3Top,
Interesting post. You sound well informed about rotaries, so forgive me if i offer some constructive criticism. I am actually in agreement about the mechanical reliability of a well designed rotary engine. Piston sideforce during reversal is the biggest headache in piston engines, causing most of the durability limitations.
Not sure that rotary fuelburn can be dismissed out of hand, since it will contribute to the running cost of a privately owned machine. The rotary only really burns Avgas well, since the modifications for A1 are to make it burn more like SI Avgas. This means that a rotary "diesel" never has any hope of approaching the efficiency of a piston engine. With emerging piston diesel combustion technologies CI particulates are becoming a thing of the past. Regardless of existing legislation i feel that engine designers have a responsability to consider CO2 emissions.
Not sure you can dismiss comparison with piston so easilly either. To my mind a two rotor rotary is equivalent to an inline 4, in that there are 2 bangs per crankshaft revolution. The displacement comparison is always a problem, so the best solution is to consider any engine as an airpump. In this way a 2 litre inline 4 pumps 1 litre per per crank revolution, and a twin rotor with 1/2 liter per rotor section (ie 3 litre by convention) also pumps 1 litre per crank revolution. The rotary combustion chamber does not offer an easy shape for either SI laminar-to-turbulent swirl for flame propagation, or DI swirl for air/fuel mixing, so i would say piston makes better use of its litre (without considering heat loss through surface_area/volume).
My final concern is that the seals need the continous addition of oil into the combustion chamber for lubrication. An oxydation catalyst will remove hydrocarbon emissions, but work better if the engine is designed to reduce emissions from the outset. The 4-stroke is definately a compromise, but the piston/ring lubrication oil stays well away from the valves/ports. Centrifugal oil conditioners now exist to remove particulate contamination. This oil loss is my main concern with 2-strokes and rotaries.
The 400 to 600 HP range for helicopters is an interesting region. It may be that rotaries can offer a good infill, but my take is that variations on the turbo charged piston theme are equally promising. For diesel the main limitation is definately combustion initiation. Getting piston engine mass down is definately an area for development, with the block being the biggest contributor. At the moment i would estimate piston to have 1/2 the power-to-weight of rotary...
Mart
Edit: Correction about 2-stroke oil injection (actually better than normal engine oil for emissions).
Last edited by Graviman; 23rd May 2006 at 21:04.