Thread: Question about AFR...

Alright, this may seem like a dumb question, but hear it out!

Does anyone here know exactly why our engines begine to knock when running lean? Yes I know it's because it gets hot, but the real question is WHY does it get hot?

Any sensible approach says that the closer you get to a stoichiometric reaction, the higher the adiabatic flame temperature becomes. Therefore, the engines get hotter as you move from rich towards stoichiometric, which is what we normally talk about when we say "running lean" I believe. So obviously, running closer and closer to stoich will cause the higher temperatures, and thus the detonation.

However, logic says then that if we move beyond stoichiometric all the way over to the true lean side and run the engine under those conditions, the temperature will drop off again just as it did on the rich side (maybe at a different rate, but it will still drop). The smaller amount of fuel means less energy is available to heat the engine, no matter how you try to swing it. Therefore, running leaner than stoich should by all means be as safe as running richer (provided you are lean enough to keep it cool).

The only reason I can think of NOT to do this, is because the increased oxygen concentration present when running lean will create a lot more NOx emissions (where this is NOT the case with running rich or with EGR).

So, my question is, does anyone actually know any reason (OTHER than NOx or reduced power) NOT to run very lean (say 17 or 18 AFR)?? I am imagining the fuel saving features of running the engine in such a way. Better yet, run it very lean but still introduce a lot of EGR instead of air to help counter the increased NOx.

Yeah, we always hear it's dangerous to run lean, but is it really if it's done correctly? I'm not sure I want to try it on my engine to find out.

Speak your thoughts!

I'll explain detonation (different from pre-ignition) and you'll see why running lean won't work. It's not really a matter of the engine getting hotter or cooler, it's what happens in the combustion chamber at the moment of ignition.

Here are a few things to keep in mind:
A rich mixture will burn faster than a lean mixture.
A rich mixture is less prone to spontaneous ignition than a lean mixture.

With that in mind, imagine the spark plug igniting a rich mixture. The flame front propagates out from the spark plug quickly (rich burns faster) as the piston is still traveling up towards top-dead-center pushing all of the available mixture towards the flame front in the hopes of a complete combustion. Now by the time the piston has reached the top, hopefully most of the mixture has burned and the resulting gasses expand to push the piston down.

Now imagine the spark plug igniting a lean mixture. The flame front propagates out from the spark plug a bit slower than the rich mixture as the piston is still traveling up towards top-dead-center pushing all of the available mixture towards the flame front in the hopes of a complete combustion. As this happens the un-burnt mixture compresses greatly causing the temperature to rise to the point where it will spontaneously combust before the flame wall has reached that part of the mixture, and before the piston has even reached the top of the cylinder. This causes extremely high/localized pressure at the point of this detonation which causes all of the bad effects (small pits in the piston and cylinder head usually far away from the spark plug and a loss in power as the detonation works against the raising piston). This also explains why if you retard ignition you can avoid detonation by starting the burn much closer to top-dead-center (later) to prevent this situation.

There's a link I'll search for the explains detonation and pre-ignition extremely well. Give me a second.

This looks like it:
Engine Basics: Detonation and Pre-Ignition: Streetrod Stuff

I'd say it's mandatory reading for everyone on this forum. Certainly all the turbo guys. Also, it might contradict what I said above. If it does let me know and I'll edit my post. I wrote it from memory.

Actually that explanation makes perfect sense, thanks. I did not even think about flame propagation rate... But would advancing the timing even further not help to correct for the slow propagation rate of a lean mixture (so as to burn most of the fuel before peak compression where the detonation is most likely to occur)? Of course, it makes sense that the lower compression associated with a more advanced spark will reduce the propagation rate even further, so it might not help much. I'll have a read of that article, thanks.

No, and I didn't fully explain all of the factors at play before so I'll hit on that now. In addition to the piston coming up which increases the pressure on the unburnt mixture, the flame front coming down and expanding gases behind it also increase the pressure on the unburnt mixture. No matter how you cut it, that unburnt mixture is going to be under a lot of pressure. The best thing to do is have the flame front get there quickly before it has time to detonate on it's own, and keep it from detonating by using high octane fuels and keeping the mixture as cold as possible from the beginning. It might be possible to advance the timing so much that you avoid this situation, but then you've ignited the mixture so early that the entire combustion will work on the piston as it is still raising which is basically what happens with pre-ignition. This is the quickest way (1 or 2 engine revolutions) to destroy your engine (melted pistons, bent rods).

Read the article. It's really easy to understand and such good info. =]