Spatial and Temporal Temperature Distributions in a Spark Ignition Engine Piston at WOT 2007-01-1436
Two coupled finite element analysis (FEA) programs were written to determine the transient and steady state temperature distribution in a spark ignition engine piston. The programs estimated the temperatures at each crank angle degree (CAD) through warm-up to thermal steady state. A commercial FEA code was used to combine the steady state temperature distribution with the mechanical loads to find the stress response at each CAD for one complete cycle.
The first FEA program was a very fast and robust non-linear thermal code to estimate spatial and time resolved heat flux from the combustion chamber to the aluminum alloy piston crown. This model applied the energy conservation equation to the near wall gas and includes the effects of turbulence, a propagating heat source, and a quench layer allowing estimates of local, instantaneous near-wall temperature gradients and the resulting heat fluxes. Model inputs include: geometry (bore, stroke, inlet valve lift, valve inlet area, piston motion equation); combustion (air-fuel ratio, gasoline calorific value, ignition timing, inlet gas temperature, pressure); and operating conditions (cylinder pressure trace, compression ratio, engine speed). The code was verified by comparing the estimated near wall temperature profiles and local wall heat flux time history to experimentally obtained values.
The temperature distribution in the piston was estimated with the second FEA model which used the heat fluxes estimated from temperature gradients calculated by the combustion chamber model as the piston crown boundary conditions. Thermal loads in the form of either surface temperature or a convective coefficient with an environmental temperature were estimated and applied to the piston underside and ring region.
The CAD resolved results estimated heat flux into the piston crown as a function of radial distance from the piston center and piston temperatures. The peak heat fluxes were 11.5 and 12.5% higher than predicted by the Woschni correlation at 2000 and 5000 rev/min, respectively. Integration of the flux over CAD gave cycle heat flows at the center of the piston 24 and 18% higher and at the periphery 32 and 30% lower than the Woschni correlation. The temperature results show surface temperature fluctuations of five to seven degrees with the depth of the fluctuation extending to 2.5 mm and 1.5 mm at 2000 and 5000 rev/min, respectively.