Rotary engines based on the Wankel principle were developed with two fundamentally different approaches to cooling the rotor. Mazda, Audi, Suzuki, Ingersoll-Rand and others used the oil-cooled rotor. It is an expensive, heavier and more complicated design, which achieved specific fuel consumption in the range of .55 to .6 lbs/hp hr. This is about 15% to 20% poorer than a typical four-stroke piston engine. The other approach taken by Outboard Marine Corporation (OMC), Fichtel Sachs and Norton was to use the incoming air-fuel mixture (“charge”) to cool the rotor. This design was much lighter, less expensive and through the use of roller bearings and very low rotor cooling losses achieved a specific fuel consumption between .45 and .5 lb/hp hr, which was close to the 4-stroke piston engine.

Historically all of the charge-cooled rotary engines that were developed used an arrangement where the fuel-air mixture passed through the rotor from one side to the other. This design cooled the rotor unevenly, which lowered engine rotor bearing life and increased friction between the rotor and the end housing.

In 1985 Moller International acquired the major rotary engine assets of OMC. OMC’s main product was the Johnson and Evinrude outboard engine and they were the world leader noted for their product’s reliability. OMC reportedly spent over $200 million between 1970 and 1985 developing a number of different rotary engine models including a 530cc displacement model that went into volume production and used in a snowmobile as a test product. Emissions requirements were one of the key motivators for this program. OMC believed that they would not be able to meet the proposed emissions standards proposed for the late 1980’s with their two-stroke engines and therefore chose to develop a lightweight low emission 4-stroke rotary engine. Fortunately for our Company the proposed emission standards were not enacted as originally planned and OMC stayed with their two-strokes, allowing Moller International to purchase their rotary engine technology.

Since acquiring the OMC charge-cooled rotary design, Moller International has spent ~$35 million on further development, testing and product integration efforts related to its rotary engine, preparing it for use in its aeronautical products as well as for use in a wide range of other suitable applications.

Rotary Engine Developments Chart



Freedom Rotapower Engine
Performance Advantages

• High power to weight ratio
-More than 2 HP per pound of installed weight in high-performance versions
-Compares with .6 HP/lb. to 1 HP/lb. for 2-strokes and .3 HP/lb. to .65 HP/lb.for 4-stroke pistons.

• High power to volume ratio
-(Power Output / Volume) > 100 HP per cubic foot of installed volume
-Compares with 36 HP/ft³ to 50 HP/ft³ for 2-strokes and 10 HP/ft³ to 20 HP/ft³
for 4-stroke piston engines.

• Few moving parts
-Moving Parts - only 2 for single rotor engine.
-Compares to 7 parts for 2-stroke and 25 parts for 4-stroke piston with same nstantaneous output torque.

• Solid fuel economy
-Specific Fuel Consumption < .45 lb./HP-hr ~ (stratified charge). Expect <.4 lb./HP-hr when both stratified charged and turbo-charged
-Compares to .65 lb./HP-hr for 2-strokes and ~ .4 lb/HP-hr for the best 4-stroke piston.

• Proven multi-fuel performer
-Demonstrated on gasoline, natural gas, alcohol and propane
-Spark-ignited diesel, kerosine and jet fuel

• Very low emissions levels
-See Emissions Performance

• Enhanced energy at exhaust
-Exhaust temperatures > 1500 °F
-Acts like a naturally occuring thermal reactor
-Ideal for turbocharge/co-generation applications

• Low vibration levels
-Hard mounted engine can be used as part of the structure

• Modular design
-Stacking of rotors easily extends range of available power


Freedom's Rotapower engine has all the inherent advantages of existing Wankel-type rotary engines. In addition, it improves the Wankel-type engine performance as follows:

Emissions and fuel consumption
Uses our proprietary low cost lean burn combustion process together with our patented combustion surface coating and pre-heated incoming fuel-air charge to greatly reduce both emissions and fuel consumption. This technology has the potential to eliminate both the lubrication and cooling systems.

Long engine life in a low-cost design
Uses a new proprietary rotor and rotor cooling technology to reduce rotor weight and related mechanical stresses throughout the engine. Uses new apex seal material and proprietary plasma sprayed internal coatings.

Maximum power with minimum weight and size
Uses our proprietary porting design and advanced component development to generate twice as much power for its weight as most four-stroke competitors.

Modular design allows low cost power growth
One- and two-rotor versions of the Rotapower 650 Series engine provide scalable power for applications with requirements from 75 to 150 horsepower.

Ability to operate on any fuel
Proprietary technology developed to lower emissions and fuel consumption also allows the use of any type fuel with modest changes in the fuel injection system for operation with diesel fuel.

While all of the above advantages make the rotapower engine attractive, the ultimate advantage of this engine is its ability to generate very low emissions.


The main advantages of the Wankel-type rotary engines are:

• Simple - with only two or three moving parts
- Low part count means less cost
- Few moving parts means less to wear out

• Powerful for their size
- Efficient packaging

• Uses the efficient four-stroke combustion process
- Solid fuel economy
- Low emissions

• Vibration-free
- Engine can be hard mounted
- Smooth operation at high RPM

• Reliable
- Reduces the cost of maintenance

The main historical disadvantage:
• Poor sealing: Mazda did have a sealing problem early in their product lifecycle. This was quickly and easily solved by going to a harder material for the apex seals.

• Poor emissions performance: The high surface to volume ratio of the combustion chamber can lead to more quenching and higher emissions. The Rotapower's charge cooled configuration increases the rotor face and incoming charge temperature which results in less quenching and better combustion.

• Poor fuel economy: In oil cooled rotary engines, fuel consumption suffers at low power due to oil sloshing losses. In the Rotapower, charge cooling eliminates this source of lost efficiency.


Fundamental hardware differences between a 4-stroke piston and a Wankel rotary
  • The Rotor replaces the piston engine's piston.
  • The Eccentric Shaft replaces the piston engine's crankshaft and connecting rods.
  • The Rotor Housing replaces the piston engine's cylinder.
  • Intake and Exhaust Ports in the housings replace valves, camshafts, rocker arms, springs, lifter rods, and timing belts.

4 - Stroke Piston vs. 4 - Stroke Rotary


Ground Applications
The Wankel rotary engine principle Rotapower engine is a perfect alternative to nearly all 2-stroke engine applications and many of the high power-to-weight applications using conventional 4-stroke piston engines today.