Combustion T Alrayyes
Fluid motion with combustion chamber Turbulence Swirl SQUISH AND TUMBLE
Combustion in SI Engines
Introduction The combustion in SI engines inside the engine can be divided into three regions 1. Ignition and flame development 2. Flame propagation 3. Flame termination 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. The vast majority of the burning, incylinder pressure rise and workout occurring at the flame propagation The final 5% (some sources use 10%) of the air-fuel mass which burns is classified as flame termination. During this time, pressure quickly decreases and combustion stops
Burning of fuel in a fuel bomb
Ignition and Flame Development Combustion is initiated by an electrical discharge (plasma discharge)across the electrodes of a spark plug. This occurs anywhere from 10 to 30 before TDC, depending on the geometry of the combustion chamber and the immediate operating conditions of the engine. Flame can generally be detected at about 6 of crank rotation after spark plug firing. When plasma discharge occur, it creates a spherical flame front that propagates outward into the combustion chamber. Combustion starts very slowly because of the high heat losses to the relatively cold spark plug and gas mixture (not enough energy to heat the surrounding). This, in turn, does not raise the cylinder pressure very quickly, and very little compression heating is experienced. It is desirable to have a rich air-fuel mixture around the electrodes of the spark plug at ignition. A rich mixture ignites more readily, has a faster flame speed, and gives a better start to the overall combustion process. Spark plugs are generally located near the intake valves to assure a richer mixture, especially when starting a cold engine.
Spark plugs and coils A stoichiometric mixture of hydrocarbon fuel requires about 0.2 mj of energy to ignite selfsustaining combustion. The discharge of a spark plug delivers 30 to 50 mj of energy, most of it is lost by heat transfer Overall spark discharge lasts about 0.001 second and gives a peak temperature of 60000K and average temperature of 6000K.
Applied potential is generally 25,000-40,000 volts, with a maximum current on the order of 200 amps lasting about 10 nsec (1 nsec = 10-9 sec).
Most automobiles use a 12-volt electrical system, including a 12-volt battery. There are different methods used to obtain the high voltage required: The low voltage is multiplied many times by the coil that supplies the very high potential delivered to the spark plug. Some systems use a capacitor to discharge across the spark plug electrodes at the proper time Most engine use a magneto driven off the engine crankshaft to generate the needed spark plug voltage
Flame propagation in SI engines By the time 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. Turbulence, swirl, and squish, flame propagation cause the combustion to occur 10 times faster than if there were a laminar flame front moving through a stationary gas mixture. In engines, The flame propagation occurs in an environment where there is a high temperature and pressure, this causes the chemical reaction time to decrease and flame speed to increase.
The high temperature and pressure is caused by: 1. Burned gas behind the flame is hotter than the unburned gas in front of the flame. This causes a reduction in unburned gas density compare to the rest of the charge. The decrease in density cause the burned charge to expand in the expense of unburned charge For example, in the figure below, it shows that 30% burned gas occupy 60% of the volume. Compressing 70% of the charge into 40% of the volume. This causes compressive heating 2. Radiation: Radiation emitted from the flame (could be up to 3000 K)further heats the rest of the charge. Temperature rise from radiation further increase the pressure. Convection contribution is minor compare to radiation, due to the short time included in each cycle.
Ideally the AFR should be 70% burned by TDC and almost completely burned at about 15 atdc. Thus the combustion in SI engine is almost but not exactly happening at constant volume The close the combustion to constant volume the higher the thermal efficiency However, constant volume combustion is not the best way to operate. A gradual increase combustion pressure might be desirable for smooth transfer of force to the face of the piston (not to damage the piston). An example of the pressure rise rate is 240 kpa/deg In addition to the decrease in thermal efficiency, a slower pressure rise rate might increase the possibility of knock The combustion process is then a compromise between the highest thermal efficiency possible (constant volume)and smooth engine cycle with less efficiency.
Flame termination At about 15 to 20 atdc, 90-95% of the air-fuel mass has been combusted. The react of reaction and flame speed will reduce during flame termination. And the combustion end up slowly dying out The reaction is happening at small volume in the corners of the combustion chamber and a long the chamber wall, mainly pushed by the burned gas (Although the cylinder start to expand). Near the walls, turbulence and mass motion of the gas mixture has been dampened out and there is a stagnant boundary layer. the large mass of metal wall will act as a sink and conduct away much of the energy being released in the reaction flame. During the flame termination period, self ignition will occur in the end gas in front of the flame front This knock might not much of an effect since most of the fuel has already burned out.
Factors influencing combustion Spark timing Engine speed Equivalence ratio Residual gas fraction Compression ratio Combustion chamber design
Spark timing Spark timing must be located relative to the TDC to obtain max power or torque. Combined duration of the flame development and propagation process is typically between 30 and 90 CA degrees. If the start of combustion process is too advanced before TDC, compression stroke work transfer (from piston to cylinder gases or negative work) increases. If the end of combustion process is delayed by retarding the spark timing, peak cylinder pressure occurs later in the expansion stroke and is reduced in magnitude. These changes reduce the expansion stroke work transfer from cylinder gases to the piston. 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.
Engine Speed Mixture burning rate is strongly influenced by engine speed. 4 Flame speed (m/s) increases with engine speed due to the higher turbulence, swirl, and squish. Although flame speed increases, the duration of combustion in crank angle degrees might increase slightly with increasing engine speed. Increase of the engine speed, reduces the time available for a complete combustion. To compensate this, ignition timing should be adjusted spark advance is increased with increasing engine speed.
Mixture properties The fuel-air equivalence ratio affects the burning rate. Lean mixtures have slower flame speeds, as shown in Figure. Slightly rich mixtures have the fastest flame speeds, with the maximum for most fuels occurring at an equivalence ratio near 1.2. Burning rate reduces for richer and leaner mixtures. The burned gas fraction in the unburned mixture, due to the residual gas fraction and any recycled exhaust gases (EGR), slows down both flame development and propagation. Fuel composition changes can be significant. Faster burning engines (high turbulence) are less sensitive to changes in mixture composition, p and T than slower burning engines.
Compression Ratio Increase in CR increases the p and T of the charge at ignition, Reduces the mass fraction of the residual gases Both are favourable conditions for ignition which reduces the first stage of combustion, and increases flame propagation rate in the main stage. Increasing CR, increases Area/Volume ratio of the cylinder, increasing the cooling effects and the quench layers. Final stage of combustion is increased.
Combustion Chamber Design Intake manifold design and combustion chamber shape effects the gas. flow and turbulence intensity. Turbulence strongly effects burning rate of the fuel. Spark plug location effects distance travelled by the flame and flame front surface area. Number of spark plugs.
Abnormal combustion Fuel composition, engine design and operating parameters, combustion chamber deposits may prevent occurring of the normal combustion process. There are two types of abnormal combustion : Knock Surface ignition
Knock Knock is the autoignition of the portion of fuel, air and residual gas mixture ahead of the advancing flame, that produces a noise. As the flame propagates across combustion chamber, end gas is compressed causing pressure, temperature and density to increase. Some of the end gas fuel-air mixture may undergo chemical reactions before normal combustion causing autoignition End gases then burn very rapidly releasing energy at a rate 5 to 25 times in comparison to normal combustion. This causes high frequency pressure oscillations inside the cylinder that produce sharp metallic noise called knock. Knock will not occur when the flame front consumes the end gas before these reactions have time to cause fuel-air mixture to autoignite. Knock will occur if the precombustion reactions produce autoignition before the flame front arrives
Surface Ignition Surface ignition is ignition of the fuel-air charge by any source other than the spark plug, for example: overheated valves spark plugs, by glowing combustion chamber deposits or by any other hot spot in the engine combustion chamber It may occur before the spark plug ignites the charge (preignition) or after normal ignition (postignition). It may produce a single flame or many flames. Surface ignition may result in knock.
Design Parameters to control knock Compression ratio increase in CR increases thermal efficiency but also increases the tendancy to knock - limits engine performance. Combustion chamber size and shape as combustion chamber volume gets smaller, surface area-to-volume ratio increases providing efficient cooling, reduces tendency to knock. In SI-engines max piston diameter is limited to 150 mm Flame propagation distance (chamber shape and spark plug location, number of spark plugs used) also effects knock Valve overlap Decrease overlap will reduce residual gases, produces cooling effect - reduces knock tendency Engine cooling efficient cooling reduces tendancy to knock - water cooling systems are more effective, in aircooled engines CR is limited
Operating Parameters Equivalence ratio Autoignition reactions occur at slightly lean mixtures - flame speed is lower (more time for autoignition to happen), pre-reaction duration is relatively short. Lean and rich mixtures - tendancy to knock is reduced. Spark advance Increasing spark advance, p and T increases, flame speed also increases reducing the time for pre-reactions, However tendancy to knock increases with increasing spark advance. Oxygen concentration in combustion chamber decreasing oxygen concentration reduces the tendency to knock humidity of intake air also cools the charge and reduces knocking tendency. Cooling water temperature cooling water T effects mean combustion chamber temperatures - tendancy to knock decreases with decrease in T
Variation in combustion Ideally, combustion in every cylinder of an engine would be exactly the same In addition, ideally, there would be no cycle-to-cycle variation in anyone cylinder. Unfortunately, this is not the case and there is both a cycle to cycle variation within the cylinder and variation between the different cylinders.
Reason for the variation between cylinders Length and geometry of intake manifold (change in the volumetric efficiency and AFR) Temperature difference in the runners (cause variation in evaporation rate and subsequently volumetric efficiency) Evaporative cooling Different components of gasoline EGR introduction Most of the above reasons will be mainly for carburettors systems and throttle position fuel injector The main reason for the variation in multipoint injectors and incylinder injection is fuel injected.
In cylinder variation: AFR variation between cycles (standard deviation is between 2 to 6%) Turbulence variation Incomplete mixing especially near the spark plug (cause a variation in the start of the combustion which will affect the subsequent combustion) Fast burn in about twice the slow burn The greatest percentage differences occur at light engine loads and low speeds, with idle being the worst condition.
As a compromise, the average burn time is used to set the engine operating conditions (i.e., spark timing, AF, compression ratio, etc.). This lowers the output of the engine from what could be obtained if all cylinders and all cycles had exactly the same combustion process. A cycle in which fast burn occurs is like a cycle with an over-advanced spark. This happens when there is a rich AF ratio, higher than average turbulence, and good initial combustion startup The result of this is a temperature and pressure rise too early in the cycle, with a good chance of knock occurring. This limits the compression ratio and fuel octane number that can be tolerated for a given engine.
A cycle with a slower than average burn time is like a cycle with a retarded spark. This occurs when there is a lean mixture and higher than average EGR. The result of this is a flame lasting well into the power stroke. This is when partial burns and misfires occur Higher heat loss occurs because of the longer combustion time and because the flame front is wider with the slower burning lean mixture. For smooth operation, engine conditions must be set for the worst cyclic variations in the worst cylinder If all cylinders had the exact same combustion process cycle after cycle, a higher engine compression ratio could e tolerated, and the air-fuel ratio could be set for higher power and greater fuel economy. Cheaper, lower octane fuel could be used.
Combustion in CI engines
Combustion in CI Engine 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 nonhomogeneous 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 very small
Compression ratios of modern CI engines range from 12 to 24. Higher compression ratio increase thermal efficiency However, because the overall air-fuel ratio on which CI engines operate is quite lean (equivalence ratio = 0.8), less brake power output is often obtained for a given engine displacement.# Only air is contained in the cylinder during the compression stroke. 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.
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 drop size 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.
Liquid Atomization at High Speeds
3. Mixing: After vaporization, the fuel vapour must mix with air to form a mixture within the AF range which is combustible. This mixing is happening because of the high fuel injection velocity added to the swirl and turbulence in the cylinder. Combustion can occur within the equivalence ratio limits of Φ = 1.8 (rich) and Φ = 0.8 (lean). Figure 7-15 shows the non-homogeneous distribution of air-fuel ratio that develops around the injected fuel jet.
4. Self-Ignition: At about 8 btdc, 6 to 8 after the start of injection, the air-fuel mixture starts to self-ignite. Before actual combustion, a secondary reaction is occurring which includes breakdown of large hydrocarbon molecules into smaller species and some oxidation These reactions, caused by the high-temperature air, are exothermic and further raise the air temperature. This finally leads to an actual sustained combustion process.
5. 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 vapour 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. Combustion cause an increase in temperature and pressure inside the engine. This increase will create a good environement for combustion even in areas where self ignition didn t start. The combustion last between 50 to 60 more than the 20 injection duration. This is because some fuel particles takes longer to mix into a combustible mixture.] 60% of the fuel is burned in the first third of the combustion Burning rate increase with increasing speed, so the burn angle remain about constant
Ignition delay, ID The period between the start of fuel injection into the combustion chamber and the start of combustion is termed as ignition delay period. Physical Factors Affecting Ignition Delay: In cylinder pressure, temperature An increase in temperature, pressure will decrease ID time. Engine speed An increase Engine speed will decrease ignition delay time Engine speed increases the air motion and turbulence, reduces ID time slightly (in ms), in terms of CA degrees ID increases almost linearly. Injection quantity (load) Reducing engine load changes AFR, cools down the engine, reduces wall temperatures, reduces residual gas temperatures and increase ID.
Injection timing: If injection is too early, ignition delay time will increase because temperature and pressure will be lower. If injection is late, the piston will move past TDC, pressure and temperature will decrease, and again ignition delay time will increase. Combustion chamber design Spray impingement on the walls effect fuel evaporation and ID increase increase compression ratio, increase p and T and reduces ID Reducing stroke volume, increase surface area to volume ratio, increase engine cooling and increase ID Swirl rate Change vaporisation rate and air-fuel mixing - under normal operating conditions the effect is small. At start-up (low engine speed and temperature) more important, high rate of evaporation and mixing is obtained by swirl Oxygen concentration Residual gases reduce O2 concentration and reducing oxygen concentration increases ID
Cetane number Both physical and chemical properties of the fuel is important. Ignition quality of the fuel is defined by its cetane number. It is important to use fuel with the correct cetane number for a given engine. If the cetane number is low, ignition delay will be too long, and a more-thandesirable amount of fuel will be injected into the cylinder before combustion starts. When combustion then does start, a greater amount of fuel will be quickly consumed, and the initial cylinder pressure rise will be greater. This causes a very large initial force applied to the piston face and a rough engine cycle. If the cetane number is high, combustion will start too early before TDC, with a resulting loss in engine power. Cetane number can be changed by blending small amounts of certain additives to the fuel. Additives that accelerate ignition include nitrites, nitrates, organic peroxides, and some sulfur compounds
Cold-Weather Problems in CI engines Starting a cold CI engine can be very difficult. The cold environment cause longer ignition delay time and even in some cause failure to start. The failure to start is mainly due lower self ignition temperature The lower temperature is caused by: Low air and fuel temperature Low cold metal cylinder wall High oil viscosity.
Lubrication oil and cold start: Lubrication oil is cold, its viscosity is high, and distribution is limited. The starter motor has to turn the cold engine that is poorly lubricated with very high viscosity oil. This results in a slower than normal turnover speed to start the engine. The slower than normal rotation of the engine, combined with the cold metal cylinder walls, promotes a large heat loss to the walls and keeps the air temperature below that needed to self-ignite the fuel.
Methods to promote cold start Glow plug Glow plug is the mostly common used method to promote cold start. 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.
The pony engine Because of the large amount of energy needed to rotate and start very large CI engines that are cold, using an electric motor powered from a battery is sometimes not practical. Instead, a small internal combustion engine can be used as the starting motor for the larger engine. The pony engine is first started and then used to turn over the large engine by engaging it to the flywheel of the large engine. The pony engine is then disengaged when the large engine is started. Higher compression ratio engine To aid in cold starting, many medium-size CI engines are built with a higher compression ratio than would otherwise be needed. Some are also given a larger flywheel for this purpose.
Preheating the lubricating oil Preheating the lubricating oil electrically is done on some engines to help the starting process. Some systems even distribute the oil throughout the engine before starting by means of an electric oil pump Late injection and a richer air-fuel mixture Late injection and a richer air-fuel mixture are also used to aid starting. Continuous Running It is not an uncommon practice to leave large CI engines running continuously during cold weather to avoid the problem of restarting them.