Internal Combustion Engines Course PDF

11/28/2020 Engine Geometric parameters Internal Combustion Engines Course E-Learning Section No.1 Engine Geometric parameters Engine Geometric parameters  11/28/2020 Engine Geom...

11/28/2020 Engine Geometric parameters Internal Combustion Engines Course E-Learning Section No.1 Engine Geometric parameters Engine Geometric parameters  11/28/2020 Engine Geometric parameters Theoretical and Real SI Otto cycle Theoretical and Real SI Otto cycle Mean Effective pressure  11/28/2020 Indicated Work and Mean Effective pressure Engine brake and friction power 𝐼 = 𝐼 = 𝑊 =𝑊 ∗ ∗𝑍 Engine brake and friction power Engine brake and friction power 𝑊 =𝑊 ∗ ∗𝑍 𝑊 =𝑊 ∗ ∗𝑍 𝑊 =𝑊 + 𝑊 𝑊 = 2 ∗ 𝜋 ∗ 𝑁 ∗ 𝐵𝑟𝑎𝑘𝑒 𝑇𝑜𝑟𝑞𝑢𝑒 = 𝐵𝑟𝑎𝑘𝑒 𝑝𝑜𝑤𝑒𝑟 𝐵𝑟𝑎𝑘𝑒 𝑝𝑜𝑤𝑒𝑟 = 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 ∗ 𝑖𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑 𝑝𝑜𝑤𝑒𝑟  11/28/2020 Specific fuel consumption Specific fuel consumption Specific fuel consumption is the amount of fuel consumed by a vehicle for each unit of power output. 𝑚 𝐼 = 𝑊 𝑚 𝐵 = 𝑊 𝑚 𝐹 = 𝑊 Engine Efficiencies Engine Efficiencies Mechanical Efficiency: Volumetric efficiency is a comparison of the actual volume of air–fuel mixture drawn into an engine to the theoretical maximum volume that could be drawn in. Thermal Efficiency: Fuel Conversion Efficiency: 𝑊 𝜂 = 𝑚 ∗ 𝐻𝑉  11/28/2020 Air To Fuel Ratio Air To Fuel Ratio Combustion Fundamentals: Basic Flame Types: Premixed flames: fuel and oxidizer are homogeneously mixed before reaction occurs. Laminar and turbulent premixed flames Non premixed flames: fuel and oxidizer come into contact during combustion process. Laminar and turbulent diffusion flames  11/28/2020 Premixed Flames Reactants are perfectly premixed prior to ignition → all components necessary for the reaction are present in the fuel → to initiate reaction one has only to ignite the mixture → complete combustion Formation of a propagation flame front - Front separates unburned from fully burned - self-sustaining Targeted to lean conditions (Advantages) → complete combustion → “no” soot → no bright flame → Visibility depends on fuel: e.g. blue glow of the premixed bunsen flame originates from excited states of CH (Methylidyne), C2 (carbon atoms in hydrocarbon, carboxylic acid) (intermediate species in oxidization) HO or HO (hydroxide), H, CO,… → Lower combustion temperatures → lower NOx Premixed Flames Bunsen Burner: Gaseous fuel enters into the Disadvantages: mixing chamber, into which air is entrained. 1- Danger of Explosion 2- Combustion instability Velocity of the jet entering the mixing chamber may be varied. Applications : Gasoline and natural gas engines, modern gas turbines, explosions Entrainment of air and mixing can be optimized. Laminar premixed flames A premixed flame is a self-sustaining propagation of a localized Mixing chamber must be long enough to generate a premixed gas issuing from combustion zone at subsonic velocities (deflagration regime) the Bunsen tube into the surroundings. The classical device to generate a laminar premixed flame is the Bunsen burner: If the fuel flow velocity is > laminar burning velocity a Bunsen flame cone establishes at the top of the tube.  11/28/2020 Outer diffusion flame Outer cone (luminous zone): reaction and heat transfer Preheating region containing fuel and air Inner cone (dark zone): fuel rich flame Typical Bunsen-burner flame is a dual flame: a fuel-rich premixed inner flame a diffusion outer flame: CO and H2 from inner flame encounter ambient air  11/28/2020 Laminar Diffusion Flames (Laminar Non- Premixed Flames) Used in certain applications (e.g., residential gas appliances). In a diffusion flame combustion occurs at the interface between - mostly partially-premixed flames the fuel gas and the oxidant gas, Used in fundamental flame research. and the burning depends more on rate of diffusion of reactants than Primary concern in design is the flame geometry. on the rates of chemical processes involved. Parameters that control the flame shape, - Fuel flow rate It is more difficult to give a general treatment of - Fuel type diffusion flames, largely because no simple, measurable parameter, - Other factors analogous to the burning velocity in premixed flames, can be defined. Premixed vs diffusion flames: The solid fuel is first heated by heat transfer induced by combustion. The liquid fuel reaches the flame by capillarity along the wick and is vaporized. Fuel oxidation occurs in thin blue layers (the color corresponds to the spontaneous emission of the CH radical) Unburnt carbon particles are formed because the fuel is in excess in the reaction zone. The soot is the source of the yellow light emission. Flow (entrainment of heavy cold fresh air and evacuation of hot light burnt gases) is induced by natural convection  11/28/2020 Kinematic balance for a steady oblique flame Theory of Zeldovich, Frank-Kamenetsky and Semenov: 2A  Tu  RTb2  exp E / RTb  Thermal expansion through the flame front 2 SL     u c p  Tb  E  Tb  Tu 2 * Normal component of velocity vector  v n u    v n b u v n ,b  v n ,u b Factors influencing flame velocity and thickness: * Tangential component of velocity vector 1- Equivalence ratio: Flame speed peaks occur at stoichiometric or slightly fuel-rich mixtures vt,u  vt,b (highest burned gases temperatures) 2- Fuel molecular structure: depends on fuel molecular weight At steady-state the burning velocity equals the flow velocity of the unburnt mixture normal to the flame front 3- Temperature and pressure ( increase with Tu and decrease with P) S L ,u  vn ,u  vu sin  Turbulent Flames Turbulent flames Unlike the laminar burning velocity, the turbulent flame velocity is not a Most of combustion devices operate in turbulent property of the gas but instead it depends on the details of the flow. flow regime, i.e. internal combustion or aircraft engines, industrial burners and furnaces. Laminar combustion The IC engine in-cylinder flow is always turbulent and the smallest eddies (Kolmogorov scale) are typically larger than the laminar flame thickness (1 mm). applications are almost limited to candles, lighters and some domestic furnaces. Turbulence increases the Under these conditions the flame is said to display a structure known as wrinkled laminar flame. mixing processes thus enhancing combustion. Also combustion influences turbulence. Heat release due to A wrinkled laminar flame is characterized by a continuous flame sheet that is distorted by the eddies passing through the flame. combustion causes very strong flow accelerations through the flame front (flame-generated turbulence). Moreover, The turbulent burning velocity depends on the turbulent intensity ut and can be up to 30 times the laminar burning velocity huge changes in kinematic viscosity associated with temperature changes may damp turbulence leading to flow St / Sl  1  a ut / Sl  b Sl St re-laminarization Laminar flame Turbulent flame  11/28/2020 Thermal theory of Mallard and Le Chatelier: TPF (Turbulent Premixed Flame) can be divided to three zones: The flame consists of two zones: – Preheat zone Pre-heat zone: the unburned gases are heated by conduction and reach ignition – Reaction zone Reaction zone: chemical enthalpy is converted into sensible enthalpy – Post flame zone The reaction zone is thin – CH layer. – fuel consumption layer Energy balance on preheat zone:  cpTi Tu    Tb Ti    uSL The reaction zone is highly wrinkled m m – Due to turbulence eddies r – Due to self-instability Laminar flame-front thickness: 1 w is a mean reaction rate, evaluated at Ti Hydrodynamic instability (Landau-Darrieus)  r  S L r  S L Diffusion-thermal instability w    Tb  Ti  T  T  Bouyancy effect (Rayleigh-Taylor) Laminar flame speed: S L    w  b i w  c  T  T  Ti  Tu   u p  i u SPARK IGNITION ENGINES  Two basic categories Pre-mixed Non-premixed (Diffusion)  Both characterized as Laminar or Turbulent  Premixed:  Results from gaseous reactants that are mixed prior to combustion  Flame propagates at velocities slightly less than a few m/s  Reacts quite rapidly  11/28/2020 Non-premixed (Diffusion) Structure of Flame Vb Vu = SL b u As in Diesel Engine: [Fuel] T Gaseous reactants are introduced separately and mix during [O2] d[F ]   Ea  combustion  [ F ]n [O2 ]m exp  dt  RT  [radicals] Energy release rate limited by mixing process Reaction zone between oxidizer and fuel zone Products Reaction Pre-heat zone Zone Zone Flame Visible part of the flame thickness  Diffusion of heat and radicals Spark ignition Engines Combustion Flame Thickness and Quenching Distance A rough estimate of the laminar flame thickness  can be obtained by: 2 2  kcond     1 mm Sl Sl    c p  As a flame propagates through a duct heat is lost from the flame to the wall Local quenching d It is found experimentally that if the duct diameter is smaller than some critical value then the flame will extinguish This critical value is referred to as the quenching distance dmin and is close in magnitude to the flame thickness. d min    11/28/2020 Spark ignition Engines Combustion  11/28/2020 Combustion produced pressure rise  11/28/2020 Burn duration Optimum Combustion Phasing Heat release schedule has to phase correctly with piston motion for optimal work extraction In SI engines, combustion phasing controlled by spark Spark too late – heat release occurs far into expansion and work cannot be fully extracted Spark too early – Effectively “lowers” compression ratio – increased heat transfer losses – Also likely to cause knock Optimal: Maximum Brake Torque (MBT) timing – MBT spark timing depends on speed, load, EGR, , temperature, charge motion, … – Torque curve relatively flat: roughly 5 to 7 o CA retard from MBT - results in 1% loss in torque Spark Timing  11/28/2020 Spark Timing Improvements  11/28/2020 The minimum energy required to ignite Control of Turbulence Level for Efficient Ignition air-fuel mixture. Effect of Various Parameters on MIE: - Distance Between Electrodes - Fuel - Equivalence Ratio - Initial Temperature -Air Movement  11/28/2020 Minimum Ignition Energy and Flammability Limits - A flame is spark-ignited in a flammable mixture only if the spark energy is larger than some critical value known as the minimum ignition energy Eign - Ignition energy is inversely proportional to the square of the mixture pressure. - A flame will only propagate in a fuel-air mixture within a range of mixture compositions known as the flammability limits. - The fuel-lean limit is known as the lower (or lean) flammability limit and the fuel-rich limit is known as the upper (or rich) flammability limit. - The flammability limit is affected by : both the mixture initial pressure and temperature. Combustion process must be properly located relative to the TDC to obtain max power or torque. Combined duration of the flame development and propagation process is typically between 30 & 90 CA degrees. If the start of combustion process is progressively advanced before TDC, compression stroke work transfer (from piston to cylinder gases) increases. If the end of combustion process is progressively delayed by retarding the spark timing, peak cylinder pressure occurs later in the expansion stroke and reduced it. - The optimum timing which gives maximum brake torque (called maximum brake torque or MBT timing) occurs: when magnitude of these two opposing trends just offset each other. - Timing which is advanced or retarded from this optimum MBT timing gives lower torque. - Optimum spark setting will depend on: 1- rate of flame development and propagation 2- length of flame travel 3- details of the flame termination process  11/28/2020 These depend on: 1- engine design, 2- operating conditions 3- properties of the fuel-air 4- burned gas mixture With optimum spark setting, A- max pressure occurs at about 15 degrees CA after TDC (10 - 15) B- half the charge is burned at about 10 degrees CA after TDC. C- In practice spark is retarded to give a 1 or 2 % reduction in brake torque from max value. Normal combustion spark-ignited flame moves steadily across the combustion chamber until the charge is fully consumed. Surface ignition is ignition of the fuel-air charge by overheated valves or Abnormal combustion spark plugs, by glowing combustion chamber deposits or by any other hot spot in fuel composition, engine design and operating parameters, combustion chamber deposits the engine combustion chamber - it is ignition by any source other than the spark may prevent occurring of the normal combustion. plug. There are two types of abnormal combustion : It may occur before the spark plug ignites the charge (pre-ignition) or after normal Knock ignition (post-ignition). Surface ignition It may produce a single flame or many flames and may leads to knock. Knock has the following effects on engine operation: B- Density factors: 1. Noise and Roughness. 2. Carbon deposits Increasing density will increase the possibility of Knock by: 3. Mechanical damage: engine wear, cylinder head and valves may be pitted 1. Increasing load. 2. Increasing compression ratio. 4. Increase in heat transfer. 5. Decrease in power output and efficiency 3. Supercharging. 4. Advancing the spark. 6. Pre-ignition: combustion Occurs before the spark. C- Time factors: To prevent Knock in the S.I. engine the end gas should have: Increasing the time of exposure of the unburned mixture to auto-ignitions will increase tendency A- Low temperature. B- Low density. C- Long ignition delay. D- Non- reactive to Knock by: 1. Increasing the distance of the flame travel. Factors that result in knock are: 2. Decreasing the turbulence of mixture. 3. Decreasing the speed of the engine. A- Temperature factors: The temperature of the unburned mixture is increased by the following factors: D- Composition: 1. Raising the compression ratio. 2. Supercharging. The probability of Knock in S.I. engines is decreased by: 3. Raising the inlet temperature. 4. Raising the coolant temp. 1. Increasing the octane rating of the fuel. 5. Increasing load. 6. Advancing the spark. 2. Either rich or lean mixtures. 3. Stratifying the mixture. 7. Raising the temperature of the cylinder and C.C. walls. 4. Increasing the humidity of the entering air.  11/28/2020 Compression Engines Combustion DIESEL COMBUSTION PROCESS Liquid fuel injected into compressed charge Fuel evaporates and mixes with the hot air Auto-ignition with the rapid burning of the fuel -air that is “premixed” during the ignition delay period Premixed burning is fuel rich As more fuel is injected, the combustion is controlled by the rate of diffusion of air into the flame in diesel COMBUSTION: Intake air not throttled Heterogeneous – Load controlled by the mass of fuel injected – liquid, vapor and air >A/F ratio: idle ~ 80 – spatially non-uniform >Full load ~19 (less than overall stoichiom.) turbulent No “end-gas”; avoid the knock problem diffusion flame – High compression ratio: better efficiency – High T and P Combustion: – Mixing limited – Overall lean Auto-ignition in different parts of C.C. Delivery rate history, injection rate history, heat release, and in-cylinder pressure  11/28/2020 Ignition delay period I. Fuel is injected when the temperature of air reaches the temperature of self- ignition (250 C). II. Induction ignition delay includes physical and chemical delay periods (0.7-3 ms). III. The physical induction is measured from the moment of fuel injection to the moment of formation of combustible F/A mixture. V. The chemical induction is measured to the moment of pressure rise indication. Two basic injection methods in CIE: Diesel Combustion 1- Direct injection (single open C.C.) 2- Indirect injection (divided chamber- fuel injected in pre-chamber) For large engine (low speed engines)- mixing time is long – DI in open or shallow bowl chamber type is used. For small (high speed) engines swirl is not sufficient so IDI is used to enhance turbulence in pre-chamber during compression and products/fuel blowdown and mix with air in the main chamber (deep bowl in piston is used)  11/28/2020 The ignition characteristics of fuel affect the ignition delay and the ignition quality of a fuel is defined by its Cetane number CN. For low cetane fuels the ignition delay is long and most of the fuel is injected before auto-ignition and rapidly burns, under extreme cases this produces an audible knocking sound referred to as “diesel knock”. For high cetane fuels the ignition delay is short and very little fuel is injected before auto-ignition, the heat release rate is controlled by the rate of fuel injection and fuel-air mixing – smoother engine operation. Cetane Number Factors Affecting Ignition Delay The method used to determine the ignition quality in terms of CN is analogous to Injection timing – At normal engine conditions the minimum delay occurs with that used for determining the antiknock quality using the ON. the start of injection at about 10-15 BTC. The cetane number scale is defined by blends of two pure hydrocarbon The increase in the delay time with earlier or later injection timing occurs reference fuels. because of the air temperature and pressure during the delay period. By definition, isocetane (heptamethylnonane, HMN) has a cetane number of 15 and cetane (n-hexadecane, C16H34) has a value of 100. Injection quantity – For a CI engine the air is not throttled so the load is varied by changing the amount of fuel injected. In the original procedures a-methylnaphtalene (C11H10) with a cetane number of zero represented the bottom of the scale. This has since been replaced by Increasing the load (bmep) increases the residual gas and wall temperature HMN which is a more stable compound. The higher the CN the better the which results in a higher charge temperature at injection which translates to ignition quality, i.e., shorter ignition delay. a decrease in the ignition delay. The cetane number is given by: Intake air temperature and pressure – an increase in either of them will result in a decrease in the ignition delay, an increase in the compression ratio CN = (% hexadecane) + 0.15 (% HMN) has the same effect.  11/28/2020 Disadvantages of Diesel Engines Diesel Engine Characteristics (compared to SI engines) Cold start difficulty Noisy - sharp pressure rise: cracking noise Better fuel economy Inherently slower combustion Lower power to weight ratio – Overall lean, thermodynamically efficient Expensive components NOx and particulate matters emissions – Large displacement, low speed – lower FMEP – Higher CR Diesel driving fuel economy ~ 30% better than SI: > CR limited by peak pressure, NOx emissions, combustion and heat transfer loss 1- 5% from fuel energy/volume 2- 15% from eliminating throttle loss – Turbo-charging not limited by knock: higher BMEP over domain of operation, 3-10% from thermodynamics: lower relative losses (friction and heat transfer) a- 2nd law losses (friction and heat transfer) b- Higher compression ratio Lower Power density c- Higher specific heat ratio – Overall lean: would lead to smaller BMEP  Dominant world wide heavy duty applications – Turbocharged: would lead to higher BMEP  Dominant military applications > not knock limited, but NOx limited  Significant market share in Europe > BMEP higher than naturally aspirated SI engine - Tax structure for fuel and vehicle – Lower speed: overall power density (P/VD) not as high as SI engines Small passenger car market fraction in US and Japan Emissions: more problematic than SI engine  Fuel cost – NOx: needs development of efficient catalyst  Customer preference – PM: regenerative and continuous traps  Emissions requirement Typical SI and Diesel operating value comparisons 11/28/2020