1. a) With a neat sketch explain the 4 phases of combustion.
Four 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 ‘ab’.
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 FUEL 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.
Uncontrolled Combustion:
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 delay 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”.
Controlled 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 previous 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.
After burning:
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”.
b) How is turbulence created within the engine cylinder?
It is known that a considerable amount of air movement takes place at the end of the compression
stroke due to piston movement. If more movement of air or turbulence is required in some particular
engine design, it is obtained by sloping the scavenge ports away from a radial direction so that the air
moves into the cylinder tangentially, causing the air to rotate within the cylinder. The same effect is
achieved in engines with air-inlet valves by means of a shield or deflector fitted on one side of the valve.
Where shields or deflectors are fitted, care must be exercised when assembling the valve to ensure that
the deflector is properly located and that the means to prevent the valve turning is correctly fitted.
Some high-speed engines with open combustion chambers are designed with a very small clearance
between the flat circumferential surface surrounding the pocket on the top of the piston and the flat
bottom face of the cylinder cover or cylinder head.
When the piston is nearing the upper part of its travel, air is forced out from between the two flat
surfaces. The air assumes a rotary motion as it is compressed in to the piston crown pocket, and a very
high degree of turbulence known as squish is obtained, Squish is necessary with high-speed engines
having direct injection, as it gives the necessary mixing of air and fuel to complete combustion in the
very limited time available.
