Under the Skin: Why Opposed Piston Engines Are Coming Back to the Roads Car News

Teaching an old dog new tricks isn’t supposed to be possible, but American company Achates Power is clearly on to something with its revamped version of an early combustion engine concept: the opposing piston engine, or OPE in abbreviated.

Instead of a single crankshaft driven by reciprocating pistons in their own bores, an OPE has two crankshafts connected by opposing gears and two pistons in each bore. Fuel is injected between the two pistons as they come together and the ignition pushes the pistons out of the way, thus driving the crankshafts.

Achates offers several versions of its OPE, diesel and gasoline, designed for everything from everyday cars to military vehicles.

The gasoline version, which is currently being tested in a Ford F-150 pickup truck, is a 2.7-liter three-cylinder unit developing 270 hp and 480 lb-ft of torque. Despite the combustion of gasoline, it is a compression ignition gasoline engine (GCI), which means that it ignites the fuel and air by squeezing it hard enough to generate heat, and this heat, along with the residual hot exhaust gases deliberately left trapped in the cylinders, ignites the fuel. As in a conventional diesel engine, there are no spark plugs and no electric ignition system. The fuel is injected directly not at the top of the two pistons but at a tangent between them.

Air enters the engine through the cylinder ports and the exhaust outlets through another set of ports in the same manner, typical of a two-stroke engine. Air is supplied to the cylinders by the combination of a compressor and a variable geometry turbocharger. The pneumatic system not only fills the cylinders with air, but also discharges the exhaust gases in a precisely controlled manner. This approach means that the engine does not act so much as a pump to push in and out gases, reducing the pumping losses of power and efficiency that are normally experienced by combustion engines.

The advantages are low emissions (thanks to the GCI) and the fact that the engine is simpler and cheaper to manufacture than a conventional counterpart, without cylinder heads or valve trains.

The loss of cylinder heads is a factor that reduces the area relative to the volume of the engine, removing less heat, so that more of the combustion energy is transformed into mechanical energy (high thermal efficiency). In fact, when tested, the efficiency of the GCI version of the engine came close to the low CO2 levels of the diesel.

Two-stroke engines are traditionally lubricated by mixing oil with gasoline, which by modern standards is an emissions nightmare called total loss lubrication, but Achates’ OPE is lubricated separately. , like any other car engine, so this is not a problem. The beauty of the two-stroke cycle is that it is much more energy dense than the four-stroke equivalent, as it pulls twice as often (every two strokes instead of every four), so over work at the same time.

OPEs have been around forever, powering ships, tanks, and railroad locomotives since around 1900. Achates claims their OPEs are 30-50% more efficient than equivalent gasoline and diesel engines and 10% cheaper to manufacture than a gasoline and diesel engine. Supercharged V6 doing the same job. The company plans to put a 1,000 hp advanced combat engine (ACE) diesel into production in 2024.

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