Comparison of injectors for compression ignition of natural gas with entrained diesel

Abstract

New fuel injector prototypes for heavy-duty engines have been developed to use direct-injection natural gas with small amounts of entrained diesel as an ignition promoter. This ‘co-injection’ is different from other dual-fuel engine systems, where diesel and gas are introduced separately. Two co-injectors were compared with a Westport HPDI injector that injects diesel and gas through separate systems into the cylinder. All injectors have identical gas-nozzle geometry and inject fuel into the cylinder near top-dead-centre, but differ in the manner of introducing the diesel. Both co-injectors introduce diesel into the gas plenum before the gas needle is actuated, causing a two-phase gas-blast injection. The first co-injector (‘B’) injects the diesel with relatively high velocity into the gas plenum, which probably disperses it over a large volume inside the injector. The second prototype (‘CS’) introduces the diesel at very low velocity so that it may remain near the needle seat prior to injection. The injectors were tested in a 2.5-litre single-cylinder engine with 17:1 compression ratio. Load varied from 6 to 13 bar gross indicated mean effective pressure. Temperature-controlled exhaust-gas recirculation of 0 or 30 per cent was used. Co-injection of natural gas and diesel can increase the ignition delay relative to the HPDI system (which uses a pure diesel pilot injection). The HPDI and CS injectors required 7–15 per cent diesel fuelling (by energy), while B required 9 to 20 per cent diesel fuelling. All injectors yielded the same fuel economy (within 2 per cent). However, premixed diesel, gas, and air can burn rapidly enough to produce knock. Knock was typically inaudible (below 3 bar intensity) and greatly reduced for conditions with exhaust-gas recirculation. With co-injector CS, all gaseous emissions could be brought very close to those of the HDPIJ36 injector, but co-injector B resulted in high hydrocarbon and CO emissions at low load. Particulate emissions from the co-injectors were slightly lower than for the J36 injector, possibly due to more fuel/air premixing prior to ignition.