I know that the question/title is a bit ambiguous, but I couldn't wrap it in a single sentence, so please let me elaborate. You can change the mixture of an engine for whatever reason. Because of multiple reasons, it's unwise to go either too lean or too rich. Going too lean will burn your valves and spark plugs, melt your head etc. Going too rich will wash away the oilfilm, cause oil consumption and contamination, soot in the exhaust, spark plug fouling etc. Not too mention a high fuel consumption.

But these are not actual limits, they're just reasons not to go that rich or lean. I'm curious how rich or lean you can go before the petrol jut won't ignite. I'm aware that this is depends(heavily) on the engine design, but let's assume we have the ideal engine optimised for either running as rich or as lean as possible.

I have read once that according to most tuners, 1.2 lambda (or 17.6 AFR) is about the leanest you can go before the engine will stall. Just above 1 would give the best economy, 0.88 has the most power, and 0.82 creates the most heat during warming up. But I haven't seen anyone really proving this and Google won't help me. So I'm wondering if any of you know more about it, or if you can back it up.

As background info: I have an aircraft engine here that i perform tests on. As part of that i need to measure the AFR with a Bosch LSU4.9 oxygen sensor. The engine is just a conventional 4Cyl, multiport injected, 16 valve boxer engine. Because of the application of the engine it's designed to run from 0.82 to 1.12 lambda, according to the specs.

I'm using an Innovate LM-2 controller, and a self designed controller to measure the mixture. The former uses a Bosch LSU4.2 sensor, the latter uses an LSU4.9 sensor. They both always give the same result. The weird thing is, the mixture goes all the way from 0.7 lambda at low RPM's to 1.6 at higher RPM's. I didn't hold that for possible, but since both sensors give the same results, it almost has to be true. A lean mixture could be caused by a leak in the exhaust drawing in oxygen, but then lambda 0.7 would never be reached. So i'm trying to figure out wether my sensors are giving garbage results, or the engine is really running at such ridiculous mixtures. The engine never stalls or runs bad though.

  • What fuel is the aircraft engine running? Regular unleaded octane or aircraft fuel?
    – Zaid
    Commented Jan 16, 2017 at 13:07
  • @Zaid It's just a conventional engine running on 95RON, but it's optimised for durability and fuel economy at high rpm's.
    – Bart
    Commented Jan 16, 2017 at 13:08
  • Because of it's nature and application, its compression ratio, ignition timing etc. are not similar to automotive engines.
    – Bart
    Commented Jan 16, 2017 at 13:14
  • Yes, the CR would have to be fairly low to sustain such a lean burn without autoigniting, and the ignition timing should be fairly advanced as well in comparison. AFR 1.6 makes it similar to diesel's operation
    – Zaid
    Commented Jan 16, 2017 at 13:36
  • @Zaid Yup, seriously everything about that engine is different from a standard automotive one. To start, being a an aircraft engine, it has two injectors and two sparkplugs per cylinder, 4 coils, two of every sensor, two ECU's, two CAN networks.. It's driving me crazy, really.
    – Bart
    Commented Jan 16, 2017 at 13:45

2 Answers 2


You can't go any leaner than injecting no fuel whatsoever. This may sound dumb but actually an engine will continue to spin for a few firing cycles without fuel. Some engines are able to actually shut down cylinders in certain operating modes. I accept this isn't technically "lean" because there is no ignition event taking place though.

With regards to too rich, the key to getting fuel to burn is that it is well atomised, that is to say a mist of fuel hanging in the air that's being squished in the cylinder. When you go very rich, the fuel can "drop out". That is to say it no longer hangs as an atomised mist in the air but pools at the bottom of the combustion chamber (i.e. on the crown of the piston) and you don't get a clean burn; unburned fuel ends up in your exhaust. The point at which this happens largely depends on your fuel system. The traditional carburettor would be the worst for this. Modern ultra-high pressure injectors such as those found in the VW TSI engines can keep a greater volume of fuel atomised although this is typically done in tandem with a turbocharger so you have more air than at atmospheric pressure available thus the mixture doesn't typically go too rich.

I'm not sure if this answers your question. I suppose you could fill an entire cylinder with petrol but it certainly wouldn't combust.

  • I get your point that the air gets vapour saturated with petrol at a certain point, just as it can with water. You can only 'dissolve' so much petrol in the air. But you can dissolve exponentially more vapour, the hotter you make the air, you can go really far. I guess you're already way past the limit of ignition then. On the other hand, lean mixtures will just not burn if the concentration is too low. If it still burns, it's probably a local rich spot that burns and not an actual lean mixture. Just like diesels mixtures who are lean as a whole, but there's a local rich spot at the injector.
    – Bart
    Commented Jan 16, 2017 at 9:55
  • It's counter intuitive because you actually want cold air for optimum power, this is why high performance turbocharged cars feature intercoolers. Cold air is more dense so therefore contains more oxygen per cubic centimetre. Commented Jan 16, 2017 at 9:57
  • True, if your goal is to get maximum performance. But the question here is how rich or lean you can physically/chemically go. Obviously you'd never go past 0.88 or 1.05 or something if performance in whatever area is your goal.
    – Bart
    Commented Jan 16, 2017 at 13:12

A good point to start are the publications about engines by people such as Ricardo who designed and built test engines which were used to test extremes of compression ratio, mixture, timing etc. One engine was the E6T.

  • Interesting tip. I have looked into his work, seems very interesting.
    – Bart
    Commented Jan 16, 2017 at 9:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .