Ignition timing determines the relationship between the time the spark plug
is fired and the time that the piston reaches the end of the piston's upward travel
in the cylinder (TDC). 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. Optimum ignition timing
is usually the earliest or most advanced firing of the spark plug that is possible
without causing detonation. An engine's optimum ignition timing is designed to
be the most advanced ignition timing possible during the most demanding conditions.
Optimum ignition timing is affected by all of the following variables:
• | Air density (atmospheric pressure) |
Ignition systems equipped with a knock sensor (KS) can be engineered for optimum
ignition timing. A knock sensor (KS) can enable the powertrain control module
(PCM) to adjust ignition timing in order to adapt to any of the variables that
affect optimal ignition timing. The KS detects when the engine is experiencing
detonation. The KS then signals the PCM to reduce spark advance until detonation
is no longer detected.
Operation
The Knock Sensor (KS) system has two major components.
• | The KS module part of the Powertrain Control Module (PCM) |
The KS detects abnormal vibration (detonation) in the engine. The KS is located
in the center of the engine block below the intake manifold. The KS module receives
the KS signal and communicates within the PCM. After the correct calculations
are made the PCM adjusts the ignition timing in order to reduce detonation.
When the KS detects detonation, the KS module opens a circuit in the PCM.
In response the PCM retards the spark advance in order to reduce detonation. The
amount of timing retard that the PCM applies is based on engine speed and the
length of time that engine detonation is detected. Once the spark timing is retarded,
the KS module performs calculations in order to determine whether more or less
spark timing advance is required. Normally the ignition timing advance is increased
until zero retard (normal ignition timing) is re-established. If detonation occurs
again the whole cycle will repeat itself. The alteration of ignition timing by
the KS often occurs continuously while the engine is running even though no detonation
is heard by the vehicle's operator.
Results of Faulty Knock Sensor (KS) Operation
Loss of the knock sensor (KS) signal or loss of ground at the KS module causes
the KS signal to the powertrain control module (PCM) to remain high. A constantly
high input to the PCM from the KS causes the PCM to control the ignition timing
as if no detonation were occurring. Failure of the PCM to retard ignition timing
when necessary could cause any of the following concerns:
• | Excessive engine detonation |
• | Engine damage during heavy engine loads |
• | Higher exhaust emissions |
A KS that falsely indicates detonation can cause the PCM to retard ignition
timing unnecessarily. Reduced spark advance can cause any of the following conditions:
• | Sluggish engine performance |
• | Higher exhaust emissions |