The Future of Advanced Air Mobility

Russia Sees Hybrid Propulsion Potential in Updated Rotary Piston Engines

Russia’s aerospace and automotive industries are looking to give rotary piston engines a new lease on life as part of innovative propulsion systems for hybrid-electric aircraft. The move follows the decision of Japanese carmaker Mazda to switch to a piston engine to extend the range of its MX-30 sports utility vehicle.

It is now almost 20 years since Russia ceased production of rotary piston-powered aircraft and automobiles, and that was a full decade before Mazda introduced its RX-8 rotary car in 2012. Now the  Russian aerospace industry is tapping new technology to overcome that vehicle’s baseline Wankel engine, which included rapid wear to the rotor and its housing (the stator) and carbon residue buildup leading to loss of compression.

The Russian Foundation for Advanced Research Projects (known by its local acronym as FPI) has been funding research on a next-generation version of the Wankel engine for five years. In 2019 it awarded Baranov’s Central Institute for Aviation Motors (СIAM) a research and development contract for a technology demonstrator called the RPD-100T. The main purpose of this work was to assess the use of modern composite materials in compact turbocharged rotary-piston engines able to work in a wide range of ambient temperatures (from -83 degrees F to +126 degrees F).

During bench testing in climatic chambers and resource laboratories, the RPD-100T confirmed design targets including a weight-to-power ratio of 1.32 pounds per unit of horsepower (hp), specific fuel consumption of between 0.55 and 0.62 pounds/hp per flight hour, an operating ceiling limit of 33,000 feet, and 3,000 hours between overhauls. Rated at 100 hp for takeoff and 120 hp for emergency situations, the RPD-100T engine is intended for both piloted and autonomous aircraft, as well as wheeled or tracked ground vehicles, speedboats, and generator units.

An operable RPD-100T unit was on display at last July’s MAKS air show in Moscow, during which the Russian government’s ministry for trade and industry awarded CIAM a new four-year contract to develop a 150-hp version designated as the RPD-150T. Initially, this is envisaged as going into mass production as a local substitute for the Rotax 912/914/915 piston engine family that powers many light aircraft.

However, CIAM also now sees the potential to apply the technology to support hybrid-electric propulsion for new VTOL aircraft. The basis for this could be the EU-RPD-350T power unit, which incorporates an RPD-350T rotary piston engine in combination with an electric motor, to generate 350 hp at takeoff and 190 hp in cruise flight.

According to CIAM, the EU-RPD-350T, which weighs 463 pounds, has a parallel layout that means either the engine or the electric motor can drive the propellers, and that it would have sufficient redundancy for an aircraft to land in the event of either part of the powertrain failing. Working in partnership with the FPI, CIAM aims to have the new powertrain in production in 2024.

The Russian partners are looking to use new advanced materials to reduce wear and tear on the propulsion system. It is proposing to build the main components from a nano-structured, metal-ceramic composite containing titanium carbon nitride and silicon carbide.

Initially, the new rotary piston engine could run on existing aviation or automobile fuel. However, CIAM says it could be adapted to run on liquefied hydrogen for future applications.