To add to PTGuy's excellent -- as always -- technical explanation, it is interesting to speculate how Mazda's anti-knock sensor works. Most sensors of this type "listen" to the engine block for the characteristic sound of detonation occurring, detonation being what is also called preignition. Upon that sound being present, the ecu backs off the timing and perhaps does other things as well. Newer BMWs with the V8 that has the turbo sitting atop the engine in the vee between cylinder banks use the spark plugs to detect detonation. Air ionizes a fraction of a second before detonation occurs. This ionization can be detected by an electrical gap inside the combustion chamber; the spark plugs provide exactly such a gap. By detecting this ionization, BMW's ecu can adjust timing and whatever to avoid detonation rather than to experience it and then back off, which is how traditional systems work. (It is possible that other manufacturers use the spark plugs in this way; I only know about BMW's use.) I mention all this because Mazda has recently announced a hybrid engine that operates partly as a normal spark-ignition engine and partly as a diesel-like compression-ignition engine. This obviously needs some pretty sophisticated engine management because at diesel-level compression ratios the engine could self destruct quickly running gasoline. I wonder what technology the company is using to detect -- and avoid -- detonation.
