Solving the Utterly Useless Fuel Economy Issue in the Mazda RX8
An interesting blog & article on an amazing body and suspension. I’d own one now if I had the plan to make improvements. Perhaps one day.. Here are some clips from the post and the link so enjoy:
35 MPG RX8 by Paul Lamar
This is a method of downsizing the rotary engine for highway cruising. Right now the RX8 engine is running at about 3250 RPM at 65 MPH and getting 25 MPG. My guess it is using about 30 HP to go 65 MPH. That would be a fuel burn of around 2.6 gallons an hour or 15.6 pounds per hour or a BSFC of around .52. BSFC is defined as the number of pounds of fuel burned for every HP generated in one hour. Here is a very old BSFC map from a NSU Wankel rotary engine. No doubt the RX8 engine BSFC is considerable improved over this engine never the less the basic principles still apply. I don’t have a corresponding map for the RX8 engine in case you were wondering.
A turbo compound rotary could achieve a BSFC of about .38. Turbo compound engines use small turbines extracting HP from the 50% waste energy in a gallon of gas and feeding it back into the output shaft. A well known technology from the 1950’s used in airliner piston engines. One of the problems with these A/C engines was the failure of an exhaust valve would take out the turbine. Needless to say the rotary has no exhaust valves. The 8 HP turbine would be geared down by at least ten to one. Working backwards a BSFC of .38 would be a fuel burn for 30 HP of 11.4 pounds per hour or 1.9 gallons per hour or a MPG of 34.2 MPG at 65 MPH.