01-03-2013, 12:33 PM
COMBUSTION IN C.I. ENGINE
COMBUSTION IN C.I..ppt (Size: 785 KB / Downloads: 76)
Review of Combustion In S.I. Engine
In the S.I. Engine a Homogeneous Carburetted mixture of petrol vapour and air, in nearly chemically correct ratio, is compressed in the compression stroke through a small compression ratio (6to 10) and the mixture is ignited at one place before the end of compression stroke by means of an electric spark.
After ignition a single definite flame front progresses through the air fuel mixture, and entire mixture being in the combustible range.
Combustion In C.I. Engine
In C.I. Engine, air alone is compressed through a large compression ratio (12 to 22) during compression stroke, so that the temp. & press. of the air becomes very high ( well enough the ignition point of the fuel).
In this highly compressed & highly heated air in the combustion chamber ,one or more jets of fuel are injected in the liquid state,(which is compressed through a high pressure of about 110 to 200 bar , by means of a fuel pump.)
In the C. I. engine the fuel is not ignited at once ,but it is spread over a definite period of time corresponding to 20-40 degree crank rotation. (This period in most cases greater than ignition delay.) The initial fuel droplets meet air, whose temp. is only little above their self ignition temp. & they ignite after ignition delay. The subsequent fuel droplets finds air already heated to much higher temp. by burning of initial droplets & therefore light up much more quickly ,almost as they injected from the injector/ nozzle. But their movement is stopped due to less quantity of air availability.
Variables Affecting Delay Period
Fuel:
A higher the self ignition temp. of the fuel , higher will be the delay period.
Injection pressure :
To achieve the complete combustion in the very short time available, the liquid fuel should be injected in the droplets of smallest size to obtain largest surface-volume ratio. But at the same time the rate of combustion also depends upon which the products of combustion can be removed from the from the surface & replaced by the fresh air. A smaller droplets have lesser momentum & hence have lesser velocity resulting in partial suffocation by its own products of combustion.
Knock in C.I. Engine
If the delay period is long ,a large amount of fuel will be injected and accumulated in the chamber. The auto ignition of this large amount of fuel may cause high rate of pressure rise high maximum pressure which may causes knocking in diesel engine.
The longer the delay period ,the more rapid and higher will be the pressure rise since more fuel will be present in the cylinder before the rate of burning comes under control. This cause rough running & may cause diesel knock. Therefore objective is to keep delay period as short as possible .but at the Same time some delay period would be necessary otherwise the droplets would not be dispersed in the air for complete combustion & this would cause the high smoke & high fuel consumption.
Short ignition delay & low self ignition temp. of the fuel is desirable ,to eliminate the knock in case of C.I. engine.