All About Aircraft Piston Engines

Aircraft piston engines, also called reciprocating engines, are mechanical marvels that have driven aviation forward since its earliest days. Though seen less in aircraft today, they are still utilized in private, vintage, and light-sport or training planes. Because of their lasting presence, this blog will cover how piston engines generally function, as well as distinguish common types that have been used over the years.

How Aircraft Piston Engines Work

Piston engines are able to convert chemical energy from fuel into mechanical energy through reciprocating pistons. These pistons perform a back-and-forth motion within cylinders, which creates the force needed to rotate the crankshaft. The crankshaft, in turn, spins the propeller, producing the necessary thrust for flight.

The Four-Stroke Cycle

1. Intake Stroke: The piston moves downward and the intake valve opens, drawing a mixture of air and fuel into the combustion chamber from the induction system.
2. Compression Stroke: The intake valve closes, and the piston moves back upward, compressing the air-fuel mixture to significantly increase its energy potential.
3. Combustion/Power Stroke: Near the top of the compression stroke, a spark plug ignites the compressed mixture. The resulting rapid expansion of hot gases forces the piston downward, turning the crankshaft and generating mechanical power.
4. Exhaust Stroke: As the piston returns upward again, the exhaust valve opens. The spent combustion gases are expelled from the cylinder and discharged through the exhaust system.

This cycle repeats continuously, powering the propeller throughout the flight.

Notable Components for Combustion

Fuel Delivery Systems

As we briefly mentioned, a vital part of piston engine operation is the delivery of a properly metered air-fuel mixture to each cylinder. Typically, one of the following systems are used:

• Carburetor: Found in smaller or legacy general aviation aircraft, a carburetor blends fuel and air before the mixture enters the intake manifold. Although they are simple and cost-effective, carburetors are less precise and may be susceptible to performance issues at higher altitudes due to air density changes.
• Fuel Injection System: Fuel injection systems deliver fuel directly into each cylinder’s intake port or combustion chamber, facilitating more accurate fuel metering, improved engine efficiency, and better performance in a wider range of operating conditions.

Magnetos and Spark Plugs

Igniting the air-fuel mixture is equally crucial for combustion, which is achieved through magnetos and spark plugs. During engine start, a battery-powered starter motor turns the crankshaft, which begins rotating the magnetos. Once spinning, magnetos generate electricity without relying on the aircraft’s electrical system during flight.

Most aircraft piston engines employ a dual ignition system, comprising two magnetos and two spark plugs per cylinder. In addition to bolstering ignition efficiency, if one magneto or spark plug fails, the engine can continue to operate on the remaining ignition components.

Types of Aircraft Piston Engines

In-Line Engines

In-line engines have all cylinders arranged in a single straight row, creating a narrow profile that reduces aerodynamic drag. However, because rear cylinders receive less airflow, these engines often face cooling challenges that need to be mitigated with liquid cooling systems.

Rotary Engines

In rotary engines, the entire engine rotates with the propeller around a fixed crankshaft. This motion provides effective air cooling and a lightweight design, but also introduces severe gyroscopic forces that makes aircraft handling difficult. Moreover, rotary engines also suffer from inefficiencies and excessive oil consumption, cementing them firmly in the past.

V-Type Engines

The cylinders in V-type engines form a “V” shape around a shared crankshaft, fitting more cylinders in a shorter engine length. Because of their greater output, they generate higher thermal loads and typically require liquid cooling. Although rare in modern certified general aviation, these engines may appear in experimental or performance-focused aircraft and unmanned aerial vehicles (UAVs) to achieve higher power density in small airframes.

Radial Engines

Radial engines arrange their cylinders in a circular pattern around a central crankshaft, usually using an odd number per row for smoother operation. Their large frontal area increases aerodynamic drag and can reduce pilot visibility, but they offer an excellent power-to-weight ratio, effective air cooling, and mechanical simplicity. Radial engines are still produced, and while they are uncommon in factory-new general aviation aircraft, they are sometimes used in aerobatic and agricultural aircraft.

Horizontally Opposed Engines

The industry standard for most contemporary general aviation aircraft, horizontally opposed engines place two banks of cylinders directly opposite each other on either side of a central crankcase. As such, these are compact, lightweight, and well-balanced engines with low vibration and effective air cooling. Moreover, horizontally opposed engines improve forward visibility and simplify maintenance, with Cessna, Piper, Cirrus, Diamond, and Tecnam aircraft almost universally using this type.

Explore Ready-to-Purchase Reciprocating Engine Parts

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