Influence of Directly Injected Gasoline and Porosity Fraction on the Thermal Properties of HCCI Combustion Chamber Deposits

The limited operational range of low temperature combustion engines is influenced by near-wall conditions. A major factor is the accumulation and burn-off of combustion chamber deposits. Previous studies have begun to characterize in-situ combustion chamber deposit thermal properties with the end goal of understanding, and subsequently replicating the beneficial effects of CCD on HCCI combustion. Combustion chamber deposit thermal diffusivity was found to differ depending on location within the chamber, with significant initial spatial variations, but a certain level of convergence as equilibrium CCD thickness is reached. A previous study speculatively attributed these spatially dependent CCD diffusivity differences to either local differences in morphology, or interactions with the fuel-air charge in the DI engine. In this work, the influence of directly injected gasoline on CCD thermal diffusivity is measured using the in-situ technique based on fast thermocouple signals. Comparison of measurements under motoring and firing conditions leads to a conclusion that directly injected fuel trapped within CCD porosity has a negligible impact on in-situ thermal diffusivity for cylinder head CCD. An analytical investigation quantifies the sensitivity of CCD diffusivity to absorbed air or fuel, respectively, and indicates that variation in CCD porosity, rather than the presence of fuel in the pores, accounts for thermal diffusivity differences between cylinder head and piston CCD.