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Originally Posted by Drweldin
OK - stupid question: If Detonation is cause by risidual mixture in the chamber, why do people automatically assume they have to richen the mixture to cure it?? It would seem the opposite wouild be true - right?
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It's not quite that simple. To understand it, you need to understand the actual mechansim of detonation. A fuel-air mixture has tendency to explode under pressure and heat. How resistant a particluar fuel is to exploding under pressure and heat determines its octane number.
In the combustion event in an ICE, the fuel-air mixture is ignited by the spark from the sparkplug and should follow the path of a controlled burn that expands outwards igniting the fuel in a widening sphere from the kernel at the sparkplug.
this flamefront of igniting air-fuel though tends to pressurise the unignited or yet to be ignited air-fuel mixture as the combustion gases expand.
Detonation/knock arises when the pressurisation of the as yet unignited air-fuel mixture detonates and explodes of its own volition before being ignited as part of the controlled burn. The force generated by the detonation of the fuel-air under severe pressure from the expanding combustion gases is much greater than the force generated from the normal controlled burn of a regular combustion event. That's why knock/detonation breaks engine parts. I don't think they've yet managed to find a material that can withstand the extreme violence of detonation, otherwise, they'ld simply build an engine to run on detonation since the energy/force generated per unit fuel is so much greater.
Detonation can be caused by a number of things, but they all boil down to pressure and heat. If your mixture is too lean, it reduces its ability to withstand pressure and heat, so richening up the mixture serves to avoid detonation in 2 ways. The higher fuel percentage reduces the inclination to knock and the added fuel also reduces the heat in the cylinders.
Using colder plugs as explained in the previous posts reduces the heat factor. Boost pressure is limited on a specific octane gasoline because of the threshold pressure (at a certain heat) afterwhich knock is encountered. A better cooling system (more efficient turbos + more efficient intercooler) reduces the heat of the intake charge, allowing more boost pressure before knock is encountered. Water and alcohol injection also reduce heat for more knock prevention.
Retarding timing advance reduce the pressure factor, as does running lower compression pistons.
Max