Development and Simulation of Pressure Reactive Pistons for Spark Ignition and Compression Ignition Engines

	Development and Simulation of Pressure Reactive Pistons for Spark Ignition and Compression Ignition Engines Researchers: Dennis Assanis Wooheum Cho Inyong Choi Andrew Ickes Jason Martz Ryan Nelson Jeff Sanko Scott Thompson John Brevick (Ford) Bruce Inwood (Federal Mogul) Sponsors: Department of Energy Ford Motor Company
Abstract: This project consisted of the design, development, and evaluation of a passively controlled, variable compression ratio piston. Experimental testing of protoype pistons was conducted, along with modeling of the piston behavior and simulation-based parametric studies.
Background: The pressure reactive piston assembly consists of a piston crown and a separate piston skirt, with a set of spring contained between them. The piston crown deflects in response to the cylinder pressure. As the piston crown deflects, the cylinder clearance volume increases, lowering the effective compression ratio and reducing peak cylinder pressure. Since the system is passively controlled by cylinder pressure, it can accomodate rapid changes in engine load. Advantages for Spark Ignition Engines: - Increase efficiency at part load operating conditions by operating at a higher effective compression ratio - Successfully avoid spark knock at full load conditions, even during rapid transients Advantages for Compression Ignition Engines: - Reduce NOx emissions by lowering cylinder temperatures through operation at reduced compression ratios under most load conditions - Maintain good cold-start performance by operating at increased compression ratios during start-up - Reduce Noise, Vibration, Harshness (NVH) by lowering cylinder pressures and cylinder pressure rise rate
Achievements: Three prototype pistons were manufactured: two pistons for a spark ignition engine and one piston for a compression ignition engine. All three were tested experimentally in single cylinder engines. Models of the piston operation were also developed to investigate the piston concept at different operating conditions. Spark Ignition Engine Test Result Summary The spark ignition PRP prototypes demonstrated the utility of the PRP concept when applied to a spark ignition engine. At low loads, the piston behaved like the elevated compression ratio, yielding a brake specific fuel consumption (BSFC) improvement of 3.5-10% during low load conditions. At high load conditions, the PRP limited the peak cylinder pressure to that of the baseline (lower compression ratio) piston. Compression Ignition Engine Result Summary The compression ignition PRP prototype did not demonstrate the desired behavior. To limit the stress in the springset, the spring rate was lower than desired. As a result, the piston deflected during compression, causing the direct fuel injection to be incorrectly targeted at the piston and leading to poor combustion. Because the upper piston was subjected to high stress levels from the springs, it had to be made from steel, which resulted in a heavy component. The resulting high mass of the piston assembly caused a decrease in efficiency. Modeling of Pressure Reactive Piston A new model for flame propagation was developed that accounted for the piston crown deflection that occurs during operation. A series of parametric studies were conducted to determine the performance of the PRP under different speed and load conditions. Additionally, the operational effects of different spring preloads were examined.
Publications: Assanis, D., Cho, Wooheum, C., Choi, I., Ickes, A., Jung, D., Martz, J., Nelson, R., Sanko, J., Thompson, S., Brevick, J., Inwood, B., "Pressure Reactive Piston Technology Investigation and Development for Spark Ignition Engines", SAE Paper 2005-01-1648 (link) Brevick, J., "Design and development of a Pressure Reactive Piston (PRP) to Achieve Variable Compression Ratio", Department of Energy Contract FC02-99EE50576 Final Technical Report