Exhaust Hydrocarbon Speciation from a Single-Cylinder Compression Ignition Engine Operating with In-Cylinder Blending of Gasoline and Diesel Fuels 2012-01-0683

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%.