Combustion in IC Engines PDF

Combustion in IC Engines Dr. Suad Alhaj Mustafa 1  Flame Propagation in SI Engine After the intake stroke the fuel-air mixture is compressed and then ignited by a spark plug just before the piston reaches top center The turbulent flame spreads awa...

Combustion in IC Engines Dr. Suad Alhaj Mustafa 1  Flame Propagation in SI Engine After the intake stroke the fuel-air mixture is compressed and then ignited by a spark plug just before the piston reaches top center The turbulent flame spreads away from the spark discharge location. Flow N = 1400 rpm Pi = 0.5 atm 2  In-cylinder Parameters Tu – unburned gas temperature k 1 Tb,e – early burning gas elements T2  P2  k Tb,l – late burning gas elements    T1  P1  3  Flame Development Flame development angle Dqd – crank angle interval during which flame kernal develops after spark ignition. Rapid burning angle Dqb – crank angle required to burn most of mixture Overall burning angle - sum of flame development and rapid burning angles Mass fraction burned CA 4  Mixture Burn Time vs Engine Speed The time for an overall burn is: Dq90% t90%   min   360  o N    60s   rev  If we take a typical value of 50 crank angles for the overall burn N (rpm) t90%(ms) Standard car at idle 500 16.7 Standard car at max power 4,000 2.1 Formula car at max power 19,000 0.4 Note: To achieve such high engine speeds a formula car engine has a very short stroke and large bore. 5  Mixture Burn Time B B/2 5 cm tcomb    0.2 s Sl 25 cm / s How does the flame burn all the mixture in the cylinder in the time available, especially at high engine speeds? 6  Mixture Burn Time vs Engine Speed Recall the turbulent burning velocity is proportional to the turbulent intensity St ~ ut, which increases with the piston speed , ut = ½ up The piston speed is directly proportional to the engine speed, up ~ N Therefore, at higher engine speeds the turbulent flame velocity is also higher and as a result need less time to burn the entire mixture Combustion duration in crank angles (40-60 degrees) only increases a small amount with increasing engine speed. f = 1.0 Pi =0.54 atm Spark 30o BTC 7  Heat Losses During Burn During combustion the cylinder volume is very narrow. Heat loss to the piston and cylinder head is very important In order to reduce the heat loss want burn time to be small (high flame velocity) accomplished by either increasing the - laminar burning velocity, or - turbulence intensity. Highest laminar burning velocity is achieved for slightly rich mixtures (for isooctane maximum Sl = 26.3 cm/s at f  1.13) Squish promotes in-cylinder turbulence 8  Optimum F/A Composition Maximum power is obtained for a f = 1.1 that gives the highest burning velocity (minimum heat loss) and flame temperature (maximum PCV) Best fuel economy is obtained for a F/A that is less than 1.0 9  Spark Timing Spark timing relative to TC affects the pressure development and thus the engine imep and power. Ignite the gas before TC to center the pressure pulse around TC. The overall burning angle is typically between 40 to 60o, depending on engine speed. Engine at WOT, constant engine speed and A/F motored 10  Maximum Brake Torque Timing If start of combustion is too early work is done against piston and if too late then peak pressure is reduced. The optimum spark timing that gives the maximum brake torque, called MBT timing occurs when these two opposite factors cancel. Engine at WOT, constant engine speed and A/F 11  Effect of Engine Speed on Spark Timing Recall the overall burn angle (90% burn) increases with engine speed, to accommodated this you need a larger spark advance. Fixed spark advance WOT Brake Torque N* MBT Fixed engine speed Brake Torque CA* 12  Effect of Throttle on Spark Timing At part-throttle the residual gas fraction increases, and since residual gas represents a diluent it lowers the laminar burning velocity. Because of lower burning velocity overall burn angle increases so need to increase spark advance. At idle, the residual gas fraction is very high  the burn time is very long  long overall burn angle requires more spark advance. In modern engines the ECU sets the spark advance based on engine data such as: throttle position, intake manifold pressure and engine speed. 13  Abnormal Combustion in SI Engine Knock is the term used to describe a pinging noise emitted from a SI engine undergoing abnormal combustion. The noise is generated by shock waves produced in the cylinder when unburned gas autoignites. 14  Engine Damage From Severe Knock Damage to the engine is caused by a combination of high temperature and high pressure. Piston Piston crown 15 Cylinder head gasket Aluminum cylinder head  Knock cycle Exhaust valve Spark plug Normal cycle Intake valve Observation window for photography 16  Knock As the flame propagates away from the spark plug the pressure and temperature of the unburned gas increases. Under certain conditions the end-gas can autoignite and burn very rapidly producing shock waves. end-gas flame shock P,T P,T P time time The end-gas autoignites after a certain induction time which is dictated by the chemical kinetics of the fuel-air mixture. If the flame burns all the fresh gas before autoignition in the end-gas can occur then knock is avoided. Therefore knock is a potential problem when the burn time is long. 17  Knock Engine parameters that effect occurrence of knock are: i) Compression ratio – at high compression ratios, even before spark ignition, the fuel-air mixture is compressed to a high pressure and temperature which promotes autoignition ii) Engine speed – At low engine speeds the flame velocity is slow and thus the burn time is long, this results in more time for autoignition However at high engine speeds there is less heat loss so the unburned gas temperature is higher which promotes autoignition These are competing effects, some engines show an increase in propensity to knock at high speeds while others don’t. 18  Knock iii) Spark timing – maximum compression from the piston advance occurs at TC, increasing the spark advance makes the end of combustion crank angle approach TC and thus get higher pressure and temperature in the unburned gas just before burnout. x End of combustion P,T T x Ignition 19  Knock Mitigation Using Spark Advance Spark advance set to 1% below MBT to avoid knock x x X crank angle corresponding to borderline knock 1% below MBT x x x x x 20  Fuel Knock Scale To provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by which fuels are assigned an octane number ON. The octane number determines whether or not a fuel will knock in a given engine under given operating conditions. By definition, normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) has a value of 100. The higher the octane number, the higher the resistance to knock. Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90. A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance. 21  Octane Number Measurement Two methods have been developed to measure ON using a standardized single-cylinder engine developed under the auspices of the Cooperative Fuel Research (CFR) Committee in 1931. The CFR engine is 4-stroke with 3.25” bore and 4.5” stroke, compression ratio can be varied from 3 to 30. Research Motor Inlet temperature (oC) 52 149 Speed (rpm) 600 900 Spark advance (oBTC) 13 19-26 (varies with r) Coolant temperature (oC) 100 Inlet pressure (atm) 1.0 Humidity (kg water/kg dry air) 0.0036 - 0.0072 Note: In 1931 iso-octane was the most knock resistant HC, now there are fuels that are more knock resistant than isooctane. 22  Octane Number Measurement Testing procedure: Run the CFR engine on the test fuel at both research and motor conditions. Slowly increase the compression ratio until a standard amount of knock occurs as measured by a magnetostriction knock detector. At that compression ratio run the engines on blends of n-hepatane and isooctane. ON is the % by volume of octane in the blend that produces the stand. knock The antiknock index which is displayed at the fuel pump is the average of the research and motor octane numbers: RON  MON Antiknock index  2 Note the motor octane number is always lower because it uses more severe operating conditions: higher inlet temperature and more spark advance. The automobile manufacturer will specify the minimum fuel ON that will resist knock throughout the engine’s operating speed and load range. 23  Knock Characteristics of Various Fuels Formula Name Critical r RON MON CH4 Methane 12.6 120 120 C3H8 Propane 12.2 112 97 CH4O Methanol - 106 92 C2H6O Ethanol - 107 89 C8H18 Isooctane 7.3 100 100 Blend of HCs Regular gasoline 91 83 n-C7H16 n-heptane 0 0 For fuels with antiknock quality better than octane, the octane number is: ON = 100 + 28.28T / [1.0 + 0.736T+(1.0 + 1.472T - 0.035216T2)1/2] where T is milliliters of tetraethyl lead per U.S. gallon 24  Fuel Additives Chemical additives are used to raise the octane number of gasoline. The most effective antiknock agents are lead alkyls; (i) Tetraethyl lead (TEL), (C2H5)4Pb was introduced in 1923 (ii) Tetramethyl lead (TML), (CH3)4Pb was introduced in 1960 In 1959 a manganese antiknock compound known as MMT was introduced to supplement TEL (used in Canada since 1978). About 1970 low-lead and unleaded gasoline were introduced over toxicological concerns with lead alkyls (TEL contains 64% by weight lead). Alcohols such as ethanol and methanol have high knock resistance. Since 1970 another alcohol methyl tertiary butyl ether (MTBE) has been added to gasoline to increase octane number. MTBE is formed by reacting methanol and isobutylene (not used in Canada). 25  Engine Stability If the fuel-air mixture is leaned out with excess air, or is diluted with increasing amounts of residual gas or exhaust gas recycle, the burn time increases  the cycle-by-cycle fluctuations in the combustion process increases. Eventually a point is reached where engine operation becomes rough and unstable, this point defines the engine’s stable operating limit. With no or little dilution combustion occurs prior to the exhaust valve opening consistently cycle after cycle. With increasing dilution, first, in a fraction of the cycles the burns are so slow that combustion is only just completed prior to the exhaust valve opening. As dilution increases further, in some cycles combustion is not complete prior to the exhaust valve opening and the flame extinguishes before all the fuel is burned. Finally misfire cycles start to occur where the mixture is not ignited. As the dilution is further increased the proportion of partial burns and misfires increase to a point where the engine no longer runs. 26  Effect of Fuel-air Dilution Set spark timing for MBT, leaner mixture needs more spark advance since burn time longer. Along MBT curve as you increase excess air reach partial burn limit (not all cycles result in complete burn) and then ignition limit (misfires start to occur). Ignition limit Partial burn limit Complete burns in all cycles MBT spark timing Partial burn regime 27  Combustion in CI Engine In a CI engine the fuel is sprayed directly into the cylinder and the vaporised part of the fuel mixes with air and ignites spontaneously. These photos are taken in a RCM under CI engine conditions with swirl 1 cm Air flow 0.4 ms after ignition 3.2 ms after ignition 28 3.2 ms after ignition Late in combustion process  In-Cylinder Measurements This graph shows the fuel injection flow rate, net heat release rate and cylinder pressure for a direct injection CI engine. Start of injection Start of combustion End of injection 29  Combustion in CI Engine The combustion process proceeds by the following stages: Ignition delay (ab) - fuel is injected directly into the cylinder towards the end of the compression stroke. The liquid fuel atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure air. Premixed combustion phase (bc) – combustion of the fuel which has mixed with the air to within the flammability limits (air at high-temperature and high- pressure) during the ignition delay period occurs rapidly in a few crank angles. Mixing controlled combustion phase (cd) – after premixed gas consumed, the burning rate is controlled by the rate at which mixture becomes available for burning. The burning rate is controlled primarily by the fuel-air mixing process. Late combustion phase (de) – heat release may proceed at a lower rate well into the expansion stroke (no additional fuel injected during this phase). Combustion of any unburned liquid fuel and soot is responsible for this. 30  Four Stages of Combustion in CI Engines Start of End of injection injecction -20 -10 TC 10 20 30 31  CI Engine Types Two basic categories of CI engines: i) Direct-injection – have a single open combustion chamber into which fuel is injected directly ii) Indirect-injection – chamber is divided into two regions and the fuel is injected into the “prechamber” which is connected to the main chamber via a nozzle, or one or more orifices. For very-large engines (stationary power generation) which operate at low engine speeds the time available for mixing is long so a direct injection quiescent chamber type is used (open or shallow bowl in piston). As engine size decreases and engine speed increases, increasing amounts of swirl are used to achieve fuel-air mixing (deep bowl in piston) For small high-speed engines used in automobiles chamber swirl is not sufficient, indirect injection is used where high swirl or turbulence is generated in the pre-chamber during compression and products/fuel blowdown and mix 32 with main chamber air.  Types of CI Engines Glow plug Orifice -plate Direct injection: Direct injection: quiescent chamber swirl in chamber Indirect injection: turbulent and swirl pre-chamber 33 Direct Injection Direct Injection Direct Injection Indirect injection quiescent chamber multi-hole nozzle single-hole nozzle swirl pre-chamber swirl in chamber swirl in chamber 34  Combustion Characteristic Combustion occurs throughout the chamber over a range of equivalence ratios dictated by the fuel-air mixing before and during the combustion phase. In general most of the combustion occurs under very rich conditions within the head of the jet, this produces a considerable amount of solid carbon (soot). 1o 5o ASI ASI Shadow graph Backlit photo Liquid fuel High soot Fuel vapour Diffusion flame 35  Ignition Delay Ignition delay is defined as the time (or crank angle interval) from when the fuel injection starts to the onset of combustion. Both physical and chemical processes must take place before a significant fraction of the fuel chemical energy is released. Physical processes are fuel spray atomization, evaporation and mixing of fuel vapour with cylinder air. Good atomization requires high fuel pressure, small injector hole diameter, optimum fuel viscosity, high cylinder pressure (large divergence angle). Rate of vaporization of the fuel droplets depends on droplet diameter, velocity, fuel volatility, pressure and temperature of the air. Chemical processes similar to that described for autoignition phenomenon in premixed fuel-air, only more complex since heterogeneous reactions (reactions occurring on the liquid fuel drop surface) also occur. 36  Fuel Ignition Quality The ignition characteristics of the fuel affect the ignition delay. 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 autoignition and rapid combustion, 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 autoignition, the heat release rate is controlled by the rate of fuel injection and fuel-air mixing – smoother engine operation. 37  Cetane Number The method used to determine the ignition quality in terms of CN is analogous to that used for determining the antiknock quality via the ON. The cetane number scale is defined by blends of two pure hydrocarbon reference fuels. By definition, isocetane (heptamethylnonane, HMN) has a cetane number of 15 and cetane (n-hexadecane, C16H34) has a value of 100. 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 HMN which is a more stable compound. The higher the CN the better the ignition quality, i.e., shorter ignition delay. The cetane number is given by: CN = (% hexadecane) + 0.15 (% HMN) 38  Cetane Number Measurement The method employed to measure CN uses a standardized single-cylinder engine with variable compression ratio The operating condition is: Inlet temperature (oC) 65.6 Speed (rpm) 900 Start of fuel injection (oBTC) 13 Coolant temperature (oC) 100 Injection pressure (MPa) 10.3 With the engine running at these conditions on the test fuel, the compression ratio is varied until combustion starts at TC  ignition delay period of 13o. The above procedure is repeated using blends of cetane and HMN. The blend that gives a 13o ignition delay with the same compression ratio is used to calculate the test fuel cetane number. 39  Cetane Number versus Octane Number The octane number and cetane number of a fuel are inversely correlated. Cetane motor method octane number Octane-heptane Alcohol-gasoline Cetane number Gasoline is a poor diesel fuel and vice versa. 40  Factors Affecting Ignition Delay Time Injection timing – At normal engine conditions the minimum delay occurs with the start of injection at about 10-15 BTC. Earlier or later injection timing results in a lower air temperature and pressure during the delay period  increase in the ignition delay time Injection quantity – For a CI engine the air is not throttled so the load is varied by changing the amount of fuel injected. Increasing the load (bmep) increases the residual gas and wall temperature which results in a higher charge temperature at injection  decrease in the ignition delay. Intake air temperature and pressure – an increase in ether will result in a decrease in the ignition delay, an increase in the compression ratio has the same effect. 41 Factors Affecting Ignition Delay (gauge) 42