Like many of these alternative architectures, the OPOC engine can run on a variety of fuels including both gasoline and diesel as well as biofuels. Hofbauer explains that the use of two pistons per cylinder allows the pistons to move only half the distance for the same compression ratio so that the engine can run twice as fast. The OPOC engine operates on a two-stroke cycle, with each piston exposing only the intake or exhaust ports, allowing better management of which ports are open by timing each piston. The pairs of pistons oscillate back and forth with a common combustion chamber between them. Within each cylinder are two pistons that are linked to a common crankshaft. Ecomotors has developed a modular configuration with each module consisting of two cylinders. The primary claimed advantage of the OPOC architecture is high power density and fuel efficiency improvements of 50 percent over current spark-ignition engines. Ecomotors includes numerous veteran auto-industry executives and engineers, including Don Runkle of General Motors and Peter Hofbauer, formerly of Volkswagen. The opposed-piston opposed-cylinder (OPOC) architecture has drawn considerable attention recently with the emergence of a new company called Ecomotors. However, newer configurations and the ability to use alternative fuels have revived interest, especially for range-extender applications where constant speed operation and low noise (due to the continuous external combustion) are beneficial.
Until recently, Stirling engines were mainly used for stationary applications-in part because they were not suitable for typical transient applications where the power delivery varied significantly over time.
As the air cools, both pistons move back to their original positions, and the process repeats. As the heated air continues to expand, it displaces the power piston, which drives a crankshaft that produces rotational torque. The air in one chamber is heated via heat transfer through the cylinder wall pushing back the displacer piston, which is linked to a second power piston in the expansion chamber. Like the Ecomotors OPOC and the Scuderi, pairs of pistons operate together to provide the complete cycle.
The heat source-which could be almost anything, including combustion-is external to the engine. In 1816, Scottish inventor Robert Stirling conceived of the closed-cycle engine with the working fluid (in this case, air) remaining contained within the device. However, that requires moving air and fuel in and exhaust gases out of the combustion chamber, all of which adds complexity and reduces efficiency. Each of the alternative engine architectures discussed here have one major feature in common with the standard piston engines that have dominated the automobile for more than a century: Fuel is burned inside a chamber to convert chemical energy into mechanical energy for propulsion.