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Moller Delivers Better than SULEV with Ethanol-Fueled Rotary Engine

Rotapower Engine

Moller International, the developer of the Skycar aircraft, the Rotapower line of Wankel-type rotary engines and the Aerobot line of Unmanned Aerial Vehicles (UAV), has achieved emission levels below California SULEV standards in recent tests of the Rotapower engine fueled by ethanol without exhaust aftertreatment.

The 530cc engine tested produced 22 hp at 4500 rpm with an air/fuel ratio of 20 and lambda of 1.35. The fuel was E95 with 5% acetone added plus a synthetic lubricant together with some castor bean oil. The toxic emission levels with ethanol were as follows:

  • Unburned hydrocarbons (HC) = .5 ppm or .0043 gm/hp-hr
  • Carbon monoxide (CO) = 9 ppm or .03 gm/hp-hr
  • Nitrous oxide (NOx) = 3 ppm or .016 gm/hp-hr

The carbon dioxide produced was 11%—25% lower than with gasoline.

Previous tests of the Rotapower engine using gasoline were witnessed by Dr. Andy Burke from the Institute of Transportation Studies (ITS) at the University of California in Davis. These tests showed that with gasoline the Rotapower engine produced toxic emissions below the ultra low emission vehicle level (ULEV) without using an exhaust aftertreatment (no catalytic converter required).

Moller developed an early version of the Rotapower engine some 20 years ago—based on the rotary snowmobile engine acquired in 1985 from Outboard Marine Corporation—for the Skycar aircraft, a four-person Personal Air Vehicle (PAV) designed for both Vertical Takeoff and Landing Vehicle (VTOL) and high-speed (350 mph) forward flight.

Moller developed a number of enhancements, including a new intake and porting arrangement, a new fuel-injection system, and a proprietary composite coating that reduces fuel consumption and cooling load.

The Rotapower engine is a multi-fuel compatible engine and in testing various fuels including gasoline, diesel, natural gas, and various types of alcohol, Moller found that the engine ran coolest and cleanest on ethanol. Moller has developed a variety of rotary engines from 1.5 to 300 horsepower, and now recommends that they all be run on ethanol when available.

By compounding this engine we are able to supercharge the engine and also add to the power output directly. The effective compression ratio can therefore be increased while the geometric compression ratio is reduced.

NASA has stated after an extensive experimental study of the rotary engine that a compound version of this engine should achieve specific fuel consumption to between .3 and .32 lb/hp hr. This would make the rotary competitive with a turbo-charged diesel, which is the gold standard of engine performance. The high octane rating of ethanol makes it possible to operate at effective compression ratios of 12+ to 1, which tends to offset the lower energy per pound of fuel from ethanol versus gasoline or diesel.

Typically with our Rotapower engine where the rotor is cooled by the incoming fuel/air charge, ethanol gives a 25% boost in power due to the lower charge temperature versus gasoline. Non-compounded versions of our Rotapower engine have fuel consumption about 10 to 15% higher than the best 4-stroke piston when using gasoline. Non-compounded ethanol fueled versions are approximately 5% poorer compared to the best 4-stroke piston engine using ethanol.

Testing is just beginning on our compound engine. We have seen a significant reduction in fuel consumption and exhaust temperature, but it is too early to quote this data without further tests and documentation, which should be available within the next 30 days.

—Paul Moller



I like to check the moller website every few months...I still dream of the day when I get to have a flying car like the Jetsons.

I believe Freedom Motors (the spin off for rotary development) is about to put into production an APU using a 530cc "rotapower" engine.

When looking at the low horsepower rating realize that this should be evaluated as an aircraft engine...it is designed for reliable constant operation in an aircraft afterall...and reliability is more important than max power in aircraft.


I wouldn't trust Moller as far as I could throw him.


well thats a problem with all startups which promise a lot to get investements

i hope it isn't the same like1999-2002 with those IT start ups, they had a lot of fantasy in shadowing and lying

i remember a east german startup which promised a algorithm that could compress video data 10 times better then divx ... :)
and so on ...

if 1 out of 3 companies will deliver what they have promised, then i think we have a future


Keep in mind that most general aviation engines turn the prop at < 2400rpm. At ~2400 RPM (or 2700, depending on the prop), the tips of the blades start going transonic -- which makes and aweful racket and ruins the efficiency of the prop. You might hear the "prop crack sound" on an old warbird or on some high performance singles -- that's what's happening.

Also, keep in mind that the smallest general aviation aircraft that you find at most airports is the Cessna 150. The smallest engine that you can find in one of those is just over 100hp. And, it has to deliver that 100hp at 2300RPM or so.

Some airplane engines (Rotax, auto conversions, etc) use a reduction drive (gearbox, giant belt with pulleys, etc) - so that the prop can spin around 2k RPMs, and the engine can spin at 4500+ (depending on the ratio you picked when you bought the gearbox). But, that's a heavy piece that bolts onto the engine block, and opens up a wholenother dimension of engineering tradeoffs.

So, in short, a 22hp engine that runs at 4500rpm isn't appropriate for the kind of aircraft that I've flown. Maybe it would be appropriate for a UAV (less HP required, and a smaller-diameter prop that can spin at more rpms without the tip going transonic (the speed of the tip of the prop is rpm*pi*r^2). But, it would not be appropriate for anything that could haul my chubby self off of the runway.


Your formula's wrong: v(fps) = 2 * pi * r * RPM / 60.

4500 RPM would be perfect for a motor swinging a 40" fan.  The blade-tip speed would be only 785 ft/sec, well subsonic.

Roger Pham

Dr. Moller,

Unless you know something that the rest of the world don't, including Madza, NSU Wankel, Cummin, Detroit, etc...the Wankel rotary has been tested and retested ad nauseum, and has been repeatedly shown to be far inferior to a piston gasoline engine in term of thermal efficiency, combustion efficiency, and exhaust emission. This is due to the wedge-shape combustion chamber that makes for less efficient combustion, and large combustion chamber surface area that increases heat loss.
The Mazda Rx7 and Rx8 are cases in point, which show horrible fuel efficiency, worse than the much heavier and far more powerful Corvette. Reliability record for the Rx7 has not been good, either. The Mazda 13-B engine when adapted for aviation use has not shown any efficiency advantage nor any gain in power-to-weight ratio over the antiquated Lycoming or Continental aircraft engines, except for cost, when the 13B came from used Rx7 cars in Japan that must have its engines replaced by law after 30,000 miles.

How can Dr. Moller say that a turbocharged otto-cycle engine with effective compression ratio of 12 (geometric ratio even less) can compare efficiency-wise with a turbocharged diesel engine with compression ratio of >18 and expect people to believe it? If this is true, Dr. Moller, the whole world will start driving your "Rotapower" rotary engines in a heart beat.


Totally agree with Roger.

However, for a long time I believe that rotary engine could find it niche as range extender (series hybrid) in PHEV. Since most of the driving such hybrid will do in pure electric mode, range extender IC engine should be as light as possible. The lightest IC engine is rotary. Higher fuel consumption will be trade-off (about 10% higher then piston engine), but lower electricity consumption in electric mode due to lighter IC engine should offset it overall. Shorter engine life also shouldn’t be a problem either, since the engine is supposed to work only occasionally.


My bad about the formula... That was dumb!

Never seen a 40" prop on a C150, though -- they're usually around 69 inches or so.

Roger Pham

Good point, Andrey,

A piston engine will need at least 3-4 cylinders to achieve sufficient smoothness in 4-stroke operation, due to the reciprocating components. Meanwhile, a single-rotor Wankel rotary behaves like a 3-4 cylinder piston engine in term of torsional vibration characteristic, but translational mode of vibration is nearly nil due to the continous rotation instead of reciprocation. The higher power-to-weight ratio is also very nice.

Best Wishes, Dr. Moller.

Harvey D.


I fully agree with you. The ICE driven on- board generator (and the vehicle body and accessories) should be as light as possible to maximize the PHEV electric mode range.

Assuming that the PHEV range extender ICE driven generator would operate about 15% of the time, a 30,000 miles generator endurance would last about 200 000 total miles. That should be sufficient for most private vehicles.

I read some times ago, that an Indian firm has produced a very light weight (24 lbs.) 24-HP diesel that could compete with the weight/power ratio of the rotaries. Any more on this motor? Is it another fantasy?

Harvey D.


I fully agree with you. The ICE driven on- board generator (and the vehicle body and accessories) should be as light as possible to maximize the PHEV electric mode range.

Assuming that the PHEV range extender ICE driven generator would operate about 15% of the time, a 30,000 miles generator endurance would last about 200 000 total miles. That should be sufficient for most private vehicles.

I read some times ago, that an Indian firm has produced a very light weight (24 lbs.) 24-HP diesel that could compete with the weight/power ratio of the rotaries. Any more on this motor? Is it another fantasy?


Luke, I believe the Skycar would have 8 150hp engines mounted in pairs in 4 ducts not a single 22hp motor. or am i missing something from your post?


The idea of blending acetone into ethanol for fuel here in Phoenix Arizona in the summer just isn't going to happen. Why? Ethanol is volatile enough by itself, but the boiling point of acetone is in the vicinity of 136 degrees F IIRC, lower than the temperature of hot pavement here 6 months out of the year.

But, that really isn't the problem. First, the tales of longevity problems for the Wankel engine are new to me; this is the only time I've heard of them. Maybe I'm just naive.

I have done some looking in regard to aircraft engines. If you think Lycoming or Continental produce bulletproof engines, better think again. As mentioned above, producing maximum horsepower at relatively low rpms is not a feat to be taken lightly. And as you take a look around the general aviation world, you start learning of lots of Lycoming and Continental piston engines that ate an exhaust valve, in spite of careful operator attention.

A Wankel will NEVER lose an exhaust valve - they don't exist!

I can't see that there would be any more difficulty with a Wankel than a piston engine.

It also seems that the new Renesis engines from Mazda work harder on recirculating the unburned HC from the exhaust. Yep, the combustion chamber shape of a Wankel is extraordinarily wide and flat, and causes quench problems with inherent inefficiencies. But I have heard that the new Mazda engines meliorate this to a large degree. Exactly how that is done is something I don't know. But it sounds like there's still some hope for the Wankel, even if Moller can't bring it about himself.


The rotary engine may well be the norm in most new vehicles within the next decade. Their positive characteristics outweigh their short comings. There are dozens of companies all over the planet currently testing rotary engines for hybrids. Because rotary engines have separate chambers for intake and combustion, they are ideal for burning hydrogen (ie: http://msnbc.msn.com/id/6343414). Particularly I have interest in the companies now looking at generating hydrogen while running on gas. In effect, a full sized car may may achieve 50+mpg via a single engine and 3 tanks - gas, hydrogen and water. Simply cannot do this with pistons.

With respect to the negative statements about Dr. Moller's efforts ... please take another look. I've been present for 3 of the test flights of the old M400 prototype. It flies rather well and that was with the smaller engines. I have no doubt there will be Moller skycars in the air over major cities within the next 5 years. Rather cool to be running 8 x 1060cc rotary engines at 7,000rpm (~1240hp) all directly mounted to a metal frame without rubber bushings and there was no vibration. These engines are showing a MTBF at a projected 8K-10K hrs (equivilant to 200K-300K miles in a car).



These new engines from China produce 22 hp and only weigh about 120 lbs. I emailed the company and their cheap.



Brett Gastelum


Is there any way a 20 year old would be able to get his hands on a rotary that is environmentally safe? I see you all are talking about ethanol rotaries and hybrids. Can someone please email me some info on how someone like me, a college student, would be able to afford one of these. I love my car, but i don't intend of giving it up any time soon. I just want something that can keep the power and be good for the environment at the same time.

Brett Gastelum

I'm sorry, intend on*

Henry Gibson

Serious consideration should be given to the APT OPOC engine. If electricity is involved then Capstone turbines with air bearings could be considered. MITI has tested a small jet propulsion engine with air bearings. There was a forgotten experimental military jet with a fan in each wing and one in the nose. The fans were driven by hot gas diverted from the main jet engine. The plane was destroyed by sucking in a cable during a lift test. ..HG..

John Burns

Eight year later and nothing. Andrey's post was good.

Electric motor traction is infinitely superior to direct internal combustion engine traction. It has max torque at zero revs. No power sapping, space consuming, heavy, complex transmission is needed. Decoupling the power source, the IC generator, and the electric traction motors, gives great benefits. Having a constant speed running Wankel engine generator provides electricity for the electric motors, up until now it was not feasible as piston engines were so inefficient.

Matters are now better with more efficient electric motors, generators and now, and only "just" some IC engines. The Chevy Volt's piston engine cannot provide efficient power via the generator having to drive the car directly at some speeds. The problem is the engine. But things are improving to the point a full electric transmission is possible.

Using supercapacitors 90% plus of kinetic braking energy can be reclaimed and then given back to accelerate improving efficiency. The generator Wankel engine can be run at a maximum, efficient, constant speed & load at its most efficient "sweet spot". The Wankel engine generators can be much smaller than a direct drive engine using a transmission. The Wrightbus for London has a 7 litre diesel engine but a 2.5 litre engine when the same bus is used as a hybrid - a third of the size. And this 2.4 engine will be running at maximum efficiency for most of the time, while the 7 litre engine will not.

A hybrid runs on the concept of an energy buffer, the battery and supercapacitors. Batteries consume about 33% of the kinetic braking energy while supercapacitors can reclaim 90% plus. Eliminating the mechanical transmission and having an electric transmission with the engine/generator providing electricity direct for the electric traction motors, without a battery (energy) buffer, is now feasible because of more efficient electric motors, generators and Wankel engines running at constant efficient speeds. The small, light, quiet and smooth Wankel engine now comes into it own in this application.

So, making very light and small Wankel engines more efficient, running at an efficient constant speed is the answer.

A piston engine and transmission plus the weight of a full tank of fuel (total car weight) is getting close to the weight of an EV and Lithium batteries and its total car weight for the same energy storage. Recall that 80% of the energy in a tank of fuel is wasted, so the energy density then is not great at all in that tank of fuel. A set of batteries will give off 90% plus of the stored energy. So, total EV vehicle weight with batteries and a normal vehicle with a full tank of fuel, can be about the same, but the EV will store more "useful" energy. And low slung batteries in car give a lower centre of gravity improving road holding.

Using a Wankel generator engine running at a constant speed and load the efficiency rises dramatically offsetting any state change losses. Only 66% of the energy in the tank is wasted while 80% is lost with current direct drive setups. Plus there are all the other gains.

Laser ignition, still under R&D but working, and direct injection should solve many of the Wankel's problems. A lean burn can be achieved with laser ignition. Running a Wankel at a constant speed will improve efficiency for sure and give the engine its niche at last as a range extender.

The BMW i3 is showing the way with direct drive electric traction motors, but the inefficient two cylinder motorcycle engine used as a range extender needs replacing ASAP with a small, light Wankel engine. The motorcycle engine was not designed for generator use, as is the case with the Chevy Volt's engine.

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