The Decoupling of the Chemical Kinetic and Heating Effects of Pilot Diesel in a Diesel-Ignited Natural Gas Engine under Various Hydrogen Blending Ratios

Abstract

Abstract

The diesel-ignited natural gas engine suffers from low thermal efficiency at low loads and knocking at high loads. Addressing these issues requires precise control of the combustion process. Pilot diesel combustion provides both a heating effect and a chemical kinetic effect for natural gas combustion. Although hydrogen addition can improve overall combustion, how these two effects vary with hydrogen blending ratio remains unclear. The objective of this study is to decouple and quantify these two effects under various hydrogen blending ratios using numerical simulation combined with the chemical inertness method and the segmented variable component method. A 6-cylinder turbocharged and intercooled diesel/natural gas dual-fuel heavy-duty engine was employed, with hydrogen introduced at pilot diesel injection timings of 15°CA BTDC and 40°CA BTDC, respectively. The results show that as the hydrogen blending ratio increases, the pilot diesel chemical kinetic effect decreases from 20.3% to 17.4% (at 15°CA BTDC) and from 62.6% to 8% (at 40°CA BTDC), while the heating effect increases correspondingly. These changes are more pronounced at the earlier injection timing (40°CA BTDC). At the start of natural gas combustion, the distribution and concentration of pilot diesel in the cylinder remain nearly unchanged. Hydrogen combustion inhibits the generation of highly active intermediates from pilot diesel, which explains the decrease in the chemical kinetic effect. With increasing hydrogen blending ratio, the CH2O concentration and distribution area decrease, while the heat release area and high-temperature region in the cylinder expand.