Compression Ignition Engines PPT

Abnormal Combustion

Delay period:

  Delay period is the time interval (measured in milliseconds) 

between the commencement of fuel injection and the beginning of 

ignition and combustion. The start of combustion is indicated 

by the deviation of point of the pressure curve above the normal 

compression pressure. In practice, this actual time is as low as 

0.006 seconds. The delay period consists of the following:

-  Physical delay period.

-  Chemical delay period

Factors influencing delay period:

 1.  Characteristics of fuel (self-ignition temperature,

        volatility and viscosity).

 2.  Temperature and pressure of compressed air.

 3.  Degree of atomization of injected fuel.

 4.  Air motion / turbulence present in the combustion

       space.

 5.  Engine speed.

 6.  Injection timing.

 7.  Characteristics of the combustion chamber.

Factors affecting knock

Knocking 

   Knocking is violet gas vibration and  audible sound produced by extreme  pressure differentials leading to the very rapid rise during the early part of uncontrolled second phase of combustion.Delay period is directly related to Knocking in CI engine. An extensive delay period can be due to following factors:

    •  A low compression ratio permitting only a 

marginal self -ignition temperature to be 

reached.

   •  A low combustion pressure due to worn out piston, rings and bad valves

   •  Low cetane number of fuel

   •  Poorly atomized fuel spray preventing

      early combustion and

     Coarse droplet formation due to malfunctioning of injector parts like spring

   •  Low intake temperature and pressure of 

      air 

METHODS OF CONTROLING DIESEL KNOCK

    We have discussed the factors which are 

responsible for the detonation in the previous 

sections. If these factors are controlled, then the 

detonation can be avoided.

     Using a better fuel. Higher CN fuel has lower delay 

period and reduces knocking tendency.

     Controlling the Rate of Fuel Supply. By injecting

 less fuel in the beginning andthen more fuel amount

 in the combustion chamber detonation can be 

controlled to a certain extent. Cam shape of suitable

 profile can be designed for this purpose. 

 Knock reducing fuel injector: 

This type of injector avoid the sudden increase in 

pressure inside the combustion chamber because 

of accumulated fuel. This can be done by 

arranging the injector so that only small amount

 of fuel is injected first. This can be achieved by

 using two or more injectors arranging in out of

 phase.

  By using Ignition accelerators : 

C N number can be increased by adding chemical

 called dopes. The two chemical dopes are used 

are ethyl-nitrate andamyle –nitrate in  concentration of 

8.8 gm/Litre and 7.7 gm/Litre. But these two 

increase the NOx emissions.

Increasing Swirl : Knocking can be greatly

 reduced by increasing swirl ( or reducing

 turbulence). Swirl helps in knock free 

combustion.

Injection systems :

Direct – Injection systems:

      In largest size engines, where mixing rate 

requirement are lease stringent, quiescent direct – 

injection systems of the type are used. The momentum 

and energy of the injected fuel jets are sufficient to 

achieve adequate fuel distribution and rates of mixing 

with the air. Additional organized air motion is not 

required. The combustion chamber shape is usually a 

shallow bowl in the crown of the piston, and a 

central multi hole injector is used.