Turbine-Compound Free-Piston Linear Alternator Engine.

Abstract

The free-piston engine (FPE) was being used on stationary power plants and automobile test back in 1950’s. The advantages of the FPE are obtained mainly from the freely moving piston, with which a variable compression ratio can be easily achieved. This gives the possibility of high compression ratio with high efficiency and the flexibility of burning different fuels. With many alternative fuels, such as biofuels under development to replace the traditional gasoline or diesel fuel, the potential of the FPE is again becoming valuable. The primary goal of the present research is to develop a numerical model of the FPE that can be used to understand the conceptual design and operation. Until now, a model for the FPE was not available, so a model is built in Matlab/Simulink with many user-defined functions and algorithms. The second goal was to integrate the FPE with a linear alternator. Historically, the FPE extracted power solely through a power turbine. Many research groups have used the linear alternator with the FPE and have claimed high efficiency. This study focused on using both power extraction devices together, namely turbine-compound free-piston linear alternator (TCFPLA) engine. It is believed that the linear alternator as the secondary power output has the potential to increase the efficiency when combined with the turbine. The most special characteristic of the TCFPLA engine is its energy-recovering configuration. With the air box fully surrounding the combustion chamber, it absorbs most of the heat from the combustion chamber. This heat recovery process was proven in the study to be a great advantage on efficiency. Two important control parameters were defined, namely the bounce chamber mass and the injection position. These two parameters have to change with load for the best performance output. A 2D engine map is generated for various linear alternator output at each given fueling rate. The brake efficiency reached 50% at the mid to high load conditions with high alternator output. This makes the TCFPLA engine very competitive with the diesel engine.