Compression ratio (r) = total volume / clearance volume
A higher compression ratio increase the pressure and temperature of the working mixture and decrease the concentration of residual gases and tendency of the engine to detonate.
Detonation.The efficiency of engine must be design for the highest compression ratio but maximum compression ratio can be used is limited by detonation.
In the auto-ignition theory it is assumed that the flame velocity is normal before the onset of auto-ignition whereas in detonation theory a true detonating wave formed by pre-flame reactions has been proposed as the mechanism for explosive auto-ignition.
Such a shock wave would travel through the chamber at about twice the sonic velocity and would compress the gases to pressure and temperature where the reaction should be practically instantaneous.
Control Detonation.
Increase engine rpm, retarding spark, reduce pressure in inlet manifold by throttling, making the ratio too lean or too reach and injection water.
Recognition of Detonation.
Detonation may be heard if the engine is not too noise, high temperature on spark plug gasket, vibration and presence of intermittent puffs of gray smoke in exhaust.
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