Calibration of a Knock Prediction Model for the Combustion of Gasoline-Natural Gas Mixtures

Gaseous fuels, such as Liquefied Petroleum Gas (LPG) and Natural Gas (NG), thank to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Moreover natural gas, which is mainly constituted by methane, whose molecule has the highest hydrogen/carbon ratio, leads also to lower ozone depleting emissions. The authors in a previous work (1) experienced the simultaneous combustion of gasoline and natural gas in a bi-fuel S.I. engine, exploiting so the high knock resistance of methane to run the engine with an ‘overall stoichiometric’ mixture (thus lowering fuel consumption and emissions) and better spark advance (which increases engine efficiency) even at full load: the results showed high improvements in engine efficiency without noticeable power losses with respect to the pure gasoline operation. With the aim to provide a knock prevision submodel to be used in engine thermodynamic simulations for a knock-safe performance optimization of engines fuelled by NG/gasoline mixtures, the authors recorded the in-cylinder pressure cycles under light knocking condition for different engine speed, loads and natural gas fraction (i.e. the ratio between the injected natural gas mass and the total fuel mass), and used the gas pressure data to calibrate a classical knock-prediction model: as shown, the results obtained allow to predict the onset of knocking in a S.I. engine fuelled with a gasoline-natural gas mixture with any proportion between the two fuels, with a maximum error of 5 CAD.