When sudden and violent knock are experienced in petrol engine, these knocks are termed as "Detonation". Just like petrol engines, a high-pressure wave set up in compression ignition engine (Diesel Engine) and this wave strikes the cylinder wall, cylinder head and piston resulting in knock. These knocks are known as "Diesel Knock".
There is power and efficiency loss along with rough running of engine is observed due to diesel knock. In severe cases it may wreck the engine.
A considerable amount of air -fuel mixture may ignite at the same instant and result in a combustion knock or heavy thud rather than pink. While it is end mixture which knocks in the spark ignition engine (petrol engine) it may fairly large part or parts of the mixture, which knock in compression ignition engine.
To prevent diesel knock, following important methods can be used very effectively.
01) Add 1% of ethyl nitrate or any nitrate to accelerate the combustion of fuel. This process in known as "Doping" and reduce delay period considerably and hence prevent diesel knock.
02) For spontaneous ignition of the fuel, we require much higher temperature this can be achieved by raising the compression ratio. The rate of pressure rise increases resulting in smooth running of the engine. If compression ratio increased in petrol engine, the engine is more liable to detonate, but in diesel engine it reduce the diesel knock.
03) Diesel knock can be reduce by increasing the turbulence of the compressed air injected promotes homogeneous mixture by stripping the fuel from the spray.
04) Arrange the fuel injector in such a way that it should start injecting only a small quantity of fuel in starting of the engine. If we increase the injection pressure, which encourages the atomization of fuel and prevent knock.
05) By raising the temperature of the coolant that of the intake air as well as cylinder head and combustion chamber.
06) By increasing the inlet pressure of air (Supercharging) we can prevent diesel knock.
Many times we noted this sudden noise but not aware about the phenomenon behind this, now above description will clear the doubt about knock in your automotive engine. What you say?
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