How does retarding timing prevent knock?
Posted by Anachronism--@reddit | askcarguys | View on Reddit | 13 comments
Most of us know knock is caused when the fuel ignites before the spark plug fires. And the solution is to delay when the spark plug fires.
Except if the fuel is already igniting without the spark plug then how does delaying when it fires help?
BoondockUSA@reddit
You’re describing two different issues.
Pre-ignition is when the fuel ignites before the spark plug fires. It’s caused by engine compression generating enough heat to ignite the fuel, much like a diesel engine. It’s very harmful to a gasoline engine and isn’t normal. A frequent cause can be too low of fuel octane for the compression ratio, carbon build up, or an engine that’s overheating from being overworked.
Too advanced of timing is igniting the fuel with spark, but it’s happening too soon. In normal operations, it takes time for the ignited fuel to propagate to the entire air/fuel charge, so the spark happens before the piston reaches Top Dead Center (TDC). The faster the engine is spinning, the more advanced the ignition timing has to be. Ideally, the burning fuel will begin pushing down on the piston the very moment the piston goes past TDC. Igniting the fuel too soon means the burning fuel builds cylinder pressure before TDC, which causes engine knocking (which is very harmful). Igniting the fuel too late means the engine isn’t producing full power and is wasting fuel because the fuel isn’t completely burned up by the time the piston reaches Bottom Dead Center (BDC). The wasted energy is just expelled out the exhaust when the exhaust valve opens. The ideal time to ignite the air/fuel charge is a very narrow window. Causes of engine pinging due to too advanced of timing can be an incorrect timing adjustment on cars that require manual timing adjustments, too low of octane for the ignition timing, faulty engine sensors, a lean air/fuel mix, or the engine running overly hot.
Back in the days of distributors and manual timing adjustments, there were specific timing specs that you had to set the distributor to using a timing light. The recommended timing spec was a good all-around setting that was slightly retarded (actual term for ignition timing, not a derogatory term) from perfect performance optimal to allow for when the engine was working hard in hot climates (such as towing). You could squeeze out a couple more horses by advancing the timing a couple of degrees from the recommended setting, but it was entering dangerous territory. Modern cars have computers and knock sensors so they’re essentially constantly finding the optimal timing setting for you with less margin of error.
oppositelock27@reddit
He's not really describing two different things. One of the methods used by an ECU to mitigate preignition caused by insufficient octane is to pull back ignition timing. It's a perfectly valid question and something I've often wondered about myself. Your rather lengthy answer fails to explain why that method works.
BoondockUSA@reddit
I did explain it.
Again, there’s two reasons for “pre-ignition”. One is actual pre-ignition, and the other is knocking from improper timing for the conditions.
The first is the fuel auto igniting like a diesel engine. That isn’t fixed by retarding the timing. This animation shows it: https://youtube.com/shorts/uB2iuia9ynw?si=-4R_z_kNKxKxjHZs If you’ve ever heard an old carbureted car “dieseling” after it’s turned off, that is pre-ignition in its most obvious form. Another example of a “dieseling” engine.
The other is caused by the ignition system igniting the fuel/air charge too soon because the ignition is too far advanced for the operating conditions. That is fixed by retarding the timing. The middle cylinder in this animation shows it.
oppositelock27@reddit
Where you’re off base is stating ECUs don’t pull timing for preignition caused low octane or other issues not related to overly advanced ignition timing. My understanding is that they actually do, it seems to be effective, and that’s the entire crux of the original question. At face value it shouldn’t work, but it does. You keep explaining the basics of how knock happens without explaining how the ECU actually keeps the engine from grenading when Joe Consumer dumps 87 octane in his BMW M3.
No-Habit-7079@reddit
When ECU retards ignition, peak cylinder pressure happens later (after the piston starts going down), pressure and heat drop slightly, the fuel burns smoother instead of exploding. It's all about cylinder pressure and cylinder temperature. The ECU firing the spark later so cylinder pressure doesn't spike too early helps spark knock as it is the cause.
BoondockUSA@reddit
In your example, it’s usually not pre-ignition/auto-ignition kabooms. It’s the fuel/air mix being so close to auto ignition that the ignition spark essentially explodes the air/fuel mix instead of smoothly burning it so the power happens after TDC. When it happens in your example, the ECU can retard the timing to delay the explosive hit until the piston is closer to TDC.
Gasoline direct injection engines are a whole different can of worms and is how some gasoline engines can have higher compression ratios that the auto-ignition compression of gasoline. The ECU has a lot more ability to stop pre-ignition in direct injection gasoline engines.
Floppie7th@reddit
Actual preignition can be improved by pulling timing, because it reduces combustion chamber temperature. It's a pretty weak effect, though.
Heavy_Performer1007@reddit
Maybe if all cylinders get less advanced of timing, it'll help that cylinder that's misfiring?
Never heard of pre-ignition being stopped by pulling timing
Unlikely-Act-7950@reddit
I prevent spark knock by using high octane fuel
blizzard7788@reddit
A lot of wrong answers here. Detonation (knock) happens AFTER the spark, not before. The octane rating is a measure of a fuel’s ability to resist, but not eliminate, detonation. Retarding the timing results in the spark plug firing later in the combustion stroke, thus reducing the temperature and pressure in the cylinder. Advancing the timing fires the plug sooner creating more heat and pressure. This is where a higher octane rating resists detonation.
https://youtu.be/qMZ7dFZvhhI?si=H__3hpfLmuoiwLTH
Pre-ignition is caused by a faulty plug, or a hot spot from carbon buildup. It is very rare in a EFI engine and almost fatal if it happens repeatedly. The octane rating has no direct impact on pre-ignition.
https://www.faasafety.gov/files/notices/2019/Jul/Preignition.pdf
Kickstart68@reddit
Knock is where the mixture spontaneously ignites due to pressure in the cylinder. When this happens it is a sudden and violent explosion rather than progressively burning.
The spark normally happens before top dead centre, when the mixture is still being compressed. Normally the burning of the mixture from the spark will be quite slow, so the extra pressure from the burning mixture is not that critical.
However if the mixture burns quickly for some reason, the cylinder pressure from both the piston going up and the burning fuel can reach a critical point and detonate the mixture. This is detonation, and can be very damaging.
If the ignition timing is retarded then the fuel starts to burn later, so pressure increase from the burning mixture happens later. Hence likely that the cylinder pressure does not reach the critical level that would detonate the remaining mixture.
horsimus@reddit
Best way I can put it is that it’s two different ways of achieving the same bad thing.
Really under-spec, low octane fuel will make your petrol engine run like a diesel.
But spark timing is interesting. The spark plug doesn’t actually fire when the piston is at top dead centre – it’s already fired a fraction of a second before. This gives the fuel more time to burn and exert maximum pressure on the piston. As the burn starts, the pressure it’s building is still easily overcome by the force of the rising piston. But fire the spark plug too early and that’s no longer the case – you’re getting huge pressure from the expanding gas while the crankshaft is still pushing the piston up.
The trick is to find the balance to fire early enough to put maximum force on the piston and late enough to avoid detonation.
wpmason@reddit
Usually just by cooling down the combustion chamber.
Timing advance exists to optimize the efficiency and performance of an engine, namely by getting the most complete combustion possible. This generates more heat than incomplete combustion.
If the chamber is hot enough to cause knocking, the computer can dial back the timing to slightly de-tune the engine and allow the chamber to cool slightly.