Introduction to combustion in CI engine

Combustion in CI engine    

Click above title for video

So, what is Combustion?

Combustion is a chemical process, in which fuel is burning in the presence of oxygen, and produces heat and light.

In the CI engine, the only air is supplied during suction stroke. This air is compressed to attain high pressure and high temperature at the end of the compression stroke, The high-pressure diesel is injected in the cylinder. Injection of diesel at the high pressure helps in the disintegration of fuel jet to achieve better combustion of fuel. Better the atomization, assure better fuel combustion because of large surface area available to absorb heat from hot air and start the ignition as soon as possible.

In the cylinder, air and diesel form a heterogeneous mixture for combustion.

After injecting the diesel in the engine, the droplet of fuel absorbs the latent heat from the hot air and gets evaporate, and form a combustible mixture to start the ignition.

Once ignition has taken place, the heat released by combustion of the first fuel droplet is helped to further evaporation and combustion.

As we have seen from the figure air swirl help to disintegrate the fuel jet into small droplets and form a combustible mixture. Air swirls ensure that the sufficient air is available to support the flame travel in the engine to achieve the complete combustion of fuel.

What happens if there is no swirl or very little swirl present?

The air swirl plays a very important role in the combustion of diesel fuel, air swirl means a controlled turbulence of compressed air which helps to form a combustible mixture and complete ignition of all the fuel inside the cylinder.

 If there is no air swirl inside the cylinder it decreases the air-fuel mixture quality, which leads to incomplete combustion. 

In a figure red circle shows a fuel droplet, it will absorb heat from surrounding air shown by a yellow ring, fuel droplet need more heat to achieve self-ignition temperature, because of no swirl the yellow ring form a barrier to transfer heat from the black ring which is at a higher temperature, to fuel droplet. Because of this barrier ignition delay increases. Here swirl helps to displace the yellow ring and provide heat to fuel droplets to achieve self-ignition temperature.

 High turbulence is also affecting the ignition; high turbulence may extinguish the flame, which also leads to incomplete ignition.

To achieve the controlled turbulence or swirl, intake manifold design plays an important role here.

In the below figure four stages of combustion are shown.

The total injection time is shown here. Injection starts at about 19⁰ BTDC and end of injection is about 10⁰ ATDC, the total injection timing is about 29⁰.

First stage is ignition delay

Second stage is Rapid or uncontrolled combustion

Third stage is controlled combustion

Fourth stage is after burning

ignition delay

It is counted from the start of injection to the point where the p-θ curve separates from the pure air compression curve (motoring curve).

The ignition delay period is divided in two parts

                1. Physical delay

                2. Chemical delay

1. Physical delay

ž  The time measured between the start of injection to the attainment of chemical reaction condition, is called Physical delay period.

ž  The physical delay period consists of the atomization, vaporization, mixing with air and rising in temperature.

2. Chemical delay

ž  Pre-flame reactions start slowly and then accelerate until local inflammation or ignition takes place.

ž  In most of CI engines, the ignition delay period is shorter than the duration of injection.

Rapid or Uncontrolled Combustion

The fuel accumulated due to ignition delay burns rapidly resulting in sudden pressure rise.

ž  It is measured between the ends of the delay period to the point of maximum pressure.

ž  In this stage pressure rise is rapid.

ž  During the delay period, the mixture of fuel and air spread over the wide area.

ž  The mixture of fuel and air come across the high-temperature fresh air, rapid ignition takes place.

Controlled Combustion

In this stage the injected fuel burns directly because of the high temperature and pressure achieved in the second stage.  

ž  In this phase, the pressure rise is controlled by the rate of injection of fuel.

ž  Some fuel is injected during the end of burning, increase the pressure.

ž  This pressure is controlled by the injection rate of fuel.

After Burning

ž  Ideally, in the first three phases, the combustion process is completed.

ž  After burning is present because of the poor mixing/ distribution of fuel particles.

The factor affecting the ignition delay is as follows

S. No.	Variable increases	Effect on delay period	Reason
1	Cetane Number	Reduces	Reduction in self-ignition temp.
2	Injection Pressure	Reduces	Reduces physical delay because of smaller droplet size.
3	Injection advance angle	Increases	Temperature and pressure is low when injection start.
4	Compression Ratio	Reduces	Air pressure and temperature increases, and reduces the auto-ignition temperature.
5	Intake Temp.	Reduces	Increase in air temperature
6	Jacket water temp	Reduces	Increase wall and air temperature
7	Fuel temperature	Reduces	Better vaporization and increases chemical reaction
8	Intake pressure (supercharging)	Reduces	Reduces auto ignition temp.
9	load	Decreases	Operating temp increases