The green circle shows the knock sensor

Though engine knocks happen fast in an engine cycle, they can still be prevented. One way of doing so is by ensuring that your engine’s knock sensor is working—that is, if your car has one. Most cars nowadays have one installed in their engine. But that doesn’t mean that car owners know they have one in their vehicle. Many car owners only realize their car has a knock sensor when their engine is already knocking.

So if you don’t know where yours is located in your engine, the picture on the right might give you an idea. In most cars, the knock sensor is bolted onto the center of the cylinder block’s wall, approximately at the level of the cylinders. But there are also some vehicles where it is bolted onto the cylinder head. Well, these differences in location are actually due to differences in estimates of where the knock sensor would work most effectively—meaning, where it would be most sensitive to engine knocks.

A knock sensor uses piezo electric mechanism

Most knock sensors look like a midget spark plug with most of its bulk in its center; it has a threaded part on one end and a small pin or pole-like projection on the other. Other knock sensor designs don’t have the threaded part, which is used to bolt the sensor to the block; instead, they have a threaded bolt hole.

When knocks occur inside a cylinder, the sensor picks up the reverberations, which have frequencies ranging from 6 to 8 kiloHertz. Inside the knock sensor, a resonating plate relays the shock waves to a piezo electric crystalline element. This element then generates a small amount of voltage. In turn, this weak electric signal goes to the vehicle’s electronic control unit and tells it to retard or delay the firing of the spark in the combustion chamber. That way there wouldn’t be two flame fronts igniting simultaneously in the cylinders and thus, knock wouldn’t occur.

Unfortunately, the knock sensor can handle only mild engine knocks. It sometimes happens that the knock frequency is way beyond the frequency range the sensor is designed to pick up, so it’s either the sensor would detect it too late or it would not detect the knock at all. But then that’s in extreme cases. So long as you take good care of your engine and ensure that it has a perfectly working knock sensor in place, you really don’t have to worry much about extreme-case engine knocks.

(Find out other ways of preventing engine knocks by reading  our other post: Knocking Out Engine Knocks.)