Engine knock is often a problem in cars and can cause serious
engine damage if allowed to continue. In severe cases, knocking can burn holes
in pistons and cause premature engine failure. And even when knocking is only
light, it can reduce engine power.
So how does knocking occur and what can be done about it?
In a typical internal combustion engine, one or more pistons
travel up and down inside cylinders to turn a crankshaft. As a piston rises
inside its cylinder during the compression stroke, a mixture of fuel and air is
compressed. In petrol and gas engines, this fuel-air mixture is then ignited to
drive the piston as it starts its downward stroke.
However, if the mixture is ignited too early, it will "push"
against the piston as it rises towards top dead centre (TDC). If this ignition
is early by only a small amount, then the engine will exhibit a knocking sound
as the piston rattles within the cylinder. This effect is called "detonation",
"pinging" or "knocking".
Knocking is typically caused by the timing being too far
advanced. It can also be caused by higher than normal operating temperatures or
by using a lower octane fuel than normal.
As a result, all modern cars with engine management systems are
fitted with one or more piezoelectric knock sensors. These monitor for engine
knock over specific frequency ranges and automatically retard the ignition
timing if knocking begins to occur.
This allows the ignition timing maps to be set close to the
advance limits to ensure best performance. In addition, the use of knock sensors
ensures maximum engine performance with fuels of different octane ratings,
without damaging the engine.
On vehicles that don’t have knock sensors, the ignition timing
advance has to be set conservatively to prevent knocking. And if it does occur
during driving, the only remedies are to ease off on the accelerator pedal or
change down a gear.
Fig.1: this diagram shows the general arrangement of the Knock Detector. The output signal from the knock sensor on the engine block is first fed to the Knock Detector circuit for processing. It's then fed to the Programmable Ignition Timing Module and displayed on the LCD Hand Controller.
Fig.2: the block diagram of the Knock Detector circuit. The incoming knock signals are first amplified and then bandpass filtered to remove unwanted engine noise signals. This processed signal is then rectified and filtered to provide a DC signal which is then fed to the Programmable Ignition Timing Module.