Future projections in global transportation fuel use show a demand shift toward diesel and away from gasoline. At the same time, greenhouse gas regulations will drive higher vehicle fuel efficiency and lower well-to-wheel CO2 production. Naphtha, a contributor to the gasoline stream and requiring less processing at the refinery level, is an attractive candidate to mitigate this demand shift while lowering the overall greenhouse gas impact. This study investigates the combustion and emissions performance of two naphtha fuels (Naphtha 1: RON59 and Naphtha 2: RON69) and one ultra-low sulfur diesel (ULSD) in a model year (MY) 2013, six-cylinder, heavy-duty diesel engine. Engine testing was focused on the heavy-duty supplemental emissions test (SET) “B” speed over a load sweep from 5 to 15 bar BMEP (brake-specific mean pressure). At each operating point, NOx sweeps were conducted over wide ranges. At 10–15 bar BMEP, mixing-controlled combustion dominates the engine combustion process. Under a compression ratio of 18.9, cylinder pressure and temperature at these load conditions are sufficiently high to suppress the reactivity difference between ULSD and the two naphtha fuels. As a result, the three test fuels showed similar ignition delay (ID). Nevertheless, naphtha fuels still exhibited notable soot reduction compared to ULSD. Under mixing-controlled combustion, this is likely due to their lower aromatic content and higher volatility. At 10 bar BMEP, Naphtha 1 generated less soot than Naphtha 2 since it contains less aromatics and is more volatile. When operated at light load, in a less reactive thermal environment, the lower reactivity naphtha fuels lead to longer IDs than ULSD. As a result, the soot benefit of naphtha fuels was enhanced. Utilizing the soot benefit of the naphtha fuels, engine-out NOx was calibrated from the production level of 3–4 g/hp-hr down to 2–2.5 g/hp-hr over the 12 nonidle SET steady-state modes. At this reduced NOx level, naphtha fuels were still able to maintain a soot advantage over ULSD and remain “soot-free” while achieving diesel-equivalent fuel efficiency. Finally, low-temperature combustion (LTC) operation (NOx ≤ 0.2 g/hp-hr and smoke ≤ 0.2 FSN) was achieved with both of the naphtha fuels at 5 bar BMEP through a late injection approach with high injection pressure. Under high exhaust gas recirculation (EGR) dilution, Naphtha 2 showed an appreciably longer ID than Naphtha 1, resulting in a soot reduction benefit.
Toluene primary reference fuel (TPRF) (mixture of toluene, iso-octane and heptane) is a suitable surrogate to represent a wide spectrum of real fuels with varying octane sensitivity. Investigating different surrogates in engine simulations is a prerequisite to identify the best matching mixture. However, running 3D engine simulations using detailed models is currently impossible and reduction of detailed models is essential. This work presents an AramcoMech reduced kinetic model developed at King Abdullah University of Science and Technology (KAUST) for simulating complex TPRF surrogate blends. A semi-decoupling approach was used together with species and reaction lumping to obtain a reduced kinetic model. The model was widely validated against experimental data including shock tube ignition delay times and premixed laminar flame speeds. Finally, the model was utilized to simulate the combustion of a low reactivity gasoline fuel under partially premixed combustion conditions.
Recent experimental studies on a production heavy-duty diesel engine have shown that gasoline compression ignition (GCI) can operate in both conventional mixing-controlled and low-temperature combustion modes with similar efficiency and lower soot emissions compared to diesel at a given engine-out NOx level. This is primarily due to the high volatility and low aromatic content of high reactivity, light-end fuels. In order to fully realize the potential of GCI in heavy-duty applications, accurate characterization of gasoline sprays for high-pressure fuel injection systems is needed to develop quantitative, three-dimensional computational fluid models that support simulation-led design efforts. In this work, the non-reacting fuel spray of a high reactivity gasoline (research octane number of ∼60, cetane number of ∼34) was modeled under typical heavy-duty diesel engine operating conditions, i.e., high temperature and pressure, in a constant-volume combustion chamber. The modeling results were compared to those of a diesel spray at the same conditions in order to understand their different behaviors due to fuel effects. The model was developed using a Lagrangian-Particle, Eulerian-Fluid approach. Predictions were validated against available experimental data generated at Michigan Technological University for a single-hole injector, and showed very good agreement across a wide range of operating conditions, including ambient pressure (3–10 MPa), temperature (800–1200 K), fuel injection pressure (100–250 MPa), and fuel temperature (327–408 K). Compared to a typical diesel spray, the gasoline spray evaporates much faster, exhibiting a much shorter liquid length and wider dispersion angle which promote gas entrainment and enhance air utilization. For gasoline, the liquid length is not sensitive to different ambient temperatures above 800 K, suggesting that the spray may have reached a “saturated” state where the transfer of energy from the hot gas to liquid has already been maximized. It was found that higher injection pressure is more effective at promoting the evaporation process for diesel than it is for gasoline. In addition, higher ambient pressure leads to a more compact spray and fuel temperature variation only has a minimal effect for both fuels.
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