Powertrain & Energy - September 26, 2012 - 17

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 • Safe cabin heating from exhaust stacks (less danger of carbon monoxide intoxication). The recent development of diesel technology has made the two-stroke, compression-ignition (CI) engine an interesting option for light aircraft manufacturers seeking a power unit of 100-300 hp, preferably not heavier than existing SI powerplants. Converting automotive four-strokes 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 (indirect injection) and weighing only 100 kg. The aircraft diesel engine market is in its infancy; several two-stroke prototypes have been built but none has achieved type certification at the time of 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. A set of 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. But the loop scavenged engine showed no major disadvantages; it did show advantages in all key areas of interest: power-to-weight ratio, fuel efficiency, altitude performance, and cooling pack size requirements. 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. 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.

SAE Powertrain & Energy

September 26, 2012

Powertrain & Energy - September 26, 2012

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