How to fix rich air-fuel mixture

Unlike a car engine, most aircraft allow the pilot a large amount of control when it comes to engine management. This is important, as the engine performs differently in different conditions. However, to a new or inexperienced pilot, it can be difficult and even dangerous if the engine management is done incorrectly during flight. One of the most common problems that new pilots face when it comes to engine management is pre-ignition or detonation, which is caused by not correctly setting the air-fuel mixture. So today we’re going to teach you how to fix rich air-fuel mixture by leaning the mixture correctly.

Why does detonation occur?

When the air-fuel mixture is set too lean, the cylinder is over charged or ‘over boosted’ (running a high manifold pressure at low RPM), the engine temperature is too high, or you’ve used a fuel with too low of an octane rating for the engines compression, a possible result is detonation. Detonation occurs when the fuel-air charge in the cylinder ignites during the compression stroke of the engine (i.e. downward movement of the piston) before the spark plugs fire*. This spontaneous ignition is due to the fuel/air charge reaching a high enough pressure to ignite by itself. The ignition before the engine is ready means that the rapidly expanding gas now works AGAINST the engine, pushing the piston up against its planned downstroke and damaging the engine, potentially catastrophically.

*Now the timing of ignition is tweaked in some engines so it isn’t quite this simple, but it works in essentially the same manner but slightly earlier to account for compressibility and flame propagation times.

Spontaneous ignition pressure will reduce with increasing temperature, such as the ambient temperature of the engine block/piston housing, meaning detonation is most likely at high engine power and low airspeeds (low cooling), such as a maximum angle climb on a hot day, or if the air that is being inducted into the manifold is very hot – such as when carburettor heat has been applied (this is why turbo/supercharges have an intercooler between the compressor and the manifold).

The octane rating of a fuel is a measure of its resistance to detonation and can be thought of as a measure of a fuels stability (to not spontaneously explode) under high pressure. A common mistake is to put car petrol into an aircraft, and due to the lower octane rating of petrol, it almost certainly causes detonation and can destroy an engine resulting in the need to make a forced landing. It’s vital to test aircraft fuel with a fuel tester before take off anyway, to ensure it is not contaminated.

What is Pre-ignition?

Pre-ignition is similar to detonation, however in pre-ignition the spontaneous ignition of the fuel/air charge in the cylinder is due to a glowing hot carbon deposit somewhere within the cylinder igniting the charge instead of the spark plugs, and can happen at anytime in the intake or compression stroke depending on the position and how hot the carbon deposit and engine are. Pre-ignition is typically caused by excess oil making its way into the cylinder and being burnt leaving behind the carbon deposit or glowing ember.

Pre-ignition and detonation, whilst different, can work together, (glowing carbon deposit and increasing pressure) meaning a premature fuel air charge ignition is most likely on the compression stroke of the engine cycle. Pre-ignition is much more catastrophic than detonation, as mild detonation alters the combustion process it can break down protective gas layers in the design burn process, and over time, lead to pre-ignition.

When is detonation more likely?

The most likely time to experience detonation issues is in an engine that is consuming large amounts of oil (one quart per hour), at maximum power with poor cooling (such as during the take-off roll and a maximum angle climb), with the carburettor heat set to ON, during a very HOT day at low altitude, with the mixture at peak EGT (ie leaned to maximum power with no excess fuel to cool the cylinder), when using an unapproved fuel that does not have a sufficient octane rating such as MOGAS/PULP

How to correctly lean the air-fuel mixture

To fix rich air-fuel mixture, you need to correctly lean the mixture. There are different methods for leaning the mixture at different stages of flight and under different circumstances. We’re going to focus on the three most common scenarios which can lead to a rich air-fuel mixture – during cruise, during high-altitude take-off, and during climb.

How to correctly lean the mixture during cruise

To correctly lean the mixture in the cruise (which may be done below 75% power setting), you should pull the mixture knob rearwards until you notice rough-running of the engine. The mixture should then be advanced slightly to clear the rough running, and once the engine is running smoothly, advanced just slightly again to ensure running rich of peak. If the aircraft is fitted with an EGT probe, this can be used to lean more efficiently. The best economy is found by leaning the engine to peak EGT. Best power is found by leaning to peak EGT and then enrichening the mixture to 100ºF rich of peak. If engine roughness is encountered before peak EGT, use the temperature corresponding to the onset of roughness as the ‘new’ peak EGT figure.

How to correctly lean the mixture for high-altitude take-off

For a high altitude take-off, with the mixture fully rich (as is normally for take-off) the air-fuel mixture may be too rich and the engine will not develop full power, resulting in rough running with full throttle applied. To ensure you are achieving maximum power, with the brakes on, apply full throttle and lean until peak RPM is obtained – at this mixture setting you should take off, otherwise your takeoff performance may be compromised so much that you cannot clear the runway.

How to correctly lean the mixture during climb

To lean during the climb, you MUST DO SO IN ACCORDANCE WITH THE POH. The aircraft typically has a figure on the fuel flow gauge corresponding to a climb power (blue radial or top of green arc). After Take-off, Throttle is reduced to climb setting, the prop is wound back to climb setting, and the mixture is reduced to climb setting. Further leaning of the mixture during the climb must be done in accordance with the pilots operating handbook.

Do you have any other tips to fix rich air-fuel mixture and to keep the engine running better in flight? let us know in the comments below.

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ken johnson


Ken is a passionate aviator, a professional pilot and flight instructor. He has over 17 years of flight experience across hundreds of aircraft ranging from recreational, aerobatic, historic, commercial and military aircraft, training hundreds of students along the way. Find out more.

Ken has 124 posts and counting. See all posts by Ken

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