Detonation or "knocking" arises from the production of an explosion wave in the combustion chamber. This is due to rapid ignition and combustion. A portion of the unburnt air/fuel mixture becomes compressed prematurely, which in turn rapidly increases in temperature until it ignites uncontrollably and almost instantaneously. The shock wave produced has a characteristic of a metallic sound and can vary in magnitude. It can range from a mild form, occurring only at relatively low speed and wide throttle positions (typically known as "pining") to such violent effects that the engine is no longer running in a controlled manner and power output dies.
Detailed view of detonation
Now that we know the definition of detonation we need to comprehend that if the temperature of an air/fuel mixture is raised to high, the mixture will eventually explode spontaneously. This is known as spontaneous ignition temperature. But, before this explosion there is an interval called ignition time-lag. If the piston approaching tdc of the combustion stroke takes longer than this period before the spark plug ignites there will be a premature explosion. The longer the time-lag the less chance of spontaneous there is of spontaneous ignition. If the air/fuel mixture is fired at the correct "timed" point and a good flame is achieved power loss through detonation is still possible. The burnt mixture behind the flame front will be at a high temperature and pressure, which will cause compression of the unburnt gas ahead of the spontaneous ignition point. The flame front must move right through the unburnt charge before the end of the ignition time-lag period if not this can lead to overheating and excessive mechanical stressing.
Fuel choice and detonation
With any fuel the brake thermal efficiency 1 will decrease if the compression ratio is increased. A properly chosen fuel can help diminish detonation. The advances in modern day fuels have extended their ignition time-lag periods and have decreased the spontaneous ignition temperatures, which allows for the use of a higher compression ratio (safely up to 11:1) to be used in the combustion process under complete control. There are also special fuels made especially for racing applications that allow compression ratios so high that the limit is actually dependent upon mechanical aspects of the cylinder head design.
The benefit of increasing compression ratio is to increase the torque output of an engine. The harm in raising the compression ratio is that cylinder pressures increases in a linear fashion as compression ratio is increased. An 8:1 compression ratio yields compression pressures of about 275 psi, where 15:1 yields compression pressure of about 575 psi. At the time of ignition the pressures will rise 3 or 4 times the compression pressure. Also, when an engine has been modified where even a slight increase in torque is achieved it is important that detonation is not allowed. The noise of detonation of a high-compression ratio engine using the correct grade of fuel is much harder to detect than that of an engine with a much lower compression ratio burning lower octane fuels. 1 Thermal Efficiency -Thermal efficiency is the measure of the efficiency and completeness of combustion of the fuel, or, more specifically, the ratio of the output or work done by the working substance in the cylinder in a given time to the input or heat energy of the fuel supplied during the same time.
Author: Dennis Adams