Knock sensors

By Kevin Ash

Pictures: Double Red, BMW Press
(click on images)

The 2009 CBR600RR has had changes made to its cylinder head and piston design in order to extract more power through improved burning efficiency, but one feature that allows it to run its high 12.2:1 compression ratio (not the highest on a sports bike but still quite a squeeze for a fuel-air mixture...) is its knock sensor.

A knock sensor last TuesdayHigh compression ratios are not just good for efficiency, they’re crucial for it – the more the fuel-air mixture is compressed before the spark ignites it, the more complete the burn and the more the energy that can be extracted. The problem is, squeezing a fuel-air mixture increases its temperature – and it’s already very hot in an engine’s combustion chamber – until a point is reached where the mixture will ignite spontaneously, before the spark sets it off. This is a more sudden and violent explosion rather than the controlled burn of normal ignition, and it happens too early in the piston’s movement up the bore, meaning power is lost and the engine can quickly be damaged.

Cylinder pressure is also affected by the ignition timing: the further the timing is advanced, the higher the pressure in the cylinder and the more likely knock is to occur. But the best power and efficiency are reached when the timing is advanced as far as it safely can be, just before knock starts to happen. A knock sensor helps the engine management to find this point: a sensor in the cylinder detects the intense shockwaves produced by knock as soon as they start to happen, and a signal is sent to the engine management, which in turn holds the ignition at a point where these shockwaves are only just occurring. They’re enough to be detectable but not to affect efficiency or cause damage.

BMW boxer twins' knock sensors are sited just behind and below the throttle bodyAs well as being efficient, another big advantage is that an engine with a knock detector is capable of running on a wide range of fuels. High octane fuels (usually rated at 98 RON, Research Octane Number, at the pumps) resist knock better than lower octane ones, so an engine with a knock sensor will advance the ignition further than when it is being fed a 95 octane fuel or lower. It will make more power and be more fuel efficient too, justifying (for some riders anyway) the extra cost of the higher octane fuel. It is also capable of running on the poor and variable quality fuels supplied in some countries without modification.

But on an engine with no knock sensor, there is no value in using a higher octane fuel than the one recommended by the manufacturer, as it can’t advance the ignition automatically to take advantage of it.