Experimental investigation on the efficiency of a diesel oxidation catalyst in a medium-duty multi-cylinder RCCI engine

Abstract

Reactivity controlled compression ignition (RCCI) combustion is one of the most promising low temperature combustion (LTC) techniques, as it is able to provide ultra-low NOx and soot emissions together with higher thermal efficiency than conventional diesel combustion (CDC) in a wide range of operating conditions. However, the unburned hydrocarbon (UHC) and carbon monoxide (CO) emission levels are orders of magnitude higher than CDC, which can result in a major problem for implementing the RCCI concept in real engines. In this sense, the high levels of UHC and CO emissions together with the low exhaust temperatures during RCCI operation could compromise the diesel oxidation catalyst (DOC) conversion efficiency.The objective of this work is to evaluate the efficiency of a conventional DOC in oxidizing the UHC and CO emissions from RCCI combustion. To do this, a medium-duty multi-cylinder diesel engine equipped with its original after treatment system has been used. First, the DOC conversion efficiency is evaluated under some steady-state conditions. Later, the influence of the thermal inertia on the DOC response has been evaluated by means of transient tests. In this sense, different engine load-speed steps as well some simplified conditions from the worldwide harmonized vehicle cycle (WHVC) and the supplemental engine transient cycle (SET) are evaluated. In steady-state conditions, with DOC-inlet temperatures of 200–300 °C, the results show conversion efficiencies of 100% for CO and 85–95% for HC. At 10% and 25% load, the DOC-outlet UHC levels are unacceptable considering the EURO VI regulation, while at 50% load the tailpipe emissions fulfill the emissions standard. The results in transient conditions are more promising thanks to effect of the thermal inertia, showing 100% conversion efficiency for CO and greater than 90% for UHC during large periods of engine operation.