The application of the two-stroke diesel concept to aircraft engines is everything but a novelty. For example, Junkers built a very successful series of these engines in the late 1930s named JUMO. The main advantage offered by such an engine is fuel efficiency; even in 1938, the JUMO engine was capable of a brake specific fuel consumption of 213 g/kW·h, an impressive figure even by modern standards.
It should be noted that fuel consumption is very important for aircraft performance, since a relevant portion of the aircraft total weight (sometimes up to 50%) is due to fuel storage.
The main reason for the outstanding fuel economy of two-stroke diesel engines is the high mechanical efficiency ensuing from the two-stroke cycle. Besides the possibility of having no poppet valves and the associated driving system, mechanical friction losses over the cycle are about halved in comparison to a four-stroke engine having the same crank and piston and crankcase design, due to the double cycle frequency.
Furthermore, the two-stroke cycle is a good match for aircraft engines, since it is possible to achieve high power density at low crankshaft speed, allowing direct coupling to a propeller without the need for a reduction drive (which is heavy and expensive, besides adsorbing energy).
Supercharging further improves power density and fuel efficiency, as well as enhancing altitude performance. Diesel combustion allows a higher boosting level, in comparison to spark ignited engines, limited by knocking. In addition, high octane aviation gasoline is expected to be subject to strong limitations, due to its polluting emissions of lead, while a diesel engine can burn a variety of fuels, including automotive diesel and turbine fuels such as JP4 and JP5, and Jet A.
Further advantages in comparison to gasoline powerplants are: reduced fire and explosion hazard, better in-flight reliability (no mixture control problems), no carburetor icing problems, and safe cabin heating from exhaust stacks (less danger of carbon monoxide intoxication).
The recent development of diesel technology, along with the above mentioned series of advantages, has made the two-stroke, compression ignition engine an interesting option for light aircraft manufacturers, seeking power unit of 100-300 hp, preferably not heavier than existing SI powerplants.
Conversion of automotive four-stroke units is generally not attractive, since they are relatively heavy. Thus, design must be carried out from scratch to achieve an acceptable power-to-weight target. The mission is not impossible: in the late 1990s, AVL developed a 1-L, two-stroke turbocharged diesel engine, with uniflow scavenging, achieving a brake power of 50 kW and a weight less than 80 kg, and Wilksch Airmotive brought to the experimental aircraft market a 90-kW three-cylinder, two-stroke unit using IDI combustion and weighing only 100 kg.
The aircraft diesel engine market is in its infancy: several two-stroke prototypes have been built but none have achieved type certification at the time of this writing. Selected cylinder configurations have included loop and uniflow scavenging as well as opposed piston uniflow.
Researchers from the University of Modena & Reggio Emilia and Wilksch studied the two most widespread scavenging designs: uniflow with exhaust poppet valves and loop scavenging with piston controlled ports to assess the potential of two-stroke high-speed diesel engines on light aircraft.
Comparisons were made between both the two-stroke CI configurations. Predictions for both uniflow and loop scavenged three-cylinder engines were calculated using GT-Power, supported by CFD-3D combustion and scavenging simulations.
The results were not anticipated at the outset of the study. The uniflow engine was significantly more complex and hence more costly and heavier than the loop scavenged engine. It also had the perceived advantage of complete flexibility of exhaust timing events via the cam-operated valves.
By contrast, the loop scavenged engine was simpler and cheaper to produce but brought with it the restriction of symmetrical inlet and exhaust event timing.
It was anticipated that the more complex uniflow engine would offer performance benefits at the cost of increased complexity.
This was not the case, with the loop scavenged engine showing advantages in all key areas of interest (power-to-weight ratio, fuel efficiency, altitude performance, cooling pack size requirements) and no major disadvantages.
This outcome can be mainly explained by the particular layout of the engines (three-cylinder), providing an exhaust manifold dynamics that helps to enhance trapping ratio, even with large exhaust closing retard. This fact almost canceled the advantage inherent to the cam-controlled valves, leaving the drawbacks, i.e. lower mechanical efficiency and smaller exhaust flow areas.
It is expected that a future study should address the loop scavenged engine's durability potential and should define a viable all-mechanical fuel system.
This article is based on SAE technical paper 2011-24-0089 by Enrico Mattarelli and Carlo Alberto Rinaldini, University of Modena & Reggio Emilia; and Mark Wilksch, Wilksch Aero.