COMPARISON OF KNOCK IN SI AND CI ENGINES (1).pptx

8/18/2019 COMPARISON OF KNOCK IN SI AND CI ENGINES (1).pptx

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COMPARISON OF KNOCK IN SI

AND CI ENGINES


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In spark-ignition engines, the autoignition ofthe end gas away from the spark plug, mostlikely near the end of the combustion causes

knocking.

But in compression-ignition engines theautoignition of the charge causing knocking isat the start of combustion. It is the rst chargethat autoignites and causes knocking in thecompression-ignition engines.


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It is clear from Fig.12.1 that e!plosi"e auto-ignition is mpre or less o"er before the peakpressure for the compression-ignition engines.But for spark-ignition engines, the condition

for e!plosi"e autoignition of the end charge ismore fa"ourable after the peak pressure.


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. In order to a"oid knocking inspark-ignition engines, it is necessaryto pre"ent autoignition of the end gas

to take place at all. In compressionign itionengine, the earliest possibleautoignition is necessary to a"oidknocking


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In spark-ignition engine, the charge that

autoignites is homogeneous and thereforeintensity of knocking or the rate of pressurerise at e!plosi"e autoignition is likely to bemore than that in compression-ignitionengines where the fuel and air are nothomogeneously mi!ed e"en when e!plosi"eautoignition of the charge occurs. #herefore, itis often called detonation in $I engines.


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In compression-ignition engines, only air iscompressed during the compression strokeand the ignition can take place only after fuelis in%ected %ust before the top dead centre.

#hus there can be no preignition incompression-ignition engines as in spark-ignition engines.


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It has already been pointed out that, the normalprocess of combustion in compression-ignitionengines is by autoignition.&nd thus normal rate of

pressure rise for the rst part of the charge forcompression-ignition are higher than those forsparkignition engine, in terms of per degree crankrotation. &nd normally,audible knock is alwayspresent in compression-ignition engine. #hus whenthe audible noise becomes se"ere and causeshea"y "ibrations in the engine, it is said that theengine is knocking. #herefore, it is-also a matter of

%udgement.


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& de nite demarcation between normal combustionand knocking combustion is "ery di'cult. #he rate ofpressure rise may be as high as 1( bar per degree

crank rotation in compressionignition engines. #he factors that tend to increase autoignition reactiontime and pre"ent knock in $I engines promote knockin )I engines. &lso, a good fuel for spark-ignitionengine is a poor fuel for compression-ignition engine.

#he spark-ignition fuels ha"e high octane rating ( to1(( and low cetane rating of about 2(, whereas dieselfuels ha"e high cetane rating of about *+ to + andlow octane rating of about (.


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COMPARISON OF KNOCK IN SI AND CI ENGINES (1).pptx

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