Laser induced fluorescence (LIF) imaging during the expansion stroke, exhaust gas emissions, and cylinder pressure measurements were used to investigate the influence on combustion and CO/UHC emissions of variations in squish height and fuel spray targeting on the piston. The engine was operated in a highly dilute, partially premixed, low-temperature combustion mode.
A small squish height and spray targeting low on the piston gave the lowest exhaust emissions and most rapid heat release. The LIF data show that both the near-nozzle region and the squish volume are important sources of UHC emissions, while CO is dominated by the squish region and is more abundant near the piston top. Emissions from the squish volume originate primarily from overly lean mixture. At the 3 bar load investigated, CO and UHC levels in mixture leaving the bowl and ring-land crevice are low.
With increasing squish height, UHC in the lower squish region increases at a faster rate than UHC leaving the bowl and also grows relative to UHC from higher in the cylinder. CO trends are similar but interactions with spray targeting are more pronounced. When the spray is targeted lower on the piston, UHC and CO levels leaving the bowl generally decrease relative to levels in the lower squish volume, indicating that combustion in the bowl remains clean. Emissions from higher in the cylinder decrease. In general, CO and UHC observed in the upper portion of the squish volume correlates well with exhaust gas emissions.