Opposed-piston engines can be much more efficient than todays gas and diesel engines. With two pistons per cylinder, working in opposite reciprocating action, these engines do not need cylinder heads which are a major contributor to heat losses in conventional engines. Ports in the cylinder walls replace the complex poppet valves and friction-creating valve trains of conventional engines. The intake ports at one end of the cylinder and exhaust ports at the other are activated by the piston motion and enable efficient uniflow air scavenging.
Two-stroke combustion cycle: A two-stroke engine produces twice as many power strokes per revolution as its four-stroke equivalent. This advantage leads to smaller displacement engines for similar performance, and lower in-cylinder pressure to lower emissions compared to four-stroke conventional engines.
In the past, these advantages were balanced by some well-documented shortcomings of two-stroke engines, which limited their scope of use. High hydrocarbon emissions (due to carburetion and over-scavenging) and excessive oil consumption (due to oil-fuel mixing in spark-ignition engines and port oil ejection in compression ignition, direct fuel injection engines) are difficult issues to tackle in these type of engines.
To improve emissions and reduce fuel consumption, Achates Power has made dozens of changes to the original design, including altering the shape of the combustion chamber and improving fuel injection. But so far, its fuel-efficiency claims are based on data from a single-cylinder engine, extrapolated using computer simulations to estimate the performance of a multi-cylinder engine. (Multiple cylinders would be needed for most applications.) The design also hasn’t been tested under all the conditions the engine would see in ordinary driving, such as transitioning from acceleration to cruising or decelerating.
Johnson contends the differences between single-cylinder and multi-cylinder engines are well known and can be accurately modeled.
The U.S. Army gave Achates and partner AVL Powertrain Engineering, $4.9 million to build a complete multi-cylinder prototype engine that the Army hopes can be the basis for a range of applications, including powering tanks.
The Army picked Achates because of the potential of its engine to be smaller and more fuel efficient than its conventional engines, while being able to burn a range of different fuels, including jet fuel and diesel. Achates says that its engine, which could also be used in passenger vehicles, could improve the fuel efficiency of a diesel engine by 20 percent.