LiquidPiston's Hyper-Efficient Engine: Turning the Rotary Inside Out

A new spin on the internal combustion engine by startup LiquidPiston aims for a leap in efficiency from 20 percent to 50 percent.

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As automakers augment the reciprocating piston engine with hybrid systems and improved accessories, independent inventors are busily working to make huge improvements to the basic efficiency of the internal combustion engine. Novel designs are popping up at engineering expos everywhere, and the newest comes from Bloomfield, Conn.-based LiquidPiston. Its X1 engine is a simple machine with just three moving parts and thirteen major components, but it aims to raise thermal efficiency from the 20 percent of a normal gas engine to more than 50 percent, with drastic reductions in weight and size. How? By wasting much less energy during the course of an combustion cycle.

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Up to 80 percent of the energy in fossil fuels is thrown away normal engines through the heat and pressure of exhaust, or dumped to the atmosphere through the radiator. LiquidPiston's design attempt to capture all of that waste within a tiny package. "We stretched the performance curves in every direction to get much higher efficiency," said Alec Shkolnik, President and CEO of LiquidPiston, "We took the best parts of many different thermal cycles and combined them." The design is theoretically capable of 75 percent thermal efficiency, but the group is targeting 57 percent in real world applications, still a huge jump.

The basic idea is similar to a Wankel rotary, but turned on its head. Where the rotor holds the seals in a normal Wankel, the housing does that job in the X1 engine. This allows significant reduction in oil consumption over a regular rotary motor. Other enhancements include direct injection, a high compression ratio at 18:1, and a dramatic change to the geometry of the combustion chamber, which maintains a constant volume during ignition. This change means the air-fuel mixture auto-ignites like a diesel, and can be burned much longer than normal. The result is a more complete combustion ending in low emissions and very high chamber pressures. This high pressure is allowed to act on the rotor until it reaches nearly atmospheric pressures, so almost all the available energy is extracted before the exhaust is physically pushed out. Again, this is different than a normal internal combustion engine, which releases very energetic, high-pressure exhaust gas.

Some other slick features: Since the engine is designed to convert so much more heat energy into mechanical force, less heat has to be removed from the block, so there's actually no water cooling system. In cases where the engine is under load and needs to cool down, it can skip an fuel injection event and just suck in cool air, which is then heated by the block and gets exhausted. Another option is to inject water into the combustion chamber. This has three effects: cooling the engine, reducing NOx emissions, and converting some of the water to steam, which increases power.

The compact design of LiquidPiston's lab engine currently tips the scales at 80 lbs for the 40-hp model. It would weigh less than 50 lbs in production, the company claims, far less than a comparable 40-hp diesel that would tip the scale at around 400 lbs. LiquidPiston's current aim is to continue developing the engine with an eye on the sub-100 hp market—compressors, hybrid range-extenders, military applications, boat engines—and license the intellectual property to manufacturing customers. We love seeing plucky inventors like these to completely rethinking the gasoline engine.

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