6. Effects of EGR on Engines Fueled with Natural Gas and Natural Gas/Hydrogen Blends

The exhaust gas recirculation can be used in a stoichiometric engine, for suppressing knocking and increasing efficiency, without a significant impact on pollutant emissions, since charge dilution is obtained with inert gases, allowing closed-loop control operations. However, relatively high EGR rates make worse the combustion process. This chapter deepens the effects of EGR on the performance of gaseous powered engines. In particular, the experimental data have been obtained fueling two engines with NG and NG/H₂ mixtures until 40% by volume of hydrogen, at steady state for different loads, measuring emissions upstream and downstream the three-way catalyst and analyzing the combustion process. A naturally aspirated light-duty spark ignition engine and a turbocharged heavy-duty one were tested. The results obtained with the two engines were consistent with each other. In particular, EGR could be utilized to have high specific power, with reduced thermal stress, but also to increase engine efficiency. Moreover, NG fueling permits a large flexibility in EGR system design, due to very clean engine-out exhaust gas, without visible particles. H₂ added to NG allows to mitigate the effect of EGR in reducing combustion speed. The positive effect of H₂ as combustion booster is more evident at EGR rate increasing. Nevertheless, with EGR, an increment of raw THC emission has been observed. Moreover, for the lower exhaust gas temperatures, oxidation of THC in the catalyst could result less effective. For these reasons, the blends with high hydrogen content, allowing a significant reduction of THC formation directly in the combustion chamber, can be usefully utilized for engines optimization with high EGR rates.