Engine tech: What are knock sensors?


Knock is literally a case of bad timing. A combustion event that can destroy your engine. Here’s how the propeller-heads keep that from happening


"So ... if knock is when the fuel ignites before the spark plug fires, how does the knock sensor telling the ECU to retard ignition timing help?" - Chris ACT

Knock - detonation - pinging - pinking - whatever you call it - is what happens when a pocket of fuel-air mixture in the cylinder starts to burn spontaneously, in response to excessive heat and pressure.

This generally happens too early, before the spark plug fires, and it upsets the precise choreography of the engine’s combustion dynamics by producing a shock wave.

Thousands of severe knock events at big throttle inputs and high revs will destroy an engine - so obviously this needs to be managed. Which is why modern engines have knock sensors and a control system.

Basically a knock sensor is a glorified microphone that listens for the unique sound that knock makes. A little piezoelectric doohickey.

When it detects that knock sound - if it’s severe enough - the computer retards the spark timing and the knock goes away.

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You have to think about the blistering speed of engine operation. Just 3000 rpm is 50 revolutions per second. That’s 25 firing strokes per second - per cylinder.

The next firing stroke is somewhat similar to the previous one, and somewhat predictable. A firing stroke right now is going to be similar, dynamically, to the next one. So if a cylinder knocks right now, that’s a good predictor that if something’s not changed, it’s going to knock again next time.

Which kinda answers Chris’s question. If detected knocking gets over a pre-programmed thresshold, the computer intervenes by retarding the timing.

The final thing there is kinda paradoxical. Engines are most powerful, and most efficient (which are flipsides of the same coin) when they are operating at the maximum possible ignition advance - just before knock.

That’s why ignition advance control algorithms and the knock sensors on sentry duty are designed to advance the timing right up to the point of incipient knock, then back off a bit, repeat, at high speed, for the whole time the engine operates.

So if knock is a line in the sand, the engine walks up to it, puts its toes over, and takes a small step back, 50 times a second.

Which is just one of the benefits of computer-controlled engine management.

"I thought engines had knock sensors that backed off timing when knock was detected, which means you can run any engine on any octane petrol without damaging it. Tell me more John, what crucial piece of information am I missing?" - Benny Banger

The missing information here for Benny is that if you put - say - 91 regular unleaded in a car that demands 98 - like a BMW M2. Here’s what happens. And you should never do this.

Say you pull out to overtake a truck at 100 km an hour, uphill. A steep hill, it is hot (meaning high ambient air temperature), you knock it back a couple of gears and pull 6000 rpm at wide-open throttle on the way past.

Both turbos are are belting the air in. The ambient temps are putting the bite on intercooler efficiency. There’s a lot of effective compression in the engine. And there’s a lot of load - against gravity, aerodynamic drag and inertia.

No amount of ignition retardation can overcome the fact that the compression and heat in the cylinder is sufficient to cause 91 to detonate. Retarding the ignition here is like putting a Band-Aid on a gunshot wound.

There is, of course, a predictable result. Your beautiful M-Division engine makes a loud noise, and it composes a letter to the UNHCR, alleging aggravated abuse by you.

The first thing the dealer principal does after you darken his doorstep in this unhappy state is phone his travel agent and book the family first class for a fortnight in the presidential suite of the Grand Isle Resort and Spa in the Bahamas…

So, no - a knock sensor and ignition retardation is simply insufficient to mitigate low octane fuel going in a car designed for high octane.


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