Knock control of modern engines
Knock sensors are accelerometers mounted on the block of an engine to “listen” for the onset of knock (or auto-ignition). If detected, the ECU retards the ignition timing to protect the engine, impacting performance. Most modern engines are calibrated to run on 95 unleaded petrol to produce the claimed maximum power outputs and will retard the timing only when a lower-octane fuel is used. There are, however, advanced knock-control systems able to advance the timing until the onset of knock. The result is a higher power output than claimed at the standard of 95 octane.