Melbourne-based Rotary Engine Development Agency (REDA) has developed a new type of balances, four-stroke rotary engine that has the potential to operate on a variety of fuels. (Image source: REDA)

New four-chamber rotary engine could supplant Wankel and piston engines for UAV applications

The Szorenyi rotary engine prototype uses a hinged rhombus rotor instead of the three-sided rotor found in traditional Wankel rotary engines.
The Wankel rotary engine has been an ideal choice for many owners and operators of small, propeller-driven aircraft. Compared to conventional piston engines, Wankel rotaries are small, lightweight, and has a high power-to-weight ratio. They’re nearly vibrationless, they can’t seize or knock, and they have fewer moving parts (to break). At this point, it’s difficult to improve on the Wankel design; that is, unless you’re considering changing the shape of the rotor… into a changing shape.

A new configuration of a rotary engine – the Szorenyi rotary engine – has been developed by the Melbourne-based Rotary Engine Development Agency (REDA). While the stator, or stationary part of the Szorenyi engine is similar to that of a Wankel engine, the geometric shape of the engine rotor is a rhombus, which deforms as it rotates inside the contour of the stator.

Szorenyi rotary engine cycle

This geometry translates to a rotary engine with four combustion chambers as opposed to a traditional Wankel rotary’s three. Each revolution of the crankshaft produces one revolution of the rotor and a complete engine cycle in each of the four chambers: or four power strokes. In contrast, the Wankel engine produces one power stroke per crankshaft revolution.

Wankel rotary engine cycle

A typical Wankel rotary engine uses a three-sided rotor to create cavities within the stator for a seamless intake, compression, ignition, and exhaust cycle. Point A marks one of the rotor’s three apices, Point B marks the eccentric shaft, and the white portion is the lobe of the eccentric shaft. (Image source: Y tambe)

According to REDA, each four-stroke Szorenyi rotary module is equivalent to an eight-cylinder reciprocating or opposed piston engine.

The Szorenyi engine is also more optimized for multi-rotor configuration than a Wankel rotary due to the use of peripheral ports when compared to the Wankel engine’s use of complex side ports. The ability to easily configure multi-rotor, four-stroke engines could result in rotary powerplants that generate power equivalent to 8-, 16-, or 24-cylinder reciprocating engines. Furthermore, the development of standardized modules could reduce the manufacturing and life-cycle maintenance costs.

Free to speed

Typically, Wankel engines are limited to a rotor speed of 3,000 revolutions per minute (rpm) because of the excessive crankshaft bending caused by the centrifugal forces of the eccentric rotor. The Szorenyi engine is not rev-limited in this regard, as it used a balanced rotor.

Higher potential rpm limits mean the Szorenyi engine has a higher power density than the Wankel engine, which could translate into greater aircraft range, endurance, and payload capacity. Furthermore, the Szorenyi engine has more space for internal cooling of the rotor and no need for a reduction gear in aircraft and unmanned aerial vehicles (UAVs) with large propellers.

According to the paper, the Szorenyi engine could be made to run on gasoline, aviation gasoline (avgas), butane, or hydrogen (as the inlet and exhaust ports are well separated).

REDA also noted that if a pre-compression phase was introduced, the engine could use diesel fuel – aligning with the U.S. military “one fuel” concept and making the engine a potential consideration for military applications.

Full details concerning the design and testing of REDA’s new engine, are available in the
SAE International Technical Paper, The Development of the Szorenyi Four-Chamber Rotary Engine.

An abridged version of The Development of the Szorenyi Four-Chamber Rotary Engine and other SAE Technical Papers concerning small aircraft and UAV engines are available in the latest book in SAE International’s So You Want to Design series, So You Want to Design Engines: UAV Propulsion Systems.

The book covers several UAV propulsion technologies such as traditional heavy-fuel engines, hybrid-electric architectures, distributed hydrogen-fueled fans, the aforementioned Szorenyi rotary engine, and experimental plasma propulsion – or dielectric barrier discharge.

Learn more


William Kucinski is content editor at SAE International, Aerospace Products Group in Warrendale, PA. Previously, he worked as a writer at the NASA Safety Center in Cleveland, OH, and was responsible for writing the agency’s System Failure Case Studies. His interests include literally anything that has to do with space, past and present military aircraft, and propulsion technology.

Contact him regarding any article or collaboration ideas by e-mail at
  Continue reading »