Supercharger

 Superchargers are pressure-boosting devices (compressors) that increase the pressure of the air before inletting it get into cylinder of the internal combustion engine, and the process of increasing the pressure OR forcing more air to get into engine is called as supercharging. What is turbocharger and supercharger? Turbochargers use the vehicle's exhaust gas; two fans – a turbine fan and a compressor fan – rotate from exhaust gas. Conversely, superchargers are powered directly by the engine; a belt pulley drives gears that cause a compressor fan to rotate Combustion in SI Engines The combustion process of SI engines can be divided into three broad regions: (1) ignition and flame development (2) flame propagation (3) flame termination. Flame development is generally considered the consumption of the first 5% of the air-fuel mixture (some sources use the first 10%). Stages of combustion in SI engine: 1. Preparation Phase The preparation phase is also called a period of ignition lag. This is the first stage in the combustion stages in SI engines. According to the experiment, there is some time interval between the first spark given to the mixture (at point A) and the first flame appears out of the mixture. This time interval is known as ignition lag and it is represented on the above map as period AB. Due to this combustion, there is a clear rise in cylinder pressure. This ignition lag represents the preflame reaction. According to the chain reaction theory of combustion, in preflame reactions chain carriers are produced. During the AB period, the angle changed by the crank between points A and B is known as the ignition delay angle. This first phase or preparation phase in stages of combustion in SI engines depends upon the different factors such as the temperature of fuel, pressure, molecular structure of fuel, density and air-fuel ratio in the combustion chamber. 2. Flame Propagation Phase When the first flame appears after the spark at point B, this flame travels surrounding and burns the fuel in different layers. This fuel burning rate and flame speed are noticeably low and there is a small but steady pressure rise in the combustion chamber. This burning of air and fuel in the combustion chamber continues further and it causes a continuous rise in pressure and temperature. It releases heat energy in the combustion chamber which is transferred from burned to unburned charge. The speed of flame propagation is becoming very high in the range of 15 to 35 m/s. Differentiating between the first and second phase i.e. Preparation phase and flame propagation phase is quite a difficult task. But you can distinguish between these two phases by observing ( P – θ ) diagram given above. The second phase i.e. flame propagation phase starts when the pressure in the combustion chamber starts rising at point B and the phase ends when the highest pressure is achieved in the cylinder at point C on ( P – θ ) diagram. Curve BC on the diagram represents the rate of pressure rise. The rate of heat transfer to the cylinder walls is very low at the beginning of the flame propagation phase. This stage is one the most important stage in the stages of combustion in SI engine. 3. After Burning We attained point C in the second stage of the combustion. But, it does not represent the completion of the combustion of the mixture. Because of the continuous burning of the remaining fuels in the cylinder and reassociation of dissociated gases in the combustion chamber. During the expansion stroke, the combustion of air and fuel mixture continues after point C. This phase is called after burning. After burning represents the third stage of combustion in SI engine up to point D on ( P – θ ) diagram. During after burning phase, flame velocity reduces to a certain level. This was the last phase of the stages of combustion in SI engine. Comparison of Knock in S.I and C.I Engines Knocking in CI engines:  In spark ignition engines (SI engines), at the end part of the gas, if flame speed and delay period are low in the combustion chamber, produce detonation in SI engine. In the case of the CI engine, it was the first part of the gas which causes the Knocking in CI engine and rough running of the engine.  In CI engines, when fuel is injected in the combustion chamber and combustion occurs by the way of the autoignition system. When the period of injection of fuel is long, an excess amount of fuel starts to inject in the combustion chamber. Then there is a large amount of fuel accumulate in the combustion chamber due to longer injection of fuel or delay period.  When there is uncontrolled combustion, the amount of fuel accumulated in the combustion chamber suddenly explodes. It increases the rate of pressure rise in the combustion chamber and ultimately high pressure. This high rate of pressure rise in the internal combustion engine is equivalent to a sudden increase in load on the engine.  Also high and increasing pressure of gases in the chamber affects the whole engine structure during the pressure equalization process. Due to this, you will hear a thudding sound from the engine, the same sound is called Knock or Knocking in CI engine.  From the above explanation, we can conclude that the knocking in CI engines occurs if the delay period of injection is longer than the usual condition. Knocking in SI engines:  In the normal combustion of the CI engine, air is sucked and then compressed by the piston. At the end of compression, the fuel injector sprays the fuel into the high-temperature compressed air. The fuel gets ignited and flames smoothly propagate throughout the cylinder with consuming the air-fuel mixture.  While In the case of detonation in the CI engine, at the end compression (BDC to TDC) of air, the fuel injector sprays the fuel into the air. But due to the high ignition lag, more unburnt fuel accumulates inside the chamber.  When this lot of fuel accumulated inside the chamber reaches to self-ignition temperature, it gets ignited, a large explosion takes place inside the chamber which applies the opposite force to the piston movement from BDC to TDC.  Therefore, this phenomenon creates a high pinging sound and pressure pulses which has a bad effect on the engine.  In the CI engine, the detonation process is observed at the starting of the combustion process. What causes detonation/knocking in engine? The factors responsible for detonation in an SI engine are as follows:-  High compression ratio.  Autoignition  Less ingition lag.  High self ignition temperature of fuel.  High temperature of inlet air-fuel mixture.  More quantity of inlet air-fuel mixture.  Hot spots (Overheated area inside chamber)  Due to used of low octane fuel. The factors responsible for detonation in a CI engine are as follows:-  High ignition lag  High ignition lag  High engine speed  Due to used of low cetane fuel  Less temperature and pressure of intake fuel and air Therefore, these are different factors that cause the detonation in both kinds of engines. What is effect knocking in engine? The effects of the detonation on the engine are as follows:  Bending of connecting rod.  Scuffing of cylinder bore.  Breaking of piston head and piston rings.  Erosion of cylinder head  Loss of engine power.  Produces the high intensity sound waves.  Causes pre-ignition due to high comustion temperature.  Damage to spark plug.  Melting of valves. How to prevent engine knocking/denotation? The detonation in an SI engine can be controlled by the following ways:  Decreasing intake manifold pressure.  Retarding spark timing.  Reducing the compression ratio.  Using the high octane fuel.  Reducing the intake temperature of air.  Reducing peak cylinder pressure.  Reducing the engine size.  Increasing the amount of intake fuel. The detonation in the CI engine can be controlled by the following ways:  Using the high cetane fuel helps to reduce the ignition lag.  Increasing engine size.  Increasing the intake fuel and air temperature.  Using the supercharging or turbocharging. Combustion in CI Engines In CI or compression ignition engine, in the compression stroke, only air is compressed at very high pressure and temperature. The compression ratio used is in the range of 12 to 120. The temperature of the air becomes higher than the temperature of the fuel which is diesel in the CI engine. Then diesel fuel is injected in the combustion chamber under very high pressure about 120 to 210 bar. The temperature of this fuel is around 20° to 35° before TDC (Top Dead Center). Point A on the above graph shows the time at which diesel fuel injection starts. Then, the process of combustion in the CI engine starts. This combustion takes place in four stages as discussed below. Stages of Combustion in CI engine: There are four different stages of combustion in CI engine where proper combustion of air and fuel takes place as follows: 1. Ignition Delay Period 2. Period of Uncontrolled Combustion 3. Period of Controlled Combustion 4. After Burning 1. Ignition Delay Period At this first stage of combustion in the CI engine, the fuel from the injection system sprayed in the combustion chamber in the form of a jet. Due to atomization and vaporization, this fuel disintegrates at the core which is surrounded by a spray of air and fuel particles. In this vaporization process, the fuel gets heat from the compressed and hot surrounding air. It causes some pressure drop in the cylinder. You can see this pressure drop (curve AB) in the above figure. After completion of the vaporization process, the preflame reaction of the mixture in the combustion chamber starts. During the preflame reaction, pressure into the cylinder starts increasing with the release of energy at a slow rate. This preflame reaction starts slowly and then speeds up until the ignition of the fuel takes place. You can see this process at point C on the diagram. This time interval between the starting of the fuel injection and the beginning of the combustion is called the delay period. This delay period can further be divided into two parts – Physical delay and chemical delay. The period between the time of injection of the fuel and its achievement of selfignition temperature during vaporization is called physical delay. When physical delay completes, the time interval up to the fuel ignites and the flame of the combustion appears is called chemical delay. 2. Period of Uncontrolled Combustion This is the second stage of combustion in the CI engine. After the above-mentioned delay period is over, the air and fuel mixture will auto-ignite as they have achieved their self-ignition temperature. The mixture of air and fuel in CI engines is heterogeneous unlike homogeneous in the SI engines. Due to this heterogeneous mixture, flames appear at more than one location where the concentration of the mixture is high. When the flame formed the mixture in the other low concentration starts burning by the propagation of flames or due to auto-ignition, because of the process of heat transfer. The accumulated fuel during the delay is now started burning at an extremely rapid rate. It causes a rise in in-cylinder pressure and temperature. So, the higher the delay period, the higher would be the rate of pressure rise. During this stage, you can’t control the amount of fuel burning, that’s why this period is called a period of uncontrolled combustion. This period is represented by the curve CD in the above figure. 3. Period of Controlled Combustion When the accumulated fuel during the delay period completely burned in the period uncontrolled combustion, the temperature and pressure of the mixture in the cylinder are so high that new injected fuel from the nozzle will burn rapidly due to the presence of sufficient oxygen in the combustion chamber. That’s the reason we can control the rise of pressure into the cylinder by controlling the fuel injection rate. Therefore, this period of combustion is called a period of controlled combustion. 4. After Burning This is the last stage out of the four stages of combustion in CI engine. Naturally, the combustion process is completed at the point when the maximum pressure is obtained in the combustion chamber at point E as shown in the figure. Practically, the burning of the fuel in the combustion chamber remains to continue during the expansion stroke. The main reason behind it is the reassociation of dissociated gases and unburnt fuel. Therefore, this last phase of combustion is called After Burning. These are the four different stages of combustion in CI engine.