Hi Nick,
a little late for a reply, but better late than never!
Congrats to your new post!
Are we goin' to see some Gulfstream-helicopters soon?
To the Rotary engine:
Seals are not a problem any more - actually since decades.
Also Mistral will probably offer Ceramic seals as an option in the turbo engines, which should outlast the TBO of the engines.
Also the Mistral gasoline engines are on par or slightly better than the Lycosauros on BSFC. If you are interested I can forward you some test data from Mistral or put you in direct contact with them.
The missing data is from Lycoming, as they do not publish any consumption data over 70% power!! But what is available shows at least even or better efficiency on the Mistral. However you probably cannot compare these numbers with anything on the road - on the other hand anything from the road would never last in the air at the required power levels.
To all:
The real problem with the diesel is that after you pass a power level of around 135- 160 hp, the engines become rather heavy.
Thielert promoted his V8-turbo diesel at 300 hp. Now it is out at max-cruise 260hp - around 600 lbs !! And no TBO, but TBR (time before replacement)
You can't compare car engines with aviation engines. The duty cycle in a car engine is roughly 35-40%. In an aircraft engine you have it running between 70-100% all the time.
Toyota certified its V8 gas engine at 300hp max cruise - TBO 300 hrs!!
The one exception is the rotary engine as it is very compact and robust - if configured it will outlast any other piston.
For now the best you can hope for is something like Mistral - Jet-A burning with ignition - potential power level that can be reached easily is around 450 hp max continious, after that the price/weight/power relation probably favors the turbines again.
Just a side note: Hartzell never made a heavier prop than the one for the SMA diesel ( for the power transmitted) the lighter props would shatter after a short time due to the power pulses from the engine!!
Cheers,
3top