Waste heat powers aircraft engines

Hot air piston engines exploit the thermal expansion of air to move pistons. In the process of converting the chemical energy in the fuel to mechanical energy, about 30 % of the fuel energy is lost in exhaust gases. In general, the heat generated is unutilised and thus wasted.

The EU-funded project WHEXPERS (Study and manufacturing of a wasted heat exchanger and a hot air piston engine recuperation system) aimed to make use of this waste heat. The technology developed has the added benefit of transferring heat from the exhaust gases to the hot pressurised air coming from and going to the piston engine.

The waste heat recovery system promises significant increases in engine efficiency that will reduce fuel consumption and pollutant carbon dioxide emissions. To be used in aircraft, the engine design has also been modified through the use of low-density materials, reduced friction and optimised piston assembly to maximise efficiency.

Hot air piston engines require superior sealing with reduced friction in the compressed air admission or intake system. Rotary valves control the flow of air. Modifications in rotary valve design following a preliminary test campaign have led to minimised pressure losses and target permeability.

The head cylinder design was also improved based on the results of friction and sealing tests, as was the base engine design. Next, the manufactured mechanical parts of a mock-up hot air piston engine were assembled as well as the data acquisition and control systems of the test rig.

Simulations were combined with mechanical studies to improve heat exchanger plates and the mechanical resistance of parts. Finally, the heat exchanger feed system was designed to ensure uniform flow distribution, minimised pressure drop and light weight. Performance analysis is ongoing.

The WHEXPERS project has advanced the state of the art of gas-gas heat recuperation. Innovations in materials and engineering designs are expected to lead to efficient aircraft engines with reduced fuel consumption and emissions for greener air transport.