In a gasoline internal combustion engine, under a certain set of circumstances, there is a metallic knocking sound. “Fingers knocking,” some drivers say. In fact, this phenomenon is called detonation and its occurrence is extremely undesirable, as it can lead to engine failure due to the tremendous speed of flame front propagation (more than 2000 m/s) and high shock loads on the cylinder walls, piston and block head. To control the level of danger, a detonation sensor is installed on the cylinder block.
The detonation sensor is used to monitor the degree of detonation during operation of a gasoline internal combustion engine. The sensor is mounted on the engine cylinder block. It is an important component of the engine management system, as it allows to realize maximum engine power and ensure fuel efficiency.
What is detonation?
Detonation or properly detonation combustion occurs when a part of the fuel-air mixture removed from the spark plug is heated by the flame front and spontaneously ignites to form an explosion. Detonation is accompanied by acoustic signs – metallic knocking in the crank mechanism. The degree of detonation refers to the part of the fuel-air mixture that burns with detonation.
Causes of detonation are:
- chemical composition of the fuel (octane number);
- engine design features (compression ratio, location of spark plugs, shape of combustion chamber, etc.);
- operating conditions (composition of the fuel-air mixture, ignition advance angle, engine load, soot on combustion chamber parts, etc.).
The consequence of detonation combustion is increased heat dissipation of crank mechanism elements, accompanied by increased wear, breakage and destruction. How often this phenomenon may occur depends on three main factors.
First of all, the probability of occurrence is affected by the chemical composition of gasoline, more precisely, its octane number. The higher it is, the more resistant it is to this phenomenon. The second factor, which affects no less, is the design features of the engine, namely the compression ratio, the shape of the combustion chamber, the location of the spark plugs, the shape of the bottom of the piston, etc. For example, an engine with a higher compression ratio is more prone to detonation and needs high-octane gasoline. Otherwise, why do manufacturers write the minimum allowable octane number on the gasoline tank hatch? The third factor is engine operating conditions. The probability of detonation is affected by the composition of the working mixture, load, selected gear, soot.
How does the detonation sensor work?
The principle of operation of the detonation sensor is based on the piezo effect. The design of the sensor includes a piezoelectric plate, in which, when detonation occurs, a voltage arises at the ends. The greater the amplitude and frequency of the oscillation, the higher the voltage. When the voltage at the sensor output exceeds a predetermined level corresponding to a certain degree of detonation, the electronic control unit corrects the ignition system’s operating characteristic to reduce the advance angle. In this way, the optimum characteristic of the system operation for specific operating conditions is achieved.
The detonation sensor senses and converts the energy of mechanical vibrations of the cylinder block into electrical impulses. The detonation sensor continuously sends signals to the electronic engine control unit, and the electronics responds by changing the qualitative composition of the working mixture and the ignition advance angle.
There is a certain safety threshold, if the voltage value exceeds it, the electronic control unit will give a command to reduce the ignition advance angle.
If the detonation sensor is malfunctioning (no signal), the corresponding signal lamp lights up on the instrument panel, the engine continues to run.
Detonation sensor failure
If the detonation sensor malfunctions, the indicator light on the instrument panel illuminates. The engine is running, and the car can be driven. The answer to the legitimate question “why this sensor is needed then” is as follows.
On old cars, not equipped with electronic control unit, the ignition advance angle was corrected manually by turning the ignition distributor breaker cap. This allowed to adjust the ignition system depending on the octane number of gasoline, which can vary greatly at different gas stations. In a modern engine, the device of the trampler is different, its cover is fixed, so this function is performed by the ECU. Accordingly, if the detonation sensor fails, the ignition advance angle cannot be corrected.
A failed detonation sensor affects the dynamics and economy of the engine. The principle of operation of the electronic control unit is such that when the sensor malfunctions, it sets the ignition to a known late ignition for safety reasons, in order to exclude the possibility of destruction of the engine. As a result, the power unit works, but begins to consume much more fuel, and the dynamics of the machine deteriorates. The second is especially noticeable at increased loads.
Checking the detonation sensor
The main symptoms that indicate that this device has failed:
- drop in power;
- deterioration of acceleration characteristics and a sharp increase in the “appetite” of the engine;
- smoky exhaust.
At the same time, the engine malfunction indicator lights up on the panel. Moreover, it can burn constantly, as well as light up briefly when the load increases.
Not always at hand is a scanner that can read and decipher the fault code. Getting to the service station is not always possible. The question arises: how to check the detonation sensor yourself? From the tools you need a digital multimeter.
First of all, it is necessary to find out what resistance a serviceable sensor should have on a particular model of car or engine, since all manufacturers have different values. If it differs from the normal, you need a replacement.
You can also check the voltage at the electrical contacts of the sensor, for which you need to disconnect the electrical connector of the sensor power supply and remove it from the engine. After that, the multimeter is switched to the voltage measurement mode in millivolts, its plus probe is connected to the signal contact, and the minus probe is connected to the sensor mass (the hole through which the engine mounting bolt passes).
Detonation sensor check consists in clamping the sensor with connected probes in the palm of the hand, which then should be tapped lightly on some surface. When tapping, the multimeter should detect the appearance of voltage (usually it is about 30-40 mV). The principle is simple: the stronger the impact, the greater the potential difference between the electrodes. Since the voltage is small, not every device is able to measure it, so it is necessary to make sure that the measuring device at hand is designed for such measurements. A complete absence of potential difference indicates that the detonation sensor is defective.