Detonation is one of the forms of abnormal combustion. It is an uncontrolled explosion, and its speeds are 10s if not 100s times faster than any normal fuel burning speed. In fact, the speeds are supersonic, and that is what makes detonation dangerous to your engine. The difference between gas oven normal burning and its explosion can give you an idea of the difference between normal combustion in an engine and detonation. Another analogy would be the difference between a gunshot and a mine burst.
Detonation happens when fuel octane rating is not sufficient for current combustion chamber conditions due to either incorrect fuel used, or excessive deposits build-up (which increases compression ratio), or, sometimes, too early ignition timing. All those things can work in concert, of course. Due to the combination of those reasons, at the end of the compression cycle air/fuel mixture gets heated too close to the temperature of its self-ignition. After spark plug ignites the initial portion of mixture, and flame front starts to propagate towards the rest of combustion chamber, pressure increases at a high rate all over the volume of the combustion chamber, causing the remaining mixture to quickly heat up beyond its self-ignition threshold. As soon as this threshold is surpassed in some point, another center of ignition is born, which produces another flame front. Those fronts work together to increase the pressure and temperatures even more, producing more ignition points. This chain reaction leads to explosive increase in pressure growth rate, which produces a shock wave, resonating all over the combustion chamber. On the outside of the engine detonation reveals itself as a high pitch metallic knocks or pings at heavy load and relatively low rpms.
Since detonation is a real explosion, it can damage your engine fairly quickly and therefore should be avoided. If you use manufacturer’s recommended gas and still hear detonation, you need to have your car checked and/or serviced ASAP. Modern cars with computer-controlled engines do have knock sensor and computer retards ignition timing to avoid detonation, if necessary. This significantly hurts your mileage, however, because your engine no longer utilizes optimal ignition timing.
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