The influence of soy- and palm-based biofuels on the in-cylinder
sources of unburned hydrocarbons (UHC) and carbon monoxide (CO) was
investigated in an optically accessible research engine operating
in a partially premixed, low-temperature combustion regime. The
biofuels were blended with an emissions certification grade diesel
fuel and the soy-based biofuel was also tested neat. Cylinder
pressure and emissions of UHC, CO, soot, and NOx were
obtained to characterize global fuel effects on combustion and
emissions. Planar laser-induced fluorescence was used to capture
the spatial distribution of fuel and partial oxidation products
within the clearance and bowl volumes of the combustion chamber. In
addition, late-cycle (30° and 50° aTDC) semi-quantitative CO
distributions were measured above the piston within the clearance
volume using a deep-UV LIF technique.
Compared with neat diesel fuel, the palm-based biofuel blends
produced the largest decrease in UHC (38-56%) and CO (28-44%), but
generated significant increases in both NOx (17-37%) and
soot (41-43%). With soy-based blends, however, UHC decreased by
30-43% and CO by 17-22%, while NOx (-1-2% increase) and
soot (0.8-8.5% increase) emissions varied slightly. The UHC and CO
emissions were found to correlate well with the combustion phasing
of each fuel at a given injection timing. The UHC PLIF measurements
demonstrated several differences in the evolution of the UHC
distributions throughout the cylinder volume during the mixture
formation, premixed heat-release, and mixing-controlled
heat-release portions of the engine cycle. However, the late-cycle
UHC PLIF and spectrally resolved CO LIF measurements showed no
significant differences in the main sources of UHC and CO when
fueling the engine with either diesel fuel or biofuel blends.