Noise, excessive vibration, and relative inefficiency are drawbacks of the piston-based internal combustion engines (ICE) that power today’s lawn and garden equipment, such as leaf blowers and lawn trimmers.
Now MIT start-up, LiquidPiston has developed a rotary ICE that it says is significantly smaller, lighter, and quieter, as well as 20 percent more fuel-efficient than the ICEs used in many such small-engine devices.
“If you think of handheld tools — for example, a chain saw or hedge trimmer — after about a half hour you don’t want to use it anymore because your hand feels like it’s going to fall off,” says Alexander Shkolnik, president of LiquidPiston and co-inventor of the engine. “Our engine has no vibration at all and it’s a lot quieter. It should be a much nicer user experience all around.”
LiquidPiston’s 70cc engine, the X Mini, produces about 3.5hp at 10,000rpm; at around 2kg, it’s also about 30 percent smaller than the four-stroke, 50cc piston ICEs it aims to replace. When fully complete, Shkolnik says the X Mini could develop about 5hp at 15,000rpm, and weigh about 1.5kg.
The engine runs the novel high-efficiency hybrid cycle (HEHC) — developed by Shkolnik and his physicist father, Nikolay — that achieves combustion at constant volume and over-expansion for greater energy extraction. With only two moving parts, a rotor and shaft, and no poppet valves — commonly used in other four-stroke ICEs to control fuel intake — the engine also has reduced noise, vibration, and harshness characteristics.
The X Mini is essentially an upgrade in design and efficiency of the compact Wankel rotary engine, invented in the 1950s and used today in sports cars, boats, and some aircraft.
In the Wankel, a rounded triangle rotor spins in an eccentric orbit within an oval chamber, with each rotation producing three power strokes. In the X Mini, an oval rotor spins within a modified, rounded triangular housing.
“We’ve inverted everything about the traditional rotary engine, and now we can execute this new thermodynamic cycle [HEHC] and solve all the problems that were plaguing the traditional Wankel engine [for small-engine application]", Shkolnik says.
A Wankel engine uses a long combustion chamber (like a thin crescent moon), which contributes to poor fuel economy as the flame can’t reach trailing edges of the chamber and gets quenched by the chamber’s large surface area. The X Mini’s combustion chamber is rounder and fatter, so the flame burns over less surface area.
Air and fuel intake and gas exhaust in the X Mini occur through two ports in the rotor, opened or closed as the rotor revolves, removing the need for valves. Asymmetrical location of these ports slightly delays the exhaust process during expansion. This allows for HEHC’s over-expansion process — from the Atkinson thermodynamic cycle, used in some hybrid cars — where gas is expanded in the chamber until there’s no pressure, allowing the engine more time to extract energy from fuel.
This design also accommodates HEHC’s 'constant volume combustion' — from the Otto thermodynamic cycle, used in spark-ignition piston engines — where compressed gas is held in the chamber for an extended period, letting the air and fuel mix and ignite completely before expanding, resulting in increased expansion pressures and higher efficiency.
“It takes a long time to burn fuel in an engine,” Shkolnik says. “In most engines, by the time you’re burning fuel, you’re expanding gases, and you’re losing efficiency from the combustion process. We keep combusting while the rotor is at the top of the chamber and force combustion under those conditions. It’s much more efficient that way.”
Additionally, the X Mini has relocated the apex seals, leading to decreased oil consumption. In Wankels, apex seals join the edges of the triangular rotor, where they slide and move. Lubricating them requires supplying the air-fuel mixture with large amounts of oil that burns and leaks, increasing emissions and oil consumption. In the X Mini, however, these seals are located in the triangular-shaped housing that stays put.
“Now we can supply tiny amounts of oil through the stationary housing, exactly how much oil the seal needs, and you’re not burning any oil and you’re not losing any oil to the environment,” Shkolnik says.