Types of Aircraft Engines: Piston, Turboprop, Turbofan and More

Aircraft engines come in two main families: reciprocating (piston) engines and turbine engines. The FAA says most small aircraft are designed with reciprocating engines. Turbines, which are smooth, light for their power and run on readily available jet fuel, do the work in faster, higher-flying aircraft.

A piston engine or a turboprop works with a propeller to produce thrust. Turbojets and turbofans produce thrust by speeding up the air that flows through them. This guide covers how each type works, what keeps a piston engine running, what goes wrong, and where each one earns its place.

Key Takeaways

Piston engines are classified by cylinder layout (radial, in-line, V or horizontally opposed), operating cycle (two or four stroke) and cooling (air or liquid). The horizontally opposed layout is the most popular on smaller aircraft.
Spark ignition engines use a spark plug. Compression ignition engines, often called jet-fueled piston engines, ignite fuel by compressing the air first.
Turbine engines come in four types: turbojet, turboprop, turbofan and turboshaft.
FADEC computers now run many engines, replacing magnetos, carburetor heat and mixture controls with a single throttle lever.

Piston Engines

A reciprocating engine converts chemical energy from fuel into mechanical energy by burning it inside cylinders. The name comes from the back-and-forth movement of the pistons. Pistons travel along the cylinders and turn that linear motion into the rotation of the crankshaft. Two designs exist: spark ignition, where a spark plug lights a pre-mixed fuel-air charge, and compression ignition, where the engine compresses air in the cylinder until it's hot enough that injected fuel ignites on its own.

The FAA says spark ignition has been the powerplant of choice for many years. Compression ignition engines, often called jet-fueled piston engines, have the added advantage of running on readily available, lower-cost diesel or jet fuel.

Cylinder layouts

Piston engine layouts (FAA Pilot's Handbook, Chapter 7)
Layout	How it's arranged	What the FAA says about it
Radial	Cylinders in a row or rows around a circular crankcase.	Widely used in World War II and many are still in service. The main advantage is a favorable power-to-weight ratio.
In-line	Cylinders in a line.	Small frontal area but a relatively low power-to-weight ratio. The rear cylinders of an air-cooled engine get very little cooling air, so these are normally limited to four or six cylinders.
V-type	Two banks of cylinders in a V.	More horsepower than in-line engines while keeping a small frontal area.
Horizontally opposed	Cylinders on opposite sides of the crankcase, always in even numbers, usually horizontal in a fixed-wing airplane.	The most popular on smaller aircraft. Mostly air cooled, with a high power-to-weight ratio because of the small, light crankcase, and a compact layout that allows a streamlined installation.

Any of these can use spark or compression ignition and run on a two- or four-stroke cycle. They can also be cooled by liquid or by air.

The four-stroke cycle

Spark ignition four-stroke engines remain the most common design in general aviation. In each cylinder, four piston strokes turn fuel into work.

The four strokes. The piston, the open valve and the spark change at each step.

Intake. The piston starts down, the intake valve opens and the fuel-air mixture is drawn into the cylinder.
Compression. The intake valve closes and the piston moves back up, squeezing the mixture to get far more power out of it once it's lit.
Power. The mixture is ignited, pressure jumps and the piston is forced down, turning the crankshaft.
Exhaust. The exhaust valve opens and the piston moves up again to push out the burned gas.

Even at a fairly low engine speed, the cycle repeats several hundred times a minute. In a four-cylinder engine each cylinder is on a different stroke, and precise timing of the power strokes keeps the crankshaft turning.

A two-stroke engine does all four jobs in two strokes. Because it has a power stroke on every revolution, it typically has a higher power-to-weight ratio than a comparable four-stroke. The earliest designs were inefficient and had disproportionate emissions, and that kept two-strokes rare in aviation. The FAA says modern versions with direct fuel injection and pressurized air make two-stroke compression ignition a viable alternative.

Jet-Fueled Piston Engines

Diesel technology reached aircraft through Thielert Aircraft Engines (TAE), formed in 1999 to build a Jet-A-burning diesel-cycle engine. By March 2001 its prototype became the first certified diesel aircraft engine since World War II. The attraction is that diesel and kerosene are physically similar, so the aircraft runs on standard aviation kerosene. The FAA lists more independence, higher reliability, lower consumption and lower operating costs.

TAE engines went on the Diamond DA40 single and the DA42 Twin Star, the first diesel engine in the type certificate of a new original equipment manufacturer aircraft. Retrofit approvals followed, with a supplemental type certificate to re-engine Cessna 172 models and the Piper PA-28 family. FADEC is standard on aircraft with these engines, and by 2007 jet-fueled piston aircraft had logged well over 600,000 hours.

What Keeps a Piston Engine Running

The engine can't run on its own. The FAA lists the induction, ignition, fuel, oil, cooling and exhaust systems as the auxiliary systems that have to work at the same time.

Induction: carburetor or fuel injection

The induction system brings in outside air, mixes it with fuel and delivers the mixture to the cylinders. Air enters through a port at the front of the cowling, normally through a filter, and an alternate air source, usually from inside the cowling, covers a clogged filter. Two types of induction are common:

Carburetor and fuel injection (FAA Pilot's Handbook, Chapter 7)
System	How it works	Trade-offs
Float-type carburetor	Air passes through a venturi, a narrow throat where pressure drops, and pulls fuel from a main jet into the airstream. A float and needle valve meter the fuel. The most common carburetor.	Doesn't work well in abrupt maneuvers, discharges fuel at low pressure so vaporization is incomplete, and has an icing tendency because the fuel vaporizes in the venturi.
Pressure-type carburetor	Delivers fuel under pressure from a pump, on the engine side of the throttle valve.	Better vaporization, practically no vaporization icing and little effect from rapid maneuvers. Usually not found on small aircraft.
Fuel injection	Injects fuel into the cylinders, or just ahead of the intake valve. Parts include an engine-driven pump, fuel-air control unit, fuel manifold, discharge nozzles, an auxiliary pump and pressure or flow indicators.	Considered less susceptible to icing, though impact icing on the air intake can happen in either system.

Carburetor ice

Vaporizing fuel and the drop in air pressure in the venturi cool the carburetor sharply, by as much as 60 to 70 degrees Fahrenheit, according to the FAA. Water vapor can then freeze on the inside and on the throttle valve. The handbook says it's most likely below 70 degrees Fahrenheit (21 degrees Celsius) with relative humidity above 80 percent, but it can form in outside air as warm as 100 degrees Fahrenheit (38 degrees Celsius) with humidity as low as 50 percent.

In an airplane with a fixed-pitch propeller, the first sign is a drop in rpm, then roughness. With a constant-speed propeller the governor holds rpm, so the sign is a drop in manifold pressure. It's particularly dangerous at reduced power in a descent, because ice can build unnoticed until power is added. Carburetor heat preheats the air and can melt ice that's already formed. The FAA says full heat should go on at once and stay on until normal power returns, even if the engine runs rougher for 30 seconds to several minutes as melted ice goes through. Carburetor heat can cut power by up to 15 percent, so the FAA advises against it when full power is required or in normal operation, except to check for or remove carburetor ice.

Mixture

Carburetors are calibrated at sea-level pressure with the mixture control full rich. Climb, and the air gets thinner but the fuel doesn't, so the mixture gets progressively richer, causing roughness and a loss of power. The FAA says the roughness normally comes from spark plug fouling and the fix is to lean the mixture. In a descent from altitude the mixture must be enriched again, or it can become too lean, which causes detonation. An exhaust gas temperature (EGT) gauge, which varies with the fuel-air ratio, is the handbook's recommended aid for setting mixture.

Ignition

A magneto uses a permanent magnet to generate current completely independently of the aircraft's electrical system. Most certificated aircraft have dual ignition: two magnetos, separate wires and two spark plugs in each cylinder. If one magneto fails, the other is unaffected and the engine keeps running with a slight loss of power.

The pretakeoff check tests this. The pilot moves the ignition switch from both to right, then from both to left, and watches the rpm drop. A small drop is normal, and the permissible amount is in the flight manual. The FAA says that if the engine stops on one magneto, or the drop exceeds the limit, the aircraft shouldn't fly. "No drop" isn't normal either. After shutdown the switch goes to off, because a magneto needs no outside power, and a moved propeller can still fire the engine with the battery and master switches off.

Oil, cooling and exhaust

Oil. Reciprocating engines use either a wet-sump system, where the oil sits in the engine, or a dry-sump system, where it's in a separate tank and circulated by pumps. Dry sumps carry more oil and suit very large engines. The FAA says oil temperature changes slowly, particularly after a cold start. High readings can mean a plugged line, low oil, a blocked cooler or a defective gauge, and low readings in cold weather can mean the wrong viscosity.
Cooling. Most small aircraft are air cooled. Air enters through the cowling, baffles route it over fins on the cylinders and it leaves through openings at the lower rear. It's less effective in ground operations, takeoffs and go-arounds, and a high-speed descent can shock-cool the engine. The cylinder head temperature gauge is the direct, immediate indicator. Higher airspeed, a richer mixture and less power all lower temperatures, and cowl flaps adjust the airflow where fitted.
Exhaust. The exhaust vents burned gas overboard, and on many light aircraft a muffler shroud heats the cabin. Exhaust gases contain carbon monoxide, which is odorless and colorless, so the system must be in good condition and free of cracks.

Detonation and preignition

Normal combustion starts at the spark plug and burns steadily outward. Detonation is an uncontrolled, explosive ignition of the mixture. It causes excessive temperatures and pressures that can quickly lead to failure of the piston, cylinder or valves. The FAA lists three common causes: a lower fuel grade than specified, very high manifold pressure with low rpm, and high power with an excessively lean mixture. Preignition is ignition before the normal event, usually from a hot spot such as a carbon deposit on a spark plug. The two often occur together and are hard to tell apart.

Boosting Power: Superchargers and Turbochargers

A normally aspirated engine can't hold manifold pressure above the surrounding atmospheric pressure, and that falls about one inch of mercury for every 1,000 feet. The FAA says a typical engine at 8,000 feet makes about 75 percent of its sea-level power. Forced induction compresses the intake air to fix that.

Superchargers and turbochargers (FAA Pilot's Handbook, Chapter 7)
Type	How it works	Notes
Supercharger	An engine-driven air pump or compressor, driven through gears at one speed, two speeds or variable speeds.	Uses a lot of engine power for the boost. At 8,000 feet a supercharged engine could still make 25 inches of manifold pressure where an unboosted one makes 22. A single-stage, two-speed unit on a large World War II radial makes an "altitude engine."
Turbocharger	An exhaust-driven turbine spins a compressor. A waste gate controls how much exhaust flows through the turbine, and an intercooler often cools the compressed air to reduce the risk of detonation.	Recovers energy from exhaust that would otherwise be lost, and holds rated sea-level power up to the critical altitude. The FAA calls it the most efficient way to raise horsepower.

Turbos demand care. The handbook says the turbine and impeller can spin above 80,000 rpm at extreme temperatures, so oil should be in the normal temperature range before high throttle settings, and the turbocharger needs time to cool before shutdown or the oil in the bearings can boil and leave hard carbon deposits. An overboost, from a waste gate left closed during a descent or cold oil in the actuator, can cause severe detonation.

FADEC

Full authority digital engine control began on turbine aircraft and is spreading to pistons. In a spark ignition piston engine, sensors monitor each cylinder, and a digital computer calculates the ideal fuel pulse for each injector and adjusts ignition timing. A compression ignition engine gets the same functions except the spark-related ones. The FAA says FADEC eliminates the need for magnetos, carburetor heat, mixture controls and engine priming. A single throttle lever is typical, and the pilot picks a detent such as start, idle, cruise or maximum power.

Because a failed FADEC could mean a complete loss of thrust, two separate identical digital channels each handle all functions, and there must be a backup electrical source.

Turbine Engines

A turbine engine has an air inlet, compressor, combustion chambers, a turbine section and an exhaust. The compressor can be centrifugal flow, which accelerates air outward, axial flow, which moves it through rotating and stationary airfoils along the engine's axis, or a combination of the two. The path the air takes and how power is produced define four types.

Four turbine layouts. The gold block is where each type delivers its power.

Turbine engine types (FAA Pilot's Handbook, Chapter 7)
Type	What it does	Strengths and limits
Turbojet	Compressor, combustion chamber, turbine and exhaust. The turbine drives the compressor, and the accelerated exhaust gas provides the thrust.	Limited in range and endurance and slow to respond to the throttle at low compressor speeds. Gives the highest maximum speed of the four.
Turboprop	A turbine that drives a propeller through a reduction gear, because propellers work best at much slower speeds than the engine runs.	Most efficient between 250 and 400 mph and 18,000 to 30,000 feet. Performs well at slow takeoff and landing speeds and is fuel efficient.
Turbofan	Diverts a secondary airflow around the combustion chamber. The bypass air adds thrust, cools the engine and helps suppress noise.	Turbojet-type cruise speed with lower fuel consumption. Bypass ratio is the airflow through the fan divided by the airflow through the core.
Turboshaft	Delivers power to a shaft that drives something other than a propeller. Most of the gas energy drives a turbine instead of producing thrust.	Many helicopters use turboshaft engines, and so do many auxiliary power units on large aircraft.

The FAA compares the four on net thrust. At the lowest speeds the piston engine beats the other three. The turboprop beats the turbofan up to a higher speed, and the turbofan beats the turbojet up to a higher one still. Beyond that the turbojet wins, and it reaches the highest maximum speed.

What pilots watch on a turbine

EGT or turbine temperature. Turbine temperature is the limiting factor, and the highest temperature is at the turbine inlet. The gauge has several names depending on where the probe sits, such as TIT, TOT, ITT or TGT.
EPR. Engine pressure ratio is turbine discharge pressure over compressor inlet pressure, and it indicates power on a turbojet or turbofan.
N1 and N2. Rotational speeds of the low pressure and high pressure compressors, shown as percentages of design rpm.
Torque. Power output on a turboprop or turboshaft, shown in percent, foot-pounds or psi.

What goes wrong

Foreign object damage. Debris ingestion can nick compressor and turbine blades, and bird strikes and ice can destroy an engine. Preflight includes a visual check for it.
Hot or hung start. A hot start is when exhaust temperature exceeds the safe limit, from too much fuel or too little turbine speed. A hung start is when the engine doesn't accelerate to idle after ignition.
Compressor stall. Compressor blades are small airfoils, and when their angle of attack goes past the critical angle, smooth airflow breaks down and air can slow, stagnate or reverse.
Flameout. The flame dies, which can happen from a weak mixture, low fuel pressure and low engine speed, such as with the throttle back in a descent from altitude.

Turbine thrust also varies directly with air density, so it falls as altitude or temperature rises.

Which Engine Where?

Light trainers and most small airplanes: piston engines, usually horizontally opposed.
Small turbine aircraft: very light jets, which the FAA describes as seating three to seven passengers, have become practical as turbine engines got cheaper to build.
Helicopters and auxiliary power units: turboshaft, with some helicopters using pistons.
Jets: turbojets and turbofans, with the turbofan's bypass air giving the lower fuel burn.

For what these aircraft cost to buy, see our guides to small plane costs and commercial plane costs. The propeller the piston engine and turboprop drive has its own guide: aircraft propellers. Fuel supply is in fuel systems, and the structure that carries the engine is in fuselage types.

Frequently Asked Questions

What are the main types of aircraft engines?

Reciprocating (piston) engines and turbine engines. Turbines include turbojets, turboprops, turbofans and turboshafts.

What's the most common engine on small planes?

The horizontally opposed piston engine, which the FAA calls the most popular reciprocating engine on smaller aircraft.

What's the difference between a turbofan and a turbojet?

A turbofan diverts a secondary airflow around the engine core. The bypass air adds thrust, cools the engine and reduces noise, giving turbojet-type cruise speed with lower fuel consumption.

What's a turboprop?

A turbine engine that drives a propeller through a reduction gear. It's most efficient between 250 and 400 mph and 18,000 to 30,000 feet.

Can an aircraft engine run on diesel or jet fuel?

Yes. Compression ignition engines, called jet-fueled piston engines, run on diesel or jet fuel. The handbook identifies them on the Diamond DA40 and DA42, and notes retrofit approvals for Cessna 172 models and the Piper PA-28 family.

What does FADEC do?

It's a digital computer that controls the engine and propeller. It removes the need for magnetos, carburetor heat, mixture controls and priming, and leaves the pilot with a single throttle lever.

What's carburetor ice?

Ice that forms inside a carburetor because vaporizing fuel and falling air pressure cool it sharply. It can form even on warm, humid days and reduces power. Carburetor heat melts it.

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