You’ll have no doubt heard people talking about engines running ‘a bit rich’ or ‘a bit lean’, but what does this mean? And what effect does it have? They refer to the amount of fuel present in the air/fuel mixture entering the engine – also known as the air fuel ratio, or AFR. ‘A bit rich’ means there is too much fuel present, and ‘a bit lean’ means there is not enough.

When discussing AFRs you will also hear people talking about Lambda 1 a lot too. Lambda 1 is generally considered the point at which the mixture is perfect for a petrol engine – just enough fuel to provide a nice, clean, and safe combustion with minimal waste. This is 14.7 parts air to one part fuel – an AFR of 14.7:1.

14.7:1 is perfect for idle and light throttle cruising conditions as it’s the most efficient mixture possible, meaning the best fuel economy and lowest emissions. However when we want the engine to start making more power we don’t want efficiency, so we need to increase the amount of fuel in the mixture and therefore reduce the AFR. Imagine an engine at idle, running at 14.7:1 – when we put our foot down and start introducing a load onto the engine we need to add more fuel content, firstly to match the increased amount of air coming through the intake, and secondly to increase the power produced from each engine revolution. Typically you would expect to see maximum power being made at around 12:1 AFR.

Another factor affected by AFR is temperature. In short, a leaner burn will be much hotter than a richer burn and can cause all manner of problems – the worst of which being melted pistons, valves, and combustion chambers. On the other hand, make the mixture too rich and the engine will burn excessive amounts of fuel – resulting in sooty spark plugs, heavy fuel bills, and a loss of power. In extreme cases, rich mixtures can also cause bore wash.

Getting the AFR correct for your engine for any given point is a skill – called mapping – so is best left for those who know what they are doing. But you can see why an AFR gauge is a handy tool – spotting any anomalies early could potentially save your engine.


Ford’s EcoBoost Engine In Action: The fuel injectors are mounted at the side of the combustion chamber. The spark plug is at the top of the chamber.