For more than a century spark-ignition and compression-ignition piston engines have powered nearly all cars and trucks. Even today’s hybrid vehicles and the new range-extenders such as Chevrolet’s Volt use small piston engines to generate electricity when they need to top off their batteries or boost performance on hills and highways.
But if a high-risk research project now under way at Michigan State University (MSU) succeeds, future plug-in hybrid vehicles could get extra power from a radically different kind of engine, one that uses shock waves rather than metal pistons to compress air in its fuel-efficient combustion cycle.
The wave disk engine, which is the size of a cooking pot, is mechanically simpler than conventional piston engines, with their many reciprocating and other components. These advantages mean that the compact unit could propel “a plug-in, series hybrid vehicle as much as five times farther on the same amount of fuel,” says its co-inventor, Norbert Müller, an Associate Professor of mechanical engineering at MSU.
“Our machine is smaller and lighter than a standard internal-combustion engine,” Müller said. “It’s also cheaper to manufacture because it’s all you need to build." He claims combining a wave disk engine with an electric generator would create a powertrain fundamentally capable of powering a full-size utility vehicle.
Right now, Müller's team is bench-testing a prototype wave disk generator in their East Lansing lab. The researchers are more than a year into a two-year, $2.5-million R&D program that the U.S. Department of Energy’s Advanced Research Projects Agency–Energy (ARPA-e) funded to spur automotive innovation. The project aims to produce a working 25-kW (33 hp) wave disk engine.
Müller expects the energy conversion efficiency of his first machine to be only about 30%, which trails the 45% mark set by advanced diesels. He believes that optimized versions will eventually have efficiencies as high as 65%.
Although the wave disk engine has a rotary configuration, it is not a gas turbine nor is it a rotary Wankel of the kind currently used by Mazda. And even though the new power plant may turn out to be revolutionary, the basic concept is not at all new. Its mechanical family, in fact, boasts a history that is almost as long as that of piston engines, says the wave disk’s other co-inventor, Janusz Piechna, an Associate Professor at the Warsaw University of Technology in Poland.
Engineers started studying what are today also known as pressure wave machines, pressure exchangers, or wave rotors as early as 1906, according to Piechna. This class of devices are distinguished by their reliance on unsteady, intermittent gas flows such as shock waves to transfer energy directly between a high-energy working fluid and a low-energy one. Rotary pressure exchangers have, for example, been used as superchargers on Ferraris and other high-end sports cars for decades. Examples include the Comprex supercharger originally offered by Brown Boveri (now ABB).
Unlike most previous pressure-exchange devices, the wave disk engine has a radial-flow layout rather than an axial-flow configuration. To visualize it, think of an enclosed, turbine wheel-like disk that is composed of many curved compression-decompression channels that extend radially out from the axis. As the disk spins, ports at the channel ends open and close to allow gas to flow in and out.
Essentially, when fuel enters a channel (from the center) and is ignited, the hot combustion gas acts as a fast-moving, lightweight piston that compresses the gas as it travels down the channel. Subsequent reflections of the shock waves off the walls further compress the air, which is then released at precisely the right moment from the previously closed-off channel via an end port for the expansion stage of the power cycle.
The force of the pressurized gas on the curved channel and that of the escaping, tangentially directed gas jet spin the wave disk rotor, which enables the next set of channels to engage.
Theorists have long recognized the potential of exploiting unsteady flow and wave phenomena in so-called intermittent pressure-gain combustion systems to provide improved performance. But large-amplitude waves propagating in nonhomogeneous compressible fluids behave in a nonlinear fashion, which requires detailed numerical calculations to predict.
Until the advent of modern digital computers, says Müller, such computation was often too costly, laborious, or imprecise to pursue. But high-fidelity simulation carried out at MSU and elsewhere is guiding the shaping of the geometries and timing of the wave disk system to extract optimal performance.
The new system “combines the advantages of the confined combustion realized in a piston engine with the high power density and low maintenance gas turbines are known for,” Müller explained. It also runs on the most efficient, practical thermodynamic cycle, the Humphrey cycle.
“Since it combines constant volume combustion and complete expansion in an ideal cycle, it provides higher efficiency than the Otto, Diesel, and Brayton (gas turbine) cycles,” he noted.
The MSU engineer has little doubt that if his team builds the wave disk engine just right, it could find its way into many real-world applications beyond just hybrid vehicles. “It’s just a matter of time and effort and imagination—and money, of course,” he said.