SI Engine Combustion Stages PDF

*Stages of SI engine combustion the mixture and also decreases the Three stages of combustion in SI engine are, concentration of residual gases. Ignition lag or preparation phase (AB): 3. Load on Engine: With increase in load, the cycle -...

*Stages of SI engine combustion the mixture and also decreases the Three stages of combustion in SI engine are, concentration of residual gases. Ignition lag or preparation phase (AB): 3. Load on Engine: With increase in load, the cycle -growth and development of a semi propagating pressures increase and the flame speed also nucleus of flame increases. In S.I. engine, the power developed by -ignition lag is generally expressed in terms of crank an engine is controlled by throttling. angle 4. Turbulence: Turbulence plays very important -Ignition lag is very small and lies between 0.00015 role in combustion of fuel as the flame speed is to 0.0002 seconds directly proportional to the turbulence of the propagation of flame (BC): mixture. -Period from the point B where the line of Engine Speed- The turbulence of the combustion departs from the compression line to mixture increases with an increase in point C, the maximum rise of pressure in P-θ diagram engine speed. For this reason the flame After burning (CD): speed almost increases linearly with -After point C, the heat release is due to the fuel engine speed. injection in reduced flame front after the starts of Engine Size- Engines of similar design expansion stroke -no pressure rise during this period generally run at the same piston speed. *Effect of engine variables on Ignition lag This is achieved by using small engines Fuel: High self-ignition temperature of fuel longer having larger RPM and larger engines the ignition lag. having smaller RPM. Mixture ratio: mixture richer than the stoichiometric 5. Other Factors: Among the other factors, the ratio provide shorter ignition lag factors which increase the flame speed are Initial temperature and pressure: increasing the supercharging of the engine, spark timing and intake temperature and pressure, increasing the residual gases left in the engine at the end of compression ratio, chemical reaction rate and exhaust stroke retarding the spark all reduce the ignition lag *Detonation or knocking Electrode gap: lower the compression ratio and -Knocking is due to auto ignition of end portion of higher the electrode gap is desirable -voltage unburned charge in combustion chamber. required at the spark plug electrode to produce *Auto ignition spark is found to increase with decrease in fuel-air -A mixture of fuel and air can react spontaneously ratio and with increase in compression ratio and and produce heat by chemical reaction in the engine load absence of flame to initiate the combustion or self- Turbulence: directly proportional to engine speed - ignition. This type of self-ignition in the absence of engine speed does not affect much ignition lag flame is known as Auto-Ignition. measured in milliseconds *Pre-ignition -Pre-ignition is the ignition of the homogeneous *Effect of engine variables on flame propagation mixture of charge as it comes in contact with hot -Rate of flame propagation affects the combustion surfaces, in the absence of spark. process in SI engines. Effects of Pre-ignition -It increase the tendency of The factors which affect the flame propagations are denotation in the engine -It increases heat transfer 1. A:F ratio: The mixture strength influences the to cylinder walls because high temperature gas rate of combustion and amount of heat remains in contact with for a longer time generated. 2. Compression ratio: The higher compression ratio increases the pressure and temperature of *Effect of detonation minimum tendency to knock as the flame travel is 1. Noise and roughness- knocking produces a loud minimum. pulsating noise and pressure waves. These waves -Location of exhaust valve: The exhaust valve should vibrate back and forth across the cylinder. be located close to the spark plug so that it is not in 2. Mechanical damage- (a)High pressure waves the end gas region; otherwise there will be a generated during knocking can increase rate of wear tendency to knock. of parts of combustion chamber. (b) Detonation is -Engine size: Large engines have a greater knocking very dangerous in engines having high noise level. tendency because flame requires a longer time to 3. Carbon deposits- Detonation results in increased travel across the combustion chamber. carbon deposits. -Turbulence of mixture: decreasing the turbulence 4. Increase in heat transfer- Knocking is of the mixture decreases the flame speed and hence accompanied by an increase in the rate of heat increases the tendency to knock. Turbulence transfer to the combustion chamber walls. depends on the design of combustion chamber and 5. Decrease in power output and efficiency- Due to one engine speed. increase in the rate of heat transfer the power (d) Composition: output as well as efficiency of a detonating engine The properties of fuel and A/F ratio are primary decreases. means to control knock : 6. Pre-ignition- increase in the rate of heat transfer (i) Molecular Structure: The knocking tendency to the walls has yet another effect. is markedly affected by the type of the fuel used. *Effect of engine operating variables on the engine Paraffins knocking Detonation Olefins (a) Temperature factors: Napthenes and Aromatics -Raising the compression ratio: Increasing the (ii) Fuel-air ratio: The most important effect of compression ratio increases both the temperature fuel-aft ratio is on the reaction time or and pressure (density of the unburned mixture). ignition delay. -Supercharging: It also increases both temperature (iii) Humidity of air: Increasing atmospheric and density, which increase the knocking tendency humidity decreases the tendency to knock of engine by decreasing the reaction time of the fuel -Coolant temperature: Delay period decreases with *SI engine combustion chamber increase of coolant temperature, decreased delay (a) T-head combustion chamber: 2 cam shaft, prone period increase the tendency to knock to detonation, average octane number 45 50 -Temperature of the cylinder and combustion (b) I-head or side valve combustion chamber: -lack chamber walls: The temperature of the end gas of turbulence depends on the design of combustion chamber. -extremely sensitive to ignition timing (b) Density factors: -prone to detonation -Advanced spark timing: quantity of fuel burnt per (c) F-head combustion chamber: -Compromise cycle before and after TDC position depends on spark between I-head and L-head timing. *Stages of combustion in CI engine (c) Time factors: 1. Ignition delay period: The period between the -Flame travel distance: If the distance of flame travel start of fuel injection into the combustion chamber is more, then possibility of knocking is also more. and the start of combustion is termed as ignition -Location of sparkplug: A spark plug which is delay period. centrally located in the combustion chamber has Physical processes- are fuel spray atomization, 7.Swirl rate - Change of evaporation rate and air-fuel evaporation and mixing of fuel vapour with cylinder mixing - under normal operating conditions the air. effect is small. Chemical processes similar to that described for auto 8. Oxygen concentration- Residual gases reduce O2 ignition phenomenon in premixed fuel air, only more concentration and reducing oxygen concentration complex since heterogeneous reactions (reactions increases ID occurring on the liquid fuel drop surface) also occur. 2. Rapid or uncontrolled or pre-mixed combustion *Diesel knock phase: Combustion of the fuel which has mixed with -CI engine detonation occurs in the beginning of air within flammability limits during ignition delay combustion period occurs rapidly in a few crank angle degrees - -In CI engine the fuel and air are imperfectly mixed high heat release characteristics in this phase. and hence the rate of pressure rise is normally cause 3. Controlled or diffusion combustion phase: Once audible knock. Rate of pressure rise may reach as the fuel and air which is pre-mixed during the high as 10 bar/ᵒCA ignition delay is consumed, the burning rate (heat release rate) is controlled by the rate at which *Combustion chamber mixture becomes available for burning. According to the swirl of air 4. After burning or late combustion phase: Heat According to speed of the engine release rate continues at a lower rate into the (a) Low speed engine (n˂1500 rpm): expansion stroke -there are several reasons for this: (b) Medium speed engine (n=1500-3000 a small fraction of the fuel may not yet burn, a rpm): fraction of the energy is present in soot and fuel-rich (c) high speed engine (n=3000-5000 rpm): combustion products and can be released. * Variables affecting delay period 1. Cetane number- Both physical and chemical properties of the fuel are important. Ignition quality of the fuel is defined by its cetane number. 2. Injection timing- At normal operating conditions min ignition delay (ID) occurs with start of injection at 10 to 15 OCA BTDC 3. Injection quantity (load)- Reducing engine load changes AFR, cools down the engine, reduces wall temperatures, reduces residual gas temperatures and increase ID 4. Intake air temperature and pressure -Reducing intake air T and p increase ID -Strong dependence of ID on charge temperature below 1000 K – above this value effect of intake air conditions is not significant. 5. Engine speed - A change in engine speed, changes “temp~time” and “pressure~time” relationships 6. Combustion chamber design- Spray impingement on the walls effect fuel evaporation and ID increase in compression ratio, increase p and T and reduces ID