Combustion Characteristics, Performance and NOx Emissions of a Heavy-Duty Ethanol-Diesel Direct Injection Engine

Abstract

Diffusive combustion of direct injected ethanol is investigated in a heavy-duty single cylinder engine for a broad range of operating conditions. Ethanol has a high potential as fossil fuel alternative, as it provides a better carbon footprint and has more sustainable production pathways. The introduction of ethanol as fuel for heavy-duty compression-ignition engines can contribute to decarbonize the transport sector within a short time frame. Given the resistance to autoignition of ethanol, the engine is equipped with two injectors mounted in the same combustion chamber, allowing the simultaneous and independent actuation of the main injection of pure ethanol and a pilot injection of diesel as an ignition source. The influence of the dual-fuel injection strategy on ethanol ignition, combustion characteristics, engine performance and NOx emissions is evaluated by varying the start of injection of both fuels and the ethanol-diesel ratio. The results are compared against two baselines, i.e. conventional diesel combustion and dual-injections of diesel. Diesel substitution ratios above 80% on an energy basis are investigated, as the objective is to minimize diesel consumption while keeping stable and complete ethanol combustion. A minimum separation between ethanol and diesel injections is found to be necessary in order to limit the degree of premixing of ethanol at high load and avoid partial ethanol misfire causing combustion instability at low load, respectively. At low load, shortening the ethanol-diesel injection separation causes an increase in HC and CO emissions leading to lower combustion efficiencies. At high load, NOx emissions grow at higher degrees of premixing of ethanol. A slight reduction in NOx emissions occurs when increasing the relative amount of ethanol injected. Higher gross indicated efficiencies are observed for the ethanol-diesel cases compared to conventional diesel combustion. In conclusion, stable mixing-controlled combustion of ethanol is achieved with minimal diesel pilot quantities within a broad engine load range.