution to complete a cycle.  There is a power stroke every revolution instead of every two revolutions as for four-stroke engines.  Two-stroke engines are mechanically simpler than four-stroke engines, and have a higher power to weight ratio. Video: https://www.youtube.com/watch?v=kWRRHRWuduk&ab_channel=Raf faeleSigismondi https://www.youtube.com/watch?v=Ve-4rJvU- Ow&pp=ygUQVHdvLVN0cm9rZSBDeWNsZQ%3D%3D Principles of a two-stroke cycle operation:  During compression of two-stroke cycle, a sub-atmospheric pressure is created in the crankcase.  A reed valve opens and letting air rush into the crankcase.  Once the piston reverses direction during combustion and expansion begins,  The air in the crankcase closes the reed valve so that the air is compressed.  As the piston travels, it uncovers exhaust ports and exhaust gases begin to leave, rapidly dropping the cylinder pressure to that of the atmosphere.  Then the intake ports are opened and compressed air from the crankcase flows into the cylinder pushing out the remaining exhaust gases.  This pushing out of exhaust by the incoming air is called scavenging.  Cycle of operation in two stroke engine  With two-stroke engines, the scavenging is not perfect as some of the air will go straight through the cylinder and out the exhaust port, a process called short circuiting. Some of the air will also mix with exhaust gases and the remaining incoming air will push out a portion of this mixture.  If the engine is a carbureted gasoline engine, there is a fuel-air mixture in the crankcase. Some of this fuel-air mixture will short  circuit and appear in the exhaust, wasting fuel and increasing the hydrocarbon emissions.  Carbureted two-stroke engines are used where efficiency is not of primary concern and advantage can be taken of the engine's simplicity; this translates into lower cost and higher power per unit weight. Familiar examples include motorcycles, chain saws, outboard motors, and model airplane engines.  With a two-stroke diesel or fuel injected gasoline engine, loss of fuel is not a problem since air only is used for scavenging. Comparison of Two Stroke and Four Stroke Engines Two stroke engines offer the following advantages:  For the same cylinder dimensions and other working conditions two stroke engines develop almost twice the power developed by four stroke engines. The reason is obvious. In case of four stroke engines there is one power stroke during every two revolutions of the crankshaft whereas in case of two stroke engines there is one power stroke during each revolution of the crankshaft.  Torque on the crankshaft is more uniform in case of two stroke engines as compared to four stroke engines.   For the same power development, two stroke engines are compact and occupy less space.  Generally valve operation of four stroke engines involves complicated mechanisms. This can be avoided in case of two stroke engines.  The initial cost of a two stroke cycle engine is considerably less than a four stroke cycle engine  As suction and exhaust strokes are eliminated in case of two stroke engines, the work required to overcome friction during these strokes is saved. Disadvantages:  Thermal efficiency of a two stroke cycle engine is less than that a four stroke cycle engine.  Scavenging problems.  The consumption of lubricating oil is large in a two stroke cycle engine because of high operating temperature.  A two-stroke engine produces a lot of pollution. Air Standard Cycles Although internal combustion engine does not operate on a thermodynamic cycle, as combustion process is not reversible and the working substance does not go through a cycle, still the concept of theoretical cycles may be useful to show the effect of changing operating conditions on the performance. Air Standard Cycle The simplest theoretical cycle for the analysis of internal combustion engine process is known as air standard cycle. In an air standard cycle the following simplifying assumptions are made:  The gas in the engine cylinder is a perfect gas; i.e. it obeys the gas laws.  The cycle is assumed to operate as a closed cycle.  The physical constants of the gas in the cylinder are the same as those of air at moderate temperature.  The compression and expansion processes are adiabatic- no heat is gained or lost during the process.  The operation of the engine is frictionless.  The specific heats of working medium remain constant within the operating range.  The working medium does not undergo any chemical change during the cycle.   Heat is supplied and rejected in a reversible manner and also heat is supplied and rejected instantaneously. Air standard cycle is used as it provides a means for: (a) Determining the maximum ideal efficiency of a specified thermal cycle, and (b) Evaluating the effect of modification of one or more of the parameters making up the cycle on the maximum efficiency. The most common air standard cycles used in connection with internal combustion engines are: (1) Otto Cycle: This cycle is also known as constant volume cycle or explosion cycle. (2) Diesel Cycle: This cycle is also known as constant pressure cycle. (3) Dual Cycle: This cycle is also known as constant volume and pressure cycle. Thermal Efficiency of a Cycle In the performance of its cycle, an engine takes in a certain quantity of heat during its operation. Part of this heat is converted into useful work, whilst the remainder is rejected during the completion of the full cycle. The amount of work done, on the piston, during the cycle must, therefore, be the difference between the heat taken in and the heat rejected. Thus,  Work done during cycle = Heat supplied - Heat rejected.   Efficiency of cycle = Work done / Heat supplied = (Heat supplied - Heat rejected) / Heat supplied  This is the theoretical efficiency of a cycle and is known as the theoretical thermal efficiency. In air-standard cycles, air is considered an ideal gas such that the following ideal gas relationships can be used: Constant Volume Heat Addition This cycle is often referred to as the Otto cycle and considers the special case of an internal combustion engine whose combustion is so rapid that the piston does not move during the combustion process, and thus combustion is assumed to take place at constant volume. The working fluid in the Otto cycle is assumed to be an ideal gas. Additionally, let us assume, to keep our mathematics simple, that it has constant specific heats. This assumption results in simple analytical expressions for the efficiency as a function of the compression ratio. Typical values of γ chosen for an Otto air cycle calculation range from 1.3 to 1.4, to correspond with measured cylinder temperature data. The Otto cycle for analysis is shown in the Fig. The four basic processes are: 1 to 2 isentropic compression process 2 to 3 constant-volume heat addition process 3 to 4 isentropic expansion process 4 to 1 constant-volume heat rejection process The reader should be able to show that the following relations are valid: Heat addition Q in  mC v (T3  T2 ) 2.1 Heat rejection Q out  mC v (T4  T1 ) 2.2 Compression stroke  P2 / P1  r  , T2 / T1  r  1 2.3 Expansion stroke P4 / P3  (1/r )  , T4 / T3  (1/ r )  1 2.4 where m = mass of gas in the cylinder C v = constant volume specific heat r = compression ratio γ = specific heat ratio The compression ratio of an engine is defined to be The compression ratio (r) is defined as the volume at the beginning of the compression stroke divided by the volume at the end of the compression stroke, i.e., r  V1 / V 2 2.5 The expansion ratio (rex) is defined as the volume at the end of expansion (power) stroke divided by the volume at the end of the heat supplied process. The expansion ratio for the Otto cycle becomes rex  V 4 / V 3 2.5 r  V1 / V 2 2.5 For a thermodynamic cycle such as this, the thermal efficiency is   W out /Q in  1  (Q out /Q in ) 2.6 If we introduce the previously cited relations for Q in and Q out , we get  (T 4  T1 ) 1  1   1   1 2.7 (T 3  T 2 ) r 1   1    1 r Otto cycle Mean Effective Pressure (MEP): The theoretical constant pressure that, if it acted on the piston during the power stroke would produce the same net work as actually developed in one complete cycle. net work for one cycle Wnet MEP   displacement volume v1  v2 o The thermal efficiency of Otto cycle is a function of compression ratio “r” and ratio of specific heats “γ”. o Efficiency is independent of heat supplied and pressure ratio. o The use of gases with higher “γ” values would increase efficiency of Otto cycle. Constant Pressure Heat Addition This cycle is often referred to as the Diesel cycle. The Diesel cycle engine is also called a compression ignition engine. The compression ratio is higher in a Diesel engine, with typical values in the range of 16 to 23, than that of an Otto engine, so that the cylinder temperature will be high enough for the air-fuel mixture to self-ignite. The duration of the combustion process is controlled by the injection and mixing of the fuel spray. Fuel is sprayed directly into the cylinder by a high pressure fuel injector beginning at about 150 before top dead center, and ending about 50 after top dead center. The other processes in the Diesel cycle are the same as in an Otto cycle. The cycle for analysis is shown in the Fig. The four basic processes are: 1 to 2 isentropic compression process  2 to 3 constant pressure heat addition process 3 to 4 isentropic expansion process 4 to 1 constant volume heat rejection process Again, assuming constant specific heats, the student should recognize the following equations: Heat addition Q in  mc p (T3  T2 ) 2.9 Expansion stroke  1  1 P4    T     , 4   P3  r  T3  r  2.10 where we have defined the parameter β as V3 T    3 V2 T2 2.11 In this case the indicated efficiency is 1   1  1  r  1 [  (   1)] Diesel cycle Dual Cycle Modem compression ignition engines resemble neither the constant-volume nor the constant-pressure cycle, but rather an intermediate cycle in which some of the heat is added at constant volume and then the remaining heat is added at constant pressure.  The Dual cycle is a gas cycle model that can be used to more accurately model combustion processes that are slower than constant volume, but more rapid than constant pressure.  The Dual cycle also can provide algebraic equations for performance parameters such as the thermal efficiency. The distribution of heat added in the two processes is something the designer can specify approximately by choice of fuel, the fuel injection system, and the engine geometry, usually to limit the peak pressure in the cycle. Consequently this cycle is also referred to as the limited-pressure cycle. The cycle notation is illustrated in the Figure. In this case we have the following difference: Heat addition Q in  m[ c v (T3  T 2 )  c p (T 4  T3 )] 2.14 The expansion stroke is still described by Equation 2.11 provided we write (cut off ratio)   V4 /V3.. In terms of   P3 / P2 , a pressure rise parameter, it can be shown that   1 1     1    1    (  1)   (   1)  2.15 r   Dual cycle Comparison of the Otto, Diesel and Dual Cycles The important variable factors which are used as the basis for comparison of the cycle are compression ratio, peak pressure, heat addition, heat rejection, and the network. 1. Same Compression Ratio and Heat Addition: All the cycles starts from the same initial state points 1 and air is compressed from state 1 to 2 as the compression ratio is same. From the T-s same heat input for the three cycles are same and heat rejection is low for Otto cycle and high for Diesel cycle.  Otto   Dual   Diesel Otto cycle allows the working medium to expand more whereas Disel cycle is least in this respect. 2. Same Compression Ratio and Heat Rejection Heat supplied in Otto cycle is more compared to Diesel Cycle. The heat rejection and compression ratio is same.  Otto   Dual   Diesel 3. Same Peak Pressure, Peak Temperature and Heat Rejection. Heat supplied in Diesel Cycle is more compared to Otto cycle. The heat rejections, Peak Pressure, Peak Temperature are same.    1  (Qout / Qin )  Diesel   Dual  Otto 4. Same Maximum Pressure and Heat Input. Heat rejection in Diesel Cycle is less compared to Otto cycle. The heat input, and Peak Pressure are same.  Diesel   Dual   Otto 5. Same Maximum Pressure and Work Output Heat rejection in Diesel Cycle is less compared to Otto cycle. The heat input, and Peak Pressure are same.  Diesel   Dual   Otto Examples and Problems Example 1 In an Otto cycle engine, the temperature and pressure at the beginning of compression are 45 0 C and 1 kg/cm2 respectively and the temperature at the end of adiabatic compression is 325 0 C. If the temperature at the end of constant volume heat addition is 1500 0 C , calculate the air standard efficiency and the temperature and pressure at the end of adiabatic expansion. Assume γ = 1.4 Solution P1= 1 kg/cm2 T1= 45 + 273 = 318 0k T2= 325 + 273 = 598 0k T3= 1500 + 273 = 1773 0k Now for adiabatic compression 1-2, T2 / T1  (v1 / v 2 )  1  r  1 r  (T2 / T1 )1/  1  (598 / 318)1/(1.41)  4.85  -1 Air standard efficiency = 1 - (1/r)  1  T1 / T 2 1  318 / 598 = 0.467 Air standard efficiency = 46.7 %   Now, p 2  p1 ( v1 / v 2 )  ( 4.85)  9.1 kg / cm 2 1.4 Now from constant volume heat addition process 2-3, we have p 2 / T2  p 3 / T3 p 3  9. 1  1773 / 598  27. 0 kg / cm 2  Now, p 4  p 3 /( v 4 / v 3 )  27.0 / 9.1  2.97 kg / cm 2 Considering adiabatic expansion 3-4,  1  1 T3 / T 4  (v 4 / v 3 )  r T 4  1773 / 1. 886  940 0 k t 4  940  273  667 0 C Example 2 The following data relate to an air standard Diesel cycle: Pressure and temperature at the end of suction stroke 1 kg/ cm2 and 30 0 C 0 respectively, maximum temperature during cycle = 1500 C, compression ratio = 16. Find (a) The percentage of stroke at which cut off takes place, (b) The temperature at the end of expansion stroke, and (c) The theoretical efficiency. Solution   1 In this case T2  T1 ( v 1 / v 2 )  303  (16 ) 0.41  946 0 k For constant pressure heat addition 2-3, we have v 3 / v2  T3 / T2  (1500  273) / 946  1.875 v 3  1.875 v 2 Hence, Now, percentage of the stroke at which cut off occurs = (Vol. at cut off- Clearance volume)/ Stroke volume = (v 3  v 2 ) /( v 1  v 2 )  100  ( 1.875v 2  v 2 ) /(16 v 2  v 2 )  100 = 5.833 % (b) Now, Expansion ratio = v 4 / v 3  ( v 4 / v 2 )  ( v 2 / v 3 )  ( v1 / v 2 )  ( v 2 / v 3 ) = 16  (1/1.875) = 8.54 Hence, T4  T3 / (v 4 / v3 )  1  1773 /(8.54) 0.41 = 734 0k (c) Now, heat supplied = C p (T3  T2 )  1.005 (1773  946)  831.14 kj / kg of air Heat rejected = C v (T4  T1 )  0.718 (734  303)  309.5 kj / kg of air Air standard efficiency = (Heat supplied- Heat rejected)/ Heat supplied st = (831.14-309.5)/831.14 st = 62.8 % Example 3 In an engine working on Dual combustion cycle, the temperature and pressure at the beginning of compression are 100 C and 1 kg/cm respectively. The compression ratio is 10: 1. If the maximum pressure is limited to 70 kg/cm and 1680 kj/kg of heat is added per kg of air, determine the temperature at salient points of the cycle and the air standard efficiency of the engine. Solution In this case p1 = 1 kg/cm2 T1 = 273 + 100 = 373 0k p3 = p4 = 70 kg/cm2 Compression ratio (v1/v2) = 10 T2  T1 (v1 / v2 )  1  373 (10) 0.4  959 0k p2  p1 ( v1 / v 2 )   1  (10)1.4  25.6 kg / cm 2 Now for constant volume heat addition 2-3 T3  T2 ( p3 / p2 )  959  (70 / 25.6)  2625 0 k Heat added at constant volume = C v ( T 3  T 2 )  0. 718 ( 2625  959 ) = 1196.2 kj/kg of air Heat added at constant pressure = Total heat added – Heat added at constant volume = 1680 -1196.2 = 483.8 kj/kg of air Hence C p (T4  T3 )  483.8 1. 005 ( T 4  2625 )  483. 8 T4 = 3110 0k Also, p3 = p4 Cut off ratio (v4/v3) = (T4/T3)= (3110/2625) = 1.185 Now for adiabatic expansion 4-5 T 4 / T 5  ( v 5 / v 4 )   1  [( v 5 / v 2 )( v 2 / v 4 )]  1  1  1 T 4 / T 5  (v 5 / v 4 )  [( v 1 / v 2 )( v 3 / v 4 )]  1 =(r / k) T5  3110 /(10 / 1.18)0.4  1300 0 k Now heat rejected at constant volume = C v ( T5  T1 )  0.718 (1300  373)  = 665.6 kj/kg of air Work done = Heat supplied- Heat rejected = 1680-665.6 = 1014.4 kj/kg of air Air standard efficiency = 1014.4/1680  100 = 60.4 % Example 4 A high speed Diesel engine working on ideal duel combustion cycle, takes in air at a pressure of 1 kg//cm2 and temperature of 50 0C and compress it adiabatically to 1/14 of its original volume. At the end of the compression the heat is added in such a manner that during the first stage the pressure increases at constant volume to twice the pressure of the adiabatic compression, and during the second stage following the constant volume heat addition, the volume is increased twice the clearance volume at constant pressure. The air is then allowed to expand adiabatically to the end of the stroke where it is exhausted, heat is rejected at constant volume. Calculate 1- the temperature at the key points of the cycle. 2- ideal thermal efficiency Solution In this case, P1 = 1 kg/cm2, T1 = 273+50 0k T 2  T1 ( r )  1  323 ( 14 ) 0. 4 = 930 0k p 2  p1 ( v1 / v 2 )   1  (14)1.4  40 kg / cm 2 p3  2  40  80 kg / cm 2 T3  T2 ( p3 / p2 )  930  (80 / 40)  1860 0k p 4  p 3  80 kg / cm 2 T 4  T 3 ( v 4 / v 3 )  1860  2  3720 0 k Now expansion ratio = ( v 5 / v 4 )  ( v1 / 2 v 3 )  (14 / 2)  7 T5  T4 /( v 5 / v 4 )  1  3720 / (7) 0.7  1710 0 k So the temperatures at key points are Point (1) = 323 0 k Point (2) = 930 0 k Point (3) = 1860 0 k Point (4) = 3720 0 k Point (5) = 1710 0 k Heat added = Heat added at const. volume + Heat added at const. pressure = 0.718(1860-930) + 1.005(3720-1860)= 2537 kj/kg of air Heat rejected = C v (T5  T1 )  = 0.718 (1710-323) = 995.9 kj/kg of air Air standard efficiency = (Heat added- Heat rejected) / Heat added = (2537-995.9)/2537 = 60.74 % Example 5 In the ideal constant volume cycle the temperature at the beginning and end of the compression are 500C and 3450C respectively. Determine the air standard efficiency and the compression ratio. A petrol engine with above compression ratio develops 25 I.H.P and consumed 5.44 kg of fuel per hour having calorific value 43260 kj/kg. Determine the indicated thermal efficiency and the efficiency ratio referred to the air standard cycle. Solution T2 / T1  ( v1 / v 2 )  1  r  1 Air standard efficiency = 1  1 /( r )   1  1  ( T 1 / T 2 ) = 1 - (323/618) = 0.477 i.e. Air standard efficiency = 47.7 % Also r  1  ( 618 / 323)  1.912 , γ-1 = 0.4 r = 5.06 Heat equivalent of indicated horse power = 25 / 1.36 = 18.38 kw Heat supplied = m f  C.V = (5.44/3600)  43260 = 65.37 kw. Indicated thermal efficiency = 18.38/65.37  100 = 28.12 % Efficiency ratio = 0.2812/ 0.477  100 = 58.9 % Fuel Air Cycle and Actual Cycle  AIR STANDARD CYCLES  Idealized processes  Idealized working fluid (air)    otto engine Corrected for fuel-air mixture FUEL AIR CYCLE Lean mixture  Idealized processes Rich mixture  Actual working fluid (air + fuel + residual gases) Otto   fuel / air  engine Corrected for losses ACTUAL CYCLE  Actual processes  Actual working fluid (air + fuel + residual gases)    actual theoretical Fuel Air Cycle The fuel-air cycle is a theoretical cycle based on the actual properties of the cylinder gases.  In this cycle, instead of air as working substance, a mixture of air and fuel is introduced and after combustion, gases expand in the engine cylinder.  If the actual physical properties of the cylinder gases before and after burning are taken into account:  a close approximation to the actual pressures and temperatures existing within the engine cylinder may be obtained.  The mean effective pressures and efficiencies calculated by this approximation are only a few percent above the actual results obtained by tests. The following assumptions are commonly made for fuel air- cycle analysis: (l) There is no chemical change in either fuel or air prior to combustion. (2) The charge is always in equilibrium.. (3) There is no heat exchange between the gases and the cylinder walls in any process. (4) Compression and expansion processes are frictionless. (5) In case of reciprocating engines, the velocities are negligibly small. (6) In fuel-air cycle it is assumed that the fuel is completely vaporized and perfectly mixed with the air. (7) Burning takes place instantaneously at top dead centre. Const. volume air cycle and fuel air cycle compared. Fuel-air cycles take into account the following:  The actual composition of the cylinder gases.  Variation of specific heat of these gases with temperature.  The effect of dissociation and incomplete chemical reactions at high temperatures.  (d) The variation in the number of molecules present in the cylinder as the pressure and temperature change. PV=N R T Note:  Dissociation of burnt gases in the cylinder of an internal combustion engine occurs at high temperatures and is mainly due to the dissociation of the CO2 into CO and O there is very little dissociation of the H2O.  There is no dissociation in the burnt gases of a weak fuel mixture as the temperature produced is too low.  Hence the maximum amount of dissociation occurs in the burnt gases of the chemically correct fuel mixture but decreases with the weaker and richer mixtures. Advantages of fuel-air cycle (1) In case of air standard cycle only effect of compression ratio on engine efficiency can be evaluated. However, in case of fuel-air cycle the effect of fuel air ratio, compression ratio, inlet pressure and temperature can be easily demonstrated, (2) Fuel air cycle is a better approximation to actual operating conditions of engine, Actual efficiency of a good engine is about 80% of the fuel- air cycle efficiency. (3) Cycle peak pressures and temperatures can be closely approximated which affect the engine structure and design. Actual Cycle A real cycle and fuel-air cycle has been shown in the Fig. showing clearly various losses. The start of pressure rise due to combustion occurs at point (a) and combustion is virtually completed at point (b) where the pressure starts to fall along a nearly isentropic line. Real cycles for internal combustion engines differ from thermodynamic cycles in many respects: (a) Time loss factor i.e. loss due to time required for mixing of fuel and air and for combustion. (b) Heat loss factor i.e. loss of heat from gases to cylinder walls. (c) Exhaust blow down loss i.e. loss of work on the expansion stroke due to early opening of valve. A real cycle and fuel-air cycle Valve Timing Introduction The exact moment at which the inlet and outlet valve opens and closes with reference to the position of the piston and crank shaft is known as valve timing diagram. It is expressed in terms of degree crank angle. As described in the ideal cycle inlet and exhaust valves open and close at dead centers, but in actual cycles they open or close before or after dead centers as in the table below.  Theoretical valve timing diagram Video: https://www.youtube.com/watch?v=Te- OltNQRUk&ab_channel=TheAutoPartsShop Actual valve timing at low and high speed The Fig. shows the intake valve timing diagram for both low speed and high speed S.I. engines.  Valve timing at low and high speed Intake valve timing  It is seen that for both low speed and high speed engine the intake valve opens 100 before the arrival of the piston at TDC on the exhaust stroke. This is to ensure that the valve will be fully open and the fresh charge starts to flow into the cylinder as soon as possible after TDC.  As the piston moves out in the suction stroke, the fresh charge is drawn in through the intake port and valve. When the piston reaches the BDC and starts to move in the compression stroke, the inertia of the entering fresh charge tends to cause it to continue to move into the cylinder. To take advantage of this, the intake valve is closed after BDC so that maximum air is taken in. This is called ram effect.   At low engine speed, the charge speed is low and so the air inertia is low, and hence the intake valve should close relatively early after BDC for a slow speed engine.  In high speed engines, the charge speed is high and consequently the inertia is high and hence to induct maximum quantity of charge due to ram effect the intake valve should close relatively late after BDC. Exhaust valve timing The exhaust valve is set to open before BDC, say about 250 before BDC in low speed engines and 550 before BDC in high speed engines.  If the exhaust valve did not start to open until BDC, the pressures in the cylinder would be considerably above atmospheric pressure during the first portion of the exhaust stroke, increasing the work required to expel the exhaust gases.  Opening the exhaust valve earlier reduces the pressure near the end of the power stroke and thus causes some loss of useful work.  The overall effect of opening the valve prior to the time the piston reaches BDC results in an overall gain in output.  By closing the exhaust valve a few degrees after TDC (about 50 in case of low speed engines and 200 in case of high speed engines) the inertia of the exhaust gases tends to scavenge the cylinder by carrying out a greater mass of the gas left in the clearance volume. This results in increased volumetric efficiency. Valve overlap  A period when both the intake and exhaust valves are open at the same time. This is called valve overlap (say about 150 in low speed engine and 300 in high speed engines).  This overlap should not be excessive otherwise it will allow the burned gases to be sucked into the intake manifold, or the fresh charge to escape through the exhaust valve. Video: https://www.youtube.com/watch?v=3JuHzRuhslA&ab_channel=MAPer formance Throttle valve  All spark ignition engines require a method of reducing or increasing engine power output during operation.  An Intake throttle valve is used to restrict airflow into the engine so that a suitable air/fuel ratio is maintained during load operation. Video: https://www.youtube.com/watch?v=eDF93dVvH14&t=159s&a b_channel=ADTWStudy Operation at full load The figure illustrates typical full throttle and part throttle indicator diagrams for a four-stroke cycle SI engine. There are two distinct areas:  Area A is a measure of the indicated net work produced in the cylinder.  Area B is usually termed the pumping loop, and is a measure of the pumping losses encountered during the cycle.   A full (open) throttle offers much less restriction to the flow of the air-fuel mixture and a greater mass of mixture, therefore, enters the combustion chamber, causing loop A to be greater at full throttle. Operation at part load  At part throttle, the restriction to flow offered by the throttle reduces the mass of fresh charge entering the combustion chamber, and therefore reduces the area of loop A and the work output.  Engines with supercharging or turbochargers have intake pressure greater than exhaust pressure, giving a positive pump work as shown in Figure (c). Typical full throttle and part throttle indicator diagrams Engine Performance Introduction Engine performance is an indication of the degree of success of the engine performs its assigned task, i.e. the conversion of the chemical energy contained in the fuel into the useful mechanical work. The performance of an engine is evaluated on the basis of the following; (a) Specific Fuel Consumption. (b) Brake Mean Effective Pressure. (c) Specific Power Output. (d) Specific Weight. (e) Exhaust Smoke and Other Emissions. Basic measurements: The basic measurements to be undertaken to evaluate the performance of an engine on almost all tests are the following: (a) Speed (b) Fuel consumption (c) Air consumption (d) Smoke density (e) Brake horse-power (f) Indicated horse power and friction horse power (g) Heat balance sheet or performance of SI and CI engine (h) Exhaust gas analysis  Schematic test arrangement for an engine Five important engine efficiencies are:  Indicated thermal efficiency (ηith)  Brake thermal efficiency(ηbth)  Mechanical efficiency (ηm)  Volumetric efficiency (ηv)  Relative efficiency or Efficiency ratio (ηr) Basic Power Measurements In general, and as indicated in the figure, the energy flow and energy losses through the engine are expressed as three distinct categories of power. They are indicated power (ip) , friction power (fp) , and brake power (bp). The indicated power output The indicated power, IP, is the power output you would calculate from a p-V indicator diagram (the net work transferred from the gas to the piston during a cycle). It is based on the gross cycle work done during the compression and expansion strokes.  W  P dV All expressions below are for 4-stroke cycle engines, which have the number of revolutions per engine cycle equal to 2, the number of cylinders, nc. net workfrom gas cycles   N  IP  Wi  numberof cylinders per cyl per cycle sec  nc   pdV   comp.expn.  2 N IP  W i  n c W i four stroke engine 2 IP  W i  n c W i N two stroke engine since the four-stroke engine has two revolutions per power stroke and the two-stroke engine has one revolution per power stroke. The brake power output The brake power, BP, is the rate at which work is done; and the engine torque, b, is a measure of the work done per unit rotation (radians) of the crank. The brake power is the power output of the engine, and measured by a dynamometer. The brake power is less than the indicated power due to engine mechanical friction, pumping losses in the intake and exhaust. The brake power is related to the brake torque,  b , which you will measure, and the angular velocity  BP  W b   b 2  N  Many modern automobile engines have maximum torque in the 200 to 300 N-m range at engine speeds usually around 4000 to 6000 RPM. The point of maximum torque is called maximum brake torque speed (MBT). CI engines generally have greater torque than SI engines. Large engines often have very high torque values with MBT at relatively low speed. Other ways which are sometimes used to classify engines are shown in Eqs. Specific power = B.P / Ap Output per displacement = B.P / Vd where: Ap = piston area of all pistons Vd = displacement volume The Friction power Friction power (fp), it was indicated that some of the power which is produced in the cylinder is not delivered at the driveshaft. This power, which is the difference between the power produced (ip) and the useful power delivered (bp), is termed friction power (fp). FP = IP - BP Mean Effective Pressure  The mean effective pressure (mep) is the work done per unit displacement volume, and has units of force/area. It is the average pressure that results in the same amount of work actually produced by the engine. The mean effective pressure is a very useful parameter as it scales out the effect of engine size, allowing performance comparison of engines of different displacement. There are three useful mean effective pressure parameters-imep, bmep, and fmep. BP BP BMEP   V cy.  2 N D L st n c 4 2  60 IP IP IMEP  .  N V cy D 2 L st n c 4 2  60 FMEP = IMEP − BMEP The mean piston speed, Up is an important parameter in engine design since stresses and other factors scale with piston speed rather than with engine speed. Since the piston travels a distance of twice the stroke per revolution, it should be clear that Up = 2 Lst N/ 60 The engine speed, N, refers to the rotational speed of the crankshaft and is expressed in revolutions per minute. Specific Fuel Consumption The specific fuel consumption is a comparative metric for the efficiency of converting the chemical energy of the fuel into work produced by the engine. As with the mean effective pressure, there are two specific fuel consumption parameters, brake and indicated. The brake specific fuel consumption (bsfc) is the fuel flow rate m f , divided by the brake power BP. It has three terms that are standard measurements in an engine test: the fuel flow rate, the torque, and the engine speed: m  f BSFC BP The indicated specific fuel consumption (isfc) is the ratio of the mass of fuel injected during a cycle to the indicated cylinder work, and is used to compare engine performance in computational simulations that do not include the engine friction. m f ISFC  IP Typical values of measured bsfc for naturally aspirated automobile engines depend on the engine load, with values ranging from about 200 to 400 g/kWh. The specific fuel consumption and engine efficiency are inversely related, so that the lower the specific fuel consumption, the greater the engine efficiency. In SI units the BSFC is expressed in kg/kWh. When expressed in these units the BSFC is related to ηb through 3600 BSFC .  b C.V  Low values of sfc are desirable,  for SI-engines 250 – 270 g/kW.h for  CI-engines, 200 g/kW.h Engine Efficiencies Mechanical efficiency The ratio of the brake power to the indicated power is the mechanical efficiency, m. The value of FP does not change greatly with load so that the mechanical efficiency increases from zero to its maximum value at a given speed, usual in the range from 0.7 to 0.9. BP IP  FP FP m    1 IP IP IP Combustion efficiency The time available for the combustion process of an engine cycle is very brief, and not all fuel molecules may find an oxygen molecule with which to combine, or the local temperature may not favor a reaction. Consequently, a small fraction of fuel does not react and exits with the exhaust flow. Combustion efficiency comb is defined to account for the fraction of fuel which burns. comb typically have values in the range 0.95 to 0.98 when an engine is operating properly. Thermal efficiency The thermal efficiency is essentially a measure of how well the engine converts the chemical energy of the fuel into shaft work. Since the water in the products leaves the engine in the vapor phase, it is conventional to use the lower heating value of the fuel, C.V, along with the mass flow rate of fuel in the expression for brake power: BP BP  b   Q in. m. f C.V Convenient units are kW for BP, kg/s for mass flow of fuel, and kJ/kg for C.V.  IP IP  i   Q in. m. f C.V The fuel flow is related to the airflow through: m f  m a F where F is the fuel-air ratio Volumetric Efficiency A performance parameter of importance for four-stroke engines is the volumetric efficiency, v. It is defined as the mass of fuel and air inducted into the cylinder divided by the mass that would occupy the displaced volume at the density ρi in the intake manifold. The flow restrictions in the intake system, including the throttle, intake port, and valve, create a pressure drop in the inlet flow, which reduces the density and thus the mass of the gas in the cylinder. The volumetric efficiency is a mass ratio and not a volume ratio. The volumetric efficiency for an engine operating at a speed N is thus. m act v  m cy. N m cy.   a V c 2 Volumetric efficiency is affected by pressure and temperature at the end of induction, residual gas pressure and temperature, and compression ratio. Volumetric efficiency is affected by the fuel, engine design and engine operating variables,  Fuel type, F/A ratio, fraction of fuel vaporized in intake system and fuel heat of vaporization  Mixture T as influenced by heat transfer  Ratio of exhaust to inlet manifold pressure  Compression ratio   Engine speed  Intake and exhaust manifold and port design  Intake and exhaust valve geometry, size, lift and timing Relative efficiency or Efficiency ratio (ηr) Relative efficiency or Efficiency ratio is the ratio of thermal efficiency of an actual cycle to that of the ideal cycle. Actual thermal efficiency  rel  Air standard effifiency Effect of fuel-Air ratio on performance The fuel to air ratio plays an important role in the combustion process. In a gasoline engine fuel intake is throttled to obtain an ideal fuel-air ratio for optimum emission, fuel economy, and good engine performance.  In spark-ignition engines the ideal air to fuel ratio (A/F ratio) is 14.7:1 which is referred to as stoichiometry. An air to fuel ratio higher than stoichiometry has an impact on fuel economy and emissions and an A/F ratio lower than stoichiometry influences the power, drivability and emissions.  A lean air/fuel mixture (equivalence ratio less than unity) will burn more slowly and will have a lower maximum temperature  than a less lean mixture. Slower combustion will lead to lower peak pressures, and both this and the lower peak temperature will reduce the tendency for knock to occur.  The air/fuel mixture also affects the engine efficiency and power output. Maximum power will be with a rich mixture when as much as possible of the oxygen is consumed; this implies unburnt fuel and reduced efficiency. Conversely, for maximum economy as much of the fuel should be burnt as possible, implying a weak mixture with excess oxygen present. When the air/fuel mixture becomes too weak the combustion becomes incomplete and the efficiency again falls. Heat balance sheet The performance of an engine is usually studied by heat balance-sheet. The main components of the heat balance are:  Heat equivalent to the effective (brake) work of the engine,  Heat rejected to the cooling medium,  Heat carried away from the engine with the exhaust gases, and  Unaccounted losses (radiation etc.). The unaccounted losses include the radiation losses from the various parts of the engine and heat lost due to incomplete combustion. The friction loss is not shown as a separate item to the heat balance-sheet as the friction loss ultimately reappears as heat in cooling water, exhaust and radiation. Heat balance sheet for CI engine Item S.I. Engine C.I. Engine Heat converted to useful work (i.p.) 25 to 32% 36 to 45% Heat carried away by cooling water 33 to 30% 30 to 28% Heat carried away by exhaust gases 35 to 28% 29 to 20% Heat unaccounted for 7 to 10% 5 to 7% Total (= Energy supplied) 100% 100% Engine Performance Test To ensure that an IC engine can constantly perform in the optimal working state, it is necessary to know the general relationships of the engine speed versus its fuel consumption and the engine speed versus its output torque. Examples and Problems Example 1 A nine cylinder petrol engine of bore 145 mm and 190 mm stroke has a compression ratio of 5.9 to 1 and develops 460 B.H.P at 2000 r.p.m., when running on a mixture 20 % lean. The fuel used has a calorific value of 46816 kj per kg and contains 85.3 % carbon and 14.7 % hydrogen. Assuming a volumetric efficiency of 70 % at 15 0 C and a mechanical efficiency of 90 %, find the indicated thermal efficiency of the engine. Solution Indicated thermal efficiency = Heat equivalent of I.H.P. / Fuel supplied m  B.P / I.P , 0.9 = 460 / I.P I.P = 511.11 H.P = 375.82 kw The fuel consumption must be deduced from the air consumption, and the air required per kg of fuel for a chemically correct mixture, thus, Volumetric efficiency = Vol. combustibles aspirated per stroke / Swept volume Swept volume ( Vst ) =  /4  (0.145)2  0.19  9  (2000/2(60)) ( Vst ) = 0.47 m3/sec  air  P / RT  air  100 / 0.287  (15  273)  air = 1.21 kg/m3 Mass of air per second = 0.47  1.21 = 0.569 kg/sec For chemically correct mixture mass of air required per kg of fuel (85.3/12) C + 14.7/2 H2 + x(O2+ 79/21N2) A CO2 + B H2O + 79/21 x N2 Balance of C: 85.3/12 = A Balance of H2: 14.7/2 = B Balance of O2: X= A + B/2 X= 85.3/12 + 14.7/2(2) X= 10.78 ( A / F) th  X  32  1000 / 100  233 ( A / F) th  14.8   ( A / F) act / ( A / F) th 1.2= ( A / F) act / 14.8 ( A / F) act = 17.76 ( A / F) act = m.air / m.fuel 17.76= 0.569/ m.fuel m.fuel = 0.032 kg/sec ith  I.P / m.fuel  C.V ith  375.82/ 0.032(46816) = 0.25 i.e. 25% Example 2 An eight cylinder automobile engine of 85.7 mm bore and 82.5 mm stroke with a compression ratio of 7 is tested 4000 r.p.m. on a dynamometer which has a 53.35 cm arm. During a 10 minute test at a dynamometer scale beam reading of 40.8 kg, 4.55 kg of gasoline for which the calorific value is 46200 kj/kg are burned, and air at 21 0C and 1.027 kg/cm2 is supplied to the carburetor at the rate of 5.44 kg per min. Find the B.H.P. delivered the b.m.e.p., the b.s.f.c., the brake specific air consumption, the brake thermal efficiency, the volumetric efficiency, and the air-fuel ratio Solution (a) B.H.P.  W  R  ( 2N / 60) = (40.8  9.8/1000)  (53.35/100)  (2   4000/60)=89.44 kw = 121.64 H.P (b) b.m.e.p. = B.P / Vst. = B.P/ (  / 4) D 2 Lst Z N / 2  60 b.m.e.p = 89.44 / (  /4) (85.7/1000)2 (82.5/1000)(8)(4000/2  60) b.m.e.p = 704.8 kpa (c) b.s.f.c.  m.f / B.P m.f = 4.55/10 (60) = 0.00758 kg/sec b.s.f.c.= (0.00758  3600)/89.44 = 0.305 kg/kw.hr (d) the brake specific air consumption = m.air /B.p b.s.a.c. = 5.44(60)/89.44 = 3.649 kg/kw.hr (e) the brake thermal efficiency= B.P. / m.f  C.V bth = 89.44/0.00758(46200) =0.255, i.e. 25.5% (f) the volumetric efficiency= m.act / m.cyl mass of air at intake conditions ( m.cyl ) = (  / 4) D2 Lst Z air N / 2  60  air = P/RT= 102.7/(0.287)(294)= 1.22 kg/m3 m.cyl =(  /4) (85.7/1000)2(82.5/1000)(8)(1.22)(4000/2  60) m.cyl = 0.155 kg/sec v  (5.44/60)/0.155 = 0.585 i.e. 58.5 % the air-fuel ratio = (5.44/60) / 0.00758 = 11.96 Example 3 A six-cylinder petrol engine develops 62 H.P at 3000 r.p.m. The volumetric efficiency at N.T.P. is 85%. The bore is equal to the stroke and thermal efficiency of 25 % may be assumed. Calorific value of petrol is 44100 kj/kg. Air-fuel ratio is to be 15:1. Calculate the cylinder bore and stroke. Solution Thermal efficiency = 25 % bth  B.P / m.fuel  C.V 0.25 = (62/1.36)/ m.fuel  44100 m.fuel = 0.0041 kg/sec Air-fuel ratio = 15:1 A/F = m air / 0.0041 =15/1 m air = 0.0624 kg/sec v  m.act / m.cyl 0.85 = 0.0624/ m.cyl m.cyl = 0.0733 kg/sec m.cyl = (  / 4) D2 Lst Z air N / 2  60  air = P/RT= 100/(0.287)(294)= 1.2 kg/m3 0.0733 = ( / 4 ) D 2  D  6  1.2  3000/(2  60) D3 = 0.0733  4  2  60/   6  1.2  3000 D = 0.08 m Cylinder diameter = Stroke = 8 cm Example 4 A four-stroke gas engine has a cylinder diameter 27 cm and piston stroke 45 cm. The effective diameter of the brake is 1.62 meters. The observation made in a test of the engine were as follows:  Duration of test 38 min 30 sec Total no. of revolutions ` 8080 Total number of explosions 3230 Net load on the brake 92 kg I.m.e.p 5.75 bar Gas used 7.7 m3 Pressure of gas at meter 135 mm of water above atmospheric pressure Atmospheric temperature 15 0C Height of barometer 750mm Hg Calorific value of gas 4400 kcal/M3 at N.T.P. Mass of jacket cooling water 183 kg Rise in temperature of jacket 47 0C cooling water Draw up a heat balance sheet and estimate indicated thermal efficiency and brake thermal efficiency. Solution I.m.e.p. = I.P / Vst. = I.P/ (  / 4) D 2 Lst Z N / 2  60 I.P = (5.75  100) ( / 4) (27/100)2 (45/100)(1)(3230/38.5  60) I.P = 20.7 kw B.P = T    W  R  2  N / 60 B.P = (92)(9.81)/1000)  (1.62/2)  2   8080/(38.5  60) B.P =16.05 kw Heat supplied Atmospheric pressure = (gh) Hg =13.6(1000)(9.81)(750/1000)= 100062 Pa =100 kPa Pressure of gas supplied= (gh) Hg  (gh) w = 13.6(1000)(9.81)(750/1000) +1000(9.81)(135/1000)=101386.35 Pa Pgas = 101.386 kPa m.gas = Vg.   g  ( Vg   g ) at N.T. P. Volume of gas used at N.T.P. = 7.7/(38.5  60)  (101.386/100)= 3.38  10-3 m3/sec Heat supplied = Q.f  Vg.  C.V = 3.38  10-3  4400  4.18= 62.16 Kw Heat lost to cylinder jacket cooling water ( Q.w ) = m.w C w t w = 183/(38.5  60)  4.18  47 = 15.56 kw Heat lost to exhaust, radiation etc.= Q.f  B.P.  Q.w  = 62.16 - 16.05 -15.56 = 30.54 kw i th  I.P / Q.f = 20.7/ 62.16 = 0.333 i.e. 33.3 % b th  B.P / Q.f = 16.05/ 62.16 = 0.258 i.e. 25.8 % Example 5 A two-stroke oil engine gave the following results at full load: Speed 350 rpm Net brake load 60 kg Indicated mean effective pressure 2.75 bar Oil consumption 4.25 kg/hour Jacket cooling water 490 kg/hour Temperature of jacket water 20 0C and 45 0C at inlet and outlet Air user per kg of oil 31.5 kg Temperature of air in test room 20 0C Temperature of exhaust gases 390 0C Cylinder diameter 22 cm  Stroke 28 cm Effective brake diameter 1m Calorific value of oil 44100 kj/kg Proportion of hydrogen in fuel oil 15 % Specific heat of dry exhaust gases 1.008 kj/kg k Specific heat of steam 2.1 kj/kg k Find I.P., B.P. and draw up heat balance sheet for the test in kj per minute and percentage. Solution I.P = I.m.e.p. (  / 4) D2 Lst Z N / 60 I.P = (2.75  100) ( / 4) (22/100)2 (28/100) (1) (350/60) = 17.07 kw I.P = 1024.2 kj/min B.P = T    W  R  2  N / 60 B.P = (60)(9.81)/1000)  (1/2)  2   350/60 = 10.78 kw B.P = 646.8 kj/min Heat supplied = Q.f  m.f  C.V = (4.25/3600)  44100 = 52.06 kw Q.f  3123.75 kj/min Heat lost to cooling water ( Q.w ) = m.w C w t w Q.w = (490/3600)  4.18  (45-20) = 14.22 kw = 853.4 kj/min  2H2 + O2 = 2H2O, 1 kg + 8 kg = 9 kg i.e. one kg of H2 produces 9 kg of H2O  Weight of H2O produced per kg of fuel burnt = 9  H  ratio of H2 in fuel = 9  0.15  (4.25/3600) = 0.00159 kg/sec Total mass of exhaust gases (wet) = mass of air + mass of fuel m.ex = (31.5  4.25 + 4.25)/3600 = 0.0384 kg/sec Mass of dry exhaust gases = mass of wet exhaust gases – mass of H2O = 0.0384 – 0.00159 = 0.03678 kg/sec Heat lost to dry exhaust gases = 0.03678  1.008  (390-20) Q.ex = 13.72 kw = 823 kj/min Assuming that steam in exhaust gases exists as superheated steam at atmospheric pressure and exhaust gas temperature, total heat of 1 kg of steam at atmospheric pressure (1 bar) and 390 0C = h sup er  h f  Heat to steam in exhaust gases = m.st ( h sup er  h f ) Q.st = 0.00159 (2951.97 – 417.46) Q.st = 4.03 kw = 241.8 kj/min Heat lost to radiation = Q.f  B.P.  Q.w  Q.st  Q.ex = 52.06 - 10.78 - 14.22 - 4.03 - 13.72 = 9.31 kw = 558.6 kj/min Heat balance sheet Heat input Kj/min % Heat expenditure Kj/min % Heat 3123.75 100 1- Heat equivalent of 646.8 20.7 supplied by B.P. combustion 2- Heat lost to 853.4 27.3 of fuel cooling water 3- Heat lost to dry 823 26.3 exhaust gases 4- Heat lost to steam in exhaust gases 241.8 7.8 5- Heat lost to radiation 558.6 17.9 Total 3123.75 100 Combustion in Spark Ignition Engines Introduction Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in fuel with oxygen in air resulting in liberation of energy in the form of heat. Video: https://www.youtube.com/watch?v=ukDnJBnqv- k&ab_channel=NAGENDRAMATTA Following conditions are necessary for combustion to take place: 1. The presence of combustible mixture Ignition of charge is only possible within certain limits of fuel-air ratio. Ignition limits correspond approximately to those mixture ratios, at lean and rich ends of scale, where heat released by spark is no longer sufficient to initiate combustion in neighboring unburnt mixture. For hydrocarbons fuel, the stoichiometric fuel air ratio is 1:15 and hence the fuel air ratio must be about 1:30 and 1:7 1 – Fuel supply 2 – Air intake 3 – Throttle 4 – Intake manifold 5 – Fuel injector 6 – Engine  Systems of fuel injection 2. Some means to initiate mixture. In S I Engines, spark plug initiates combustion of combustible mixture of petrol and air. 3. Stabilization and propagation of flame in Combustion Chamber. The design of combustion chamber has an important influence upon the stabilization and propagation of flame inside the combustion chamber. In an ideal cycle it can be seen from the diagram, the entire pressure rise during combustion takes place at constant volume i.e., at TDC. However, in actual cycle this does not happen. Theoretical diagram of pressure crank angle diagram is shown. a → b : Compression b → c : Combustion c → d : Expansion  Theoretical p-v diagram Theoretical p-θ diagram Combustion in S.I engine may roughly divide into two general types: 1. Normal Combustion In a conventional spark-ignition engine, the fuel and air are homogeneously mixed together in the intake system, inducted through the intake valve into the cylinder where it mixes with residual gases and is then compressed. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge (electrodes exceeds 10,000 ᵒC).  The pressure/crank-angle history 2. Abnormal Combustion The combustion gets deviated from the normal behavior resulting in loss of performance or damage to the engine. Abnormal combustion can take several forms, principally pre-ignition and self-ignition.  Pre-ignition is when the fuel is ignited by a hot spot, such as the exhaust valve or incandescent carbon combustion deposits.   Self-ignition is when the pressure and temperature of the fuel/air mixture are such that the remaining unburnt gas ignites spontaneously. . Pre-ignition can lead to self-ignition and vice versa. Normal Combustion Stages of combustion on actual p-θ diagram  Ignition lag (A→B): Flame front begins to travel.  Spreading of Flame (B→C): Flame spreads throughout the Combustion Chamber.  After burning (C→D): C is the point of maximum pressure, a few degrees after TDC. Power stroke begins. In the below figure, you can see the curve ABM which is a motoring curve (When the engine is not firing) and the curve ABCD which is a combustion curve (When the engine is firing).  Pressure diagram for a spark ignition engine 1. The delay period stage (A→B):  When the piston approaches the end of the compression stroke, a spark is discharged between the sparking plug electrodes. The spark leaves a small nucleus of flame.  There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called ignition lag or 'delay period'.  The Fig. compares the pressure diagrams for the cases when a mixture is ignited and when it is not ignited. The point at which the  pressure traces diverge is ill-defined, but it is used to denote the end of the delay period.  Ignition lag is time interval in the process of chemical reaction during which the molecules gets heated up to self-ignition temperature and produce nucleus of flame.  The delay period is typically of 1-2 ms duration and only approx 1% of the charge is burned during that period.  Delay period depends on temperature, pressure, nature of fuel, composition of fuel-air mixture, proportion residual gases, the energy applied at the spark plug and the duration of the spark. 2. Flame propagation stage (B→C):  After the ignition, cylinder pressure continues to rise while the flame front travels at a certain flame speed and peak pressure is obtained at 5 –20O ATDC. This is essential for maximum thermal efficiency.  Combustion process takes place in a turbulent flow field, and can be defined as the 90-95 percent mass fraction bum duration. 3. After burning stage (C→D):  Final stage covers the period from the maximum cylinder pressure to the termination of the combustion process.  This generally happens when the rich mixture is supplied to engine.  Maximum temperature value is reached during this stage (after maximum pressure).  Usually 70 –75% of the total energy is released until maximum pressure is obtained, and 85 –90% of the total energy is released until maximum temperature is obtained.  Since expansion stroke starts before this stage of combustion, with the piston moving away from the top dead centre, there can be no pressure rise during this stage. Abnormal Combustion There are two types of abnormal combustion:  Surface ignition (pre-ignition).  Knock (self-ignition). Surface ignition Surface ignition is caused by the mixture igniting as a result of contact with a hot surface, such as an exhaust valve. Surface ignition is often characterized by running-on as the engine continues to fire after the ignition has been switched off. If the surface ignition occurs in advance of the spark, then it is called pre-ignition. Pre-ignition is initiated by some overheated part such as the sparking plug electrodes, exhaust valve, metal corners in the combustion chamber, carbon deposits or cylinder head gasket rim etc. Effects of pre-ignition (1) Due to early ignition of the charge, pressure begins to rise inside the cylinder when the piston is still moving up, thereby increasing negative work done by the piston during compression stroke. (2) Pre-ignition generally results in increased pressure and temperature of the charge and hence leads to detonation. (3) Sometimes detonation may also lead to pre-ignition due to overheating of spark plug tip etc. (4) Pre-ignition leads to power loss of the engine which goes on increasing successively. (5) Sometimes excessive heating of engine parts may result in piston and cylinder damage, particularly in heavy duty engines. (6) In multi-cylinder engines when pre-ignition develops in one or more cylinders, then the remaining cylinders will carry on at full speed and power, dragging pre igniting cylinder after them and may lead to engine seizure or even breakage of parts. Video: https://www.youtube.com/watch?v=UfqXhnr2Ho4&ab_channel=MNos ko2011 Knocking in spark ignition engines Video: https://www.youtube.com/watch?v=zxDHXXHJ6rE&ab_channel=Mech anicalBoost As shown in figure, flame travels from a-d and compresses the end charge gas (end gas) bb’d and raises its temperature. Temperature also increases due to heat transfer from the flame front. Now, if the final temperature is less than the auto-ignition temperature, normal combustion occurs and charge bb’d is consumed by the flame itself.  Self-ignition occurs when the pressure and temperature of the unburnt gas are such as to cause spontaneous ignition. As the flame front propagates away from the sparking plug, the unburnt (or 'end') gas is heated by radiation from the flame front and compressed as a result of the combustion process. If spontaneous ignition of the unburnt gas occurs, there is a rapid pressure rise which can be characterized by a 'knocking'. The 'knock' is audible, caused by resonances of the combustion chamber walls. A second flame front develops and moves in opposite direction, where the collision occurs between the flames. This causes severe pressure pulsation, and leads to engine damage or failure. Results of Knocking The harmful effects of detonation are as follows: 1. Noise and roughness. 2. Mechanical damage. 3. Carbon deposits. 4. Increase in heat transfer. 5. Decrease in power output and efficiency. 6. Pre-ignition.  Combustion in compression ignition engines Phases of Combustion Although the chemical reactions during combustion are undoubtedly very similar in compression ignition and spark ignition engines, the basic physical aspects of the two combustion processes are quite different.  In case of spark ignition engines, fuel is in the gaseous form, homogeneously mixed with air and ignition is initiated by means of a precisely controlled spark.  In case of compression ignition engines, only air is compressed during compression stroke and fuel is injected in the form of a fine spray near the top dead center of the piston stroke, the fuel and air are not homogeneously mixed. Thus in case of compression ignition engines, the time and place where ignition occurs is not fixed. In compression ignition engines, only air is compressed during the compression stroke and the ignition can take place only after fuel is injected just before the top dead centre. Thus there can be a no preignition in compression ignition engines as in spark ignition engines.  Only air is contained in the cylinder during compression stroke, and a much higher compression ratio (16 to 24) are used in CI engines.  Air intake into the engine is un-throttled and engine torque and power output controlled by the amount of fuel injected per cycle.  The diesel engines have a higher maximum efficiency than the spark ignition engine as compression ratio is higher.  Comparison of load and SFC of spark ignition and diesel engines  This ensures good part load fuel economy as there are no throttling losses.  The fuel/air mixture is always weaker than stoichiometric, as it is not possible to utilize all the air.  Compression ignition engines can operate over a wide range of air/fuel mixtures with equivalence ratios in the range 0.2 - 0.9.  The power output of the engine is controlled by the amount of fuel injected. In an ideal diesel cycle, the heat is added at constant pressure. The combustion process is best approximated as a constant-pressure heat addition in an air-standard cycle. Theoretical P-V diagram  In diesel engines, only air is send into the combustion chamber during induction.  The fuel is sprayed directly into the cylinder and the fuel-air mixture ignites.  Fuel is sprayed directly into the cylinder by a high pressure fuel injector (200 bar) beginning at about 150 before top dead center, and ending about 50 after top dead center. The fuel atomizes into small droplets and penetrates into the combustion chamber, the droplets vaporize and mix with high-temperature and high- pressure cylinder air. Sometimes the fuel jet is designed to impinge on to the combustion chamber wall; this can help to vaporize the fuel.  Actual P-V diagram Stages of combustion in C.I engines The combustion process in a compression ignition engine takes place in four separate stages. During the first period there is no visible pressure rise, and is known as delay period. This is followed by rapid combustion during which rapid pressure rise occurs. After this, there is period of gradual or controlled combustion. The fourth stage is known as after burning during which fuel burns after injection, terminates. The Fig. shows the pressure diagram for a compression ignition engine; there are four stages of combustion:  Ignition delay, AB.  Rapid or uncontrolled combustion, BC.  Controlled combustion, CD.  After burning Pressure diagram for a compression ignition engine 1. Delay Period (Ab) The delay period also known as ignition lag. After injection there is initially no apparent deviation from the unfired cycle. It is the time period from the start of fuel injection and the sensible pressure rise.  Points “a” represents the time of injection  Point “b” represents the time of combustion. During this period fuel constantly goes inside the engine cylinder through the nozzle, but it does not ignite immediately. There is a definite period of apparent inactivity between the time when the first droplet of fuel hits the hot air in the combustion chamber and the time when it starts through the "actual burning" phase.  The ignition delay period can be divided into two parts, the physical delay and the chemical delay. Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air.  Good atomization requires: high fuel-injection pressure, small injector hole, optimum fuel viscosity, high cylinder pressure.  Rate of vaporization of the fuel droplets depends on droplet diameter, velocity, pressure and temperature of the air. Chemical processes similar to that described for auto ignition phenomenon in premixed fuel-air, only more complex since heterogeneous reactions also occur. Chemical delay is more effective for the duration of the ignition delay period. Ignition delay period is in the range of  0.6 to 3 ms for low-compression ratio DI diesel engines,  0.4 to 1 ms for high compression ratio, turbocharged DI diesel engines.  In practice it is difficult to distinguish between these two periods, so the delay period is measured from the beginning of the injection to the moment of ignition.  Delay period has got considerable influence on the operation of an engine. A shorter delay period gives a smoother operation; as longer period results in a rougher and noisier running engine. 2. Period of Rapid Combustion (Bc) The period of rapid combustion is counted from end of delay period or the beginning of the combustion to the point ‘C’. The period of rapid combustion also called the uncontrolled combustion, is that phase in which the pressure rise is rapid. Combustion of the fuel which has mixed with air within flammability limits during ignition delay period occurs rapidly in a few crank angle degrees. The rate of heat release is maximum during this period. This is also known as uncontrolled combustion phase, because it is difficult to control the amount of burning during the process.  If the amount of fuel collected in the combustion chamber during the ignition delay is much - high heat release rate results in a rapid pressure rise which causes the diesel knock.  If the pressure gradient is in the range 4-5 bar/oCA, engine operation is not smooth and diesels knock starts.  This value should be in the range 2 to 3 bar/oCA for smooth operation (max allowable value is 10 bar/oCA) of the engine.  A high rate of pressure rise means a sudden application of load to the engine structure, which often results in damage of the parts.  A high rate of pressure rise also produces a violent pounding noise which is known as "Diesel knock". 3. Controlled Combustion (Cd) The rapid combustion period is followed by the third stage, the controlled combustion. The temperature and pressure in the second stage are so high that fuel injected burn almost as they enter and find the necessary oxygen and any further pressure rise can be controlled by injection rate. The period of controlled combustion is assumed to end at maximum cycle temperature ‘D’. 4. After-Burning Combustion does not stop with the completion of the injection process. The un-burnt and partially burnt fuel particles left in the combustion chamber start burning as soon as they come into contact with the oxygen. This process continues for a certain duration called the after burning period. This burning may continue in expansion stroke up to 70 to 80% of crank travel from TDC. Diesel Knock Knock in SI and CI engines are fundamentally similar.  In SI engines, it occurs near the end of combustion;  In CI engines, it occurs near the beginning of combustion. Knock in CI engines is related to delay period. When delay period is longer, there will be more and more accumulation of fuel droplets in combustion chamber. This leads to a too rapid a pressure rise due to ignition, resulting in rough engine operation. When the delay period is too long, the rate of pressure rise is almost instantaneous with more accumulation of fuel. Homogeneous Charge Compression Ignition (HCCI) Homogeneous charge combustion ignition is a type of engine. HCCI is an engine which neither uses a spark plug like a petrol engine nor fuel injectors like diesel engines it uses compression to combust and igniter the engine. In a spark ignition engine,  The combustion process produces less emission than that of a diesel engine, but its compression ratio is knock limited. The diesel engine is more fuel efficient,  it operates at a higher compression ratio and is unthrottled, but it has higher NOx and particulate emissions.  (HCCI) have been developed to combine the best aspects of spark and diesel combustion, that is, combining the homogeneous fuel- air mixture of a spark ignition engine with the higher compression of a diesel engine to achieve lower emissions with much higher engine efficiencies.   Basic principle:  HCCI is characterized by the fact that the fuel and air are mixed before the combustion starts and the mixture auto-ignites as a results of the temperature increase in the compression stroke.  The HCCI combustion initiates simultaneously at multiple sites within the combustion chamber and that there is no flame propagation. In HCCI engine:  Very lean mixtures are used such that the peak flame temperature is below 1800 K to prevent large amount of thermal NOx formation.  The lean premixed charge helps minimize particulate emissions.  Advantages of HCCI Combustion  They can operate at diesel-like compression ratios (>15), thus achieving 30% higher efficiencies than S.I gasoline engines.   Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions.  HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.  HCCI avoids throttle losses, which further improves efficiency. Disadvantages of HCCI Combustion  Auto-ignition is difficult to control, unlike the ignition event in S.I and diesel engines, which are controlled by spark plugs and in- cylinder fuel injectors.  High pressure rise rates contribute to engine wear.  Carbon monoxide (CO) and hydrocarbon (HC) emissions are higher. Self-Ignition Caractéristiques of Fuels Self-Ignition If the temperature of an air-fuel mixture is raised high enough, the mixture will self-ignite without the need of a spark plug or other external igniter. The temperature above which this occurs is called the self- ignition temperature (SIT).  This is the basic principle of ignition in a compression ignition engine.  The compression ratio is high enough so that the temperature rises above SIT during the compression stroke.  Self ignition then occurs when fuel is injected into the combustion chamber.  Self-ignition or auto-ignition is not desirable in an SI engine, where a spark plug is used to ignite the air-fuel at the proper time in the cycle.  The compression ratios of gasoline-fueled SI engines are limited to about 11:1 to avoid self-ignition.  When self-ignition does occur in an SI engine higher than desirable, pressure pulses are generated.  These high pressure pulses can cause damage to the engine and quite often are in the audible frequency range. This phenomenon is often called knock.  Pressure crank-angle diagram Self-ignition of fuels The Fig. shows the basic process of what happens when self- ignition occurs.  If a combustible air-fuel mixture is heated to a temperature less than SIT, no ignition will occur and the mixture will cool off.  If the mixture is heated to a temperature above SIT, self-ignition will occur after a short time delay called ignition delay (ID).  The higher the initial temperature rise above SIT, the shorter will be ID.  The value for SIT and ID for a given air-fuel mixture are depending on many variables which include:   temperature,  pressure,  density,  turbulence,  fuel-air ratio,  presence of inert gases. Ignition delay is generally a very small fraction of a second. During this time, preignition reactions occur, including oxidation of some fuel components and even cracking of some large hydrocarbon components into smaller HC molecules. These preignition reactions raise the temperature at local spots, which then promotes additional reactions until, finally, the actual combustion reaction occurs.  The Fig. shows the pressure-time history within a cylinder of a typical SI engine. With no self-ignition the pressure force on the piston follows a smooth curve, resulting in smooth engine operation.  When self-ignition does occur, pressure forces on the piston are not smooth and engine knock occurs.  By limiting the compression ratio in an SI engine, the temperature at the end of the compression stroke where combustion starts is limited. The reduced temperature at the start of combustion then reduces the temperature throughout the entire combustion process, and knock is avoided.   On the other hand, a high compression ratio will result in a higher temperature at the start of combustion. This will cause all temperatures for the rest of the cycle to be higher. The higher temperature of the end gas will create a short ID time, and knock will occur. The pressure-time history Table : Ignition temperature and compression ratio for various fuels Table : Stoichiometric air/fuel ratio Octane Number and Engine Knock The fuel property that describes how well a fuel will or will not self- ignite is called the octane number.  The higher the octane number of a fuel, the less likely it will self- ignite.  Engines with low compression ratios can use fuels with lower octane numbers,