rupak knocking

7/30/2019 Rupak Knocking

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Knocking

Knocking (also called pinking or pinging)technicallydetonation ininternal combustionenginesoccurs whenfuelin the cylinder is ignited by the firing of thespark plugand smooth burningproceeds but some of the unburned mixture in the combustion chamber explodes before the flamefront can reach it, combusting suddenly before the optimum moment of thefour-stroke cycle. Theresulting shockwave collides with the rising piston, creating a characteristic metallic "pinging" sound.

Detonation

The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow asmall time for the flame-front of the burning fuel to expand throughout the mixture, so that maximumpressure occurs at the optimum point. The flame-front moves at about 1100feet/second duringnormal combustion. It is only when the remaining unburned mixture is heated and pressurized by theadvancing flame front for a certain length of time that the knocking effect occurs. It is caused by aninstantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinderpressure rises dramatically beyond design limits. If allowed to persist detonation will cause vibrationand damage to engine parts.

Detonation can typically be prevented by:

The use of higheroctanepetrol The addition of octane-increasing"lead",isooctane, or other fuel additives Reduction of cylinder pressure by increasing the engine revolutions or reducing the load on the

engine, or both

Reduction of in-cylinder temperatures (such as through cooling orcompressionratioreduction), or

Retardation of spark plugignition. Adding water in the form of humidity to the air. Proper combustion chamber design which concentrates mixture near the spark plug and

generates high turbulence to promote fast even burning.

Correct ignition timing is essential for optimum engine performance andfuel efficiency. Modernautomotive and small-boat engines havesensorsthat can detect knock and delay the spark plug firingto prevent it, allowing engines to safely use petrol of lower octane ratings, with the consequence ofreduced power and efficiency.

These knock sensors work by using a smallpiezoelectricmicrophone connected to the ecu. Spectralanalysis is used to detect the trademark frequency produced by detonation at various RPMs. Whendetonation is detected, ignition timing is retarded, reducing the knocking and protecting the engine.

Pre-ignition

Detonation is a different phenomenon from pre-ignition, which occurs when the air/fuel mixture inthe cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition iscaused by an ignition source other than the spark. Heat can buildup in engine intake or cylindercomponents due to improper design or maintenance, or overheating of the air/fuel mixture duringcompression. This heat buildup can only be prevented by eliminating the overheating (throughredesign) or the compression effects (by reducing the load). As such, if pre-ignition is allowed tocontinue for any length of time, power output reduces drastically and engine damage can result.

Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may existseperately.
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Knocking order falls with branching, unsaturation, aromatization etc. The knocking order is: Straight

chain paraffins > branched chain paraffins > olefins > naphthenes > Arenes. The knocking may be

prevented by adding compounds like TEL or tetra ethyl lead. The gasoline from petrol pump which we

get contain anti knock compounds (TEL).

Octane no. of gasoline is the no. for rating anti knock qualities of gasoline. The higher the octane

no.,lesser the tendency for knocking and better being the fuel. Octane no. is derived by assuming 100

arbitrary value for isooctane and zero for n-heptane. Thus no. is the % of isooctane in a mixture of

isooctane and n-heptane whose knocking resembles with the knocking of fuel used.

The octane no. of fuel is increased by two to three units by adding 2 to 4 ml of TEL in one Gallon

petrol. Tetra ethyl lead is supposed to dissociate as

(C2H5)4Pb 4C2H5 + Pb

Ethyl free radical

The ethyl free radical converts some of the straight chain hydrocarbons into branched chainhydrocarbons and thus increases the octane number. In addition to TEL, a little ethylene dibromide is

also added which converts Pb to PbBr2, which comes out through exhaust and prevents its deposition

in engine due to decomposition of TEL.

Pb + CH2BrCH2Br PbBr2 + CH2=CH2

Volatile

A gasoline fuel with octane no. 30 or above is good fuel. The efficiency of fuel increases as the octane

no increases.

Addition of TEL in petro, no doubt improves the octane number of fuel but it also causes lead pollution

in atmosphere. The attempts are therefore been made to improve octane number of fuel by someother means. Unleaded or lead free gasoline marketed today is the gasoline obtained by isomerisation

and alkylation blended with BTX (Benzene, toluene, xylene) or methyl-t. butyl ether. Its octane

number is 90. Thus following processes are used for getting lead free gasoline with better octane

number.

(i) Isomerisation (Reforming) : By passing gasoline vapours over aluminium chloride (AICI3)at 200oC

(ii) Alkylation : Isobutylene formed during cracking on alkylation with isobutene forms iso-octane

in presence of concentrated H2SO4.


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rupak knocking

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