A study on pressure reactive piston for spark ignition engines

The thrust toward improving vehicle fuel economy has stimulated the development of engine technologies including Variable Compression Ratio (VCR) piston designs. Pressure Reactive Piston (PRP) technology separates the piston into two pieces with a spring set located between the upper and lower pistons. The unique feature of PRP is that the upper piston reacts to the cylinder pressure during the power stroke, accommodating rapid engine load changes passively. This mechanism effectively limits the peak cylinder pressures at high loads without an additional control device, while allowing high compression ratio under low loads.; The maximum compression ratio without knocking was examined, and the preload and spring constant of spring set were obtained by using a quasi-dimensional simulation program. Belleville spring was selected as the spring set of the PRP because of its compactness and ability to carry high load with small deflection. Dynamic analysis of the piston crown was performed to calculate spring deflection and instantaneous chamber volume.; The baseline and PRP engine were tested on single cylinder SI engine. Dynamometer test results demonstrated that BSFC improvement of the PRP engine over the baseline ranged from 8 to 18% at part loads. Full load torque was developed with the PRP engine without knocking at a similar magnitude as the baseline. The PRP engine combustion is characterized by Reverse and Flattened Motion of piston crown near the Top Dead Center (TDC) and higher thermal efficiency.; An analytic model for the geometric interaction between the spherical flame and the combustion chamber was newly developed in order to calculate the flame entrainment rate of unburned charge. The program was modified for simulating PRP motion to investigate the effect of the spring set on engine performance and emissions over various operating conditions. It was found that BSFC improvement of the PRP engine increased with engine speed and fuel conversion efficiency was gradually increased with the increase of the spring set preload until the spring set could not be compressed any more due to high preload. However, NO emissions were increased at part loads compared to the baseline due to higher compression ratio.