Combustion and Emissions of Paired-Nozzle Jets in a Pilot-Ignited Direct-Injection Natural Gas Engine

This paper examines the combustion and emissions produced using a prototype fuel injector nozzle for pilot-ignited direct-injection natural gas engines. In the new geometry, 7 individual equally-spaced gas injection holes were replaced by 7 pairs of closely-aligned holes (“paired-hole nozzle”). The paired-hole nozzle was intended to reduce particulate formation by increasing air entrainment due to jet interaction. Tests were performed on a single-cylinder research engine at different speeds and loads, and over a range of fuel injection and air handling conditions. Emissions were compared to those resulting from a reference injector with equally spaced holes (“single-hole nozzle”). Contrary to expectations, the CO and PM emissions were 3 to 10 times higher when using the paired-hole nozzles. Despite the large differences in emissions, the relative change in emissions in response to parametric changes was remarkably similar for single-hole and paired-hole nozzles. Compared to the reference injector, the paired-hole nozzle produced larger soot aggregates and larger numbers of particles; interestingly, soot primary particle size did not change significantly. In addition to the experimental results, select experiments were modelled using reacting-flow computational fluid dynamics. These simulations suggested that the paired-hole nozzle did enhance air and fuel mixing during some stages of the injection and combustion event, but the net effect was to increase the total residence time of natural gas in the rich, moderate-temperature conditions needed to form soot.