The Influence Of Natural Gas And Hydrogen Co-combustion With Diesel Fuel On Engine Exhaust Emissions And In-cylinder Gas Composition

Abstract

The development of future ultra-low emission combustion strategies is necessary in order to strengthen the security of energy supply and address the rising concerns over the health and environmental effects due to pollutant combustion emissions. The imposition of legislation, which places strict limits on pollutant emissions from combustion systems around the world, is designed to reduce the dependence on petroleum based fossil fuels, and indicates the need to diversify towards sustainable and cleaner burning fuels. Natural gas and hydrogen stand out as two potential alternatives for the currently used petroleum based fuels. Natural gas has the highest carbon to hydrogen ratio than any other fossil fuel and, therefore, releases relatively few by-products (CO2 and particulates) into the atmosphere as pollutants on combustion. Hydrogen is the ultimate a ‘zero carbon emission’ fuel with a relatively high energy density; however, hydrogen is not naturally available in the way that natural gas is and requires and investment to produce, and can therefore be used to complement natural gas combustion. The work presents an experimental investigation of diesel fuel co-combustion with both natural gas and hydrogen, carried out on a modern, naturally aspirated, direct injection diesel engine. The engine was supplied with a range of methane-diesel fuel, hydrogen-diesel fuel and methane-hydrogen-diesel fuel mixtures and the effect on combustion and exhaust emissions was investigated. At low to medium engine loads, the results showed a decrease in particulates, NOx and CO2 exhaust emissions as diesel fuel was replaced by methane-hydrogen mixtures. At high engine loads and relative to diesel only combustion, NOx emissions increased steeply, which was attributed to the combined diesel fuel and methane-hydrogen mixture co-combustion temperatures exceeding the threshold temperature for NOx emissions. In addition, an in-cylinder gas sampling technique was utilised with the research engine to gain a greater level of insight into the process of energy release and emissions formations in the gas of hydrogen methane diesel co-combustion than is afforded by the measurement of engine exhaust. The engine was supplied with a range of hydrogen-diesel fuel and methane-diesel fuel mixtures, and in-cylinder gas sample composition was investigated at two sampling locations; within the diesel fuel spray and between adjacent spray cones. Concentrations of NOx were found to be higher between the two diesel sprays relative to within the spray cone for both hydrogen and methane addition. In the case of hydrogen-diesel fuel co-combustion, the measured particulate levels were observed to be higher in the diesel fuel spray relative to between two sprays; however, in the case of methane-diesel fuel co-combustion, higher particulate levels were measured in the region between the two sprays. This was attributed to methane contributing significant quantities of particulates (unlike hydrogen) to the total particulate concentration produced from the methane-diesel fuel mixture in between two sprays.