Knock Sensor
The 1.8L engine is equipped with a flat design is non-resonant design Knock Sensor (KS) that is constructed of a piezoelectric element which generates an AC signal when vibrated. KS is located on the engine block behind the intake manifold, on the engine block between cylinders 2 and 3. Normal engine operation will cause the KS to generate signals of known frequencies. When engine knock is present, the KS frequency changes. This signals the Powertrain Control Module (PCM) to retard ignition timing. The PCM sends a bias voltage of 5 volts to the KS on the KS signal high circuit. The PCM expects approximately 2.5 volts back on the KS Signal Low circuit. The KS generated AC signal rides on top of the bias voltage provided by the PCM. The PCM uses the input from the KS to adjust the ignition timing in order to control detonation and enables the PCM to adjust the ignition timing in order to adapt to any of the variables that affect the optimal ignition timing. Ignition timing determines the relationship between the time the spark plug is fired and the time that the piston reaches top dead center (TDC). TDC is the point in time when the piston achieves maximum upward travel in the cylinder. If the spark plug ignites the compressed air/fuel mixture too late, not all of the air/fuel mixture has time to burn while the fuel is highly compressed. Late ignition causes a decrease in fuel efficiency, decreased power, and increased exhaust emissions. If the spark plug fires too soon, too much of the air/fuel mixture starts burning before the piston reaches the top of the compression stroke. Early ignition of the air/fuel mixture causes detonation, commonly referred to as spark knock. Constant spark knock in the motor is undesirable. Excessive spark knock can reduce engine performance. If severe enough, detonation can cause engine damage.
Every engine has an optimum ignition timing value. The optimum ignition timing is usually the earliest or most advanced firing of the spark plug that is possible without causing detonation. An engines optimum ignition timing is designed to be the most advanced ignition timing possible during the most demanding conditions. The optimum ignition timing is affected by all of the following variables:
- Engine load.
- Engine temperature.
- Atmospheric pressure.
- Fuel quality.
- Fuel's octane rating.
The KS detects when the engine is experiencing detonation. The sensor then signals the PCM to reduce the spark advance until detonation is no longer detected. A conventional resonant type KS uses a vibration plate that has the same resonance point as the expected knocking frequency of the engine. The non-resonant type KS has a steel weight with an insulator separating the weight from a piezoelectric element. See Fig 1 . The vibration caused by engine detonation is transferred to the weight, whose inertia applies pressure to the piezoelectric element. The weights action against the element, generates an electromotive force that is modified and transmitted to the PCM. The non-resonant design sensor is able to detect detonation vibration over a wide frequency band, from 6-15 kHz. In comparison, the resonant type KS detects only vibrations that are within a narrow frequency band.
In response to the KS signal, the PCM retards the spark advance in order to reduce the detonation. The amount of timing retard that the PCM applies is based on the engine speed and the length of time that the engine detonation is detected. Once the spark timing is retarded, the KS circuitry in the PCM performs calculations in order to determine how much spark advance should be re-introduced. Normally, the ignition timing advance is increased until zero retard, or normal ignition timing, is re-established. If detonation occurs again, the whole cycle will repeat. The alteration of the ignition timing by the KS often occurs continuously while the engine is running, even though no detonation is heard by the vehicle's operator.
Loss of the KS signal causes the PCM to operate in fail safe mode. In fail safe mode the PCM commands maximum spark retard. A KS that falsely indicates detonation can cause the PCM to retard the ignition timing unnecessarily. Reduced spark advance can cause poor fuel economy, sluggish engine performance and/or higher exhaust emissions.
A KS that fails to detect detonation can cause the PCM to control the ignition timing as if no detonation were occurring. Failure of the PCM to retard the ignition timing when necessary could cause An excessive engine detonation, engine damage during heavy engine loads and/or higher exhaust emissions.