Sandia National Laboratory led by Sandia researcher Peter Van Blarigan that has been testing physical components of free-piston engines which could be 50% efficient. This is about twice as good as current gas combustion engines and about as efficient as fuel cells. Fuel cells are too expensive. He is assembling a complete free-piston engine prototype, a project that he expects to complete within a year. There are two working ends instead of current single active end pistons. It would use two combustion chambers at the of a dumb-bell [see pictures]. Each movement would drive a double two stroke combustion process.
Potential advantages of the free-piston concept include:
Simple design with few moving parts, giving a compact engine with low maintenance costs and reduced frictional losses. The operational flexibility through the variable compression ratio allows operation optimisation for all operating conditions and multi-fuel operation. The free-piston engine is further well suited for HCCI (Homogenous Charge Compression Ignition) operation.
The main challenge for the free-piston engine is engine control, which can only be said to be fully solved for single piston hydraulic free-piston engines. Issues such as the influence of cycle-to-cycle variations in the combustion process and engine performance during transient operation in dual piston engines are topics that need further investigation.
25KW version of the FP3 free piston engine
Piston power: In an unconventional engine design, a rod with a piston at either end shuttles between two combustion chambers. Magnets at the center of the rod move past metal coils (orange) to create an electrical current. Credit: Peter Van Blarigan, Sandia National Laboratory
In conventional internal combustion engines, multiple pistons are connected via rods to a crankshaft that, via the transmission, drives the wheels. Free-piston engines do away with the crankshaft: the pistons aren’t connected to anything. Instead, two opposing pistons just shuttle back and forth inside a chamber. To generate electricity, the pistons could be equipped with rows of magnets that shuttle past metal coils to create an electrical current.
A paper at the American Chemical Society: Simulation of a Two-Stroke Free-Piston Engine for Electrical Power Generation by Qingfeng Li, Jin Xiao, and Zhen Huang [the quote below about free-piston linear alternator is from this paper which describes all acronyms]
The free-piston linear alternator (FPLA) is a newly emerged energy converter. It is small, has only one moving part, and is free of friction and noise, using a two opposed two-stroke cylinder as the driver, and each stroke is a power-generating process. It can be used as the power pack for hybrid electric vehicle (HEV). It is regarded as a further forward step in producing a new generation of ICE [Internal combustion engine – regular gas engines].
(1) During most of the piston strokes, the velocity of FPLA keeps constant. It reverses to the maximum values in the opposite direction just before the top dead center (TDC). This makes the acceleration at least 3 times larger than the traditional ICE (Internal combustion engine – regular gas engines). In addition, it avoids the longer residence time at TDC (top dead center), which seems a better way to reduce the peak temperature.
(2) The TDC of FPLA can change with the external loads as well as other parameters. It can make sure that the ignition occurs right around TDC. Under this way, there will be more energy used in the expansion process. Besides, using lean mixture to acquire a higher compression ratio can improve the indicated thermal efficiency effectively.
(3) The FPLA has the shorter combustion duration under a high compression ratio. The peak temperature and pressure are lower than the traditional ICE. This could reduce the temperature-dependent emissions. It is an environmentally friendly power source for the future.