How does the mixture of an engine affect the engine's operation?

Question

I know that in most piston aircraft that there is a mixture control for the engine. My question is, how does the mixture control affect the engine's operation and what type of situations dictate a particular mixture setting? Would an incorrect mixture setting be detrimental, a minor effect or nothing at all?

Answer 1

Simple Answer: Running the engine lean or rich at different altitudes.

Longer Answer: There are a couple of reasons you want to adjust the mixture of a aircraft engine.

1. Since air density changes with altitude, it's a good idea to be able to change the fuel/air mixture at different altitudes. If you did not adjust the mixture at high altitudes you may not get full power because the air is thinner and now there is same amount of fuel being mixed with less air. All the unburnt fuel can cause issues like loss of power and/or fouling of plugs.
2. Being able to lean the mixture at higher altitude lets you save fuel, so you can go further on a full tank. (hence saving money)
3. Running a engine rich can help cool engine. Fuel acts like a coolant in this manner. Typically on climbing you want to run the engine a bit rich because your going slower and less air is able to help cool the engine.
4. Traditional piston powered aircraft engines are shut off by pulling the mixture to the off position. This starves the engine and burns off remaining fuel in the cylinders.

This topic can also bring up other discussions. Like EGT, CHT, running an engine rich of peak or lean of peak, monitoring fuel flow rate with a flow meter, and fuel injectors on fuel injected systems.

Hope this helps.

Answer 2

The mixture control allows you to increase (to run "richer") or decrease (to run "leaner") the amount of fuel in the fuel-air mixture. As a general rule, you want to "lean out" the engine at higher altitudes because the air is thinner, and it takes more air (by volume) to burn the same amount of fuel.

Mixture controls are used with many piston engines. However, turbine engines and some piston engines with fuel injection or altitude compensating carburetors do not have mixture controls.

In addition to zingle-dingle's points, it is important to lean an engine for takeoff at high elevations. At a 7,000 ft (or higher) runway, for example, an engine with a full-rich mixture setting will not generate nearly as much power as one properly leaned out.

Since the normal mixture setting at low-elevation airports may be full-rich, this may come as a surprise to someone visiting the mountains. It's not uncommon for a plane to crash at a high elevation airport because the engine was not leaned properly for takeoff.

Conversely, if an engine is properly leaned out at 14,000 ft., for example, the mixture should be adjusted to be more rich on or before a descent to 1,000 ft. to prevent engine overheating or possible stoppage due to too little fuel in the fuel/air mixture.

Answer 3

The stoichiometric mixture is the ideal chemical ratio of air:fuel, that will burn all the fuel in the mixture with no excess oxygen left. It is located at somewhere around 15 parts air and 1 part fuel per unit of mass. Mind you: the ideal ratio is based on mass, not on volume so if air density changes (as you climb/descend), as the carburetor is basically mixing volumes, your air:fuel ratio will change with altitude.

Mixture control will help you adjust that ideal 15:1 ratio, between what is called "lean" (more than 15 parts air), and "rich" (less than 15 parts). Most power is available at the ideal ~15:1 so normally that is where you would want to aim your mixture. However:

• Running a bit on the "rich" side of ideal will consume more fuel, generate less power, and might cause carbon deposits, but the extra vapor-fuel that remains unburned will serve as a coolant and reduce the temperature of your cylinders. That is why most of the times the pilots will set mixture on the "rich" side of ideal.
• Running a bit on the "lean" side of the ideal will consume less fuel, will generate a little bit less power, but the lack of cooling from the extra fuel will eventually overheat the cylinders, leading to all sort of nasty stuff like detonation and overheating the oil.

As for the particular situations calling for operations: 99% of the times you simply want a good/ideal mixture, which is why mixture control is pretty much getting automated these days. There are very few exceptions I can think of: for long cruise at cold temperatures I might try a leaner mixture to see if I can skim a couple of pounds of fuel. Or in an emergency engine-coughing situation I might cram the mixture to full rich to prevent the engine from starving or overheating. But then again, if mixture is automated the FADEC will do that anyway.