I'm learning about vacuum leaks on engine intakes and I'm hearing this terminology of lean air fuel mixture. I thought I heard of a rich mixture before too. What's a normal mixture referred to as? A balanced mixture?

Also just for clarity, this terminology doesn't apply to anything else in the vehicle does it? Like I wouldn't hear someone speaking of lean/rich oil, or lean/rich battery acid or some other substance the car uses right?

  • Did you really have to ask here when the answer is in the first sentence of "Air–fuel ratio" article in wikipedia? Commented Aug 15, 2016 at 7:15
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    "Lean" refers to meat which is low in fat, as well as to anything limited (as opposed to "rich," something overprovided). Commented Aug 15, 2016 at 15:55
  • @AaronBrick Yeah; you know, I was kinda wondering if, historically, the choice of those terms is somehow related to e.g. the use of animal fats or oils as fuels or something for like, old lanterns or whatever. It isn't common, but if you search around you do occasionally see people use "fat" instead of "rich" when talking about air-fuel ratios. It's obscure but it's definitely out there (one example, it's subtly mentioned in parentheses in the middle of that article).
    – Jason C
    Commented Aug 15, 2016 at 23:40

3 Answers 3


I do not know the etymology of these words or the history of their use in the context of air-fuel ratios, although their dictionary definitions (lean [adj.], rich, too bad I don't have an OED subscription) and usage in other contexts does parallel their usage here. I did some cursory research but gave up, the folks on the English site may be able to help there. However, answering your basic question:

The terms are relative in the same sense that "dim" and "bright" are relative:

  • "Lean" means a lower fuel-to-air ratio; that is, less fuel in the mix; compared to something context-dependent.
  • "Rich" means a higher fuel-to-air ratio; that is, more fuel in the mix; compared to something context-dependent.

If you say, "the air-fuel mixture is rich" (or "too rich", same thing), it is implied to mean that there is more fuel in the mix than there should be in whatever situation you are talking about.

Now, there's a commonly quoted stoichiometrically ideal ratio of 14.7 air:gasoline. "Stoichiometrically" meaning that 14.7 masses of air react with 1 mass of gasoline according to the chemical reaction equation. This is often the context. You can find an interesting discussion of the finer points of this number here, mostly centered on the definition of "air".

So in that sense, when speaking of gasoline, if you said the mixture was "rich" then it means the ratio is < 14.7:1 air:gas, and if you say it's lean, you mean the ratio is > 14.7:1 air:gas.

But 14.7 isn't the magic number for other types of fuels, and may not be the magic number even for gasoline depending on other conditions in the engine. If you're talking about those situations, "rich" still means more than whatever amount of fuel is ideal for the given amount of air, and "lean" means less.

I guess if it was neither "rich" nor "lean" you'd call it... just right?

Still, I do not know why "rich" and "lean" were the chosen words for this, and wouldn't mind finding out, myself.

Also just for clarity, this terminology doesn't apply to anything else in the vehicle does it?

I have never heard it used in any other context. The words "lean" and "rich" are so strongly associated with the air-fuel ratio that if you told a mechanic that the battery acid was "rich" I suspect they would either wonder why you had gasoline in your battery acid, or why you were trying to build an engine that burned battery acid as a fuel, heh.

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    Your car doesn't burn battery acid? Commented Aug 15, 2016 at 6:06
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    Looks like another subtlety involved is the order of the ratio, AFR vs FAR. Sounds like AFR is used in the automotive context.
    – jxramos
    Commented Aug 15, 2016 at 7:00

A lean air/fuel mixture is when there is more air than required to burn the fuel. A rich mixture is when there is too little air for the given quantity of fuel.

The ideal mixture, where there is exactly enough air to burn the fuel is known as the stoichiometric mixture and is about 15:1 for gasoline. That means 15 parts air to 1 part fuel.

You can have a read of the wikipedia page that discusses this:


No, this terminology is not used elsewhere for a vehicle.

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    Not that there's anything wrong with this answer, but I do think it's worth bearing in mind that there are situations where you may want a lean or rich mixture and that the ideal burn isn't necessarily ideal for the vehicle. For example, in aviation, it's common practice to lean out the mixture to increase the temperature when having problems with magnetos . And I've heard of people running cars deliberately on the rich side to prevent overheating.
    – Dan
    Commented Aug 15, 2016 at 9:02

Ideal air fuel mixture (ratio) is 14.7:1. That is 14.7 parts of air ignites with 1 part of fuel. This will archive the best fuel economy and lowest emissions. This figure is also known as Lambda 1.

Lean air fuel ration refers to additional air (more than 14.7) being burned with 1 part of fuel. Vacuum leak will cause lean air-fuel mixture because unmetered air is entering the engine.

Rich air-fuel ration is the opposite, less then 14.7 part of air ignites with 1 part of fuel. Example can be faulty fuel pressure regulator causing too high fuel pressure.

Terminology of "lean" and "rich" is only being used in air-fuel ratio.

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