Sensitivity Analysis and Control Methodology for Linear Engine Alternator

Linear engine alternator (LEA) design optimization traditionally has been difficult because each independent variable alters the motion with respect to time, and therefore alters the engine and alternator response to other governing variables. An analogy is drawn to a conventional engine with a very light flywheel, where the rotational speed effectively is not constant. However, when springs are used in conjunction with an LEA, the motion becomes more consistent and more sinusoidal with increasing spring stiffness. This avoids some attractive features, such as variable compression ratio HCCI operation, but aids in reducing cycle-to-cycle variation for conventional combustion modes. To understand the cycle-to-cycle variations, we have developed a comprehensive model of an LEA with a 1kW target power in MATLAB^®/Simulink, and an LEA corresponding to that model has been operated in the laboratory. This MATLAB^®/Simulink numerical model has been used to examine the sensitivity of the LEA dynamics and performance parameters to changes in the design and operating inputs. The sensitivity analysis provides insight into the pathway for improving and optimizing the design, as well as an assessment of the effects of modeling assumptions on the reliability of predictions. A difficulty during the modeling is associated with the cycle-to-cycle energy balance for the LEA, and it is clear that this difficulty is reflected in real-world LEA control. If the alternator consumes more energy in a cycle than the engine provides, the system moves towards a stall. If the alternator consumes less energy, then the stroke, compression ratio and maximum translator velocity must rise steadily from cycle-to-cycle until efficiency losses curb the increase. The authors have recognized that the control of this energy balance in the model affects sensitivity analysis and must, therefore, mimic the real world intended control methodology. To understand the LEA behavior further, a control methodology was developed based on the basic feedback control systems in order to monitor the compression ratio of the single cylinder LEA system from cycle-to-cycle, with a view of keeping compression ratio substantially constant. Initially, the LEA system behavior was analyzed with and without the external controller, mainly to highlight the importance and need for an external control methodology. Further, two different control strategies were implemented and investigated. Finally, the cycle-to-cycle variations were studied as spring stiffness increased, by introducing combustion stochastics. With the proposed controller strategies and the addition of stiff springs, the cycle-to-cycle variations were reduced, and the LEA system operated steadily.