China has flown its first hydrogen-powered four-seat aircraft, the Liaoning Ruixiang RX4HE, on March 25. The airplane is somewhat unique in having an internal combustion engine (ICE) that runs on the liquid hydrogen used as fuel. Developed with the FAW (First Automobile Works), the engine displaces two liters and runs on the 4.5 kilograms (9.9 pounds) of highly-pressurized H2 carried on board.
This enables one hour endurance at a cruising speed of 180 kilometers per hour (112 mph). FAW claims 43-percent efficiency for the powertrain and an overall thermal efficiency “greater than 40 percent.” (The video shows the RX4E, no videos of the HE model yet available.)
According to Wikipedia, “China FAW Group Corp., Ltd. is a Chinese state-owned automobile manufacturer headquartered in Changchun, Jilin. Founded in 1953, it is currently the second largest of the “Big Four” state-owned car manufacturers of China, together with SAIC Motor, Dongfeng Motor Corporation and Changan Automobile.”
e-Flight Journal reports the engine is “turbocharged and specifically developed to use hydrogen as fuel potentially for automobile use as well.”
Green Car Congress reported in June, 2022 that the direct-injection engine can achieve a ‘peak power of 120 kW at 4400 rpm and a maximum torque of 340 Newton ·meters at 2,000 rpm.”
Combusting H2 directly in an engine allows lower-quality fuel to be used, a necessary requirement in fuel cells. Although the engine can achieve almost zero emissions, there is a residual nitrogen oxide (NOx) output of 20 parts per million (ppm).
Ruixiang began the project in 20 and has been working with multiple Chinese entities including the FAW auto company, Beijing Institute of Technology and a research institute. Ruixiang intends to use the demonstrator to further the research and application of hydrogen aviation by using hydrogen as the direct fuel source.
A paper by the group, “Development of a turbocharged direct-injection hydrogen engine to achieve clean, efficient, and high-power performance, appears in the September 15, 2022 issue of the journal Fuel.
The abstract explains part of why the direct injection technology appeals to its developers.
“Hydrogen, as clean and renewable energy, is an ideal fuel for internal combustion engines. The direct-injection (DI) hydrogen engine can offer large power with low cost and rely less on hydrogen purity. In this study, a 2.0L DI turbocharged hydrogen engine is implemented to achieve clean, efficient, and high-power performance. Peak power of 120 kW @ 4400 rpm and a maximum torque of 340 N·m @ 2000 rpm can be achieved with the matched turbocharger. Appropriate retarded injection can suppress abnormal combustion and broaden the dynamic boundaries. A maximum brake thermal efficiency (BTE) of 42.6% is obtained with the slightly lean excess air coefficient (λ) of 1.91 @ 2000 rpm and 40.4% BTE with the λ of 2.47 @ 3000 rpm. The high conversion efficiency of NOx emissions of over 99.5% is reached at low speeds (below 2000 rpm) and drops to 90% at 4400 rpm with the use of the NH3-SCR after-treatment system. The NOx emissions of approximately two-thirds of the whole working conditions can be reduced below 20 ppm. The optimized DI hydrogen engine can achieve large power (Brake mean effective pressure = 17 bar), high efficiency (Brake thermal efficiency = 42.1%), and near-zero emissions (NOx < 20 ppm) simultaneously.”
Achieving certification for the two-seat XRX1E in 2015, “Ruixiang has been developing the four-seat full electric RX4E based on the certification and operation experience of RX1E since 2017. RX4E made the first flight in October 2021 and is currently undergoing CAAC type certification under Part 23 normal category.”
With options to use battery power, hydrogen fuel cells, hybrid power and now direct combustion H2 powered engines, future aircraft designers will have broad approaches to explore.