Modeling and Control of Hydraulic Linear and Free-Piston Engines.

Abstract

The EPA has developed a free-piston engine (FPE) and a hydraulic linear engine (HLE) for application as hydraulic power plants in a hydraulic hybrid vehicle. Both engines extract power from the piston motion using a linear hydraulic pump. This dissertation's objective is to compare HLE and FPE performance trends through modeling while developing the control tools necessary to enable reliable engine operation.

A physics-based engine model combines dynamics, thermodynamics, and hydraulics correlations to evaluate performance trends and assist with control development. Preliminary simulations show that asymmetric piston behavior causes variations in cylinder-to-cylinder HLE efficiency that necessitate cylinder balancing.

An adaptive control scheme estimates and adjusts for HLE cylinder performance discrepancies. A control-oriented model captures HLE behavior using an estimate of rotational kinetic energy sampled at the turnaround points. State feedback control ensures that the HLE tracks a set point and a recursive least squares algorithm estimates periodic differences in HLE response. An extremum seeking algorithm exploits the adaptive scheme to optimize injection timing of each cylinder individually.

Precise control of piston turnaround location is paramount to reliable FPE operation. Combining an energy balance and the Otto cycle, a control-oriented model implicitly describes FPE clearance height evolution. A linearization of the control-oriented model suggests open-loop unstable operating conditions at high load. State feedback using dynamic inversion stabilizes the FPE system. In order to constrain piston motion, a reference governor manages load changes. When implemented on the physics-based model with the feedback control law, the reference governor successfully enforces a position constraint of 0.5 mm.

Using the proposed control and modeling methods, a series of physics-based simulations explore HLE, FPE, and conventional engine performance. The primary difference in engine behavior is friction. While the FPE exhibits low frictional losses and the highest relative hydraulic conversion efficiency, it also suffers from a restricted power range compared to the HLE and the conventional engine due to engine speed limitations. The HLE has lower friction than the conventional engine at most operating conditions. However, inertial forces resulting from a large piston assembly mass increase HLE bearing loads and friction at high speeds.