Environmental Sustainability Benefits of Natural Gas and Liquefied Petroleum Gas in a Diesel Engine Retrofitted for Spark Ignition Operation

In the European context of a planned transition to a zero-carbon future, the initial question at the origin of this study was to assess the energy and environmental performance of a retrofitted diesel engine converted to stoichiometric spark ignition (SI) operation using two gaseous and sustainable fuels: natural gas (NG) and liquefied petroleum gas (LPG). Further to a parametric study regarding the spark advance and injection timing, this paper delivers experimental results obtained at the engine test bed for different operating points, which are compared with the results of a baseline commercial gasoline engine. The results showed a clear improvement in terms of CO2 for the NG (especially) and LPG with a knock-free operation with respect to gasoline. Cyclic variability also improved. As for the engine-out pollutants, in the case of carbon monoxide (CO) and particle number (PN), the results are in favor of gaseous fuels. Concerning the nitrogen oxides (NOx), as expected, a higher emission was obtained with the retrofitted engine due to its higher compression ratio.