Chapter 7: Combustion Dr Ali Jawarneh Department of Mechanical Engineering Hashemite University
Outline In this lecture we will discuss the combustion process: The characteristics of the process. The different phases of the process. The factors affecting it.
COMBUSTION IN SI ENGINES The combustion process of SI engines can be divided into three broad regions: (1) ignitionand flame development, (2) flame propagation, and (3) flame termination. Flame development is generally considered the consumption of the first 5% 10% of the air fuel mixture. During the flame development period, ignition occurs and the combustion process starts, but very little pressure rise is noticeable and little or no useful work is produced. Just about all useful work produced in an engine cycle is the result of the flame propagation period of the combustion process. This is the period when the bulk of the fuel and air mass is burned (i.e., 80 90%). During this time, pressure in the cylinder is greatly increased, and this provides the force to produce work in the expansion stroke. The final 5% 10% of the air fuel mass which burns is classified as flame termination. During this time, pressure quickly decreases and combustion stops. In an SI engine, combustion ideally consists of an exothermic subsonic flame progressing through a premixed homogeneous air fuel mixture. The spread of the flame front is greatly increased dby induced dturbulence and swirl ilwithin ihi the cylinder. The right ih combination of fuel and operating characteristics is such that knock is avoided or almost avoided.
Spark: 10-30 0 btdc Start End of combustion: 15 btdc -15 0 atdc Max press.: 5-10 atdc Flame can generally be detected at about 6 of crank rotation after spark plug firing. Burn angle: 25 0
(1) Ignition and Flame Development Combustion is initiated by an electrical discharge across the electrodes of a spark plug. This occurs anywhere from 10 to 30 btdc. Combustion starts very slowly because of the high heat losses to the relatively cold spark plug and gas mixture. Flame can generally be detected at about 6 of crank rotation after spark plug firing. Overall spark discharge lasts about 0.001 second, with an average temperature of about 6000 K. A stoichiometric mixture of hydrocarbon fuel requires about 0.2 mj of energy to ignite self sustaining combustion. This varies to as much as 3 mj for non stoichiometric mixtures. The discharge of a spark plug delivers 30 to 50 mj of energy, most of which, however, is lost by heat transfer.
Ignition and Flame Development Power is supplied through an electrical system which operates usually on low voltage (12 volts battery). The low voltage is amplified to generate the required high voltage using a coil or a capacitor. The gap distance between electrodes on a modern spark plug is about t0.7 to 1.7 mm. Normal quasi steady state temperature of spark plug electrodes between firings should be about 650 to 700 C. Some quality spark plugs with platinum tipped electrodes are made to last 160,000 km (100,000 miles) or more.
(2) Flame Propagation in SI Engines After the first 5 10% of the air fuel mass has been burned, the combustion process is well established and the flame front moves very quickly through the combustion chamber. Due to induced turbulence, swirl, and squish, flame propagation speed is about 10 times faster than if there were a laminar flame front moving through a stationary gas mixture. The flame at this phase is no longer spherical and is highly distorted. As a result of the fast burn rate, a sharp rise in the temperature and pressure of gases occurs.
As the gas mixture burns, the temperature, and consequently the pressure, rises to high values. Burned gases behind the flame front are hotter than the unburned gases before the front, with all gases at about the same pressure. This decreases the density of the burned gases and expands them to occupy a greater percent of the total combustion chamber volume. Figure 7 3 shows that when only 30% of the gas mass is burned the burned gases already occupy almost 60% of the total volume, compressing 70% of the mixture that is not yet burned into 40% of the total volume. Compression of the unburned gases raises their temperature by compressive heating. In addition, radiation heating emitted from the flame reaction zone, which is at a temperature on the order of 3000 K, further heats the gases in the combustion chamber, unburned and burned. A temperature rise from radiation then further raises the pressure. Heat transfer by conduction and convection are minor compared with radiation, due to the very short real time involved in each cycle. Flame Propagation in SI Engines
Flame Propagation in SI Engines Fig. 7-4. Fig. 7-5. Flame speed depends on the type of fuel and the air fuel ratio. Lean mixtures have slower flame speeds, as shown in Fig. 7 4. Slightly rich mixtures have the fastest flame speeds, with the maximum for most fuels occurring at an equivalence ratio near 1.2. Exhaust residual and recycled exhaust gas slows the flame speed. Flame speed increases with engine speed due to the higher turbulence, swirl, and squish (Fig. 7 5) 5).
Flame Propagation in SI Engines
Flame Propagation in SI Engines Burn angle: The angle through which the crankshaft turns during combustion, is about 25 for most engines. Figure 7 6 The increase in the angle of the ignition and flame development period (5% burn) is mainly due to the almost constant real time of the spark ignition process. During flame propagation (5% burn to 95% burn) both combustion speed and engine speed increase, resulting in a fairly constant burn angle of about 25 for the main part of combustion. If combustion is to be completed at 15 atdc, then ignition should occur at about 20 btdc. If ignition iti is too early, the cylinder pressure will increase to undesirable levels btdc, and work will be wasted in the compression stroke. If ignition is late, peak pressure will not occur early enough, and work will be lost at the start of the power stroke due to lower pressure. Actual ignition timing is typically anywhere from 10 to 30 btdc, depending on the fuel used, engine geometry, and engine speed.
Flame Propagation in SI Engines Spark Timing For any given engine, combustion occurs faster at higher engine speed. Real time for the combustion process is therefore less and the real time for the engine cycle is also less, and the burn angle is only slightly changed. This slight change is corrected by advancing the spark[the spark to occur sooner (relative to crankshaft degrees) as RPM increase.] as the engine speed is increased. This initiates combustionslightlyearlier slightly inthe cycle, with peak temperature and pressure remaining at about 5 to 10 atdc. At part throttle, ignition timing is advanced to compensate for the resulting slower flame speed. These not only use engine speed to set timing but also sense and make fine adjustment for knock and incorrect exhaust emissions. Engine at WOT, constant engine speed and A/F Modern engines that are controlled by an engine control unit to control the timing throughout the engine's RPM range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM range.
(3) Flame Termination The last 5% or 10% of the mass has been compressed into a few percent of the combustion chamber volume by the expanding burned gases behind the flame front. Although at this point the piston has already moved away from TDC, the combustion chamber volume has only increased on the order of 10 20% from the very small clearance volume. This means that the last mass of air and fuel will react in a very small volume in the corners of the combustion chamber and along the chamber walls. Due to the closeness of the combustion chamber walls, the last end gas that reacts does so at a very reduced rate. Near the walls, turbulence and mass motion of the gas mixture have been dampened dout, and there is a stagnant tboundary layer. The large mass of the metal walls also acts as a heat sink and conducts away much of the energy being released in the reaction flame. Both of these mechanisms reduce the rate of reaction and flame speed, and combustion ends by slowly dying away. Although very little additional work is delivered by the piston during this flame termination period due to the slow reaction rate, it is still a desirable occurrence.
Cyclic variations in Combustion Figure 7 9 Pressure as a function of time for 10 consecutive cycles in a single cylinder of an SIengine, showing variation that occurs due to inconsistency of combustion. 1-cycle-to-cycle 2-cylinder-to-cylinder Cyclic variations are mainly caused by Variation in mixture motion within the cylinder at the time of spark, Variation in the amounts of air and fuel fed to the cylinder each cycle, Variation in the mixing of fresh mixture and residual gases within cylinder each cycle.
ENGINE OPERATING CHARACTERISTICS Power Operation Cruising Operation Idle and Low Engine Speed ClosingThrottle at HighEngineSpeed Starting a Cold Engine Power Operation: For maximum power at WOT (fast startup, accelerating up a hill, an airplane taking off), fuel injectors and carburetors are adjusted to give a rich mixture, and ignition systems are set with retarded spark (spark later in cycle). This gives maximum power at a sacrifice of fuel economy. The rich mixture burns faster and allows the pressure peak to be more concentrated near TDC, with the probable compromise of rougher operation. At high engine speeds, there is less time for heat transfer to occur from the cylinders, and exhaust gases and exhaust valves will be hotter. To maximize flame speed at WOT, no exhaust gas is recycled, resulting in higher levels of NOx.
Cruising Operation: For cruising operationsuchassteady freeway driving or long distance airplane travel, less power is needed and brake specific fuel consumption becomes important. For this type of operation aleanmixtureis supplied to the engine, high EGR is used, and ignition timing is advanced to compensate for the resulting slower flame speed. Fuel usage efficiency (miles/liter) will be high, but thermal efficiency of the engine will be lower. This is because the engine will be operating at a lower speed, which gives more time per cycle for heatlosses from thecombustion ti chamber. Idle and Low Engine Speed: At very low engine speeds the throttle will be almost closed,, resulting in a high vacuum in the intake manifold. This high vacuum and low engine speed generate a large exhaust residual during valve overlap. This creates poor combustion, which must be compensated for by supplying arich mixture to the engine. The rich mixture and poor combustion contribute to high exhaust emissions of HC and CO. Misfires and cycles where only partial combustion occurs in some cylinders are more common at idle speeds. A 2% misfire ifi rate would cause exhaust emissions i to exceed acceptable standards by 100 200%.
Closing Throttle at High Engine Speed: When quick deceleration is desired and the throttle is closed at high engine speed, averylarge vacuum is created in the intake system. High ihengine speed wants a large inflow of air, but the closed throttle allows very little air flow. The result is a high intake vacuum, high exhaust residual, a rich mixture, and poor combustion. Misfires and high exhaust emissions are very common with this kind of operation. The controls on engines with fuel injectors shut the fuel flow down under these conditions, and this results in much smoother operation. Starting a Cold Engine: When a cold engine is started, an over rich supply of fuel must be supplied to assure enough fuelvapor to create a combustible gas mixture. When the walls of the intake system and cylinders are cold, a much smaller percentage of the fuel will vaporize than in normal steady state operation. Thefuelis also cold and does not flow as readily. The engine turns very slowly, being driven only by the starting motor, and a greater amount of the compressive heating during compression is lost by heat transfer to the cold walls. This is made worse by the cold viscous lubricating oil that resists motion and slows the starting speed even more. All of these factors contribute to the need for a very rich air fuel ratio when starting a cold engine. Air fuel ratios as rich as 1:1 are sometimes used.
ABNORMAL COMBUSTION: KNOCK AND SURFACE IGNITION There are two primary abnormal combustion phenomena: Knock is the engine sound that results from spontaneous ignition of the unburned fuel air mixture ahead of the flame (the end gas ). Heating of unburnt mixture it by compression and radiation initiates premature combustion. Surface ignition is the ignition of the fuel air mixture by anyhotsurface surface, other than the spark discharge, prior to arrival of the flame.
Engine Knock Pressure variation in the cylinder during knocking combustion for normal combustion, light knockandhea heavy knock, respectively. el
Engine parameters that effect occurrence of 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.
SWIRL The main macro mass motion within the cylinder is a rotational motion called swirl. It is generated by constructing the intake system to give a tangential ti component to the intake flow as it enters the cylinder (see Fig. 6 2). This is done by shaping and contouring the intake manifold, valve ports, and sometimes even the piston face. Swirl greatly enhances the mixing of air and fuel to give a homogeneous mixture in the very short time available for this in modern highspeed engines. It is also a main mechanism for very rapid spreading of the flame front during the combustion process.
SWIRL Swirl ratio is a dimensionless parameter used to quantify rotational motion within the cylinder. Itisdefined in two different ways in the technical literature: (SR)l = (angular speed)/(engine speed) = w/n (SR)2 = (swirl tangential ti speed)/(average piston speed)= u t /Up Angular larmotion is very non uniform niform within the cylinder, being a maximum away from the walls and being much less near the walls due to viscous drag. Figure 6 3 Average cylinder swirl ratio as a function of crank angle for a typical SI engine. Swirl is high during the intake process, with a maximum near TDC. It then is reduced by viscous drag during the compression stroke. There is a second maximum near the end of compression when the radius of rotation is decreased near TDC and expansion from combustion occurs. Viscous drag with the cylinder walls during the expansion stroke quickly reduces this again before blowdown occurs.
COMBUSTION IN CI ENGINES Combustion in a compression ignition engine is quite different from that in an SI engine. Whereas combustion in an SI engine is essentially a flame front moving through a homogeneous mixture, combustion in a CI engine is an unsteady process occurring simultaneously at many spots in a very non homogeneous mixture at a rate controlled by fuel injection. Air intake into the engine is unthrottled, with engine torque and power output controlled by the amount of fuel injected per cycle. Because the incoming air is not throttled, pressure in the intake manifold is consistently at a value close to one atmosphere. This makes the pump work loop of the engine cycle shown in Fig. 3 9 very small, with a corresponding better thermal efficiency compared to an SI engine.
A: injection start (15 0 btdc) B: ID= 8 0 btdc and start of combustion C: end of injection 5 0 atdc X: end of combustion (30 0-40 atdc) Figure 7 16 Cylinder pressure as a function of crank angle for a CI engine. Point A is where fuel injection starts, A to B is ignition delay, and point C is the end of fuel injection. If the cetane number of the fuel is too low, a greater amount of fuel will be injected during ignition delay time. When combustion then starts, the additional fuel will cause the pressure at point B to increase too fast, resulting in a rough engine cycle.
COMBUSTION IN CI ENGINES Only air is contained din the cylinder during the compression stroke, and much higher compression ratios are used in CI engines. Compression ratios of modern CI engines range from 12 to 24. Compared to normal SI engines, highthermal efficiencies (fuel conversion efficiencies) are obtained. However, because the overall air fuel ratio on which CI engines operate is quite lean (equivalence ratio ϕ = 0.8), less brake power output is often obtained for a given engine displacement. Fuel is injected into the cylinders late in the compression stroke by one or more injectors located in each cylinder combustion chamber. Injection time is usually about 20 of crankshaft rotation, starting at about 15 btdc and ending about 5 atdc. Ignition delay is fairly constant in real time, so at higher engine speeds fuel injection must be started slightly earlier in the cycle. In addition to the swirl and turbulence of the air, a high injection velocity is needed to spread the fuel throughout the cylinder and cause it to mix with the air.
COMBUSTION IN CI ENGINES After injection the fuel must go through a series of events to assure the proper combustion process: 1 Atomization. Fuel drops break into very small droplets. The smaller the original dropsize emitted by the injector, the quicker and more efficient will be this atomization process. 2. Vaporization. The small droplets of liquid fuel evaporate to vapor. This occurs very quickly due to the hot air temperatures created by the high compression of CI engines. High air temperature needed for this vaporization process requires a minimum compression ratio in CI engines of about 12:1. About 90% of the fuel injected into the cylinder has been vaporized within 0.001 second after injection. As the first fuel evaporates, the immediate surroundings are cooled by evaporative cooling. This greatly affects subsequent evaporation. Near the core of the fuel jet, the combination of high h fuel concentration ti and evaporative cooling will cause adiabatic saturation of fuel to occur. Evaporation will stop in this region, and only after additional mixing and heating will this fuel be evaporated.
3. Mixing. After vaporization, the fuel vapor must mix with air to form a mixture within the AF range which is combustible. This mixing comes about because of the high hfuel injection velocity added d to the swirl and turbulence in the cylinder. Figure 7 15 shows the non homogeneous distribution of air fuel ratio that develops around the injected fuel jet. Combustion can occur within the equivalence ratio limits ofϕϕ = 1.8 (rich) and ϕ = 0.8 (lean). Non-homogeneous distribution of AF Self-ignition starts 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) (mainly Zone A &B).
4. Self Ignition. At about 8 btdc, 6 8 after the start of injection, the air fuel mixture starts to self ignite. 5. Combustion. Combustion starts from self ignition simultaneously at many locations in the slightly rich zone of the fuel jet, where the equivalence ratio is ϕ = 1 to 1.5 (zone B in Fig. 7 15). At this time, somewhere between 70% and 95% of the fuel in the combustion chamber is in the vapor state. When combustion starts, multiple flame fronts spreading from the many selfignition sites quickly consume all the gas mixture which is in a correct combustible air fuel ratio, even where self ignition wouldn't occur. This gives a very quick rise in temperature and pressure within the cylinder, shown in Fig. 7 16. Combustion lasts for about 40 to 50 of engine rotation, much longer than the 20 of fuel injection. This is because some fuel particles take a long time to mix into a combustible mixture with the air, and combustion therefore lasts well into the power stroke. This can be seen in Fig. 7 16, where the pressure remains high until the piston is 30 40 atdc. Burning rate increases with engine speed
Diesel Combustion Process Spontaneous combustion (auto ignition) due to temperature increase of reactants. Ignition triggered by compression heating of fuel-air mixture. Ignition initiated at random point in combustion chamber Fast combustion process Less complete combustion process
Fuel Injection The nozzle diameter of a typical fuel injector is 0.2 1.0 mm. Velocity of liquid fue1leaving a nozzle is usually about 100 to 200 m/sec. This is quickly reduced by viscous drag, evaporation, and combustion chamber swirl. Evaporation occurs on the outside of the fuel jet while the center remains liquid. Figure 7 15 shows how the inner liquid core is surrounded by successive vapor zones of air fuel that are: A. too rich to burn B. rich combustible C. stoichiometric t i D. lean combustible E. too lean to burn Factors that affect droplet size include pressure differential across the nozzle, nozzle size and geometry, fuel properties, and air temperature and turbulence. Higher nozzle pressure differentials give smaller droplets.
Cold Weather Problems Glow plug is used when starting most CI engines. A glow plug is a simple resistance heater connected to a battery with the heated surface located within the combustion chamber of the engine. For a short time, 10 15seconds, before starting the engine, the glow plug is turned on and the resistor becomes red hot. Now, when the engine is started, combustion in the first few cycles is not ignited by compressive heating but by surface ignition off the glow plug. After just a few cycles the cylinder walls and lubricant are warmed enough, so more normal operation of the engine is possible, the glow plug is turned off, and self ignition caused by compressive heating occurs.
See Examples: 7 1 7 2 7 3 7 4 7 5 7 6