Stages of combustion:
Stages of combustion
Stages of combustion can be divided into four stages namely,
1. Pre-flame combustion
2. Uncontrolled combustion
3. Controlled combustion and
4. After burning.
Pre flame combustion:
In actual engine cycle, the fuel injection starts at the point ‘a’ shown in fig. As soon as the fuel jet is known into a fine spray, the fuel starts absorbing heat from the surrounding high temperature air and vaporization of fuel starts. But in the absence of flame, therefore it is known as pre-flame reaction. At the beginning of pre-flame combustion, the energy release rate is very less than rate of heat absorption by the fuel because the amount of fuel vapour is small. As a result, the pressure in the cylinder decreases with the progressive fuel vaporization. This decrease in pressure attains a maximum value when the energy release due to pre-flame reaction is equal to the rate of heat absorption by the fuel. This process of fuel vaporization and subsequent decrease in pressure in the cylinder is shown on fig by paths
As the energy release rate due to pre-flame reaction is more than the rate of heat absorption, the pressure inside the cylinder starts increasing. This rising pressure intersects the pressure curve without the fuel injection at the point ‘c’. At point ‘c’, the pressure drop caused by the fuel vaporization is completely recovered by the energy released due to preflame combustion. The pressure inside the cylinder after the point ‘c’ rapidly increases as the ignition takes place somewhere around the point ‘c’ and flame appears. The actual flame (actual combustion) starts at the point ‘c’ where as the fuel injection starts at point ‘a’. The time required to start the actual combustion of after starting the fuel injection is known as
“delay period” and the crank angle required for this is known as “delay period angle” and it is shown in the fig by an angle α.
The time and place where ignition will stop is not fixed by anything in compression ignition engine as in SI engines.
The air fuel mixture in the combustion chamber before starting the combustion is very heterogeneous and the concentration of the fuel may vary from 0 to 100%. The first ignition (flame) generally occurs in the region of chemically correct A:F mixture because it requires minimum reaction time. Once the ignition takes place, the flame formed propagates through the mixture of air and vaporized fuel and ignites the adjacent part of the charge or it may initiate the auto ignition in the part of A:F mixture away from the flame front by transferring the heat by radiation.
A considerable amount of fuel is accumulated in the combustion chamber during the relay period (time between the start of injection of fuel and start of ignition of fuel). This accumulated fuel burns very rapidly causing a steep raise in the cylinder pressure. The rate of pressure raise increases with the increase in delay period because of the amount of fuel taking part in this combustion increase with an increasing delay period. This phase of combustion causing rapid pressure raise in the cylinder is known as “period of uncontrolled combustion”.
All the accumulated fuel during the delay period generally burns during the period of controlled combustion. The fuel injected after this (after point d) burns at the same rate at which it is injected because, the vaporisation of fuel, mixing with the air and burning takes place almost instantaneously as the fuel leaves the nozzle. This is because, the temperature and pressure inside the cylinder are sufficiently high and sufficient turbulence is created due to precious burning, thus the delay period for the fuel injected after point “d” is almost zero. This period of combustion is known as “controlled combustion” because the rate of burning can be controlled by controlling the rate of injection. This is confirmed until the supply of fuel ceases. This process is shown by the path “de” on the fig.
The thermal decomposition of the part of fuel takes place during uncontrolled and controlled combustion. The decomposed fuel molecules contain enough number of hydrocarbons and carbon particles which has lower reaction rates. Some carbon and hydro carbon, decomposed from fuel are left in the combustion product because the rate of decomposition during uncontrolled and controlled combustion is more than the rate of reaction of these molecules during that period. These unburned hydrocarbons and carbon generally burn after stopping the fuel injection during the expansion stroke. This process of combustion of decomposed carbon atoms is known as “after burning”.