Future projections in global transportation fuel use show a demand shift towards 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. In this work, low cetane and high volatility gasoline-like fuels have shown potential to achieve high fuel efficiency with low engine-out emissions in a production commercial vehicle engine.
This study investigates the combustion and emissions performance of two low cetane naphtha fuels (Naphtha 1: RON59; Naphtha 2: RON69) and one ultra-low sulfur diesel (ULSD) in a model year (MY) 2013, six-cylinder, heavy-duty diesel engine. The engine is equipped with a single-stage variable geometry turbocharger (VGT) and a fuel injection system that is capable of 2500 bar fuel injection pressure. The engine has a stock geometric compression ratio of 18.9. To date, most studies in this area have been conducted using single-cylinder research engines. Aramco aims to better understand the implications on hardware and software design in a multi-cylinder engine with a production engine air system.
Engine testing was focused on the Heavy-Duty Supplemental Emissions Test (SET) “B” speed over a load sweep from 5 to 15 bar BMEP. At each operating point, NOx sweeps were conducted over wide ranges (e.g., 0.2 → 3 g/hp-hr) to understand the implications of fuel reactivity as well as other properties on combustion behavior under both high temperature mixing-controlled combustion and low temperature premixed combustion.
At 10–15 bar BMEP, mixing-controlled combustion dominates the engine combustion process. Under a compression ratio of 18.9, cylinder pressure and temperature are sufficiently high to suppress the reactivity (cetane number) difference between ULSD and the low cetane naphtha fuels. As a result, the three test fuels showed similar ignition delay under high temperature and pressure conditions. 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 led to longer ignition delays than ULSD. As a result, the soot benefit of naphtha fuels was enhanced. Overall, naphtha fuels and ULSD had similar fuel efficiency.
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 twelve non-idle 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” (smoke ≤ 0.2 FSN) while achieving diesel-equivalent fuel efficiency.
Finally, partially premixed compression ignition (PPCI) low temperature combustion (LTC) operation (NOx ≤ 0.2 g/hp-hr; 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 EGR dilution, Naphtha 2 showed an appreciably longer ignition delay than Naphtha 1, resulting in a soot reduction benefit. Early injection PPCI operation cannot be attained with the stock engine compression ratio due to excessive pressure rise rates. Although the late injection PPCI operation offered a significant NOx benefit over mixing-controlled combustion operation, it led to lower fuel efficiency with undesirably late combustion phasing. This points the research towards a lower engine compression ratio and an air system upgrade to promote high efficiency PPCI LTC operation.