Mixture Control and Abnormal Combustion

A piston engine burns fuel only when it is mixed with air in the right proportion, and the right proportion changes with altitude, power setting and what the pilot wants from the engine: maximum power, maximum economy or cooling. The mixture control lets the pilot adjust it. Used well, it saves fuel, keeps the spark plugs clean and gives full power at high-elevation aerodromes; used badly, it wastes power or, at high power with the mixture too lean, overheats the cylinders.

The mixture is also central to abnormal combustion. Detonation, pre-ignition and backfiring are ways in which the charge burns at the wrong time or in the wrong way, and each can damage an engine within minutes. How the charge is metered in the first place is covered in carburettors and fuel injection; the engine cycle itself in piston engine principles.

Fuel/air ratio and stoichiometric mixture

Mixture strength is expressed as the ratio of the mass of air to the mass of fuel. The chemically correct, or stoichiometric, mixture is about 15:1: just enough oxygen to burn all the fuel, with nothing left over of either. A mixture with more fuel than that is rich; one with less fuel is weak or lean. Well outside these values the mixture will not burn at all.

The chemically correct mixture is not the practical operating point. It burns hottest of all, so it brings the engine closest to detonation. Engines are therefore run somewhat rich for power, and weaker for economy:

Surplus fuel in a rich mixture does not burn; it absorbs heat and cools the charge, which is why full rich is used at high power even though it wastes some fuel. A weak mixture burns more slowly, so the spark must occur earlier for peak cylinder pressure to arrive at its optimum point just after top dead centre; the timing is explained in piston engine ignition systems.

Mixture control and idle cut-off

A float-type carburettor meters fuel according to the volume of air flowing through its venturi, not the mass. As the aeroplane climbs and the air thins, each volume holds less oxygen but draws almost the same fuel, so the mixture grows steadily richer. Heated induction air, from carburettor heat or an alternate air source, has the same effect. Left alone, an over-rich mixture costs power, raises fuel consumption and fouls the spark plugs with lead and carbon.

The mixture control corrects this. In a carburettor it varies the fuel flow reaching the main metering system; in the indirect fuel injection system of light aircraft, a mixture control valve linked to the lever bleeds off part of the fuel pressure applied to the metering valve. Moving the lever back leans the mixture; forward, to full rich, enriches it.

At the fully lean end of its travel is idle cut-off (ICO), also called mixture idle cutoff, which stops the fuel flow completely. The engine then stops through fuel starvation, and this is the normal way to shut down a piston engine. Idle cut-off also appears in abnormal procedures:

• An induction fire on start: keep cranking with the mixture at idle cut-off and the throttle open, so the flames are drawn into the engine.
• An engine fire: mixture to idle cut-off and fuel off, to starve the fire.
• The hot start of a fuel-injected engine: the electric boost pump is run briefly with the mixture at idle cut-off to purge vapour from the lines, as the flight manual describes.

Because the ignition switch only earths the magnetos, the mixture control is also how an engine is stopped when a dead cut check shows a broken P-lead. A propeller is always treated as live, whatever the position of the mixture and ignition controls.

Best power and best economy mixtures

Best power mixture is the setting that gives the most power for a given throttle position and rpm, somewhat richer than chemically correct. Best economy mixture is weaker than chemically correct and gives the most power for each unit of fuel, the lowest brake specific fuel consumption, at the cost of some power. The comparison is usually summed up as best power versus best economy: power for climbing or a fast cruise, economy for range.

Neither is the full-rich setting. At take-off and in a high-power climb the engine is normally run fully rich, richer than best power, because the extra fuel protects the cylinders against overheating and detonation. The exception is a normally aspirated engine at a high-elevation aerodrome: in thin air full rich drowns the engine, and flight manuals call for the mixture to be leaned before take-off to a specified maximum static rpm or fuel flow.

Leaning: rich and lean of peak

Mixture leaning is the routine adjustment of the mixture in climb and cruise. The most precise reference is the exhaust gas temperature (EGT) gauge. As the mixture is leaned from full rich, the EGT rises to a peak at roughly the chemically correct mixture, where all the fuel and all the oxygen are consumed, and then falls again as the mixture becomes leaner, because surplus air absorbs heat just as surplus fuel does.

• Rich of peak (ROP): the pilot leans to peak EGT and then enriches by the temperature drop the flight manual specifies. This is the best-power side of peak, and it keeps the mixture away from the hottest region around peak itself.
• Lean of peak (LOP): the pilot continues leaning past peak until the EGT has fallen by the specified amount. Fuel flow and cylinder temperatures are lower, and so is power. It works only if every cylinder receives nearly the same mixture, which usually means fuel injection and a multi-probe engine monitor, and only at power settings low enough to avoid detonation. Some engines and flight manuals permit rich of peak only.

Without an EGT gauge, pilots lean by the method given in the flight manual. In a descent the air becomes denser and the mixture leaner, so it is enriched progressively; before landing it is normally set as the checklist requires so that full power is available for a go-around.

Detonation and its causes

In normal combustion, a flame front spreads smoothly outwards from the two spark plugs across the cylinder. Detonation occurs when the pressure and temperature of the unburnt charge ahead of that flame front, the end gas, rise so high that it ignites spontaneously, all at once. The result is a violent pressure spike and shock waves that hammer the piston crown, rings and valves and overheat the cylinder. Detonation takes place after normal ignition.

A car driver hears it as "knock"; in an aeroplane the sound is masked by propeller and airframe noise, so the first clues are a rising cylinder head temperature, loss of power and, when severe, rough running. The conditions that promote it are:

• fuel of a lower grade than specified: the octane or performance number measures resistance to detonation, and a higher grade may be used where the flight manual permits it, a lower one never; see aviation fuel grades and properties;
• high manifold pressure with low rpm, which gives high cylinder pressures at low crankshaft speed;
• a lean mixture at high power;
• a hot charge, such as carburettor heat at full throttle, or induction air heated by a supercharger or turbocharger;
• high cylinder temperatures from poor cooling, such as a long, slow climb on a hot day.

The remedies reverse these conditions: enrich the mixture, reduce manifold pressure or power, open the cowl flaps and lower the nose to increase the cooling airflow, and make sure carburettor heat is cold. Reducing rpm while leaving the manifold pressure high does the opposite. For the same reason, on an engine with a constant-speed propeller the rpm is increased before the manifold pressure when power is added, and the manifold pressure is reduced first when power is taken off.

Pre-ignition and hot spots

Pre-ignition is ignition of the charge before the spark, by a hot spot in the cylinder: a glowing carbon deposit, an overheated exhaust valve, or a cracked or damaged spark plug. It is a timing fault. The charge starts to burn while the piston is still rising, so the piston is opposed by already expanding gas, pressures and temperatures soar, and power is lost. It shows as rough running and a rising cylinder head temperature.

Pre-ignition and detonation feed each other. Detonation overheats parts of the cylinder until they become hot spots; pre-ignition raises temperatures until the end gas detonates. General overheating of the engine can cause either, together with structural failure and breakdown of the oil. The pilot's response is the same as for detonation: reduce power, enrich the mixture and increase cooling, then have the engine inspected.

Backfiring through the induction

A backfire is combustion that travels back from the cylinder into the induction system. A very weak mixture burns so slowly that it may still be burning when the inlet valve opens for the next cycle, and the flame then ignites the fresh charge in the induction manifold. An inlet valve that does not close properly, for example because its clearance is too small, has a similar effect, and the mixture can be heard "popping back" into the carburettor.

Backfiring is most dangerous during starting. If the engine has been over-primed, or the throttle pumped so that the accelerator pump squirts fuel into the intake, liquid fuel lies in the induction system, and a backfire can set it alight. The resulting induction fire, and the drill of continuing to crank with the mixture at idle cut-off, are described in carburettors and fuel injection.

The opposite event, afterfiring, is the burning of unconsumed fuel from an over-rich mixture in the exhaust system, seen as flames or heard as bangs from the exhaust.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.