Effects of Low Temperature Combustion on Particle and Gaseous Emission of a Dual Fuel Light Duty Engine

In recent years the use of alternative fuels for internal combustion engines has had a strong push coming from both technical and economic-environmental aspects. Among these, gaseous fuels such as liquefied petroleum gas and natural gas have occupied a segment no longer negligible in the automotive industry, thanks to their adaptability, anti-knock capacity, lower toxicity of pollutants, reduced CO2 emissions and cost effectiveness. On the other hand, diesel engines still represent the reference category among the internal combustion engines in terms of fuel consumptions. The possibility offered by the dual fuel systems, to combine the efficiency and performance of a diesel engine with the environmental advantages of gaseous fuels, has been long investigated. However the simple replacement of diesel fuel with natural gas does not allow to optimize the performance of the engine due to the high THC emissions particularly at lower loads. Increasing the injection timing of pilot diesel fuel helps to reduce THC, but cause an increase of the nitrogen oxides. Therefore more complex combustion strategies should be realized to meet vehicles emission standards. In this paper, the benefits obtainable through the activation of the low combustion temperatures have been evaluated. LTC can be activated by means of very early diesel injection timings and with the maximum by natural gas share tolerable for stable combustion. The experimental activity was also focused to analyze the particle emissions which, as is well known, represent together with the nitrogen oxide emissions, the main pollutants resulting from the combustion of diesel fuel. The activation of LTC has shown the potential to simultaneously reduce both THC and NOx emissions as well as ensuring ultra-low particle emissions. Therefore LTC should be considered as a keystrategy to make DF engines compliant with the limits imposed for the vehicles approval.