Aero engine

Piston engines, also called reciprocating engines, were the first type of aero engine. They operate on the four-stroke Otto cycle, burning fuel in cylinders to drive a crankshaft that turns a propeller. Early examples, such as the Wright brothers' 12-horsepower inline engine, were low-powered and unreliable, but by the 1930s radial and V-12 engines like the Pratt & Whitney R-1830 and Rolls-Royce Merlin produced over 1,000 horsepower and powered iconic aircraft of World War II.¹

Piston engines are still used in light aircraft, where their low cost and simplicity outweigh their lower power-to-weight ratio compared to turbines. They are also used in some unmanned aerial vehicles and homebuilt aircraft. Notable variants include opposed, inline, V, and rotary configurations, with the latter being an early design where the entire engine rotates with the propeller.

Gas turbine engines, or jet engines, generate thrust by accelerating a mass of air rearward. The basic components are a compressor, combustion chamber, and turbine. The first operational jet engine was developed independently by Frank Whittle in Britain and Hans von Ohain in Germany in the 1930s, leading to the first jet-powered flight in 1939 (Heinkel He 178).

Turbofans, which add a ducted fan ahead of the core, are the dominant type for commercial aviation because they offer high thrust and low fuel consumption. Turboprops, which drive a propeller through a reduction gearbox, are efficient at lower speeds and altitudes. Turbojets, once common, are now used mainly in supersonic aircraft. Turboshafts power helicopters. Each type trades off efficiency, speed, and complexity.²

Rocket engines carry both fuel and oxidizer, allowing operation in space. They are used for launch vehicles, spacecraft, and some experimental aircraft like the Bell X-1, which broke the sound barrier in 1947. Rockets produce high thrust but have low efficiency in the atmosphere due to their high exhaust velocity.³

Ramjets and scramjets are air-breathing engines that compress incoming air through the engine's forward motion rather than a mechanical compressor. They operate at supersonic speeds, with scramjets achieving hypersonic speeds above Mach 5. The SR-71 Blackbird used a turbojet-ramjet hybrid, and experimental vehicles like the X-43A have demonstrated scramjet flight.

Beyond the mainstream types, aero engines include several niche designs. The pulsejet, used on the German V-1 flying bomb, is a simple, noisy engine with no moving parts. The motorjet, an early hybrid, used a piston engine to drive a compressor. Electric aircraft engines, powered by batteries or fuel cells, are emerging for short-range urban air mobility.

Engine cooling has been a persistent challenge. Early rotary engines relied on air cooling, while liquid-cooled inline engines used radiators. Modern turbine blades are made of single-crystal superalloys and are cooled by internal air channels. The Pratt & Whitney J58, used on the SR-71, was a unique continuous-bleed turbojet that operated as a ramjet at high speeds.

Turbofan

A jet engine with a large ducted fan that provides additional thrust and improves fuel efficiency.

Turboprop

A gas turbine engine that drives a propeller through a reduction gearbox.

Ramjet

An air-breathing engine that compresses incoming air using the engine's forward speed.

Scramjet

A supersonic-combustion ramjet that operates at hypersonic speeds.