Extending a Wankel Future on Hydrogen Fuel
Proven in thousands of military drone aircraft, the lightweight and power-dense Wankel rotary is aiming for a future as a zero-emission range extender.
Could the Wankel rotary engine have a major future as a hydrogen-fueled, zero-emissions range-extender engine? Alternative fuel cells are still under development and there are technical hurdles and production costs to overcome. Many experts have pegged reciprocating engines as generally too heavy and complex for range-extender applications. Furthermore, hydrogen fuel – necessary for zero emissions – presents many difficulties when used in reciprocating engines. These issues are evaded in the rotary engine where the cool intake chamber is remote from the combustion zone.
“The overall objective is to create a range-extender power unit that exceeds the efficiency of the best fuel cells; with lower weight, higher TBO [time between overhauls] and at a fraction of the manufacturing cost,” said veteran Wankel development engineer David Garside. He expects that after 2030, the majority of the world’s larger electric vehicles will be fitted with range extenders of this type. The proposed basic technology for the rotary has been used extensively for many years in engines powering military UAVs (unmanned aerial vehicles, or drones), but further key technical advances now are incorporated.
Garside has been involved with rotary-engine R&D since 1964, initially working at Rolls-Royce. He notes the design can make a major contribution as a hydrogen-fueled range extender. Having developed successful rotaries for Norton motorcycles and UAVs, Garside, now formally retired, has continued his involvement in rotary engine technologies. The Norton and UAV engines used air to cool the rotor, in place of oil-cooling systems used in automotive applications. Particularly in the UAV engines, this resulted in some wet oil mist being emitted, making them unsuitable for ground use.
In 2008, Garside patented a self-pressurized air-rotor cooling system (SPARCS) which eliminated that problem. “The internal cavity of the rotor becomes part of a closed cooling system” he explained. “A small engine-shaft-mounted centrifugal fan circulates the air at high speed through the rotor, where heat is gathered and through an air/water heat exchanger where it is rejected. The advantage is that the circulating gas becomes pressurized to the average pressure in the engine working chambers via the very small two-way leakage of gases past the rotor side seals.”
In an engine at wide-open throttle, the cooling system is pressurized and hence densified, to about five to six bar (72.5 psi - 87psi), Garside noted. “Since the rate of heat transfer is proportional to gas density to the power of 0.8, we obtain a gain of more than two or three in the rate of rotor cooling, meaning we have a large margin of cooling of the rotor in UAV applications,” he said. “Furthermore, there is now zero loss of wet oil particles out of the sealed system.”
Garside noted that this technology is key to the air-cooled-rotor Wankel’s future successful application as a hydrogen-fueled range-extender engine, especially when compared to rotary designs using oil-cooled rotors. He lists these as more compact geometry, higher mechanical efficiency, reduced size, weight and production cost, improved combustion efficiency and hence overall brake thermal efficiency (BTE).
This fits well with the engine characteristics that Garside has identified as necessary in providing a lightweight, low-cost range-extender engine with zero NOx and negligible HC emissions. All sizes of the engine use just a single rotor, which only ever operates at a single load and single speed. Provision of a weak (50% excess air) hydrogen fuel /air mixture results in both zero NOx and excellent thermal efficiency.
A single-rotor design with a larger chamber size has fewer components and reduced heat losses and gas leakage, contributing to the high BTE. Such an engine still has zero radial vibrations and the same instantaneous torque vibrations as a 3-cylinder reciprocating engine. It can be rigidly mounted, saving cost and space, and is ideal for driving an integrated electrical generator. “A single-rotor Wankel’s large-diameter output shaft allows the generator rotor to be compactly cantilever-mounted on the engine shaft without additional bearings,” Garside detailed.
Since speed and load remain constant in operation, the quantity and timing of hydrogen injected per cycle also remain constant, permitting the use of a rotary valve operating at half engine speed to control the supply of hydrogen gas to the engine. The valve opens precisely as the engine air intake port closes, with injection operating at about three bar (43.5 psi) and completed within approximately the first 15% of the succeeding compression stroke. The valve inlet is fed hydrogen direct from the supply tank with no requirement for an additional pressure pump.
Garside’s plan is to develop a modular series of range-extender engines sizes aimed at different vehicle sectors, starting with a 225-cc engine with an initial test power rating of 12kW (16 hp) at 5,000 rpm, based on a 30-kW UAV engine with a core engine weight of 10 kg (22 lb). Applications would be urban taxis and smaller cargo vans.
A 650-cc variant is planned for urban buses, local distribution vehicles, garbage trucks and off-highway applications. Dry weight is approximately 24 kg (53 lb) and it would produce 34 kW (46 hp) at 4,000rpm. A 2.0-L engine with an output of about 93 kW (125 hp) also is proposed for trucks with one, two, or three engines based on vehicle size.
Even higher swept volumes may be practical, Garside suggested, and that additional power could be extracted from this engine by using a turbocompound system, harnessing exhaust gases to drive a turbine which in turn would drive a high-speed generator. “The exhaust port opens much faster than a [conventional IC engine’s] valve lifts and all the exhaust gases would be exiting from a single port, not multiple ports,” explains Garside. “Hence the turbine could be fitted very close to the exhausting chamber and collect some of the blowdown energy, as well as the expansion energy.”
Roller bearings are used for both the rotor and main shaft and lubrication is by total loss. The quantities of oil involved are extremely small because the total areas requiring lubrication are far smaller than in a reciprocating engine and experience no violent accelerations or direction changes. Rotor apex seals are made from extremely hard, low-friction Si3N4 material. The engine requires neither filter nor oil changes, reducing engine servicing to infrequent sparkplug changes.