Radial Aircraft Engine

Aircraft Engine Types

Aircraft will need some form of power to keep them flying, to postpone the inevitable return to mother earth. Since the first flights by the Wright Brothers numerous types and models of engines have been developed. Some very successful others less. Not all engine types are suitable to be installed in LSA or experimental type aircraft. Big four row nine cylinder engines like the Pratt & Whitney Wasp Major are not commonly seen on homebuilt aircraft.
Although smaller versions like the Australian ROTEC or the M-14 nine cylinder radial by the Vedeneyev Design Bureaux are available to aircraft builders.

On these pages we discuss the two main classes of aero engines: piston and turbine engines.

Piston engines

Aircraft piston engines can be divided in two types and that depends on the fuel they use, diesel and avgas. Both can be found in the same basic form or shape: radial, horizontally opposed (boxer), inline (sometimes inverted) and V setting (inverted V too: Deltahawk engines).

These days, piston engines in aircraft can be found in configurations of two cylinders inline up to eight cylinder opposed and nine cylinder radials also in diesel and with fuel injection (diesel or avgas) or carburetted (avgas) fuel systems. Engine cooling is usually by air or a mix of water and glycol which means that hoses and a radiator are applied to dump the excess heat in the atmosphere.

In General Aviation today the most predominant powerplant is the air-cooled piston engine which is horizontally opposed, with four, six and sometimes even eight cylinders arranged in two lines on both sides of the crankshaft. This is the best design compromise in terms of streamlining, cooling, power to weight ratio, thrustline and good forward visibility from the cockpit.

This piston aero engine is a horizontally opposed, reciprocating, with internal combustion, operates on the four stroke principle, uses aviation gasoline (with a STC from the manufacturer mogas too) and it drives a constant speed or fixed pitch propeller to convert the engine power into useful thrust.

Operating principle

The basis of these engines lies in a fundamental law of physics which states that a gas expands if it is heated. If a certain amount of gas is contained then the result of that is a increase in pressure when the gas is heated. There is a formula which explains it a bit: { pv = RT }. Where p = pressure, v = volume, R = a gas constant and T = temperature.
This is mathematically valid for a perfect gas as in real life there will be heat losses. It is clear from the formula that if the temperature is increased (by combustion) the pressure must rise if the volume is constant.

A mixture of fuel and air is added to the cylinders of our aero engine and after the piston has compressed the mixture (small v), this is ignited by a spark and the temperature (T) will rise. The result is in increase in pressure in the cylinder which will push the piston down thereby rotating the crankshaft and everything attached to it.

Four stroke cycle

Most aero engines are of the four stroke cycle type: intake, compression, power and exhaust. The above explained process takes place in these four strokes and the crankshaft makes two revolutions to accomplish this. In a four cylinder engine (O-235, O-230 or O-360) this comes down to two power strokes per revolution (every 180°). In a six cylinder engine this would be three power strokes per revolution (every 120°) and in an eight cylinder there will be four power strokes per revolution (every 90°). Thus, the more cylinders the more evenly spread the power pulses are.

Engine manufacturers

There are a number of manufacturers for the experimental and general aviation market: Rotax Bombardier, Jabiru, Hirth, Lycoming, Teledyne Continental, Superior / Mattituck and radial types from Vedeneyev and Rotec Engineering. Each of these companies have their own line of engines ranging from 50 bhp to 400 bhp.

Some are installing car engines (or converted types) in their aircraft, engines like VW and Chevy are used for these, usually one time installations. One successful converted car engine and sold commercially is the Subaru H6 from Eggenfellner Aircraft, available as four and six cylinders with an turbo as option. These beautiful created engines are able to run on autofuel. As are the Rotax and Jabiru's.

Piston powered engines are normally used in the lower altitudes, unless fitted with a turbocharger. Turboprops are much less efficient at these altitudes due to thicker air, they usually can be found from 10000 ft and up. Turbofans are used from roughly 25000 ft and higher, same reason as for the turboprop.