How do aircraft engines work?

Aircraft engines convert fuel or electrical energy into thrust. In real-world aviation, piston engines and turboprops usually turn a propeller, while turbojets and turbofans accelerate air rearwards. Every type depends on controlled energy conversion, cooling, lubrication and reliable fuel or power delivery across wide changes in altitude, temperature and demand.

The main aircraft engine families

Most aircraft engines either produce shaft power for a propeller or rotor, or generate thrust by accelerating air through a jet.

The engine and the device producing thrust are not always the same component. A piston engine supplies torque to a propeller; a turboshaft powers a helicopter transmission; and a turbofan combines a gas-turbine core with a large ducted fan.

Designers generally choose piston power for simplicity and lower-power applications, turboprops when more power and good lower-speed efficiency are needed, and turbofans for fast, high-altitude flight. Our comparison of piston, turboprop and turbofan installations across Cessna aircraft shows how those choices apply within one manufacturer’s range.

The piston engine’s four-stroke cycle

A conventional spark-ignition aircraft piston engine converts pressure inside its cylinders into crankshaft rotation through a repeating four-stroke cycle.

1. Intake: The piston moves down and draws a fuel-air charge through the open intake valve.
2. Compression: Both valves close and the rising piston compresses the charge.
3. Power: Magneto-powered spark plugs ignite the charge. Expanding gases force the piston down and turn the crankshaft.
4. Exhaust: The piston rises again and pushes burnt gases through the open exhaust valve.

Multiple cylinders perform these strokes at different times, smoothing the power delivered to the propeller. Flat-opposed, radial and inline layouts arrange the cylinders differently but retain the same basic process. Some light and experimental aircraft use two-stroke, rotary or other less common designs.

Most traditional aviation petrol engines use two spark plugs per cylinder, fed by independent magnetos. The duplication improves combustion and provides ignition redundancy. Compression-ignition aero engines instead ignite fuel through heat created by compression and commonly burn kerosene-based turbine fuel.

The throttle regulates airflow and therefore power, while the mixture control adjusts fuel relative to that air. These controls are often confused in simulators; our explanation of Cessna 172 throttle, mixture, magnetos and engine instruments connects the cycle to a familiar cockpit.

The gas-turbine cycle in jets and turboprops

A gas turbine uses continuous airflow rather than separate intake, compression, power and exhaust strokes inside cylinders.

1. Intake: The inlet delivers smooth airflow to the engine.
2. Compression: Axial or centrifugal compressor stages raise the air pressure.
3. Combustion: Fuel is sprayed into compressed air and burns continuously inside the combustor.
4. Turbine: Hot gas expands through turbine stages, which drive the compressor and any connected fan, propeller gearbox or output shaft.
5. Exhaust or shaft output: The remaining energy produces jet thrust or leaves as useful shaft power, depending on the engine type.

The combustion is a controlled, continuous burn rather than a series of explosions. Igniters are normally required for starting and may be used during selected adverse conditions, but established combustion is generally self-sustaining while fuel and compressed air continue to arrive.

A turbojet obtains most of its thrust from its fast exhaust. A high-bypass turbofan uses much of the turbine’s output to turn a fan that moves a larger mass of air more slowly, improving efficiency and reducing noise at typical airliner speeds. A turboprop extracts still more energy through a turbine and reduction gearbox to turn a propeller. We cover the compressor, combustor, turbine and exhaust stages of jet engines separately in greater detail.

How propellers and jets create thrust

Both propellers and jet engines create forward thrust by accelerating air rearwards, but they move different quantities of air at different velocities.

A propeller blade is a rotating aerofoil. Its changing blade angle along the span allows each section to meet the airflow appropriately despite travelling at a different rotational speed. Fixed-pitch propellers use one compromise blade angle; variable-pitch and constant-speed systems adjust blade angle to suit take-off, climb, cruise or reduced-power operation. Our detailed explanation of propeller blade angle and thrust covers that process.

A common mistake is to treat propeller RPM as a direct power indication. That may be reasonable in some fixed-pitch installations, but a constant-speed governor can hold nearly the same RPM while engine power changes substantially. Manifold pressure, fuel flow and the aircraft’s power-setting tables then become essential.

What do aircraft engine controls actually control?

Aircraft engine controls regulate airflow, fuel flow, propeller loading or an electronically managed power request, depending on the installation.

• Conventional piston engine: The throttle controls airflow, the mixture lever controls fuel-air ratio, and carburettor heat or alternate air manages induction icing or blockage where fitted.
• Constant-speed propeller: The propeller control selects a target RPM; a governor changes blade pitch to maintain it.
• Turboprop: Power, propeller and condition functions may have separate levers or may be partly combined and electronically managed.
• Turbofan or turbojet: The thrust lever requests an engine setting. A fuel control or full-authority digital engine control schedules fuel and keeps the engine within operating limits.
• Electric powerplant: A controller regulates motor torque or speed while monitoring battery, inverter and motor limits.

Engine instruments differ just as much. Piston pilots monitor combinations of RPM, manifold pressure, fuel flow, cylinder-head temperature, exhaust-gas temperature, oil temperature and oil pressure. Turbine displays may show fan and core speeds, exhaust or inter-turbine temperature, fuel flow and engine pressure ratio, but the exact set depends on the engine.

In a simulator, an apparently ineffective mixture or propeller lever is not automatically a fault. The aircraft may have a fixed-pitch propeller, compression-ignition engine, turbine, electric motor, automatic mixture system or FADEC. Identify the installed powerplant before troubleshooting the controls.

Why do aircraft engines lose power at altitude?

Aircraft engines and propellers usually produce less thrust as air density falls, although temperature, turbocharging and engine control schedules complicate the exact result.

A naturally aspirated piston engine takes in less oxygen with each induction stroke as it climbs, so its maximum available power falls. The mixture must also be adjusted where the aircraft’s procedure requires it; leaving an engine excessively rich at altitude can reduce power, foul plugs and waste fuel. A turbocharger can maintain intake pressure up to its design limits, but it adds temperature and mechanical constraints.

Turbine engines also ingest less mass at lower air density. Colder air can partly offset that loss, while FADEC or the fuel control schedules the engine against rotational, temperature and pressure limits. A fixed thrust-lever position therefore does not guarantee identical thrust at every altitude, speed and outside-air temperature.

What commonly causes an aircraft engine to fail?

An aircraft engine fails or loses power when fuel, airflow, ignition, lubrication, cooling or mechanical integrity can no longer support safe energy conversion.

• Fuel starvation: Fuel may remain aboard but be unavailable because of tank selection, pump, vent or fuel-system problems. This is different from fuel exhaustion.
• Restricted airflow or icing: Carburettor ice, induction ice, filter blockage or inlet contamination can reduce the air reaching the engine. Installed heat or anti-ice must be used according to the aircraft checklist.
• Ignition or combustion trouble: Fouled spark plugs and magneto faults affect piston engines; flameouts, compressor stalls and unstable combustion affect turbines.
• Lubrication or overheating: Low oil pressure, rising temperature or abnormal vibration can precede severe mechanical damage.
• Foreign-object or mechanical damage: Birds, debris, damaged blades, bearing faults and component fatigue can prevent an otherwise normal combustion process from producing usable thrust.
• Incorrect starting: A turbine hot start, hung start or no-light condition requires the start to be stopped according to the engine limits and checklist. Continuing to add fuel can cause damage.

There is no universal restart or fault-clearing sequence across aircraft types. Randomly moving fuel, ignition and power controls can make the problem worse; the approved aircraft checklist determines which source to verify, which controls to change and when a restart is appropriate.

Does an aircraft engine need electrical power to keep running?

Some aircraft engines can continue running after the main electrical system fails, while others depend on electrically powered control or ignition systems.

A traditional spark-ignition piston engine uses engine-driven magnetos, so combustion does not depend on the battery or alternator once the engine is running. Electronic ignition, electronic fuel injection and FADEC-controlled engines may require electrical power, usually with redundant or engine-driven sources. Electric aircraft naturally require a continuous supply from their batteries, fuel cells or generators.

Turbine engines need electrical or pneumatic energy to start, but once established they drive their own compressors and generators. The exact consequences of an electrical failure depend on whether fuel scheduling and engine control are mechanical, electronic or a combination of both.