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Getting 35 mpg from a Mazda RX8

August 3, 2009 · Print This Article

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.

An RX8 engine running at the best BSFC point should be around .47 according to the measurements some have made in aircraft service. This high BSFC problem in common with all cars. The best BSFC point for the engine is rarely at highway speeds or highway RPMs. The main reason smaller engines get better MPG on the highway is the small engines are running in a higher efficiency levels. Namely wider throttle openings.

Putting smaller engines in cars with higher axle ratios (lower engine RPM) to improve the MPG is called down sizing and down speeding. So a smaller rotary engine would get better MPG in an RX8 car at steady speeds of around 65 MPH.

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.

The trouble with turbo compounding a 240 peak HP engine while running at 30 HP is it does not appear to work. What we need is a small rotary running at 80% of peak power or more to achieve a BSFC of .38 with a turbo compound configuration. The solutions is to add a small turbo compounded rotor to the front of the current engine.

One of the unique features of a wankel engine is the possibility of drilling a straight hole through the output shaft from end to end. If we design a small single rotor turbo compound engine of around 50 HP we can achieve 35 MPG on the highway at a steady speed of 65 MPH.

What you want to do is stop the not needed rotors as they consume friction HP. This beats the current Daimler/Chrysler 4-8 engine as the deactivated pistons don’t have to go
along for a free ride. Less overall engine friction.This engine can feed its power through the large two rotor engine shaft while the two rotor engine is shut down. The large engine is only needed for acceleration and climbing hills. All accessories such as water pump and alternator would be driven by the small engine.

Approaching this from another direction. My guess on the frontal area of the RX8 is 25 square feet. Five feet high and six feet wide by .8 (old rule of thumb). The C sub d is around .3 so flat plate drag is 7.5 square feet. Aero drag will be 7.5 X .0026 X 80 MPH ^2
or 125 pounds. Drag HP will then be: (117 FPS X 125 ) / 550 ft/lb sec. or 25.5 HP.
Give it another 11 HP for rolling drag, gear eff. and accessories. The attached charts are some measurements I made of total RX8 drag and HP required using the coast down technique so this number correlates rather well.

Since we now know the optimized BSFC of the leaned rotary engine is .47 and a car requires 36 HP at a steady 80 MPH and if we optimized the SIZE of the rotary engine for cruise at 80 MPH and 36 HP we should be burning 17 pounds per hour or 2.8 GPH. That equates to 31 MPG. Right now the RX8 is getting about 20 MPG at a steady 80 MPH. I have a Car Chip to measure it at 80 MPH but I haven’t got around to measure that accurately yet.


I have measured it at 55 MPH, 65 MPH and 70 MPH. It appears to be about 25 MPG in this range. The MPG numbers assumed the mixture was set at 14.7:1 which I think the RX8 computer maintains. To calculate fuel burn you take the pounds of air burned per minute from these charts and calculate the fuel required.

So I am guessing that down sizing the rotary engine is worth 11 MPG at 80 MPH. The turbo compound feature will only have to provide an additional 4 MPG to achieve our goal of a 35 MPG RX8 at a steady 80 MPH. Four MPG is worth a lot in these days of $3 US a gallon gas.
In the rest of the world where gas is $5 or $6 a gallon it should be worth a lot more.

BTW this approach results in no loss of RX8 acceleration or top speed. In fact both may be increased. It is not a free lunch however as the cost of the engine will be increased.

Copyright: Paul Lamar


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