Alternative Fuels & Advance in IC Engines IIT Kanpur Kanpur, India (208016) Combustion in SI Engine Course Instructor Dr. Avinash Kumar Agarwal Professor Department of Mechanical Engineering Indian Institute of Technology Kanpur, Kanpur 1
Introduction of SI Engines Fundamentals of Combustion Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in the fuel with the oxygen in the air resulting in liberation of energy in the form of heat. The conditions necessary for combustion are: The presence of a combustible mixture Some means of initiation of combustion Stabilization and propagation of flame in the combustion chamber In SI engines the combustible mixture is generally supplied by the carburetor and the combustion is initiated by an electric spark given by a spark plug. 2
ESSENTIAL FEATURES OF PROCESS In a conventional spark-ignition engine the fuel and air are mixed together in the intake system, inducted through the intake valve into the cylinder, where mixing with residual gas takes place, and then compressed. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge. Following inflammation, a turbulent flame develops, propagates through this essentially premixed fuel, air, burned gas mixture until it reaches the combustion chamber walls, and then extinguishes. Combustion normally begins at the spark plug where the molecules in and around the spark discharge is activated to a level where reaction is self sustaining. This level is achieved when the energy released by combustion is slightly greater than the heat loss to the metal & gas surroundings. Initially, flame speed is very low as the reaction zone must be established, and heat loss to the spark plug is high as it is located near the cold walls. During this period, pressure rise is also small because the mass of mixture burned is small. Unburned gas ahead of flame front, and the burned gas behind the flame front, are raised in temperature by compression, either by a moving piston or by heat transfer from advancing flame. In the final stage, flame slows down as it approaches the walls of the combustion chamber (from heat loss & low turbulence) and is finally extinguished (wall quenching). During combustion, cylinder pressure rises due to release of fuel s chemical energy. Gas expansion compresses unburned mixture ahead of flame & displaces it towards the walls, and burned gases are pushed towards the spark plug. Elements of unburned mixture which burn at different times have different pressure and temperature just prior to combustion and thus end up at different thermodynamic states after combustion. Thermodynamic state & composition of burned gas is non-uniform within cylinder 3
Experimental observation: Flame structure and speed Combustion in SI engine takes place in turbulent flow field. The approximately spherical development of flame from the vicinity of the spark plug. The geometry of combustion chamber and spark plug govern the flame front surface area. Larger the surface area greater the mass of fresh charge that cross the surface and enters the fame zone. The center plug location gives twice the flame area of side flame geometry and burn about twice as fast. Mixture burning rate is highly influenced by the engine speed. Fig. Flame at the fixed crank angle after the ignition Flame structure a. Flame front, b. flame back in square cross section of cylinder Intensity of light scattered as a function of distance from flame 10 mm diameter section of flame with increasing turbulent Reynolds's number 4
Laminar burning speeds Laminar flame velocity for several fuels as a function of equivalence ratio Flame propagation relation Variation of flame geometry and velocity parameters during four individual combustion cycle 5
Flame propagation relation Variation of flame geometry and velocity parameters during four individual combustion cycles Cyclic variation in combustion, partial burning and misfire Observation and definition: Cycle by cycle variable parameters as follows: 1. Pressure related parameters 2. Burn rate related parameters 3. Flame front position parameters Individual cycle a. maximum pressure versus crank angle, b. mean pressure versus crank angle Measured cylinder pressure and calculated gross heatreleased rate for 10 cycle 6
Cause of cycle by cycle and cylinder to cylinder variation Cycle by cycle variation seen from beginning of the combustion process. Dispersion in burning rate is evident throughout the combustion process. The cause of variation in dispersion are as follows: 1. The variation of gas motion in cylinder during the combustion cycle by cycle 2. Variation in amount of fuel air and recycled exhaust gas supplied to a given cylinder each cycle. 3. Variation in mixture composition within cylinder. a. Air fuel ratio variation for 50 cycle, b.co2 and unburned HC concentration just prier to ignition Partial burning, misfire and engine stability Combustion phenomena at the engine stable operation insoles the following term: 1. Ignition limited spark timing ( The ignition limit) 2. Partial burn limited spar timing ( The partial burn limit) 3. Lean mixture limit at MBT spark Actual combustion regimes for lean operating engine Three possible combinations of ignition limit as function of fuel/air equivalence ratio 7
Abnormal Combustion: Knock & Surface Ignition The abnormal combustion phenomena are of concern because: 1. when severe, they can cause major engine damage; 2. even if not severe, these are objectionable source of noise. Abnormal Combustion: Knock & Surface Ignition 8
Ignition fundamentals Ignition of the charge is only possible within certain limits of air-fuel ratio. For hydrocarbon fuel, the stoichiometric air-fuel ratio is ~15:1. Ignition limit for hydrocarbon fuel is must lie between 7:1 to 30:1. The lower and upper ignition limits of the mixture depend upon mixture ratio and temperature. The ignition limits are wider at increased temperatures because of higher rates of reaction and higher thermal diffusivity coefficients of the mixture. Conventional Ignition Systems Spark plug A spark plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. Fundamental requirements of the ignition source: A high ignition voltage to break down in the spark-gap A low source impedance or steep voltage rise A high energy capacity to create a spark kernel of sufficient size Sufficient duration of the voltage pulse to ensure ignition 9
For a spark to jump across an air gap of 0.6 mm in an engine cylinder, having a compression ratio of 8:1, approx. 8 kv is required. Ignition system has to transform battery voltage of 12 V to 8-20 kv and, has to deliver the voltage to the right cylinder, at the right time. About 0.2 mj of energy is required to ignite a stoichiometric mixture at normal engine operating conditions by means of a spark. Over 3 mj is required for a rich or lean mixture. In general, ignition systems deliver 30 to 50 mj of electrical energy to the spark. 10
Alternative Ignition Approaches Laser ignition Laser ignition is an alternative method for igniting compressed gaseous mixture of fuel and air. The method is based on laser devices that produce short but powerful flashes regardless of the pressure in the combustion chamber. Advantages of Laser ignition over conventional spark plug : Possibility of free positioning of plasma Absence of electrode erosion effects Feasibility to ignite leaner mixtures together with higher ignition pressures, which ultimately result in higher efficiency, and Higher precision in ignition timings The possibility of multipoint ignition allows combustion to be initiated by two or more plasma sparks at multiple locations at the same time, which leads to Reduction in maximum distance of flame travel, and Shorten the combustion duration significantly. 11
Ignition in SI engines When the mixture could be ignited: (a) Spark energy must be higher than the minimum energy of ignition of the mixture, (b) Distance between electrodes is larger than the extinguishing distance for a given mixture, ( c) Local gradient of velocity is smaller than the critical for a given mixture. Improvement of spark ignition effectiveness 1. Energy of spark generated by spark-plug is in the range of 50-100 mj. This is enough for ignition of stoichiometric mixture, but it could be not enough for the lean mixtures. 2. To improve the effectiveness of spark ignition of lean mixtures a few modifications of SI engines ignition systems have been proposed: 2- spark-plug systems (twin-spark), Increase of ignition energy by: o Increase of spark energy o Laser ignition Increase the distance between electrodes, Stages of Combustion in SI Engines Detailed process of combustion in actual engine is different from the theoretical process. Sir Ricardo describes the combustion process in an actual SI engine as consisting of three stages: A shows the point of the passage of the spark (about 20 BTDC), B the point at which the first rise of the pressure can be seen (about 8 BTDC) and C the attainment of the peak pressure. AB represent the first stage (Ignition lag) BC represent the second stage (Propagation of flame) CD represent the third stage (Afterburning) 12
Stages of Combustion in SI Engines Ignition Lag 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. Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self ignition temperature, get ignited and produce a self propagating nucleus of flame. Ignition lag is very small and lies between 0.00015 to 0.0002 seconds. An ignition lag of 0.002 seconds corresponds to 35 deg crank rotation when the engine is running at 3000 RPM. This is a chemical process depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or burning. Propagation of flame Once the flame is formed, it should be self sustained and must be able to propagate through the mixture. This is possible when the rate of heat generation by burning is greater than heat lost by flame to surrounding. The starting point of the second stage is where first measurable rise of the pressure can be seen on the indicator diagram. This stage is also called as main stage as about 87% energy evolved in this stage. Stages of Combustion in SI Engines Afterburning Combustion does not terminate at this point and afterburning continues for a long time near the walls and behind the turbulent flame front. The combustion rate in this stage reduces due to surface of the flame front becoming smaller and reduction in turbulence. About 10% or more heat is evolved in after-burning stage. 13
Factors Influencing The Flame Speed Turbulence Turbulence increases the heat flow to the cylindrical wall. It also accelerates the chemical reaction by intimate mixing of fuel and oxygen so that spark advance may be reduced. This helps in burning lean mixture also. The increase of flame speed due to turbulence reduces the combustion duration and hence minimizes the tendency of abnormal combustion. However, excessive turbulence may extinguish the flame resulting in rough and noisy operation of the engine. Temperature and Pressure Flame speed increases with an increase in intake temperature and pressure. A higher initial pressure and temperature may help to form a better homogeneous mixture which helps in increasing the flame speed. This is possible because of an overall increase in the density of the charge. Engine Speed The flame speed increases almost linearly with engine speed. Since the increase in engine speed increases the turbulence inside the cylinder. Factors Influencing The Flame Speed Fuel-Air Ratio The Fuel-air ratio has a very significant influence on the flame speed. The highest flame velocities (minimum time for complete combustion) are obtained with somewhat richer mixture. When the mixture is made leaner or richer the flame speed decreases. Less thermal energy is released in the case of lean mixtures resulting in lower flame temperature. Very rich mixtures lead to incomplete combustion which results in the release of less thermal energy. 14
Factors Influencing The Flame Speed Engine Output The cycle pressure increases when the engine output is increased. With the increased throttle opening the cylinder gets filled to a higher density. This results in increased flame speed. Compression Ratio A higher compression ratio increases the pressure and temperature of the working mixture which reduces the initial preparation phase of combustion and hence less ignition advance is needed. Thus engines having higher compression ratios have higher flame speeds. Engine Size The size of the engine does not have much effect on the rate of flame propagation. In large engines the time required for complete combustion is more because the flame has to travel a longer distance. This requires increased crank angle duration during the combustion. This is one of the reasons why large sized engines are designed to operate at low speeds. Abnormal Combustion : Knock and pre-ignition Knocking (also called knock, detonation, spark knock, pinging or pinking) in sparkignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder does not start off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. Detonation can be prevented by any or all of the following techniques: Using a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate enriching the air fuel ratio which alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin above detonation reducing peak cylinder pressure decreasing the manifold pressure by reducing the throttle opening or boost pressure reducing the load on the engine retarding (reduce) ignition timing Knocking should not be confused with pre-ignition they are two separate events. However, preignition is usually followed by knocking. 15
Abnormal Combustion : Knock and pre-ignition Pre-ignition in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Causes of pre-ignition include the following: Carbon deposits form a heat barrier and can be a contributing factor to pre-ignition. Other causes include: An overheated spark plug (too hot a heat range for the application). Glowing carbon deposits on a hot exhaust valve (which may mean the valve is running too hot because of poor seating, a weak valve spring or insufficient valve lash) A sharp edge in the combustion chamber or on top of a piston (rounding sharp edges with a grinder can eliminate this cause) Sharp edges on valves that were reground improperly (not enough margin left on the edges) A lean fuel mixture An engine that is running hotter than normal due to a cooling system problem (low coolant level, slipping fan clutch, inoperative electric cooling fan or other cooling system problem) Auto-ignition of engine oil droplets Insufficient oil in the engine The Phenomenon of Knock in SI Engines Fig (a) shows the cross-section of the combustion chamber with flame advancing from the spark plug location A without knock. Fig. (c) shows the combustion process with knock. In the normal combustion the flame travels across the combustion chamber from A towards D. The advancing flame front compresses the end charge BB D farthest from the spark plug, thus raising its temperature. The temperature is also increased due to heat transfer from the hot advancing flame front. 16
Phenomenon of Knock in SI Engines The temperature of the end charge had not reached its self-ignition temperature, the charge would not auto-ignite and the flame will advance further and consume the charge BB D. This is the normal combustion process. If the end charge BB D reaches its auto-ignition temperature and remains for some length of time equal to the time of pre flame reactions the charge will auto-ignite, leading to knocking combustion. When flame has reached the position BB, the charge ahead of it has reached critical autoignition temperature. During the pre flame reaction period if the flame front could move from BB to only CC then the charge ahead of CC would auto-ignite. Because of the auto-ignition, another flame front starts traveling in the opposite direction to the main frame front. When the two flame fronts collide, a severe pressure pulse is generated. The gas in the chamber is subjected to compression and rarefaction along the pressure pulse until pressure equilibrium is restored. This disturbance can force the walls of the combustion chamber to vibrate as the same frequency as the gas. This phenomenon is called knocking in SI engines. Effect of Engine Variables on Knock Time factor Increasing the flame speed or reducing the exposure of unburned charge to auto-ignition will reduce knocking. Increase in turbulence increase the flame speed and reduce time for auto-ignition of unburned charge Increase in engine speed increase the turbulence and reduce knocking. In larger engine flame require more time to travel across combustion chamber therefore it has greater tendency of knocking. Combustion chambers are made as spherical as possible to minimize length of flame travel. Spark plug is centrally located in in combustion chamber to reduce flame travel. Composition factor Fuel-air ratio and octane number play important role to reduce knock. Reaction rate and flame temperature are affected by fuel air ratio. Maximum tendency of knock takes place for fuel-air ratio which give minimum reaction time. Higher octane number fuels have lower tendency for knocking. 17
Effect of Engine Variables on Knock Density factors Reducing the density of charge tends to reduce knocking. Increase in compression ratio will increase the density of charge which increases knocking tendency of engine. Reducing the amount of charge induced by throttling or by reducing supercharging will reduce density. Increase in inlet temperature of mixture makes compression temperature higher which increase tendency to knock. End gas should not be compressed against spark plug and exhaust wall as they are the hottest parts. Decrease in output of engine will decrease the temperature of cylinder. 18
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 a shock wave P,T end-gas flame shock P,T 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! Parameters Influencing Knock 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. 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. 19
Exhaust valve Spark plug Knock cycle Normal cycle Intake valve Observation window for photography 39 Effects of Detonation Noise and Roughness When the intensity of the knock increases a loud pulsating noise is produced due to the development of a pressure wave which vibrates back and forth across the cylinder. The presence of vibratory motion causes crankshaft vibrations and engine runs rough. Mechanical Damage In most cases of knocking a local and very rapid pressure rise is observed with subsequent waves of large amplitude, which results in to increasing the rate of wear and erosion of crown. The cylinder head and valves may also be pitted. Detonation is very dangerous in engines having high noise level. In small engines the knocking noise is easily detected and the corrective measures can be taken. In large heavy duty engines, it is difficult to detect knocking noise so corrective measures cannot be taken. If severe detonation may persist for a long time which may result in complete wreckage of piston. Carbon Deposit Increase in carbon deposit. 20
Effects of Detonation Increase in Heat Transfer The increase in heat transfer is due to two reasons: The minor reason is that the maximum temperature in a detonating engine is about 150 C higher than in a non-detonating engine, due to a o de o a g e g e, o rapid completion of combustion. The major reason is the scouring away of protective layer of inactive stagnant gas on the cylinder walls due to pressure waves. The inactive layer of gas normally reduces the heat transfer by protecting the combustion chamber walls and piston crown from direct contact with flame. Decreases power output and efficiency Increase in heat transfer decrease power output and efficiency. Pre-Ignition Increase in local heat transfer cause local overheating especially of sparking plug which may reach a temperature high enough to ignite the charge before the passage of spark. It causes preignition. Unexpected Engine Damage Damage to the engine is caused by a combination of high temperature and high pressure. Piston Piston crown Cylinder head gasket Aluminum cylinder head 21
Dangerous Accidents Major differences between the SI and CI engines: 1) Type of cycle used: In the case of SI engines, the Otto cycle is used. In this cycle, addition of heat or fuel combustion occurs at a constant volume. The basis of working of CI engines is the Diesel cycle. In this cycle the addition of heat or fuel combustion occurs at a constant pressure. 2) Introduction of fuel in the engine: In the case of SI engines, during the piston's suction stroke, a mixture of air and fuel is injected from cylinder head portion of the cylinder. The air-fuel mixture is injected via the carburetor that controls the quantity and the quality of the injected mixture. In the case of CI engines, fuel is injected into the combustion chamber towards the end of the compression stroke. The fuel starts burning instantly due to the high pressure. To inject diesel in SI engines, a fuel pump and injector are required. In CI engines, the quantity of fuel to be injected is controlled but the quantity of air to be injected is not controlled. 3) Ignition of fuel: By nature petrol is a highly volatile liquid, but its self-ignition temperature is high. Hence for the combustion of this fuel a spark is necessary to initiate its burning process. To generate this spark in SI engines, the spark plug is placed in the cylinder head of the engine. The voltage is provided to the spark plug either from the battery or from the magneto. With diesel, the self-ignition temperature is comparatively lower. When diesel fuel is compressed to high pressures, its temperature also increases beyond the self-ignition temperature of the fuel. Hence in the case of CI engines, the ignition of fuel occurs due to compression of the air-fuel mixture and there is no need for spark plugs. 22
Major differences between the SI and CI engines: 4) Compression ratio for the fuel: In the case of SI engines, the compression ratio of the fuel is in the range of 6 to 10 depending on the size of the engine and the power to be produced. In CI engines, the compression ratio for air is 16 to 20. The high compression ratio of air creates high temperatures, which ensures the diesel fuel can self-ignite. 5) Weight of the engines: In CI engines the compression ratio is higher, which produces high pressures inside the engine. Hence CI engines are heavier than SI engines. 6) Speed achieved by the engine: Petrol or SI engines are lightweight, and the fuel is homogeneously burned, hence achieving very high speeds. CI engines are heavier and the fuel is burned heterogeneously, hence producing lower speeds. 7) Thermal efficiency of the engine: In the case of CI engines the value of compression ratio is higher; hence these engines have the potential to achieve higher thermal efficiency. In the case of SI engines the lower compression ratio reduces their potential to achieve higher thermal efficiency. 23