Variable composition hydrogen/natural gas mixtures for increased engine efficiency and decreased emissions

It is well known that adding hydrogen to natural gas extends the lean limit of combustion and that in this way extremely low emission levels can be ob tained: even the equivalent zero emission vehicle (EZEV) requirements can D e reached The emissions reduction is especially important at light engine l oads. In this paper results are presented for a GM V8 engine. Natural gas, pure hydrogen and different blends of these two fuels have been rested. The fuel supply system used provides natural gas/hydrogen mixtures in variable proportion, regulated independently of the engine operating condition. The influence of rite file( composition oil the engine operating characteristi cs and exhaust emissions has been examined mainly but not exclusively for 1 0 and 20 percent hydrogen addition. At least 10 percent hydrogen addition i s necessary for a significant improvement in efficiency. Due to the conflic ting requirements for low hydrocarbons and low NOx, determining the optimum hythane composition is not straight-forward. For hythane mixtures with a h igh hydrogen fraction, it is Sound that a hydrogen content of 80 percent or less guarantees safe engine operation (no backfire nor knock), whatever th e air excess factor. It is shown that to obtain maximum engine efficiency f or the whole load range while taking low exhaust emissions into account, th e mixture composition should be varied with respect to engine load.