The main reason for the utilization of methane gas, in the form of natural gas or purified biogas, is as substitute of the diesel oil, aiming to the reduction of the pollution by the exhaust gases from the combustion engines of the urban buses. The largest pollutants of diesel engines are the particulates, originated by the combustion process, and the sulphur oxides. The latter, further, to act as nucleants for the particulates, contribute to the so called “acid rain”.
As the natural gas is free from sulphur, and the spark ignition cycle does not originate particulates, these are obvious advantages of the utilization of the methane as fuel in the urban vehicles. But the spark ignition engines burning methane originates much more NOx, nearly twice as much, than the diesel engine equivalent, particularly at high loads.
Several processes are being studied for the reduction of the NOx emissions. In this work it was considered the water injection in the intake manifold, obtaining a reduction of 2/3 in the amount of NOx from the exhaus gases.