MYSTERY OF ENGINE KNOCK.

Hokitika Guardian, 25 October 1924, Page 4

MYSTERY OF ENGINE KNOCK.

I’ISTON-TAI’ TIIEOBY. (Hv a .Motor-cyclist in flic “Daily .Mail.” I.IINIKIN, August 11. Ever since ihe invention of the inlerual combustion engine a mechanical phenomenon has baffled engineering experts. I icier to engine " knock ” not, tlie heavy thud which denotes a worn big end bearing, bill the irritating tapping noise which oci hits when the engine is labouring oil a bill, during rapid aiielenition with the ignition fully advanced, or when the engine is throttled ilowti to its :-lowest revolutions. This tapping sound is sometimes called "pinking.”

It is a fault which has not disappeared with advance in the design of motor-engines, ami experts when asked for an explanation state quite frankly that the mystery has never been cleared up satisfactorily. It is now definitely established that when benzol is used as a fuel in place of petrol the tendency of an engine to “ knock ” is lessened. As every mot-or-cyclist knows, there is a similar result when Ihe ignition control is re tank'd to its full extent. I low do these fuels lit in with one another? Normally, with the ignition advanced the explosion in the cylinder takes place approximately when the piston is about In descend the cylinder. If the ignition is retarded the explosion takes place when the piston is some distance down the cylinder barrel. Uenzol is a slow-cnmbustiou spirit—slower than the modern grade of petrol—and it is quite possible that its delaveil action has very m.llch the same effect as a retarded ignition.

This seems to exjikiiu why benzol is a palliative for a “pinking” engine. But what causes the “ jiinkhig”? Emm the above it might be deduced that -the piston itself is to bhime. A certain clearance has to be left between the piston and llio cylinder walls. AVI ten the engine is labouring and the explosion is taking place hi" the top of its stroke, piston “slap” oei'titTs —that is, the piston strikes the wall of the cylinder sideways, setting up a tapping noise. AVhen the explosion takes place with the piston

lower down the cylinder, there is less room for side-play and the tapping disn ppcars.

This is not put forward as a definite solution to the problem but as a suggestion backed up by recent knowledge of the effect of benzol on a “pinking” engine*