Combustion Control of an Opposed-Piston Free-Piston Engine during the Cold Start Process

A free-piston engine is a high-efficiency energy converter that eliminates the mechanical crankshaft. Its unique structure brings some potential advantages, but also breeds a few technical difficulties, such as cold start. To solve this problem, this paper proposes a staged combustion control strategy based on cylinder wall temperature for an opposed-piston free-piston engine. The entire cold start process is divided into three stages, and a matching control method is formulated for the objectives of each stage. A simulation model that can reflect the cold start process was developed and verified by prototype test data. The performance of the control strategy at each stage was discussed in turn through simulation analysis. The results show that the initial ignition timing and fuel injection quantity are calibrated to 7 mm and 8.23 mg, and the in-cylinder pressure gradually rises to about 65 bar as expected, which indicates that the engine has excellent cold start characteristics under the designed control strategy.