In the article “Research Needs for Future Internal Combustion Engines,” (Physics Today, November 2008, page 47), authors Dawn Manley, Andrew McIlroy, and Craig Taatjes examine some of the physics issues involved in improving the efficiency of the internal combustion engine. However, some alternative approaches may be more efficient yet.
For 20th-century automobiles, internal combustion engines had several advantages: They could produce power from a cold start and were reasonably efficient across a wide range of output power. For the 21st century, other, potentially more efficient heat engines, such as the Brayton gas-turbine cycle and the Stirling engine, are also worth noting. Hybrid cars eliminate most of the disadvantages of such heat engines by using electric motors for the primary drive function, so that the heat engine is needed only for the production of electrical power to charge the battery. The heat engine can thus be run under its most efficient conditions, not at a compromise setting to maximize torque.
A high-efficiency heat engine concept on which we have done considerable work at NASA’s John Glenn Research Center is the free-piston Stirling engine. This remarkable design eliminates the kinematic couplings and thus reduces the engine to only two moving parts, the displacer and power pistons; a linear alternator produces power directly as electricity. Small, light, and efficient, the Stirling engine—and not the internal combustion engine—may be the real future of the automobile engine.
