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Aero Piston engines

Any aircraft will need some form of power to keep them flying, to postpone the inevitable return to mother Earth as it were. Since the first flights by the Wright Brothers numerous types and models of engines have been tested, developed and flown.

Some of these designs were very successful, others somewhat less or even failed to go to any production stage. Not all engine models are suitable to be installed in LSA or experimental type aircraft due to their size and or weight limitations of the airframe.

You are not going to see a big four row nine cylinder engine like the Pratt & Whitney Wasp Major or Wright Cyclones that often in a 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 for those pilots wanting to fly a real radial engine in their project.

Some manufacturers for the experimental and general aviation market are: Rotax Bombardier, ULPower, Jabiru, Eggenfelner, Hirth, Lycoming, Teledyne Continental, Mattituck and radial engines from Vedeneyev and Rotec Engineering of Australia. Each of these companies have their own line of engines ranging from 50 bhp to 400 bhp up for sale.

On these pages we will take a look the two main classes of aero engines: Diesels/JET and AVgas models and we will be discussing how they operate and are used.

Basic Piston Engine Principle

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

Piston engines in aircraft can be found in configurations of two cylinders inline up to eight cylinder opposed and nine cylinder radials and also in diesel and with fuel injection (diesel or avgas) or carburetted (avgas only) 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.

Experimental Aircraft Engine

Aircraft piston engines can be divided in two types depending on the type of fuel they use: diesel or gasoline/petrol. Or put differently: is it a compression ignition or spark ignited engine? Both types can be found in the same basic form or shape: radial, horizontally opposed (boxer), inline (sometimes inverted, Wilksch or Gipsy Major) and the V setting (inverted V too: Deltahawk engines).

Auto Ignition

Sonex Research is developing a combustion process called Sonex Combustion System (SCS), which improves the combustion of fuel in internal combustion engines.The SCS achieves the in-cylinder control of ignition and combustion through chemical/turbulent enhancement using patented combustion chamber designs. Suitable for gas and diesel engines enabling them to run without a spark and on medium compression 12,5:1 and with multi-fuel capability and improved fuel mileage (25%-30%).

This aero piston engine is a horizontally opposed, reciprocating, with internal combustion, operates on the four stroke principle, uses aviation gasoline (and with a STC from the manufacturer on Mogas too) and it drives a constant speed or fixed pitch propeller to convert engine horse 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 an amount of gas is held in a constant volume container and that gas is heated then the result will be an increase in pressure in that container. And in the case of an engine, that container is the cylinder/ piston combination.

Ideal gas law

There is a formula which explains it a bit: p × V = n × R × T. Where p = absolute pressure, V = volume, n = the amount of gas in moles, R = a gas constant (8.314472 J K−1 mol−1) and T = absolute temperature in Kelvin.
Naturally, this is mathematically valid for a perfect gas, but in real life there will be heat losses and losses due to molecular interaction. Regardless, it is clear from the formula that if the temperature is increased (by combustion) the pressure must rise if the volume is to remain constant.


A mixture of fuel and air is added to the cylinders of our aero engine and after the piston has compressed the mixture (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. JET or Diesel engines only need to compress the air after which fuel is injected into the very hot air for combustion.

We need to emphasize the fact that there is never an explosion of fuel, it is combustion. Fuel burns at a much slower rate during combustion than when it explodes. Explosions would kill the engine in no time.

Rotec Engineering radials

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 and the smoother the engine will run.

Two stroke cycle

Very small ultralight aircraft still use gasoline two stroke engines, mainly because of the high power to weight ratio. All of the engine strokes are accomplished in one revolution of the crankshaft. This type of engine uses a mixture of oil and fuel (ratio 1:50) and there are no intake or exhaust valves. In stead, intake and exhaust ports are used and the mixture flows through the engine crankcase pushed by the downward going piston in a form of pressurization of the cylinder and scavenging of exhaust gases. At the same time the fuel/oil mixture lubricates the engine internally.

Some diesel engine manufacturers are using the two stroke cycle to enjoy the same high power to weight ratio advantage. A major difference is that two stroke diesel engines do not use fuel/oil crankcase lubrication but have a separate oil pump you see in the four stroke engines. They also must use a separate pump to get air into the cylinder as these engines do not have an intake stroke inhaling air.

Written by EAI.

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