Natural gas has cost and environmental advantages compared with conventional liquid fuels, and liquefied natural gas can provide useful vehicle range for heavy-duty applications. Westport Innovations’ High Pressure Direct Injection (HPDI) system, developed over the past decade, uses late-cycle injection of natural gas, resulting in an engine with performance and efficiency comparable to that of a conventional diesel engine but with reduced emissions of NOx, particulate matter (PM), and CO2. Fumigation dual-fuel engines and spark-ignited natural gas engines find widespread use, but their power and efficiency are often limited by end-gas knock and high CH4 emissions can result from premixed gas near walls and crevices. HPDI produces relatively higher torque and efficiency, with low CH4 emissions, but the injection system is more complex. HPDI uses a dual-needle, dual-actuator fuel injector to inject both diesel and gas into the combustion chamber near the end of the compression stroke. The ‘co-injector’ has been developed as a simpler way of implementing the HPDI strategy, using only a single needle and a single actuator. 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.
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 HDPI J36 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.
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. All injectors yielded the same fuel economy. Therefore it is relaxant to compare the injectors used in natural gas engines
Co-injection appears to be a viable method for efficient, low-emissions combustion of natural gas in engines, comparable in many respects to the HPDI process with separate gas and diesel injection. The fuel economy of a diesel engine can be achieved using less than 10 per cent diesel (by energy). The nature of the co-injection is very important, however, and co-injection can produce undesirable knock and emissions if not tuned properly. Co-injection with diesel slowly introduced into the injector plenum (prototype CS) is preferable to rapid injection into the plenum. In the former case, it appears that the diesel is injected early in the pilot injection, perhaps ahead of most of the gas. This produces a high diesel/gas ratio in the initial injection with small ignition delay. Furthermore, because most of the diesel is injected in the first injection, the magnitude of the second (load-controlling) injection has no effect on the pilot injection. If instead the diesel is sprayed into the gas plenum (prototype B), it will be injected throughout both injections, greatly complicating the tuning process as well as increasing emissions. The finding that co-injection works better when the diesel is concentrated near the start of injection is consistent with low-speed marine engine tests performed by Miyakeet. Although engine-out PM emissions with co-injector B were slightly less than with co-injector CS, higher levels of CO, THC, and CH4 were emitted from the engine with B. The higher engine-out gaseous emissions are correlated with the longer ignition delays observed in co-injector B. The longer delays occurred despite the use of more diesel fuel. This increased diesel flow required to operate co-injector B compared with CS is likely the result of the way in which the diesel fuel is introduced into the gas plenum. For low-load cases, emissions are very sensitive to the injector type, but at high load, and especially with EGR, all three injectors showed very similar emissions. The co-injector prototypes used were modified HPDI injectors, not designed for the multiple injections required for co-injection operations. Considering this, the results of the current study are encouraging but much further research is required to determine whether co-injection might have commercial advantages over the incumbent technologies for natural gas engines.
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Natural gas has cost and environmental advantages compared with conventional liquid fuels, and liquefied natural gas can provide useful vehicle range for heavy-duty applications. Westport Innovations’ High Pressure Direct Injection (HPDI) system, developed over the past decade, uses late-cycle injection of natural gas, resulting in an engine with performance and efficiency comparable to that of
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