A Computational Investigation of the Impact of Multiple Injection Strategies on Combustion Efficiency in Diesel–Natural Gas Dual-Fuel Low-Temperature Combustion Engines

Abstract

Dual fuel diesel-methane low temperature combustion (LTC) has been investigated by various research groups, showing high potential for emissions reduction (especially oxides of nitrogen (NOx) and particulate matter (PM)) without adversely affecting fuel conversion efficiency in comparison with conventional diesel combustion. However, when operated at low load conditions, dual fuel LTC typically exhibits poor combustion efficiencies. This behavior is mainly due to low bulk gas temperatures under lean conditions, resulting in unacceptably high carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions. A feasible and rather innovative solution may be to split the pilot injection of liquid fuel into two injection pulses, with the second pilot injection supporting CO and UHC oxidation once combustion is initiated by the first one. In this work, diesel-methane dual fuel LTC is investigated numerically in a single-cylinder heavy-duty engine operating at 5 bar brake mean effective pressure (BMEP) at 85% and 75% percentage of energy substitution (PES) by methane (taken as a natural gas (NG) surrogate). A multidimensional model is first validated in comparison with experimental data obtained on the same single-cylinder engine for early single pilot diesel injection at 310 CAD (Crank Angle Degrees) and 500 bar rail pressure. With the single pilot injection case as baseline, the effects of multiple pilot injections and different rail pressures on combustion and emissions are investigated, again showing good agreement with experimental data. Apparent heat release rate and cylinder pressure histories as well as combustion efficiency trends are correctly captured by the numerical model. Results prove that higher rail pressures yield reductions of HC and CO by 90% and 75%, respectively, at the expense of NOx emissions, which increase by ~30% from baseline still remaining at very low level (under 1 g/kWh). Further, it is shown that post-injection during the expansion stroke does not support the stable development of the combustion front as the combustion process is confined close to the diesel spray core.

A Comparative Experimental Analysis of Natural Gas Dual Fuel Combustion Ignited by Diesel and Poly OxyMethylene Dimethyl Ether

“…The differences may have been caused by the close-coupled (or short-dwell) strategy chosen in the present work for multiple injections and the relatively lower PES conditions chosen for the rail pressure sweep. For example, Bartolucci et al [30] observed a decrease in the CO emissions for higher rail pressures, albeit for a long dwell between the multiple injections as opposed to close-coupled multiple injections used in the present work. In a similar fashion, Hariharan et al [31] showed an improvement in the efficiency-emissions tradeoffs when increasing the rail pressure.…”

“…Rail pressure has been shown to impact dual-fuel emissions in the literature [30,31]. The AHRR and emissions values for both fueling combinations are shown in Figure 18.…”

“…A post-injection quantity of 1.5 mg/cycle, with the injection occurring 9.7 CAD after the main injection, was found to be optimal. Bartolucci et al [30] numerically investigated the benefits of using a split-injection strategy with diesel and NG at 5 bar brake mean effective pressure on a 1.8 L single-cylinder research engine (SCRE) operating at 1500 RPM. After validating an early injection timing of 310 CAD with experimental results, the effects of using multiple injections and different rail pressures were studied.…”