|Basic design of a free-piston engine as generator (not the engine modeled). Click to enlarge. Source: Prof. Valeri Golovitchev, Chalmers
Researchers at Chalmers University of Technology, Sweden, are exploring the development of a two-stroke, free–piston compression-ignited engine employing uniflow scavenging as a genset in hybrid applications.
Free-piston engines (engines without a crank) are under consideration by a number of developers as a potential high-efficiency engine for use in hybrids. The EPA, for example, is exploring the use of a free-piston engine in its heavy-duty hydraulic hybrid system.
The Chalmers researchers, as described in a paper presented at FISITA 2006, have developed a numerical analysis for their free-piston engine. The engine under exploration consists of a block with a double-ended cylinder and four exhaust valves at each end. Air flow is supplied through two belts with twelve intake ports in each. The researchers used a diesel fuel model in their calculations.
Combustion at alternating cylinder ends drives the piston assembly back and forth, thereby driving permanent magnets fixed on the connecting rod back and forth through the coils of the linear generator. The engine uses and HCCI-like combustion mode—Partially Premixed Compression Ignition (PPCI)—combustion to deliver high efficiency and low emissions.
The researchers varied engine design and spray parameters such as fuel injection timings and spray included angles to optimize the engine operation in terms of indicated efficiency and exhaust mixture composition.
To select high efficiency low emissions operating conditions for Diesel and HCCI combustion modes the parametric map analysis has been employed. Based on the map analysis, the optimal one- and multiple-pulse injection timings and profiles and external EGR levels were found for the engine under study. Two-cycle free – piston engine modeling demonstrated nearly similar pressure vs CAD histories in each cycle indicating the engine stable operation.