Strategies for Reduced Engine-Out HC, CO, and NOx Emissions in Diesel-Natural Gas and POMDME-Natural Gas Dual-Fuel Engine

Dual-fuel engines employ precisely metered amounts of a high reactivity fuel (HRF) such as diesel at high injection pressures to burn a low reactivity fuel (LRF) such as natural gas, which is typically fumigated into the intake manifold. Dual fuel engines have demonstrated the ability to achieve extremely low engine-out oxides of nitrogen (NOx) emissions compared to conventional diesel combustion at the expense of unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. At low engine loads, due to low in-cylinder temperatures, oxidation of HC and CO is very challenging. This results in both compromised combustion and fuel conversion efficiencies. The experimental campaign discussed in this paper involved a set of six engine control parameters that were strategically varied to find the best possible efficiency-emissions trade-offs for both diesel- and poly-oxy methylene dimethyl ether (POMDME)-natural gas dual fuel combustion on the University of Alabama single-cylinder research engine (SCRE) based on a PACCAR MX-11 heavy-duty engine platform. The control parameters investigated include: ((1)) Start of Injection (SOI1) of HRF, ((2)) percentage energy substitution (PES) of LRF, ((3)) introduction of the second HRF injection (SOI2), ((4)) split ratio, i.e., ratio of the duration of the first injection to the duration of the second injection, ((5)) rail pressure, and (6) intake pressure. At a fixed gross indicated mean effective pressure (IMEPg) of 5 bar (representative of typical low load operation) and an engine speed of 1339 rpm (“B speed” of the SCRE), SOI1 was varied to determine the lowest engine-out NOx point. Using that SOI1 as baseline and not-to-exceed (NTE) limits of: ((1)) Indicated specific NOx < 1 g/kWh, ((2)) maximum pressure rise rate (MPRR) < 10 bar/deg, and ((3)) coefficient of variation (COV) of IMEPg < 10%, the rest of the five control parameters were systematically varied. Based on HC and CO vs NOx emissions trade-offs, the best PES, SOI2, and split ratio were determined to achieve the lowest possible HC emissions. Subsequently, rail pressure sweeps showed minimal impact on performance and emissions between 500 bar and 1500 bar. Finally, reducing the intake pressure significantly reduced CO emissions to achieve the best overall set of operating parameters. Compared to the baseline diesel-natural gas case, HC and CO were reduced by ~88% and ~82%, respectively, in addition to ~21% improvement in the indicated fuel conversion efficiency. Whereas for POMDME-natural gas dual fuel combustion, HC and CO were reduced by ~85% and ~92%, respectively, in addition to ~20% improvement in the indicated fuel conversion efficiency. Furthermore, due to the high oxygen content of POMDME (47% m/m), engine-out soot emissions were reduced to zero measurable filter smoke number (FSN) for all conditions investigated.