Combustion and Emissions Comparison between a Diesel and a Dimethyl Ether (DME) Off-Highway Compression Ignition Engine

Dimethyl ether (DME) is a promising substitute for diesel as a fuel in heavy-duty engines. This article presents the comparison between a diesel- and a DME-powered compression ignition engine. The diesel-powered version was initially characterised at a range of operating points before being converted to operate on DME. This was achieved by replacing fuel system components with bespoke DME-compatible engine parts. An off-board fuel pressurisation and conditioning system was designed to replace the existing high-pressure fuel pump, while maintaining all other engine hardware and components.

Engine behaviour, in terms of combustion and emissions on both fuels was examined. Firstly, the effect of varying recirculated exhaust gas (EGR) concentration at constant excess air ratio, combustion phasing (CA50) and equal fuel delivery rate (by energy input) was interrogated. DME combustion was significantly faster, as combustion duration was reduced by around 30%, in some cases, when comparing to diesel. The DME-powered version of the engine was also found to produce lower carbon monoxide (CO) and unburned hydrocarbon (uHC) emissions. Up to a threefold reduction was measured, depending on engine load. NO_x emissions worsened, when transitioning to DME, for the medium load case. The low-load EGR sweep showed minimal changes in NO_x emissions. High-pressure EGR can significantly lower DME NOx emissions to below the diesel baseline levels, depending on engine load and speed, as demonstrated by the results of the 8-mode test runs. Given the extremely low particulate emissions, higher EGR concentrations can be utilised by engines operating on DME. Despite resorting to the use of bespoke equipment in this study, the challenges faced during the engine conversion were deemed manageable with the currently available technology.