Stages of Combustion in SI Engines

Combustion in SI Engine

The combustion process is defined as a rapid chemical reaction between the H₂ and C with oxygen in the air and liberates energy in the form of heat. It is not the purpose of this article to study the combustion of fuel in details as it is very complicated phenomenon. The purpose of this article is just to introduce the combustion in SI engines and effects of different parameters on the combustion and ultimately the effects on engine performance.

It is absolutely essential to burn the fuel supplied completely for the economical working of the engine and also for the safety of the engine and environment. Therefore, the mixture supplied to the engine should possess A:F ratio otherwise, combustion cannot be initiated or if initiated it cannot be sustained. In addition to this, there must be some means to initiate the combustion and the generated flame should be able to propagate through the mixture and burn the mixture completely.

It is known fact that the fuel vary with A:F ratio cannot be burned. There is a ignition limit for any fuel to start the combustion and sustain it till the complete fuel burns by the flame generated with spark plug. In addition to this, the temperature of the mixture to initiate the ignition is equally important. It is also known that the flame will propagate if the temperature of the burnt gases exceeds 1500 K for SI engine fuels. The ignition limits of hydrocarbon fuel when temperature of mixture reaches to 1500 K are shown in the figure given below:

The upper and lower limit of A:F ratio for ignition depends upon the temperature of a particular fuel. The limit becomes wider at higher temperatures because of higher reaction rate and higher thermal diffusivity coefficients of the mixture. Therefore, it is very essential to see that the A:F ratio of the mixture supplied to the engine should lie in the practical limit as shown in the figure.

Stages of Combustion in SI Engines:

It is assumed that the heat is added instantaneously of constant volume in the ideal air-cycle of SI engine. To achieve this, the burning of the fuel in the SI engine should be instantaneous. In actual engines, combustion occurs over a finite period of time as the flame starting around the spark plug has to propagate through the entire mixture of the air and fuel.

The pressure variation in the SI engine combustion chamber during the crank rotation is shown in the figure given below. This is really an unfolded p-v diagram.

The ignition is timed to take place at the point ‘a’ but the burning commences only at the point ‘b’. The time interval between these two points is known as “Ignition lag”. The major disadvantage of the ignition lag is that it reduces the power developed. If the ignition lag is too long, the peak pressure occurs during the expansion stroke, and therefore full advantage of expansion is not achieved. All consideration should be taken into account in the design of the combustion chamber and selecting the fuel used to reduce the ignition lag.

The theoretical diagram of combustion is shown in the figure:

But actual process differs from theoretical as instant combustion is not possible as shown by bc. The combustion to start takes small time after giving the spark as the surrounding mixture is to be heated up to ignition temperature and then formed nucleus of flame starts propagating through the surrounding mixture.

The pressure variation in the engine with respect to the crank angle is shown in the figure. There are mainly three stages of combustion in SI engines as shown in the figure.

1.      First Phase:

This phase is considered between the point of ignition and point of combustion. As shown in figure, ignition is timed at the point ‘a’ and combustion starts at the point ‘b’. The period of ignition lag is very small and lies between 0.00015 to 0.002 seconds. An ignition lag of 0.002 sec corresponds to 35⁰ crank rotation when the engine is running at 3000 RPM and crank angle required (which is also known as angle of advance) increases with an increase in engine speed.

2.      Second Phase:

Once the flame formed at the point ‘b’, it should be self-sustained and must to able to propagate through the mixture. This is possible when the rate of heat generation by burning the surrounding mixture of the flame nucleus must be higher than the heat lost by the flame to the surrounding. As the difference between heat generation and heat lost is higher, the rate of flame propagation is higher and complete combustion will occur earliest possible this is most desirable requirements of combustion in SI engines. The propagation of flame also depends upon the flame temperature as well as temperature and density of the surrounding mixture as its propagation is directly proportional to these factors. Weak spark and low compression ratio (as density of mixture is less) gives low propagation of the flame.

After the point ‘b’, the flame propagation is abnormally low at the beginning as heat loss is more than the heat generated. Therefore, the pressure rise is also slow as mass of mixture burned is small. Therefore, it is necessary to provide angle of advance 30 to 35⁰ if the peak pressure is to be attained 5 to 10⁰ after TDC.

After the point ‘c’, the pressure starts falling due to the fall in the rate of heat release when the flame reaches the wall in the last part of combustion and cannot compensate for its fall due to gas expansion, and heat transfer to the walls.

The time required for the flame to travel 95% of the chamber length with respect to speed of the engine is shown in the figure below:

It is obvious, the crank angle required for 95% travel increases with increasing RPM (the time available is decreased), therefore, if the combustion is to be completed at point ‘c’, the angle of advance must be increased with increasing RPM. The flame speed increases with increasing RPM because of increase in turbulence of the mixture.

The time required for the crank to rotate through an angle Ѳ₂ is known as combustion period during which the propagation of the flame takes place.

The stage I and II are not entirely distinct. The starting point of stage II is measurable as rise in pressure can be seen on p- Ѳ diagram. This is the point where the line of combustion departs from the line of compression.

3.      Third Phase:

Although the point ‘c’ represents the end of flame travel, it does not assure the complete combustion of fuel. In this case, the combustion still continues after attaining the peak pressure also and this combustion is known as After Burning. This is continued throughout the expansion stroke. This generally happens when the rich mixture is supplied to the engine.