Mixing-Controlled Combustion of Conventional and Higher Reactivity Gasolines in a Multi-Cylinder Heavy-Duty Compression Ignition Engine

Paper #:
  • 2017-01-0696

  • 2017-03-28
This research investigates the combustion characteristics and engine performance of a conventional gasoline (RON 92) and a higher reactivity gasoline (RON 80) under mixing-controlled combustion. The work builds upon previous Aramco published studies on the combustion of straight-run naphtha fuels. With a forecasted shift in global demand toward middle distillate fuels, there is potential in the market for light-end fuels such as gasoline. Combusting light-end fuels as efficiently as diesel fuel in a compression ignition engine may mitigate such a demand shift while providing the flexibility needed to accommodate future transportation scenarios. A six-cylinder, Cummins ISX15 heavy-duty diesel engine equipped with a common-rail fuel system capable of 2500 bar fuel injection pressure and with a geometric compression ratio of 18.9 was used in this study. Engine experiments were conducted at a medium engine speed, 1375 rpm, with a load sweep from 2 to 15 bar BMEP. At each load, engine-out NOx emissions were swept across a wide range, while CA50 was kept constant among the different test fuels. Compared to the baseline ultra-low sulfur diesel (ULSD), both of the gasoline fuels showed longer ignition delay times, stronger premixed combustion, and consequently higher maximum pressure rise rate (MPRR) under single injection operation. To mitigate excessive MPRR at medium-to-high load and maintain good combustion stability at low load, a two-injection strategy (i.e., pilot + main) was developed for the two fuels by utilizing their low reactivity and tailoring the proportion of pilot-injected fuel at different engine loads. Moreover, the early temperature build-up through pilot injection was also found beneficial for improving fuel efficiency. Across the load range investigated, both of the gasoline fuels consistently exhibited lower soot levels than ULSD with the reduction more pronounced at low to medium loads. Under mixing-controlled combustion, this is likely due to the higher volatility and lower viscosity of the gasoline fuels. As suggested by 3-D CFD combustion simulation, the higher volatility and lower viscosity can help improve the air utilization and therefore reduce the presence of fuel-rich regions in the combustion chamber. In addition to the steady-state NOx sweeps, 12-mode non-idle SET steady-state tests were carried out for both gasoline fuels using the two-injection strategies. Test results showed that the two low reactivity fuels were able to achieve “soot-free” (smoke ≤ 0.2 FSN) operation across the 12 modes with diesel-equivalent engine-out levels of NOx.
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