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Turbo Compounding the Rotary


  • Turbo Compounding the Rotary


    Range is important to me. Having the range to turn back has saved my butt more than once when on a long cross country's over remote areas of Canada, Alaska and Baja. A high performance airplane with a small thin wing has a real problem with fuel volume. I don't think I would be happy with a Questair Venture if I had to stop every 400 or 500 miles. Short range also cuts into long trip times and limits your weather options making the airplane slightly less safe.

    Almost all 300 MPH airplanes use thin wings. The fuel volume of a Questair Venture is 55 gallons.

    Questair Venture

    Let me sharpen my pencil a bit. 300 HP times 75% = 225 HP cruise at 275 MPH less 35 MPH head wind (One always has a head wind. A tail wind is a rare occurrence.) equals 240 MPH. 0.47 BSFC for a normally aspirated three rotor is 106 pounds per hour divided by six pounds per gallon is 17.6 gallons per hour.That gives me a max endurance of about three hours or about 720 mile range. Not really enough to go anywhere serious. That is also BTW $60 dollars an hour fuel cost at 3 dollars a gallon for 100LL. 20% more range than that would be nice or 864 miles and a $1200 a year savings at 100 hours. That is a BSFC of about 0.38 or about 10% better than some of the best aircraft piston engines at around .42 notwithstanding some extraordinary .39 claims made by certain individuals for aircraft piston engines.


    The first V 1710 produced 750 hp. By the beginning of World War 11 it was producing 1050 hp, then 1125, then 1325, and late model P-38's got 1475. But engineers recognized that there was a lot of energy going out the exhaust pipes.

    An attempt to recover some of this energy resulted in the turbo-compound V1710 shown at the bottom of the page. It was identified as the V1710-E22 by Allison, and as the V1710-127 by the government. A turbo-compound engine collects all of the exhaust gasses and runs them through a turbine, with all of the power generated going back into the crankshaft and ultimately to the propeller. It differs from a turbo-supercharged engine, which uses exhaust gas energy to increase the pressure of incoming air. Work on this engine began in about 1944 and continued until 1946, when Allison asked that it be canceled because turbine engines had greater promise. It was the first successful turbo-compound engine, and probably one of only three to ever be built. This engine was designed to power the XP63H, which, as it turned out, never flew. The V1710-E22 had a military rating of 2320 hp, and a War Emergency Rating with water/alcohol injection of 3090 hp.

    The Allison engine collected the exhaust gas from all 12 cylinders and routed it to the turbine at the rear of the engine through two exhaust tubes. The shaft from the turbine ran through the center of the first stage supercharger impeller, back to the engine and put its power directly into the crankshaft. The turbine could not be connected to the supercharger impeller because the supercharger was driven by a variable speed transmission, which did not run at a fixed speed ratio with respect to either the crankshaft or the turbine. Since this engine was to power a P-63, it used an extension shaft and a remote gearbox attached to the crankshaft at the front of the engine at lower left in the picture below. This engine represented quite an advancement over the original 750 hp engine.

    Allison Compound V1710TC

    The most successful turbo-compound engine was a version of the air-cooled, 18-cylinder, two-row radial Curtiss Wright R3350-TC. This engine had three turbines, each fed by 6 cylinders, that were geared to the crankshaft. The normal version produced 2700 hp and weighed 2850 lbs as used in the B29. The turbo-compounded version produced 3500 hp and weighted 3440 lb. This engine was used on the Douglas DC-7c and some versions of the Lockheed Constellation as well as several other aircraft. Best BSFC = .325 at 60% power at 30,000 feet.

    Curtis Wright R3350-TC (Turbo Compound

    Cutaway of R3350 TC turbine and gear box. Picture by Bill Freeman.

    Cutaway of R3350 TC turbine and gear box. Picture by Bill Freeman

    This is from the 1954 SAE Transactions. Development of the R3350 Turbo Compound Engine. The book in the Caltech library had a 1 mm coating of dust.

    R3350TC cruise heat balance

    The power recovery heat balance chart knocked me out of my chair. I had NO IDEA.... NONE WHAT SO EVER, prior to now, so much energy was wasted by the exhaust valves in a piston engine turbo compounds. Need I mention yet again THE WANKEL ROTARY HAS NO BLEEDING EXHAUST VALVES!!!. More evidence we will eventually get more than a 20% efficiency improvement with a turbo compound rotary.

    Lets look at the heat balance chart with out the exhaust valves. Total heat available to the turbine now over doubles to 34.7% from 14.9%. The losses to gears and bearings also doubles to say 3%. Windage and cooling loss goes to 3.2% from 1.6% and aero and thermal losses got to 12% from 6%. Total loss percentage is then 18.2%. 34.7% kinetic energy available for recovery minus 18.2% losses is 16.5%! Up from only 6% with exhaust valves.

    Total fuel energy burned in a 200 HP rotary is about 760 HP worth. With exhaust valves you would have only about 6% or 45.6 HP available for the turbine.

    Without exhaust valves you would have about 16.5% or 125 HP!!!! Almost three times the HP available to drive the turbine!!! No wonder you can boost a 13B rotary to 800 HP!! There is lots of kinetic HP to be had in the rotary engine exhaust. Much, MUCH more than I had at first thought. We always knew there was a lot of waste heat in the rotary exhaust. Up to 50% of the total heat. What we did not know up until now was what we could expect in terms of kinetic energy.

    R3350TC takeoff heat balance

    Chart "R3350TC-take-off-heat-balance". Out of a gross total of 3750 HP the supercharger was consuming 385 HP. Subtracting 470 HP mechanical losses from the gross total gave us 3630 net HP to the prop. Now look over on the lower right side on the chart. The basic engine was 2750 HP sans turbines. Same as the B29 engine. All three turbines generated the takeoff HP increase or 880 HP. Divide by three to get individual turbine HP's and you get 293 HP per turbine for this R3350TC engine version.

    Overall efficiency with a 10:1 air fuel ratio for take off was 17% without turbo compounds and 22.3% with the turbines. Or an overall engine efficiency gain of 5.3 percent at high power. Now the sensible power going to the turbines is with the huge losses to the exhaust valves. About THREE TIMES AS MUCH will be going to the turbines WITHOUT the exhaust valve losses. Chart three. "R3350TC Power recovery heat balance."

    R3350TC power recover heat-balance

    Curtiss Wright continued to develop this engines as the years passed up until the turbo jet took over the air line industry.

    R3350TC test hours

    More and more power was extracted from the engine.

    R3350TC power to weight ratio

    The overall engine power to weight ratio continually improved.

    R3350TC Improvements

    An excellent BSFC was achieved. Particularly at high altitudes where the back pressure on the power recovery turbine was much reduced.


    The worlds record prior to the Rutan Voyager for un-refueled distance was held by a P2V that flew from Australia to Ohio. That airplane used the CW R3350 turbo compound which had a BSFC of 0.38. Super G Connies with this engine were able to fly non-stop LA to London over the pole in the 1950's.

    I highly recommend the book "Lockheed Constellation" by Stringfellow & Bowers Motorbooks ISBN 0-87938-379-8 Therein the TWA nineteen hour non-stop flights from LA to London and twenty three hours, nineteen minute flights
    London to San Francisco are described.

    The other turbo-compound engine is the Napier Nomad, which was built in England. It was a 12-cylinder, horizontally opposed, liquid cooled, valve-less, 2-stroke cycle Diesel engine with a 3-stage turbine. It reached flight test but not production. Best BSFC = .327 at 2,027 HP.

    Napier Nomad

    NACA REPORT No. 822




    Calculations based on dynamometer test-stand data obtained on an 18-cylinder radial engine were made to determine the improvement in fuel consumption that can be obtained at various altitudes by gearing an exhaust gas turbine to the engine crankshaft in order to increase the engine-shaft work. The calculations indicted that, for turbine and auxiliary supercharger efficiencies of 85 percent minimum net brake specific fuel consumption improvements of 0.357 pound per brake horse hour at an altitude of 10,000 feet and of 0.323 pound per brake horsepower hour at 30,000 feet can be obtained by gearing the exhaust-gas turbine to the engine crankshaft and operating that engine at a speed of 2,000 rpm, an inlet-manifold pressure of 40 inches [5 psi boost] of mercury absolute, and a fuel-air ratio
    of 0.063 [12:1].

    The reduction in net brake specific fuel consumption that can be obtained if the exhaust-gas turbine supplies all the auxiliary supercharger power and if its residual power is transmitted through gears to the engine crankshaft as
    compared with auxiliary turbo supercharging, is approximately 14 percent at an altitude of 10,000 feet and 21 percent at 30,000 feet. The net brake specific fuel consumption is a minimum is for engine exhaust gas pressure approximately 25 percent above inlet-manifold pressure and varies only slightly from that minimum for a range of exhaust gas pressure from 6 to 45 percent above inlet-manifold pressure.


    The use of an exhaust-gas turbine to drive a supercharger at high altitudes is an effective method of maintaining sea level engine power at altitude. Analysis has shown, however, that the waste energy of exhaust gases is recovered more effectively by maintaining an engine exhaust pressure higher than the minimum required for turbo supercharging and thus increasing the work output of the exhaust-gas turbine. The extra turbine power beyond that required for supercharging Can be supplied to the engine crankshaft through suitable gearing (compound operation).

    The purpose of the analysis reported is to determine the improvement in net brake specific fuel consumption that can be obtained if an engine is equipped with a geared turbine and supercharger as compared with the engine using a standard turbo supercharger. The calculated values specific fuel consumption presented for an engine-turbine combination were based on NACA test data obtained on an 18-cylinder radial engine. Operating conditions for which the brake specific fuel consumption of the combination is a minimum are given. The required turbine-nozzle area is also calculated to indicate the size of turbine suitable for geared operation.

    Because the engine, the turbine, and the supercharger have different characteristics, elements designed to give maximum efficiency at some operating conditions are incorrectly matched at other conditions. Provision must therefore be made to obtain satisfactory performance over the entire operating range. The problem of obtaining a wide operating range is briefly discussed.

    The investigation reported was conducted at the NACA Cleveland Laboratory in the fall of 1944.

    turbine and supercharger powers

    The full 9 page report is obtainable in PDF form from:



    The power in the Mazda Wankel rotary engine exhaust has been talking to us for 30 years. It has been saying. "Hook me to turbine, hook me to a turbine, hook me to a turbine." Anybody that has been around a Mazda rotary powered race car without a muffler can understand the tremendous kinetic energy in the exhaust. I have a race car driver friend by the name of John Morton who claims he has permanent hearing damage from driving rotary powered race cars.

    It was not until I read that SAE paper on the R3350 TC that I could put a number on it. It then hit me like a supersonic shock wave. Yes of course it has plenty of kinetic energy in the exhaust.

    Now comes the time to harness all those super sonic horses.

    Think of the Wankel rotary as an ideal gas generator for a turbine engine. A marriage made in heaven. The simplest configuration is a large diameter blow down turbine running on the Mazda e-shaft.

    3D Graphic of a Turbo Compound Rotary Engine

    We are now building a traction drive device to extract the excess HP from a stock turbo charger. Input speed RPM can be up to 120,000 RPM and output RPM will be one tenth of that. The power from the traction drive output shaft will be transfered to the e-shaft of the rotary by a multi v belt to absorb the torsional vibrations from the rotary engine. We hope to recover at least 50 HP from the waste heat in the rotary exhaust improving the BSFC from the current .47 to .376 at cruise altitude. The range of any airplane using this engine will increase by 25%. We hope to test the engine soon on a dyno and a SAE or AIAA technical paper will be published reporting the details of the traction drive and the dyno test results.

    Part One

    Part Two

    Part Three

    Part Three

    Part Five

    Part Six

    3D graphic traction drive exploded

    3D graphic complete reductionless eng transp

    3D graphic complete reduction engine


    First the rotary is a very mechanically robust engine that tolerates high boost pressures. The Achilles heel of this technique proved over time to be the failure of the piston engine exhaust valves when the exhaust gas pressure was 25% higher than the intake pressure.

    Guess what? The rotary has no exhaust valve.

    I had an idea on how to implement a turbo compound arrangement using a gear box out of a Paxton Centrifugal supercharger. The HP, gear reduction and RPM is about right for hooking a turbine and feeding the power back into the engine instead of removing it. Hopefully we will get 20% more power with no more fuel and no additional load on the engine cooling system.

    The rotary has a lot of waste chemical energy in the exhaust that could be put to good use. Perhaps some additional air could be pumped into the exhaust system before the turbine to burn the waste hydro carbons. The fact of the matter is you can get more energy out of the exhaust than you really need for supercharging. That is why the BSFC goes down on a turbo compound as opposed to merely just turbo-charging.

    This is sort of an exponential effect so the real HP limit is the detonation and structural strength of the core engine. And here is where the rotary engine scores big time. The structural strength to weight ratio is exceptional. I also suspect the otherwise bad surface to volume ratio of the rotary combustion chamber is an asset at high boost levels.

    The thing to remember is there is always as much or more potential HP in the exhaust as there is available from the output shaft. The more you pump it up the more HP is available to pump it up. Since you don't need all the HP in the exhaust for supercharging the sky appears to be the limit. This excess exhaust HP fed back into the shaft will result in better and better BSFC. The beauty of this HP is it does not add to the thermal load on the engine.

    The big question is; how much can you get and for how long before the engine starts coming apart? I would monitor the metal content in the oil, the rotor housing metal and turbine inlet temperature very very carefully. I would not rely on coolant temperature alone. Perhaps use a bore scope from time to time to look into the housing and see if the oil is coking on the surface of the housing. The other ting I would do is find a knock sensor that would work.

    My compound setup for the rotary would use one Inconel blow down turbine feeding power into the eccentric shaft and an optional centrifugal blower using modern aerodynamics either from a turbocharger impeller, an off the shelf gear driven supercharger (there are several on the market) or a late model gear or ball driven Paxton taking energy out of the eccentric shaft to supercharge the engine. To get the full altitude benefit you would have to fabricate a variable speed drive for the centrifugal supercharger as well.

    We will start with just a one speed blower with a gear ratio that peaks the boost to what the engine will stand at sea level and at an e-shaft RPM of 6000. That e-shaft RPM assumes you are using a PSRU with a 2.2:1 ratio.

    ATI manufactures a centrifugal supercharger the gearbox of which is a candidate for my turbo compound scheme. They also have some nice looking water to air heat exchangers if someone wants to go that route for their inter-cooled turbo rotary aircraft engine.

    If you just want to fly without a supercharger and you can live with 20% more power and a 20% lower BSFC at that power at sea level you can forget the supercharger and the inter-cooler and go with just the blow down turbine.


    The full wammy is variable ratio gear supercharger with inter-cooler for altitude and variable ratio geared blow down turbine for a lower BSFC. This is several steps above the 20% more efficient 1950's R3350TC by the way, as it only had a two speed centrifugal blower (no inter-cooler) and a fixed gear ratio blow down turbine. A variable ratio drive can be made from a wide V belt and movable flange pulleys controlled by a linear electric servo and a computer

    Now... if you just want to supercharge, a turbocharger with inter-cooler is the best way to go as no HP is extracted from the eccentric shaft to drive the supercharger.


    We also discussed a roots blower and a turbocharger and an inter-cooler blowing air through the roots. As Jeff pointed out he felt the roots would be horribly inefficient as an air motor feeding energy back into the eccentric shaft. (We solved the problems of its not working at all by under gearing or pop off valves) I am not sure I agree with Jeff but I am not going to spend the money to find out.

    A real draw back to the roots as an air motor is its size and weight as it operates at close to eccentric shaft speed and is therefor big and heavy. Another real drawback is; it is a third device to suck power in the form of its efficiency and is therefor in the, eff.A X eff.B X eff.C = total eff. chain.

    A is the eff. of the turbine.

    B is the eff. of the centrifugal blower

    C is the eff. of the roots as an air motor.

    This remains however as the simplest and easiest turbo compound engine to build as all parts are off the shelf.

    Paul Lamar

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