How a knock sensor works
What a knock sensor does and why it’s so important
A knock sensor plays a pivotal role in the prevention of engine damage due to pre-ignition and detonation or knock. This article delves into the mechanics of how a knock sensor works, its significance, and its contribution to engine performance and longevity.
What is Engine Knock?
Before exploring the workings of a knock sensor, it is essential to understand what engine knock is. Engine knock, also known as pre-ignition or ping, or detonation, also called knock, occurs when the air-fuel mixture in an internal combustion engine’s cylinder ignites prematurely or unevenly. Instead of a smooth and controlled burn initiated by the spark plug, knock results in a rapid and uncontrolled explosion. This creates shock waves within the cylinder, producing a characteristic pinging or knocking sound.
A knock sensor receives a quick bias voltage from the ECM at startup. That’s a test voltage to prove that the wiring harness and sensor are working. From that point on, the sensor doesn’t require any voltage. Here’s how a knock sensor works. There’s a piezoelectric crystal inside the sensor that generates an AC voltage when it’s compressed by engine vibrations. The vibrations occur if the engine encounters preignition or detonation. So, in a sense, the knock sensor acts as the ECM’s ears to listen for abnormal engine noise. The sensor is tuned to the discrete frequency range of preignition and detonation so it doesn’t false alarm from normal engine noise.
Why does the engine need a knock sensor?
To get maximum power from each combustion event, engineers have determined that the combustion should reach its maximum at around 14° after top dead center (TDC). To make sure the air/fuel mixture reaches maximum force at that point, the ignition system initiates the spark early so the flame front can grow. The timing of the spark varies depending on the engine RPM, so it may occur at a few degrees before the top dead center (BTDC) at idle but advance to as much as 20° BTDC at 3,000 RPM. The car’s ECM watches the values from the knock sensors to make sure the combustion events occur as planned.
If the sensor detects knock, the ECM adjusts ignition timing, often retarding the spark to shorten burn time. If that doesn’t work, the ECM can adjust the air/fuel mixture. If the knock continues, the ECM will switch to SAFE MODE. At that point, it uses the misfire monitor to track how the engine is performing. The driver will often notice a lack of power when the engine is in safe mode.
How does a knock sensor fail?
The knock sensor can be mounted in the valley of a V-style engine or on the side of the engine block. It’s exposed to strong vibration, oil, and moisture. Due to where it’s mounted, it can be contaminated with moisture, preventing the piezoelectric crystal’s ability to produce an AC voltage. If that happens, the sensor must be replaced.
Types of knock sensors?
Knock sensors can be screw-in or bolt-on. If you’re installing a bolt-on style sensor, tightening to the correct torque is critical, so always use a torque wrench. In addition to the proper torque, many carmakers specify a certain orientation in order to capture vibrations. Orienting the sensor at the wrong angle can render it useless in detecting knock.
