AviTech Reader

Let us say…we are revisiting this idea, because it is not just out of the woods. This is something that was there many years ago. Let’s take for instance, the late 1930’s when the Junkers built JUMO-the series of 2-stroke diesel engines whose advantage of fuel economy and mechanical efficiency made them very popular. Talking of such a 2-stroke engine, no poppet valves and its junky operating and timing system, no friction losses due to valves –compare this to the 4-stroke engine and you will surely visualize what I am talking about. Again, why fuel? Can you imagine that depending on size and type of aircraft, the largest portion of it is fuel, sometimes can be as high as 50%!

With the 2-stroke diesel engine, it is possible to attain a high power density and fuel efficiency while at the same time enhancing altitude performance. Another salient feature of diesel engines is that it allows a higher boosting level in comparison to spark-ignited(SI) engines whose capabilities are limited by knocking-yet we know that high octane rating fuel like JP4, JP5 and Jet A is now being subject to very strong limitations due to the damage to mother nature. We haven’t exhausted the list of advantages yet; there is the low volatility of diesel compared to other fuels meaning less explosions and fires. It also has better reliability in flight-no mixture control issues, no carburetor icing issues and safe cabin heating due to reduced exposer to carbon monoxide. Let us also not gainsay the fact that the 2-stroke engine cycle has the exemplary ability to maintain a high power density at a very low crankshaft speed, hence can be directly engaged to a propeller shaft even without the reduction gearbox hence cheaper.

These advantages, coupled with the recent developments in the 2-stroke, compression ignition(CI) diesel engine technology make this an auspicious choice for small planes that only need 100-300 hp, at a lower weight than the existing SI engines. This is very pragmatic owing to the trials by AVL in the late 1990’s when they developed a 1-L, 2-stroke turbocharged diesel engine with uniflow scavenging, achieving a brake power of 50kW and a weight less than 80kg. Another proof of possibility is given by Wilksh Airmotive that brought to the experimental aircraft market a 90kW 3-cylinder 2-stroke engine with IDI (indirect injection)-only weighing 100kg.

Generally, we can say that so far, none of the models has reached certification stages as I am writing this article but we are sanguine that more research and trials with several engine configurations will yield expected victory. Some of the selected engine configurations are like the loop and uniflow scavenging type, as opposed to the piston uniflow type.

Research at the University of Modena & Reggio Emilia and Wilksh pointed out that the uniflow type was actually more intricate (would be heavier and more expensive to manufacture) though had flexibility of exhaust timing events via cam-operated valves. On the other hand, though the loop scavenged engine had the trappings of “simple and cheap”, it brought with it symmetrical inlet and exhaust event timing. It also has more to offer in terms of Power-to-Weight ratio, Fuel efficiency, altitude performance and cooling.

For now, at least we leave the rest for research to decide.

This article has been influenced by another article from SAE Aerospace magazine August 2012.