Effects of hydrogen addition under lean and diluted conditions on combustion characteristics and emissions in a spark-ignition engine

Experimental investigations on the effects of hydrogen addition to spark-ignition (SI) engines running under lean and diluted conditions are presented in this paper. Experiments were carried out in a mono-cylinder SI engine with different rates of nitrogen dilution (0 to 20 per cent by volume in the total mixture) and hydrogen/iso-octane blends (from 0 to 80 per cent by volume in the fuel). The study of the impact of hydrogen addition on combustion characteristics and emissions was performed for two different engine speeds and loads. The equivalence ratio, the rate of dilution, and the intake pressure were varied either separately or simultaneously in order to maintain a constant engine load at a fixed hydrogen fraction in the fuel. The lean and dilution operating limits were also determined for all the iso-octane/hydrogen/air/nitrogen mixtures investigated, and results show that these limits are extended only when the hydrogen percentage in the fuel is higher than 40 per cent by volume. At a fixed engine load, hydrocarbon (HC) and CO emissions decrease with an increase in the hydrogen fraction in the intake mixture, while NO(x) emissions are mainly affected by the equivalence ratio and by the amount of dilution. Pumping losses, combustion efficiency and indicated efficiency are also improved with the addition of hydrogen. High values of indicated engine efficiency with low values of HC, NO(x), and CO emissions can be achieved by combining hydrogen addition with lean and/or diluted conditions.