Learning hub · Aircraft General Knowledge · Lesson 1

The piston engine: ignition, mixture and carburettor icing

Most training aeroplanes use a simple air-cooled piston engine. It is reliable when you understand it and unforgiving when you do not. This lesson covers the cycle, the ignition, the mixture, and the one fault that can stop the engine on a mild day: carburettor ice.

About 13 minutes, plus the quiz. Part of Aircraft General Knowledge.

1. The four-stroke cycle

Each cylinder makes power in a repeating cycle of four piston strokes. The piston moves up or down in the cylinder, and the valves open and close at the right moments.

Induction (intake). The piston moves down, the inlet valve is open, and a mixture of air and fuel is drawn in.
Compression. Both valves are closed. The piston moves up and squeezes the mixture into a small space, which makes it hotter and easier to burn.
Power (combustion). Near the top of the stroke the spark plugs ignite the mixture. It burns quickly, the hot gas expands, and it pushes the piston down. This is the only stroke that makes power.
Exhaust. The exhaust valve opens and the piston moves up, pushing the burnt gas out.

One full cycle takes two turns of the crankshaft but only one turn of the camshaft, which opens the valves. The cylinders of a multi-cylinder engine fire at different moments, so the power arrives smoothly. The spark is timed to happen slightly before the piston reaches the top, because the mixture needs a short time to burn.

2. Magnetos and dual ignition

A magneto is a small generator driven by the engine that makes the high voltage for the spark. It has its own magnets and coils, so it needs no battery and no aircraft electrical system. If the battery or alternator fails, the engine keeps running.

Most aircraft piston engines have two magnetos and two spark plugs in each cylinder, each magneto serving one plug per cylinder. This gives two benefits:

Safety. If one magneto or one set of plugs fails, the other still works.
Better combustion. Two flame fronts burn the mixture faster and more completely, so the engine gives a little more power and runs a little more smoothly.

On the ground you do a magneto check: at the power setting in your checklist, you switch from BOTH to the left magneto, back to BOTH, then to the right, and watch the RPM. A small drop is normal, because only one plug is burning the mixture. A big drop, no drop at all, a rough engine, or a large difference between the two sides is a reason not to fly. The allowed drop and difference are in your flight manual.

A magneto makes its spark whenever the engine turns, and the ignition switch works by grounding the magneto. A broken switch lead can leave a magneto live with the switch at OFF. Always treat a propeller as live, and never touch it without the correct procedure, even when the engine is shut down. Follow your own aeroplane’s flight manual and your school's procedures.

3. The mixture control

The engine burns fuel and air in a ratio. Too much fuel is a rich mixture, too little is a lean mixture. The mixture control lets you change the ratio.

Why would you need to? As you climb, the air becomes less dense, so each intake stroke draws in less air, but the fuel system still delivers about the same amount of fuel. The mixture becomes richer and richer, and the engine loses power and wastes fuel. Leaning the mixture restores the balance.

Rich (full rich). Used for take-off, the initial climb and landing in most training aeroplanes. Extra fuel helps to cool the engine when the power is high. Some engines at high airfield elevations need leaning for take-off. Follow the manual.
Leaning in the cruise. You lean using the method in the manual, normally with an exhaust gas temperature (EGT) gauge or by watching the RPM. A mixture that is too lean makes the engine run hot and can damage it, so never lean beyond what the manual allows.
Before a go-around or a descent. Enrich the mixture again. You may need full power, and a lean mixture at full power is dangerous.
Idle cut-off. Pulling the mixture control fully out stops the fuel and shuts the engine down.

4. Carburettor icing

A carburettor mixes fuel into the incoming air. Two things cool the air inside it. The fuel evaporates, which takes heat from the air. The air also speeds up and loses pressure where it passes the narrow throat (the venturi), which cools it further. Together these can cool the air by tens of degrees Celsius.

If the air contains moisture, and the inside of the carburettor falls below freezing, the water vapour turns to ice on the walls and on the throttle valve. The ice narrows the passage, reduces the airflow and can block it.

Why a warm, humid day is dangerous. The cooling happens inside the carburettor, so the outside air does not need to be cold. Ice can form on a day of +15°C, and even higher, if the humidity is high: in mist, in haze, near cloud, after rain, or on a damp morning. The risk is highest at low power settings, such as a descent or a long final approach, where the throttle is nearly closed.

Symptoms.

With a fixed-pitch propeller: a fall in RPM with no change in the throttle.
With a constant-speed propeller: a fall in manifold pressure.
A rough-running engine and, if you do nothing, a loss of power and then the engine stopping.

Carburettor heat. This control directs hot air, warmed round the exhaust, into the carburettor instead of the cold outside air. The ice melts. When you apply heat, expect the RPM to fall first, because hot air is less dense. The engine may run roughly for a moment as the melted ice passes through. Then, if there was ice, the RPM rises again, above the value it had just before you applied heat. If the RPM falls and then stays steady, there was probably no ice. Apply full heat and hold it until normal power returns, following the manual. Partial heat can be worse than none, because it can leave the air at a temperature where ice still forms, without melting what is already there.

Carburettor heat reduces power and raises the engine temperature, so do not use it when full power is needed (for example at take-off) or in normal operation, except to check for ice or to clear it. Heated air may also bypass the air filter, so avoid it on dusty ground. Good practice is to check the carburettor heat in the pre-take-off checks, to apply heat before you reduce the power for a descent, and to check for ice from time to time during a long descent. Follow the sequence in your manual.

Carburettor heat reduces power and changes the mixture, and the correct use differs between aeroplanes. Use your flight manual and the checklist for your aeroplane for every flight. Engines with fuel injection have no carburettor, but they can still suffer induction icing at the air intake. They usually have an alternate air source for this.

5. Engine checks and limits

Engine instruments show you the health of the engine. The markings on them are colour-coded: a green arc is the normal range, a yellow arc is caution, and a red line is the limit you must not exceed. The exact values are in the manual.

Oil pressure. It should rise within seconds of the start. If it does not rise within the time stated in the manual, shut the engine down. Oil lubricates, cools and cleans the engine. A low pressure means damage is coming.
Oil temperature and cylinder head temperature (CHT). A high reading can mean too lean a mixture, too little cooling airflow (for example in a long climb at low speed) or too little oil. Lower the nose to cool the engine, enrich the mixture, and reduce the power.
RPM and manifold pressure. Stay inside the green arc and the limits for take-off and continuous power.
Before flight. Check the oil quantity against the minimum in the manual, and drain a small sample from the fuel drains to look for water and dirt. Water in the fuel can stop the engine.
Run-up. Do the magneto check and the carburettor heat check, check the idle and the engine instruments, and confirm the engine is healthy before you take off.

Two kinds of abnormal combustion can destroy a piston quickly. Detonation is when the mixture explodes instead of burning smoothly. Pre-ignition is when the mixture ignites too early from a hot spot. Detonation is commonly caused by a lower fuel grade than specified, too lean a mixture at high power, or very high manifold pressure with low RPM. Pre-ignition is usually caused by a hot spot in the cylinder, such as a carbon deposit or a cracked plug insulator. The two often occur together. This is why you use the fuel grade the manual specifies (often AVGAS 100LL) and do not lean beyond the limits.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

You are descending at idle power on a humid day at +16°C, in a fixed-pitch aeroplane. The RPM slowly falls by about 150 with no change in the throttle, and the engine begins to run roughly. What is the likely cause? What do you do, and what do you expect to see when you do it?
Show the answer
The likely cause is carburettor ice. It does not need cold air: the cooling happens inside the carburettor, and the risk is highest at low power in humid conditions. In a fixed-pitch aeroplane the symptom is a fall in RPM with no change in the throttle.
Apply full carburettor heat and hold it, following the manual. The RPM will fall first because hot air is less dense, and the engine may run roughly for a moment as the melted ice passes through. If there was ice, the RPM then rises again, above the value it had just before you applied heat. If the RPM falls and stays steady, there was probably no ice.
Good practice is to apply heat before you reduce power for a descent. Partial heat can be worse than none.
Magneto check (all limits invented here, your manual has the real ones: maximum drop 150 RPM on either magneto and maximum difference 50 RPM between them). With BOTH selected, the engine runs at 1,800 RPM. On LEFT it drops to 1,740. On RIGHT it drops to 1,730. Is that acceptable? What if the RIGHT reading were 1,620 instead?
Show the answer
First reading: left drop = 1,800 − 1,740 = 60 RPM, right drop = 1,800 − 1,730 = 70 RPM. Both drops are under 150, and the difference between them is 10 RPM, under 50. Acceptable. A small drop is normal because only one plug in each cylinder is burning the mixture.
Second reading: right drop = 1,800 − 1,620 = 180 RPM, which is more than the 150 limit, and the difference between the two sides is 120 RPM, more than the 50 limit. That is not acceptable, a reason not to fly.
A big drop, no drop at all, a rough engine, or a large difference between the two sides are all reasons not to fly. The real limits are in the flight manual.
A four-cylinder, four-stroke engine runs at 2,400 RPM. One full cycle takes two turns of the crankshaft. How many power strokes does one cylinder make per second? How many does the whole engine make per second? Which stroke is the one that makes power?
Show the answer
One cycle takes two crankshaft turns and contains one power stroke. At 2,400 RPM the engine makes 2,400 ÷ 2 = 1,200 cycles per minute in each cylinder, which is 1,200 ÷ 60 = 20 power strokes per second per cylinder.
Four cylinders: 4 × 20 = 80 power strokes per second for the whole engine. The cylinders fire at different moments, so the power arrives smoothly.
The stroke that makes power is the power (combustion) stroke: near the top of the compression stroke the spark plugs ignite the mixture, and the expanding gas pushes the piston down. The other three strokes (induction, compression, exhaust) only prepare for it or clear up after it.

7. Watch

A short explanation of how carburettor ice forms and what to do about it. The channel is American, so it says “carburetor” and quotes some US practice. European (ICAO/EASA) wording and some rules differ, but the physics is the same.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A short video that explains why carburettor ice can form even on a warm day, and how carburettor heat is used. It uses British spelling and a UK point of view.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Back to the learning hub.