COMPRESSION RATIO

Manawatu Times, Volume LIII, Issue 6538, 21 February 1928, Page 10

COMPRESSION RATIO

The tem “compression ratio” is self-explanatory, hut perhaps it is desirable to amplify the subject a little. In the earliest internal combustion engines the mixture was just sucked into the engine and exploded. It was soon discovered that if the mixture was compressed somewhat before the explosion took placo more power was obtained, and in this discovery lay the secret of the success of the petrol engine. The necessary compression is carried out automatically in a motor cycle engine by the piston as it rises on tch compression stroke after the inlet valve has closed, and the degree of compression obtained is obviously dependent upon the ratio between the volume above the piston at the beginning of tho compression stroke and the volume of the combustion chamber, or clearance volume, as it is sometimes called, when the piston is at the top of its stroke. . .If this compression ratio is too high, the mixture will become so hot ns a result of the compression that it will ignite spontaneously, and we have then the condition called preignition, which is usually accompanied by mechanical tapping noise from the cylinder and a considerable loss of power. A usual compression ratio on present-day engines is 5 to 1, but on racing engines it may go so high as 7 to 1, and if special fuels, such as alcohol mixtures, are used it may be even 9 or 10 to 1.