Conventional and Low Temperature Combustion Using Naphtha Fuels in a Multi-Cylinder Heavy-Duty Diesel Engine

The regulatory requirements to lower both greenhouse gases and criteria pollutants from heavy duty engines are driving new perspectives on the interaction between fuels and engines. Fuels that lower the burden on engine manufacturers to reach these goals may be of particular interest. Naphtha, a fuel with a higher volatility than diesel, but with the ability to be burned under traditional mixing-controlled combustion conditions is one such fuel. The higher volatility promotes fuel-air mixing and when combined with its typically lower aromatic content, leads to reduced soot emissions when compared directly to diesel. Naphtha also has potential to be less energy-intensive at the refinery level, and its use in transportation applications can potentially reduce CO2 emissions on a well-to-wheels basis. This work investigates the combustion characteristics and emissions of two naphtha fuels (Naphtha 1: RON59; Naphtha 2: RON69) together with one ultra-low sulfur diesel (ULSD) in a model year (MY) 2013, six-cylinder, Cummins ISX15, heavy-duty truck engine. Engine geometric compression ratio (CR) was at 17.3 in the present study to promote partially premixed combustion. Engine tests were focused on 1375 RPM, a medium speed, over a load sweep from 5 to 15 bar BMEP. At all tested operating conditions using the production hardware set, both naphtha fuels showed a substantial reduction in soot levels at an equivalent level of NOx emissions and fuel efficiency. At 15 bar BMEP, cylinder pressure and temperature are sufficiently high to suppress the reactivity difference between ULSD and the naphtha fuels. Consequently, the three test fuels exhibited similar ignition delay times, but the naphtha fuels were still observed to produce less soot than ULSD. Under mixing-controlled combustion, this is likely due to their higher volatility, lower viscosity, and less aromatic content. At 10 bar BMEP with a less reactive thermal environment, naphtha fuels started to show longer ignition delays while retaining a soot benefit over ULSD. 3-D CFD combustion simulation at 10 bar BMEP suggests the higher volatility and lower viscosity of naphtha facilitate better air utilization and a reduced presence of fuel-rich regions in the combustion chamber. When further reducing engine load to 5 bar BMEP, the ignition delay difference between naphtha fuels and ULSD became more notable. As a result, compared to ULSD, the naphtha fuels exhibited markedly enhanced premixed combustion and substantially reduced soot emissions. The benefits of the naphtha fuels allowed the engine to be recalibrated from 3-4 g/hp-hr NOx (production base level) down to 1.5-2 g/hp-hr over the 12-mode non-idle SET steady-state test cycle while maintaining "soot-free" (smoke <= 0.2 FSN) operation and diesel-equivalent fuel efficiency. In addition to conventional mixing-controlled combustion, partially premixed compression ignition (PPCI) low temperature combustion (LTC) operation was studied. Both early and late injection PPCI approaches were investigated. Through late injection coupled with high injection pressure, LTC operation was achieved up to 10 bar BMEP for Naphtha 1 and 11 bar BMEP for Naphtha 2. Under high EGR dilution, Naphtha 2, with its lower reactivity, had longer ignition delay times than Naphtha 1, thereby resulting in lower soot and a moderately improved LTC operation range. CFD-guided combustion analysis identified two PPCI concepts at 10 bar BMEP: PPCI1 at 0.79 g/hp-hr NOx and PPCI2 at 0.19 g/hp-hr NOx. PPCI1 used single injection, while PPCI2 utilized two injections. For PPCI2, in addition to a main injection occurring at -9 degrees ATDC, a post injection with small fuel quantity was employed at 7 degrees ATDC. This effectively enhanced the late-stage air utilization and led to improved fuel efficiency over PPCI1.