A 300 cc gasoline engine has been experimentally and numerically
studied to compare PFI and DI operation on naturally-aspirated and
turbocharged full load operating points. Experiment outlines the
benefits from DI operation in terms of volumetric efficiency, fuel
economy and knock propensity but also clearly indicates worse raw
engine-out CO emissions. The latter is an indication of the
survival of a large scale mixture heterogeneity in this downsized
GDI engine even when early injection and intense induced fluid
motion are combined.
For such a full load operation, the application of optical
diagnostics to study mixture heterogeneity cannot be considered
because pressure and temperature exceed sustainable levels for
transparent materials. Therefore, 3D CFD RANS computations of the
intake, injection, combustion and pollutant formation processes
including detailed chemistry information are performed to
complement the experimental data. The results at the end of
compression stroke confirm the existence of a large scale mixture
stratification. Moreover, the degree of mixture stratification can
be directly related to the measured CO emissions. Combustion and
pollutant formation modeling with detailed chemistry information
allows quantifying the influence of locally rich mixtures on flame
propagation and post-flame chemistry (CO and NOx
emissions).
Computations for various injection timings on a unique operating
point explain why the experimentally chosen injection timing is
optimal. The interaction of intake-induced fluid motion with
injection-induced fluid motion gives rise to a trade-off between
turbulence and mixture homogeneity near TDC. Increased turbulence
favors combustion speed and reduces knock propensity, which should
lead to a better efficiency. But the gain in turbulence is
counter-balanced by an enlarged mixture heterogeneity producing
larger CO emissions and therefore impeding global efficiency.
