Extreme Power Free-Piston Engine
6.0 hp/lb - Highest efficiency - Small & Robust
The pistons of free-piston engines transfer the combustion pressure directly into hydraulic pressure, without a crankshaft mechanism. They are therefore inherently very simple, robust, light and efficient. Their major drawback is the insufficient control of the piston velocity and compression ratio, determined by the combustion and hydraulic forces and the mass of the piston (spring/mass system).
The new compression ignition, 2-stroke, hydraulic free-piston engine consists of a cylinder with two opposing pistons, an exhaust-gas-driven impulse charger, and an ultra-high pressure peripheral fuel injection system. Additional new sub-concepts improve the capabilities of the free-piston engine significantly.
The pistons have a hydraulic section with two opposing faces, variably supplied with varying hydraulic pressure by high speed 2-way valves with electro-hydraulic pilots during each stroke. Velocity and compression ratio are independently controlled by varying the size and timing of the hydraulic piston force. At the end of the stroke, the pistons control the opening and closing of the inlet and outlet ports, achieving uni-flow scavenging with low turbulence and highest efficiency.
The combustion gases expand further in the impulse charger, providing highly pressurized intake air (60 psi) for increasing the amount of fuel efficiently combusted, and pressurized fluid for storing hydraulic energy in the accumulator. The high compression ratio (35:1) increases the efficiency. The high thermal and mechanical efficiency reduces the energy the heat-sensitive parts are exposed to. Unlike valve controlled engines, the critical exhaust ports are here only briefly exposed to cooler combustion gases at the end of the stroke. The combustion chamber surfaces are of steel and cooled by the fuel in the channels in the combustion ring and the air flow during scavenging.
The pump of the fuel system transfers hydraulic pressure directly into ultra-high pressurized fuel (50,000 psi) and injects it through a large number (20) of micro-slots in the combustion ring directly into the combustion chamber. The very high atomization, minimal impingement of fuel, low air turbulence, and compact chamber (s/b >3) result in very high combustion efficiency and low emissions and heat losses.
Without a crank mechanism, the piston forces are not limited by allowable bearing loads and create no side forces between piston and cylinder. Nearly unlimited combustion pressures and higher temperatures can be applied since lubrication and piston rings are not required.
Very high operating pressure (b mep 650 psi), thermal efficiency (65%, with exhaust energy recuperation), mechanical efficiency (96%), small size, and simple mechanics provide drastically improved fuel efficiency (0.23 lb/hp•h) and power to weight ratios at reduced costs.
Sample: Helicopter engine (Titanium), 3-cyl., 1,970 hp, weight 326 lbs., specific weight 6 hp/lb, specific power 5.9 hp/cu in, size 38” x 25” x 17”. Compared to turboshaft engines, 45 % lower fuel consumption and 35% less weight. Best crankshaft engines have 2.96 hp/cu in, b mep 464 psi (Mercedes) and 0.243 lb/hp•h (Lund University/Scania, Sweden)
U.S Patent (2), International patents, Patents pending.