Simultaneous diesel and natural gas injection for dual-fuelling compression-ignition engines

The introduction of alternative fuels such as natural gas is likely to occur at anincreasing rate. The dual-fuel concept allows these low cetane number fuels to be usedin compression-ignition (CI, diesel) type engines. Most CI engine conversions havepre-mixed the alternative fuel with air in the intake manifold while retaining dieselinjection into the cylinder for ignition. The advantage is that it is simple for practicaladaptation; the disadvantage is that good substitution levels are only obtained at midload.A better solution is to inject both the alternative and diesel fuels directly into thecylinder. Here, the fuel in the end-zone is limited and the diesel, injected before thealternative, has only a conventional ignition delay. This improves the high-endperformance. Modern, very high pressure diesel injectors have good turndowncharacteristics as well as better controllability. This improves low-end performance andhence offers an ideal platform for a dual-fuel system. Several systems already exist,mainly for large marine engines but also a few for smaller, truck-sized engines. For thelatter, the key is to produce a combined injector to handle both fuels which has thesmallest diameter possible so that installation is readily achieved. There exists thepotential for much improvement.A combined gas/diesel injection system based on small, high pressure common-railinjectors has been tested for fluid characteristics. Spray properties have been examinedexperimentally in a test rig and modelled using CFD. The CFD package Fluent wasused to model the direct-injection of natural gas and diesel oil simultaneously into anengine. These models were initially calibrated using high-speed photographicvisualisation of the jets. Both shadowgraph and schlieren techniques were employed toidentify the gas jet itself as well as mixing regions within the flow. Differentorientations and staging of the jets with respect to each other were simulated. Salientfeatures of the two fuel jets were studied to optimise the design of a dual-fuel injectorfor CI engines. Analysis of the fuel-air mixture strength during the injection allowedthe ignition delay to be estimated and thus the best staging of the jets to be determined