What is a Knock Sensor and What Does It Do?

Looking for a detailed explanation as to what a knock sensor is and what it's for? Look no more! There are hundreds of things that can go wrong with a car engine. In the past, if the car doesn't start, you'd have to either pay a mechanic to do a (pricy) troubleshooting or do it by yourself.

Luckily today, a car can detect problems on its own. Most cars now feature sensors that continuously track their health and functionality. One such sensor is the knock sensor. If you want to understand what a knock sensor is, you should first understand what an engine knock is. So let's get rolling!

What is an Engine Knock?

What is an Engine Knock?

If you hear a repetitive tapping sound coming from your engine, instead of the usual smooth rumble, and it gets louder and faster when you accelerate, there's a high risk that it's because of engine knock.

Engine knock, also known as engine detonation or ping, is a type of reaction and sound which occurs if there is an unplanned, second ignition in the cylinder after you start the car. In other words, an engine knock is not good for your car.

According to the experts, this is how it actually happens: when you start the car, there is ignition in the spark plug. The flame created by the spark plug burns up the air and fuel mix inside each cylinder of the engine. When that is done in a single, controlled detonation, the engine will run with a smooth rumble. The knocking sound that is characteristic of engine knock appears when there are instead multiple detonations inside the cylinder. You can learn more about the cylinder here.

In case you are not that interested in learning more about the knock sensor and you prefer to just go ahead and buy a new one, check out our blog post that included reviews of the best knock sensors here.

What is a Knock Sensor?

knock sensor

Knock sensors detect knocking sounds coming from the engine. Modern cars can feature single or even multiple knock sensors. While humans can easily hear full-blown pinging or knocking, the sensor is able to detect them at much less perceptible levels as soon as the sensor detects knocking, it generates an electric signal and sends it to the ECU.

The ECU then decides whether to alter spark plug timing, to prevent the knock. You should be able to see the CEL (Check-Engine Light) when this process is happening.

In those cases where the knock sensor fails, you'd eventually be able to hear the knocking sound coming from the engine. If the engine knock keeps repeating itself, it could result in diminishing acceleration and falling gas mileage. By the way, if you are interested in learning how to test a knock sensor, you should check out this blog post here.

How Does the Knock Sensor Work?

How is the Knock Sensor Able to Do What It Does?

Now that you know what a knock sensor is and what it's for, you might also be interested in how it actually operates. The engine crankcase is where you'll mostly see the knock sensor. These sensors make use of piezoelectric elements or ceramics. Because of this element, the sensor can convert mechanical effects (such as movement, vibration, or pressure) into electric signals.

Moreover, it is also able to convert electrical signals into the said mechanical effects. Due to this, the piezoelectric elements are also commonly found in watches, motion sensors, lithotripters, ultrasonic welding, etc.

What are the Main Reasons That Cause Engine Knock?

bad knock sensor

Apart from understanding what a knock sensor is, it's good to know what causes engine knock. There are four common factors that can cause the engine knocking:

Bad Timing

Most modern cars have computers controlling when the spark plug fires up the engine during the ride. If the spark does not fire when it should, it can result in multiple detonations inside the engine's cylinders. As you know by now, the outcome would be engine knock.

Lean Fuel/Air Mixture

Lean Fuel/Air Mixture

Any problems related to fuel pumps, fuel injectors, mass air flow sensors, or oxygen sensors can result in a lean fuel/air mixture in the car engine. In other words, the mixture will have a higher amount of air and a lower amount of fuel than it should. As the fuel levels are low, the mixture will not be able to burn as fast as it should. This can cause multiple detonations and knock.

Carbon Deposits Inside Cylinder

Fuels sold in most countries generally have a type of detergent for carbon cleaning added to it. But it's not uncommon for this detergent to not be able to keep the carbon deposits away from the inside cylinder walls. As the deposits thicken the walls, the volume of the cylinder begins to reduce. This results in an unplanned change which increases compression and leads to engine knock.

Using fuel with too Low Octane

Low Octane

If you happen to put fuel with too low octane rating in the tank, this can lead to engine knock as well - such as if the car demands high-octane, premium fuels, but you use regular unleaded fuel. This is because high octane fuels burn more uniformly than low octane fuels. Be sure to check what your car manufacturer or dealer specifies for your car model.

Knock Sensor Failure

While this is very uncommon, a malfunctioning engine knock sensor can because of engine knock. This is rare as most components of a modern car are computer-controlled. However, worth testing the knock sensor if all the mentioned reasons above do not seem to help you. To learn more regarding knock sensor failure, check out our other blog post here.

What Should I Do if I Hear Engine Knock?

engine knock

Well, there are more reasons than the above-mentioned ones as to why an engine would give away knocking or pining sounds. Worn rod bearings can cause the pistons to rattle against the crankshaft. The rod bearings' function is to make sure the piston's movements are smooth, but they can become worn out or get out of position after some time. By the way, speaking of the crankshaft, check out our other article that covers that topic in more detail here.

Another common issue is that the sound could be coming from the accessory belt, and not the engine (the accessory belt drives the AC compressor, power steering pump, alternator, and water pump). If the belt has become stretched out or one of the pulleys it's connected to becomes bent, it will create a rattling and snapping noise that is very similar to engine knock.

Here's a video of how to diagnose a car engine when detecting knocking sounds:

Final words

A knock sensor detects abnormal sounds from the engine, which are called engine knock. Engine knock can be caused by several different issues (and some of them are not necessarily even due to engine issues), but most of them related to how the fuel & air mix is burned (detonated) inside the cylinders of the engine.

An engine knock sensor will make sure to keep the most common issues that generate engine knock in check. When detecting knocking sounds, it sends signals to the ECU to adjust spark plug timing in order to eliminate the engine knock. However, keep in mind that If you happen to have a faulty engine knock sensor, it can be replaced quite easily.

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