Diesel aided by gasoline low temperature combustion offers low
NOx and low soot emissions, and further provides the potential to
expand engine load range and improve engine efficiency. The
diesel-gasoline operation however yields high unburned hydrocarbons
(UHC) and carbon monoxide (CO) emissions. This study aims to
correlate the chemical origins of the key hydrocarbon species
detected in the engine exhaust under diesel-gasoline operation. It
further aims to help develop strategies to lower the hydrocarbon
emissions while retaining the low NOx, low soot, and efficiency
benefits.
A single-cylinder research engine was used to conduct the engine
experiments at a constant engine load of 10 bar nIMEP with a fixed
engine speed of 1600 rpm. Engine exhaust was sampled with a FTIR
analyzer for speciation investigation. The results indicated that
under diesel-only operation, with the increase of EGR rate, C₁~C₃
hydrocarbons gradually became the dominant hydrocarbon species in
the engine exhaust. Under diesel-gasoline operation, UHC were
primarily present as mono-aromatics and C₄~C₇ alkanes, which are
likely either unoxidized or lightly oxidized gasoline constituents.
In addition, experiments were conducted to explore suitable
strategies to reduce the UHC emissions from diesel-gasoline
operation. The increase of diesel fraction reduced UHC emissions
but increased soot emissions, as the overall fuel reactivity was
enhanced but the cylinder charge heterogeneity deteriorated. The
results also showed that advancing the diesel SOI led to
significant drop in UHC emissions without compromising the low NOx
and low soot emissions. With the improved control strategies in
diesel-gasoline operation, UHC emissions were reduced by 65% while
the engine NOx and soot emissions remained low and the engine cycle
efficiency was improved by 8%.