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

Abstract

The introduction of alternative fuels such as natural gas is likely to occur at an increasing rate. The dual-fuel concept allows these low cetane number fuels to be used in compression-ignition (CI, diesel) type engines. Most CI engine conversions have pre-mixed the alternative fuel with air in the intake manifold while retaining diesel injection into the cylinder for ignition. The advantage is that it is simple for practical adaptation; 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 the cylinder. Here, the fuel in the end-zone is limited and the diesel, injected before the alternative, has only a conventional ignition delay. This improves the high-end performance. Modern, very high pressure diesel injectors have good turndown characteristics as well as better controllability. This improves low-end performance and hence 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 the latter, the key is to produce a combined injector to handle both fuels which has the smallest diameter possible so that installation is readily achieved. There exists the potential for much improvement. A combined gas/diesel injection system based on small, high pressure common-rail injectors has been tested for fluid characteristics. Spray properties have been examined experimentally in a test rig and modelled using CFD. The CFD package Fluent was used to model the direct-injection of natural gas and diesel oil simultaneously into an engine. These models were initially calibrated using high-speed photographic visualisation of the jets. Both shadowgraph and schlieren techniques were employed to identify the gas jet itself as well as mixing regions within the flow. Different orientations and staging of the jets with respect to each other were simulated. Salient features of the two fuel jets were studied to optimise the design of a dual-fuel injector for CI engines. Analysis of the fuel-air mixture strength during the injection allowed the ignition delay to be estimated and thus the best staging of the jets to be determined.