Originally Posted By: Virtus_Probi
Originally Posted By: camrydriver111
Normally the air/fuel mixture is ignited by the spark plug when the piston reaches dead center.
When you have an engine knock the mixture is still ignited by the spark plug, however the pressure wave causes a secondary ignition inside the combustion chamber, producing the pinging sound.
When the knock sensor detects a knock, the computer "retards" the spark plug timing so the spark plug fires later when the piston starts moving away. When it does this there is less compression and less power.
Where I live premium gas is often ethanol free, while regular contains ethanol. Gasoline has a higher power density than ethanol. I'm not sure how much this contributes but it could be a factor as well.
This is a good description of the basic concept, but the knock control system for which I designed electronics some time ago was a little more sophisticated than that.
It was a closed loop system that tried to keep the engine on the edge of knock, or even knocking very lightly, whenever it could. The engine guys told us that knocking so lightly that the driver could not sense it was the sweet spot for power and fuel economy. Got to watch a prototype of the system running in a car on a dyno and it was pretty darned impressive.
Haven't really kept up on developments in that field for some time, but
I'd imagine knock control is quite a bit more sophisticated now.
Yep. Now they have ion sensing, where the plugs double as an ion sensor. If there is any (or no) combustion happening before plug fires, the ECU will know about it due to the conductivity of combusting fuel. First heard about it on SAABs. Later generations are using the ion current signal to roughly determine the peak cylinder pressure and combustion quality