The influence of hydrogen content in hydrogen-natural gas fuel mixtures on the emissions of a lean-burn spark ignition engine has been examined under representative operating conditions, a mid load and a high load. The hydrogen content in the fuel gas mixtures was varied from 0 to 30% with the balance made up of natural gas. The primary effect on emissions was to influence the tradeoff between NOx and hydrocarbon emissions. At the mid-load condition, increasing the hydrogen content from 0 to 15% at constant equivalence ratio reduced the HC emissions by 80% with little change in NOx emissions. Increasing from 15 to 30% hydrogen content reduced the HC emissions a further 50% but increased the NOx emissions by 16%. At the high load condition, the overall result of increasing the hydrogen content was to increase the NOx emissions substantially without significantly reducing the HC emissions. The impact of increasing hydrogen content on engine efficiency is similar to the impact on hydrocarbon emissions. At the mid-load condition, engine efficiency was increased by increasing hydrogen content, but with diminishing returns. An increase from 0 to 5% hydrogen content provides a significant benefit under marginal combustion conditions but further increases in hydrogen content are less effective.

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