Combustion

Combustion

1  Combustion In SI Engines

Combustion in SI engines is divided into three categories. Ignition and ame development is the rst phase of combustion where only about 5% of the airfuel mixture is consumed. During ame development combustion has barely started and there is very little pressure rise, so there is no signi cant work done. The second phase consists of the propagation of the ame. This phase consumes about 80-90% of the air-fuel mixture. During this phase there is signi cant pressure rise, which provides the force that produces the work in the expansion stroke. The third and nal phase of the combustion process is the ame termination. This phase consumes only about 5% of the air-fuel mixture. During this phase the pressure quickly decreases and combustion ends. Figure 7.1 shows the pressure as a function of the crank angle. The maximum pressure is reached after TDC which supplies the force necessary in the expansion stroke to provide useful work

Ignition and Flame Development

Ignition of the air-fuel mixture is initiated by an electrical discharge across the electrodes of a spark plug. This explosion usually occurs 10-30 degrees before TDC. When this ignition takes place, combustion reactions follow in an outward direction. The voltage necessary to cause the plug to spark is 25,000-40,000 volts with a maximum current o fabout 200 amps. This amount of energy dissipation only last about 10 nsec since the spark of the plug is instantaneous. This amount af current and voltage cause a maximum peak temperature of about 60,000 K. The actual temperature of the plug at time of ignition is only about 6000 K with spark discharge lasting around 0.001 second. The energy delivered by the spark plug is on the order of 30 to 50 mJ, which is su cient to start combustion since only about 0.2 to 0.3 mJ of energy is necessary for stoichiometric mixtures and non-stoichiometric mixtures, respectively. One method to produce this high voltage is by using a coil in the battery. Most automobiles use a 12-volt battery that is not su cient enough to cause the spark. A coil in combination with the battery is used to multiply the voltage many times and provide the necessay voltage.

The ring of the spark plug and ame propagation are related by how much pressure is in the cylinder. When the electrical discharge causes the air-fuel mixture to ignite the ame is very small and it travels very slow because of its size. Since the ame is very small and does not propage fast it does not generate enough energy, the pressure in the cylinder is not high enough to cause combustion. When 5-10% of the mixture is burnt and the rise in pressure due to the compression stroke is high then ame propagation starts. Having a rich air-fuel mixture around the electrodes of the spark plug speeds up ignition, gives a higher ame speed, and gives a \better start to the overall combustion process." (Pulkrabek, p.232) For this reason spark plugs are generally placed near the intake valve to assure a rich mixture.

Flame Propagation

Flame propagation causes the ma jority of the combustion in the cylinder. By the time that the rst 5-10% of the air-fuel mixture has been burnt ame propagation has been well established. The ame has the speed to move quickly throughout the combustion chamber and cause the rest o fthe combustion to occur. Flame propagation is increased 10 times when induced turbulance, swirl, and squish are introduced, which always is because of engine design. If the propagation of the ame was laminar then the speed would be decreased. That is why characteristics like swirl and squish are desired; they induce the turbulent ow of the ame front. 

When the gas mixture burns, the pressure and temperature rise. Burnt gases behind the ame front are hotter than the unburnt gases in front of the ame front. When this occurs the density of the burnt gases decreases while increassing the volume, and hence occupying a larger percentage of the volume of the combustion chamber. This can be seen from equation.

                                                                    p= m /V

When the density decreases the volume increases.  the relationship between the percent of mass burnt and the percent of volume burnt. When 30% of the gas mass is burnt the burnt gases occupy 60% of the total volume. This makes 70% of the total unburnt gas make only 40% of the total volume.

Since the volume of both burnt and unburnt fuel is kept constant in the combustion chamber, and the volume of burnt gases keeps increasing as the ame propagates the unburnt mixture is compressed, therefore incresing its temperature and hence the combustion rate. In addition to the rise in temperature from the compression of the uid there is a rise in temperature due to radiative heat transfer. Radiation is emmitted from the ame reaction zone, which is at an approximate temperature of 3000 K, and transfered to the burnt and unburnt gases. As the ame propagates throughout the combustion chamber the temperature and pressure increase constantly. This rise in temperarure and pressure cause the chemical reaction to increase which causes the ame front to increase.

The e ects on combustion not only come from turbulance, swirl, and squish, but also from the type of fuel used and the air-fuel ratio. Lean mixtures have slower ame speeds than rich mixtures.  the relationship between the ame speed and air-fuel ratio. The ame speed will be the greatest when there is a rich mixture but not too rich otherwise the ame speed will decrease again. The ame speed also increses with engine speed since turbulace, swirl, and squish are increased.  an \average combustion chamber ame speed as a function of engine speed for a typical SI engine." The burn angle is de ned as the angle the crakshaft makes when combustion occurs. For most SI engines the crankangle is 25 before TDC. If ignition is too early then the pressure inside the cylinder will be high and the piston will do extra work. If however ignition is late then the pressure will be low, which will reduce the amount of work done on the engine. Actual ignition timing occurs anywhere from 10-30 before TDC. describes the relationship between the burn angle and the engine speed. Flame propagation starts at 5% and ends at 95% consumption. The typical crankangle during the main part of combustion is 25 .

Flame Termination

At about 15-20 after TDC 90-95% of the combustion process has taken place and the ame front has reached all corners of the combustion chamber. The termination of the ame consumes roughly 5% of the air-fuel mixture. that when there is only about 5-10% of the gas mass left, it has been compressed into a small percent of the combustion chamber volume. The small volume left at the end of the combustion process makes the endgas combust with the combustion chamber walls and corners. 

Because the end gas is so close to the cylinder walls the reactions are reduced in rate. Apart from being a slow reaction the large mass of the metal walls \acts as heat sink and conducts away much of the energy being released in the reaction ame."  Since the nal mixture has been minimized in energy content the rate of reaction and ame speed are reduced. This is wanted out of the last phase of combustion. Since the pressure is low at the last stage of combustion the forces exerted on the cylinder head are minimized and therefore a smooth ending to combustion is accomplished.  

2  Combustion In CI Engines

The main di erence between combustion in CI and SI engines is the way in which combustion occurs. In SI engines the combustion process rst occurs  by igniting a homogeneous mixture using a spark plug. The main di erence arises when the ame is initiated and the ame travels at a certain direction, dictated by the ame propagation, whereas combustion in CI engines there is no ame propagation with a direction. Combustion in a CI engine is a nonsteady process where a nonhomogeneous mixture is controlled through fuel injection. The mixture is nonhomogeneous since air is the only substance being compressed until late in the compression stroke. Injection of the fuel occurs at about 15 bTDC and ends at about 5 aTDC. Following are the steps that the fuel goes through, after injection, in order to cause the proper combustion.

1.atomization: the fuel droplets break into smaller droplets. 

2.vaporization: the small droplets of fuel vaporize in the chamber due to high temperatures. About \90% of the fuel injected into the cylinder has been vaporized within 0.001 second after injection."

3.mixing: after vaporization of the fuel, the fuel mixes with the air to form a combustible air-fuel mixture. 

4.self-ignition: self-ignition usually starts around \8 bTDC, 6-8 , after the start of injection."  At this point some of the mixture will ignite. These small reactions are caused by high temperature within the chamber. They are exothermic and further raise the temperature of the combustion chamber.

5.combustion: combustion nally takes place after the self ignition of the air-fuel mixture, throughout the combustion chamber. At the time of combustion, around 70-95% of the fuel in the combustion chamber is in the vapor state. At this time many ame fronts develop at di erent places throughtout the combustion chamber, with the aid of the self-ignited mixture. When all of the combustible air-fuel mixture has been used the temperature and pressure rise. The increase in temperature and pressure further increase self-ignition points and combustion increases. Throughout this process liquid fuel is still being injected into the combustion chamber. The amount of fuel that is injected dictates the rate of combustion since the fuel has to be atomized, vaporized, mixed and nally combusted. Figure 7.6 shows the relationship between cylinder pressure and crank angle.