Piston Top Shapes and Their Influence Upon Otto Cycle Combustion 921465

Combustion in reciprocating engines relies on in-cylinder gas motion to speed up the process and enhance burning efficiency. A number of factors capable of influencing such motion can be listed. Among the most important are intake port and piston top geometry. The latter normally referred to as combustion bowl shape is a subject of intense research activity in the Diesel field,in stark contrast to the mild endeavours prevailing in the Otto-cycle partisanship. However, today's environmental and energy resources availability considerations demand that cleaner and more fuel efficient engines have to be made available. Homogeneous mixture Otto-cycle powerplant improvements in fuel economy are hampered by knock incidence, among other factors. It is known that piston top features do interact with the combustion flame front in both negative or positive fashion. In order to better understand how sensitive this interaction is, four piston top shapes were tested in a 4-cyl. SI engine that is representative of the current, high production volume generation. A single cylinder research engine was also used for preliminary testing. The influence of slight differences in piston top geometry upon the Brake Specific Fuel Consumption (BSFC), the Lean Burning Limit (LBL), the Peak Combustion Pressure (Pk) and the Knock Avoidance Margin (KAM) were investigated. Tops were shaped as follows:

The flat topped pistons performances of both engines were taken as references. From the stand-point of BSFC, the 4 cyl. fitted with the convex top piston showed no improvement, the wedge squish-enhancing exhibited about 1% less fuel consumption while the concave, radial squish-enhancing version decreased BSFC by as much as 7.3%. The convex proved itself to have the highest KAM and leanest LBL burning tolerance of all versions tested. Under WOT condition, all versions developed about the same power (within +/- 3%) at 5200 rpm. Maximum torque, developed at 2800 rpm, was also consistent and within +/- 2% except for the concave piston. For this version, the incidence of knocking limited max. torque to 93% of the reference value. The research unit exhibited the same trends except for the BSFC. In this case, it was 4% lower with the convex topped piston. Conclusions of this work are: