Evaluation of the Two-Step Hiroyasu Soot Model over a Broad Range of Diesel Combustion Systems 2018-01-0242
Single-cylinder engine experiments and computational fluid dynamics (CFD) modeling were used to evaluate the classic two-step Hiroyasu soot model. A broad range of direct-injected (DI) combustion systems were investigated to assess the predictive accuracy of the soot model as a design tool for modern DI diesel engines. Experiments were conducted on a 2.5 liter single-cylinder engine. Combustion system combinations included three unique piston bowl shapes and seven variants of a common rail fuel injector. The pistons included a baseline “Mexican hat” piston, a reentrant piston, and a non-axisymmetric piston similar to the Volvo WAVE design. The injectors featured six or seven holes and systematically varied included angles from 120 to 150 degrees and hole sizes from 170 to 273 μm. A single nominal operating condition was studied: 100% load at 1800 rpm. Variations in the start of injection, injection pressure, intake pressure, and exhaust gas recirculation (EGR) level were also studied. These broad hardware and operational variations were modeled using Reynolds Averaged Navier Stokes (RANS) CFD simulations with direct combustion chemistry integration. The focus of the work was to assess the ability of the Hiroyasu soot model with Chalmers n-heptane combustion chemistry to predict the soot emissions from the various combustion systems. The soot modeling results show that while the Hiroyasu model predicts some general trends in regard to start of injection and injection pressure, it tends to fail as a predictive simulation tool for designing DI diesel combustion systems in regard to selecting the hardware that yields the lowest soot emissions.