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A Knock Detector For The Programmable Ignition

The Programmable Ignition System would not be complete without the addition of engine knock sensing. This Knock Detector is useful for adjusting ignition timing maps and can also automatically retard the ignition timing if engine knock is detected.

by John Clarke

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.

Click for larger image
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.
Click for larger image
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.

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