Lean burn versus stoichiometric operation with EGR and 3-way catalyst of an engine fueled with natural gas and hydrogen enriched natural gas

Engine tests have been performed on a 9.6 liter spark-ignited engine fueled by natural gas and a mixture of 25/75 hydrogen/natural gas by volume. The scope of the work was to test two strategies for low emissions of harmful gases; lean burn operation and stoichiometric operation with EGR and a three-way catalyst. Most gas engines today, used in city buses, utilize the lean burn approach to achieve low NO_x formation and high thermal efficiency. However, the lean burn approach may not be sufficient for future emissions legislation. One way to improve the lean burn strategy is to add hydrogen to the fuel to increase the lean limit and thus reduce the NO_x formation without increasing the emissions of HC. Even so, the best commercially available technology for low emissions of NO_x, HC and CO today is stoichiometric operation with a three-way catalyst as used in passenger cars. The drawbacks of stoichiometric operation are low thermal efficiency because of the high pumping work, low possible compression ratio and large heat losses. The recirculation of exhaust gas is one way to reduce these drawbacks and achieve efficiencies that are not much lower than the lean burn technology. The experiments revealed that even with the 25 vol% hydrogen mixture, NO_x levels are much higher for the lean burn approach than that of the EGR and catalyst approach for this engine. However, a penalty in brake thermal efficiency has to be accepted for the EGR approach as the thermodynamic conditions are less ideal.