COMPRESSION RATIO

Shannon News, 2 March 1928, Page 4

COMPRESSION RATIO

The term "-compression ratio" is self-explanatory, but perhaps it is deniable to amplify the subject a little. Tn the earliest internal combustion engines the mixture was just sucked into the engine and exploded. It was soon discovered that if the mixture was con pressed somewhat before tho explosion took place 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 i n the 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 the compression stroke andthe volume of the combustion ebambei, or clearance volume, as it is sumotimes called, when the piston is at the top of its stroke. If this compression ratio is too high, the mixture will become so hot as a result of the compression that it will ignite spontaneously, and wo have then the condition called proignition, which is usually accompanied by mechanical tapping noise from tho cylinder and a ■considerable loss of power. A usual compression ratio on pf'sent-cuiv engines is 5 to. 1. but on racing engines it may go so high a= 7 to 1, and if special fuels such as alcohol mixtures, are used it may be even 9 or 10 to 1.