Two-stroke engines have a high power-to-weight ratio but are dirty, inefficient, and short-lived. That could change with a new design by Alpha-Otto. Their RevForce engine brings one revolutionary change: a rotary exhaust valve.
This is a minor modification that solves the key shortcomings of conventional two-stroke engines. In a typical two-stroke engine, fuel and air are introduced and exhaust is expelled, usually at the same time. This overlap causes unburned fuel to escape through the exhaust, wasting fuel and adding to emissions. There is also the problem of lubrication—since the crankcase is used as an intake chamber, oil has to be added to the fuel, which is partly burned, causing further pollution and wear.
The rotary valve of Alpha-Otto is located slightly behind the conventional exhaust port. It only opens when required and does not allow the fuel/air mixture to escape too early. More to the point, it enables the crankcase to be closed and utilized as a lubricant, such as in a four-stroke engine, which lowers emissions and increases engine life.
This design does away with poppet valves and introduces electronically controlled timing of the exhaust valve, air compression, and fuel injection. This versatility enables the engine to switch in real-time to various fuels, such as natural gas, hydrogen, gasoline, and even biofuels. It is demonstrated to switch fuels in the middle of operation without any problem.
The RevForce engine is a 200-horsepower straight twin. In July 2025, Alpha-Otto will start a WeFunder campaign to transition the design to production. The first uses will be in fixed power generation, but the ultimate aim is to include transportation applications.
The RevForce design has the potential to revive an old idea and perhaps even end up in the engine bay of your next vehicle by addressing the fundamental weaknesses of the two-stroke engine and retaining its advantages.
