Measurement and control of natural gas mass flow in a dual-fuel engine operating at partial load through sonic nozzles

In this paper, the part load operation of a dual fuel engine operating at 1500 m above sea level and its control using sonic flow meters for natural gas fuelling are presented. Dual fuel operation was established by retrofitting the intake system of a commercial Diesel engine with a port-injected gaseous fuels system, which was comprised by calibrated orifices working under sonic flow conditions. Natural gas mass flow rates throughout the orifices were estimated assuming the isentropic one-dimensional theory for perfect gases. The effect of natural gas compressibility factor on the measured flow rates was established according to the recommendations of American Gas Association Report No. 10, evaluating the accuracy for the operating pressures and temperatures. Natural gas intake temperature was kept constant and the required flow rate was controlled by absolute intake pressure variations from 2 to 8.5  bar, according to the required diesel substitution. The maximum diesel substitution was close to 60% at 43% of full load, additional values were established for 6, 18, and  of full load. The absolute differences between the measured natural gas mass flow rates and those achieved by using the American Gas Association Report No. 10 were lower than 2%. A predictive correlation for natural gas mass flow rate was fitted depending on the required diesel substitution and the power output using the recommended methodology by the International Energy Agency Annex 42 for micro-cogeneration systems. The absolute errors between the measured flow rates and those achieved by the predictive correlation were lower than 3%.