Variable Compression Ratio (VCR) technology has long been recognized as a method of improving Spark Ignition (SI) engine fuel economy. The Pressure Reactive Piston (PRP) assembly features a two-piece piston, with a piston crown and separate piston skirt which enclose a spring set between them. The unique feature is that the upper piston reacts to the cylinder pressure, accommodating rapid engine load changes passively. This mechanism effectively limits the peak pressures at high loads without an additional control device, while allowing the engine to operate at high compression ratio during low load conditions.

Dynamometer engine testing showed that Brake Specific Fuel Consumption (BSFC) improvement of the PRP over the conventional piston ranged from 8 to 18 % up to 70% load. Knock free full load operation was also achieved. The PRP equipped engine combustion is characterized by reverse motion of the piston crown near top dead center and higher thermal efficiency.

The cycle simulation was modified to investigate the effect of the spring set of PRP engine over a wide range of operation. In addition, the spherical flame interaction with the combustion chamber was newly modeled to calculate the flame entrainment rate of unburned charge. It was found that fuel conversion efficiency was gradually increased with spring preload until the spring could not be compressed any more and peak cylinder pressure was effectively maintained at the level of conventional engine at high loads. In addition, BSFC improvement of PRP engine over the conventional engine increased with engine speed. Overall, the full potential of PRP technology was evaluated and discussed favorably.