As a contribution to the reduction of greenhouse gas emissions in the
transportation sector, the indicated efficiency of SI engines can be increased
via thermal swing coatings. Thereby, a decrease in greenhouse gas emissions can
be achieved, although not at all operating conditions. Here, the often-observed
increased hydrocarbon emission partially overcompensates the reduced wall heat
losses. The main root cause is always attributed to the increased surface
roughness and porosity, leading to an increased crevice volume.
Further investigations were performed at a single-cylinder engine equipped with a
FTIR for species analysis of hydrocarbon emissions. A comparison of direct
injection and port fuel injection were performed for RON95 E10 and methanol to
assess the influence of mixture preparation. 3D CFD was used to additionally
investigate the in-cylinder processes.
The comparison of port fuel injection and direct injection showed a significant
influence on the fuel hydrocarbon emissions for the direct injection when the
thermal swing coating was applied. The effect is more pronounced for methanol.
For port fuel injection nearly the same or reduced fuel hydrocarbon emissions
can be observed. This is mainly attributed to an increased wall film
agglomeration at the piston for the thermal swing coating in case of direct
injection, which can be observed in 3D CFD. Due to the low thermal effusivity of
the coating, the droplet impingement leads to a notable decrease in the surface
temperature. This results in lower evaporation of the fuel and a longer droplet
lifetime. Consequently, a fuel wall film is still present at top dead center
after ignition leading to additional hydrocarbon emissions.
