Gasoline Internal Combustion Engine Detonation

[tposey7591]

The following information applies to gasoline engines and is best processed with your more primitive reptilian brain instead of your higher order cerebral cortex brain. Doing so will avoid the confusion, misunderstanding, conflicting conclusions and erroneous interpretations associated with over thinking an issue.

In basic terms combustion detonation and fuel/air mixture burn are one in the same except for the higher velocity of the expanding gasses associated with an explosion. As most people know, after ignition it is preferable that expanding gasses in the cylinder during combustion should reach a high level of pressure soon after the piston begins its down stroke on the power stroke cycle. Timing of the burn or explosion in each cylinder is everything in terms of extracting maximum energy from the fuel/air mixture. Should it occur too early on the compression stroke, expanding combustion gasses will exert an opposing force on the piston as it moves upward in the cylinder. Too late and less force time is applied to the piston as it travels downward on the power stroke cycle. Yep, basic stuff.

What should be understood about combustion and undesirable detonation is that the fuel/air mixture in a gasoline engine will not detonate unless it is squeezed/compressed by the piston on the upward compression stroke after an ignition source has started the fuel/air mixture burn. Yeah I know, if the compression is high enough as found in a diesel engine it will self ignite but that is getting off the subject. The moral of this story is do not highly compress the fuel/air mixture after an ignition source has started the burn. If sufficiently compressed, it will detonate, rapid expansion of the combustion gassed. Obviously there are many factors that cause a given gasoline fuel/air mixture to be prone to detonation such as compression ratio, advanced ignition timing, cylinder volume fill controlled by throttle opening or as some say dynamic compression, temperature of the cylinder and intake air, hot spots in the head/cylinder/piston, fuel octane rating, combustion chamber shape, weak ignition spark intensity causing slow flame propagation. etc., etc., and the list goes on but I think you get the picture. The good news is there is a happy balance in getting the combustion timing right for a given engine to get the most from it but only an experience and knowledgeable tuner can find it, probably more an art than a science.

[Black Lotus]

Seems to me detonation is an uncontrolled combustion event anywhere in the combustion cycle. The flame front for the normal event triggers an uncontrolled "explosion" of unburned gasses elsewhere in the combustion chamber. The two pressure waves collide and the piston gets a wallop equivalent to hitting it with a hammer.
Look for melted aluminum on the spark plugs, and pitting on the pistons--or a hole melted into it.
Don't ask me how I know about the aluminum on the spark plugs' ceramic, and pitting...
Also, couldn't hear it because of the exhaust noise from my mighty '65 Corvair.

Now we have Knock Sensors that sense the specific vibrations of detonation and preignition and retard the ignition timing, and/or cut boost levels, to save the engine.

[tposey7591]

Detonation occurs only on the upward travel of the piston on the compression stroke while compressing the fuel/air mixture and only if mixture burn has started from an ignition spark or an unintended source. This applies to gasoline engines and not diesel engines. Keep in mind that the term detonation is used to identify a very quick expansion of combustion gasses that produces an audible sound. In theory if it were possible to have a very quick expansion of combustion gases, that associated with the term detonation at the moment of crank angle crossover, just after zero degrees on the power stroke cycle and not a microsecond before you would have the most efficient engine possible. Also it should be kept in mind that just about everything done from a tuning perspective, valve timing, ignition timing, valve/port size, static and dynamic compression and fuel mixture, etc., is an attempt to create maximum combustion gas pressures at the very beginning of the power stroke cycle throughout the entire usable RPM range. A tall order indeed.