Inside a Car - Knock Sensor

Introduction

The ignition system to a vehicle is controlled by a computer that determines whether the engine timing is likely to cause a knock or detonation. During combustion, if there is a knock, the computer connected to ignition system will need to limit the spark advance to prevent detonation from occurring. Every vehicle is fixed with a knock sensor in order for the control unit to the ignition system to anticipate a knock or detonation.

Any knock to an engine manifests as a small vibration that is detected by the knock sensor. This sensor works by changing the vibration to an electrical signal, which is then transmitted to the computer controlling the ignition system where the change in vibration to this voltage signal alters the timing adjustments on the ignition.

The knock Sensor

The knock sensor is made up of a piezoelectric element. A working principle to piezoelectric elements involves the transmission of an electrical current in response to detecting a change in pressure or vibration by these elements. The piezoelectric element inside the knock sensor is tuned to detect the engine knock/detonation frequency. Figure 1 illustrates the basic structure to a knock sensor. As demonstrated in figure 1, the knock sensor is made up of piezocrystals (piezoelectric elements), a shunt resistor and a thread at one end of the knock sensor which allows for the device to be threaded into the block near the pistons.

Figure 1. Basic structure to a knock sensor. Source: Schwaller A.E. (2005). Total Automotive Technology. USA, New York: Thomas Delmar Learning.

During combustion, a knock in the combustion chamber sends a vibration to the silicone rings attached to the piezoelectric crystals in the knock sensor (in the form of mechanical stress),  accelerating the silicon ring, forcing this sensor to generate an electrical voltage and a pressure wave through the cylinder block. Voltage output from the knock sensor will be high during a knock to the ignition system.

A typical voltage signal generated by the knock sensor can range between 300 millivolts to approximately 500 millivolts; however, this will depend purely on the intensity of the knock during combustion. The following video animates operation of the knock sensor in a vehicle system.

Types of Knock Sensors

There are a number of knock sensors that can be used to detect a knock during combustion, with the main type of knock sensor discussed in this article being the piezoelectric sensor. The main purpose of a knock sensor is to reduce the ignition system to inhibit any damage to the engine. Knocks to an engine can be caused by a number of reasons including overheating of the engine and poor fuel quality. As mentioned previously, ignition timing is manipulated by a knock sensor. Future engine management systems promote the idea of allowing the engine to run close to detonation because this give the engine better control force and is more economical.

An acoustic sensor is often used as a knock sensor in vehicle engines. The device is a small microphone positioned against a cylinder block and detects vibrations during the running of an engine. Detection of vibrations from a microphone is followed by an electrical signal to an engine control unit (ECU) to prepare the unit for a knock during combustion.

A novel method for detecting a knock in an engine is known as ion sensing and works without the use of a sensor. This method is new and is not typically used on a wider scale in vehicle engines. This method requires a spark plug. Imagine a spark during combustion, the product of combustion (i.e., the burning mixture) converts the fuel and air into power to control the engine. By applying an electrical current across a pair of spark plugs during the ignition of an engine, a change in the conduction of the voltage can be used to measure the status of the combustion.