A virtual-cam based control methodology for free-piston engines

Abstract

In conventional internal combustion engines, valves are opened and closed using a cam surface. The cam is kinematically related to the piston positions through the crankshaft and timing belt. In contrast, there is no crankshaft or kinematic cam surface in a free-piston engine to physically realize this mechanism. As a consequence, a free-piston engine has variable stroke lengths, which presents a challenge for active piston motion and precise stroke length control. For instance, the free-piston engine configuration is well suited for HCCI engines because the requirements for accurate ignition timing control are very flexible due to the fact that the piston motion is not restricted by crank-rod mechanism. However, accurate piston motion control is a challenge for HCCI free-piston engines [4]. This chapter presents a virtual-cam based approach to relate free-piston motion to actuated engine valve control within a clear and familiar intuitive physical context. The primary functionality of the virtual cam control framework is to create a repetitive index, which can be modified from cycle to cycle, for the exhaust/injection valves and spark timing similar to the function of physical cams in conventional engines. Since the cam is virtually created, it can be dynamically rebuilt to comply with cycle-to-cycle variations such as amount of the air/fuel supply, engine load and stroke length. This index rebuilding process is based on a cycle-to-cycle adaptive control method that uses the knowledge obtained from previous cycles to adjust the cam parameters. Experimental results are presented for a novel liquid-piston free-piston engine intended as a compact and efficient energy source for untethered power dense pneumatic systems such as untethered robots.