Well folks, it's about time i ate some humble pie:
Thanks Ian, this pm was so informative that it thought it unfair not to share it! Must admit i had noticed some similarity of RR300 with the 250...
Originally Posted by Ian Corrigible
Mart,
Don't want to flog this to death on a public forum, but you should probably take another look at your diesel vs. turbine calcs. The Thielert Centurion diesel (which Frank R. is reported to still be looking at) comes in at 0.32 lb/shp/hr, which equates to 110 g/kw/hr, not 200. The OPOC diesel is even more frugal.
The RR300 actually offers very little improvement over the Allison 250, which was already way behind newer designs from P&WC, Honeywell and TM in terms of SFC and MTBR. As I understand it, RR was originally intending to sell the C20 to Robinson (which has been in discussions with Allison since 2000/1), and it was the current President who decided to launch a whole 'new' engine as an ego trip to get some personal publicity (allegedly...). Which means that if/when Frank decides the R66 actually needs 400 shp, we'll either see a reversion back to the C20 or the development of another smoke & mirrors 'new' RR400.
Go figure !
I/C
Doing some proper bookwork, i find that aircraft fuels have a range of energy values:
Avgas: 42.0 to 43.71 MJ/kg
Diesel: ~42.6 MJ/kg
Jet A1: 42.5 to 43.23 MJ/kg
So if we choose roughly 43MJ/kg for all three fuels, this gives 100% engine efficiency with BSFC 83.7 g/kW-hour. Now the most efficient truck engine i know is about 45% efficiency, which corresponds to 186 g/kW-hour (200 g/kW-hour would be 42%).
This also give a figure for the RR300, with its cruise consumption of 255.4 g/kW-hour, of 32.8% efficiency. In fairness this is probably comparable to the Lycombing O-540, but not vastly better. The Thielert Centurion diesel, at 110 g/kw/hr, is thus running at 76% efficiency!
There was me being all cynical about CO2.
Mart