25-06-2012, 05:57 PM
Knocking and knock rating of fuels
Knocking.pptx (Size: 87.23 KB / Downloads: 65)
Methods to control diesel knock
Knocking can be controlled by having,
Higher compression ratio.
Higher inlet temperature and higher pressure air.
Lower self ignition temperature fuel.
Higher cylinder wall temperature.
Larger diameter cylinder.
Higher cetane number fuel.
Knock rating of diesel fuels
The property of ignition delay in case of CI engine is generally measured in terms of number.
It is arbitrary assigned a rating of 100 cetane number (CN), while hepta methyl nonane (HMN), C12H34 is given the value of 15.
Typically engines are designed to use fuels with cetane number of 40 to 55.
The higher cetane number, shorter the ignition delay.
Cetane number can be improved through the refining process or the blending of combustion improving additives.
Cetane number improvers can be alkyl nitrates, primary amyl nitrates, nitrites or pre oxides.
The higher the cetane number, then the lower the octane number. A good fuel or diesel engine is bad for gasoline engine.
The cetane number of other fuels is obtained by comparing the ignition delay of that fuel to the ignition delay of a mixture blend of the two reference fuels, CN of fuel = (percent of n-cetane) + (0.15) (percent of HMN)
The cetane number of the fuel is measured using a standard CFR pre-chamber engine which has a variable compression ratio.
Test procedure
Test fuel is injected into the CFR engine cylinder late in the compression stroke at 13 deg bTDC. The compression ratio is then varied until combustion starts at TDC giving an ignition delay of 13 deg of engine rotation.
Without changing the compression ratio, the test fuel is replaced with a blend of two reference fuels. The blend of the fuel is varied until combustion is again obtained at TDC at an ignition delay of 13 deg.
The CN of the test fuel is then defined as the percentage by volume of cetane in a mixture of cetane and HMN that produces the same ignition delay as the test fuel.