50 MPG RX8 by Paul Lamar

This is a method of down sizing the rotary engine for highway cruising. Right now the RX8 engine is running at about 3250 RPM at 65 MPH and getting 25 MPG.

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.

This high BSFC problem at cruise power is a common problem 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. This chart shows BSFC verses load. As you can see the fuel burn or BSFC goes way up at low loads.

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.

Lets assume the RX8 car has a frontal area of 20 square feet. The rule of thumb is 80% of the height times the width estimates the frontal area. Lets say 6 feet wide and 4 feet high. Or 5.5 feet wide and 4.4 feet high. Lets also assume a C sub d of .30! Typical for a modern car. Drag would then be 20 x .3 x 65 x 65 x .0026 or 66 pounds at 65 MPH. 65 MPH is 95.3 FPS. So required HP is then drag x FPS or 6270 pound foot per second or 11.4 HP. Double that for tire rolling resistance and you get 22.8 HP needed to maintain 65 MPH on a level road at sea level.

The current RX8 gets 25 MPG at a steady 65 MPH. 25 MPG means a fuel burn of 2.6 gallons per hour. 2.6 gallons is 16.25 pounds of fuel per hour. Divide the pounds of fuel burned by the HP generated and you get the actual RX8 engine BSFC at 65 MPH. 16.5/22.5 = .73. WAY TOO HIGH!!! A Mazda rotary is capable of a BSFC of .47 when running at 75% max power. We have proved that in aircraft service. We need to run a smaller rotary engine at a higher power level for cruising on the freeway.

Automotive cruise MPG is a real challenge. 90% of the time a car cruises on the free-way or autobahn at 20 or 30 HP while the engine may be rated at 250 HP. In order to make that highly efficient you need a small engine running at about 75% of it's max power at best torque RPM. This is the rational for shutting down some of the cylinders in a piston engine. It is also the partial rational for a Hybrid.

The trouble with a 250 peak HP engine while running at 22.8 HP is it is highly inefficient. What we need is a small rotary running at 75% of peak power or more to achieve a BSFC of .4 with a turbo compound configuration. The solutions is to add a small turbo compounded one rotor to the front of the current engine. This would be far cheaper to manufacture and far lighter weight than a hybrid vehicle. It requires no exotic and/or rare materials like lithium ion.

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 cruising speeds of around 65 MPH.

What would be great is a 2.5 rotor Wankel engine. In other words 2 rotors for acceleration and hill climbing and a much smaller one rotor for cruise. The unique configuration of the e-shaft makes that possible.

A small turbo compound rotary could achieve a BSFC of about .36 to .40. 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 .36 would be a fuel burn for 22.5 HP of 8.10 pounds per hour or 1.30 gallons per hour or a MPG of 50 MPG at 65 MPH.

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 and about 50 pounds added for the small rotary.

50 MPG is worth a lot in these days of $4 US a gallon gas justifying the cost of these modifications. In the rest of the world where gas is $5 or $6 a gallon it should be worth a lot more of course.

The small one rotor engine would have a tiny turbine to get the BSFC below .36! Diesel range or below.

The advantage over a hybrid is less extra weight and cost for electric motor and battery. Not to mention the limited availability of lithium. The advantage over the diesel is better acceleration and less cost and weight. No carcinogenic black smoke problems either.

The rear axle ratio could be the same as the current RX8 at around 4.77:1 for acceleration and hill climbing reasons.

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.

To summarise 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.

The attached charts are some measurements I made of total RX8 drag using the coast down technique so this 22.5 HP number correlates rather well. If you know the weight of a vehicle you can calculate the total drag on the vehicle by measuring the rate at which it coast downs from high speeds using the OBD II data recording capabilities.

How do the numbers correlate with reality for the stock car? Since we now know from aircraft use the optimised BSFC of the full size leaned rotary engine is .47 and the RX8 car requires 22.5 HP at a steady 65 MPH cruise. Therefore at 65 MPH and 22.5 HP we should be burning 10.6 pounds per hour or 2.8 GPH. That equates to 23.2 MPG. Right now the RX8 is getting about 25 MPG from coast down measurements at a steady 65 MPH so the theory is correlating well with the real world data.

The MPG calculations assume the mixture was set at 14.7:1 which I think the RX8 computer maintains at steady speeds. To calculate fuel burn you take the pounds of air burned per minute from these OBD II charts and calculate the fuel required.


Here is a cheaper and quicker way of testing this concept. Get an AIXRO Kart engine and mount it on the back of the RX8 differential. Remove the stock rear cover. Use a second pinion gear and run the AIXRO only at cruising speed with a one way clutch to disconnect it at lower speeds.

Assume a 30 inch diameter tire over all and that is 94.248 circumference so that is 7.85 feet in circumference. At 95 feet per second or 65 MPH that is 12 revs per second or 720 axle RPM times 4.77 is 3434 AIXRO engine RPM.

So you need a 2.17 planetary given a 7452 RPM for the AXIRO engine. That is also 114 revs per mile per hour or 96 MPH at 11,000 AIXRO max RPM.

My guess is the MPG running on the AIXRO alone will be .73/.52 X 25 will be about 35 MPG. Converting the AIXRO to an oil cooled rotor and EFI and the RX8 MPG could jump to 38.8 MPG.

Going the full turbo compound route on the AIXRO and getting the BSFC down to .4 or below the RX8 MPG would jump to .73/.4 X 25 or 46 MPG at a steady 65 MPH.

The cost of testing this is not beyond the means of an individual.

Paul Lamar