UCL Discovery

Abstract

Bio-gas is being used increasingly as a fuel in spark-ignition engines; however, most applications are produced without recourse to a systematic development programme. Furthermore, the published laboratory data show wide variations. Compared with methane, bio-gas is said to give up to a 20% reduction in output and up to a 39% reduction in efficiency. It is argued here that such large reductions occur only when the air:fuel ratio and ignition timing have not been correctly optimised. A model is used to show how the change of fuel composition can affect the air:fuel ratio. A computer simulation of a spark-ignition engine is then used to model the effects of non-optimised ignition timing and air:fuel ratio, and the effects of the slower combustion of bio-gas. In a fast-burn combustion system at stoichiometric, the model predicts a 1.3 percentage point reduction in efficiency and a 9% reduction in output when the fuel is changed from methane to bio-gas. The simulation is also used to show the effects of fuel composition, burn rate, ignition timing and equivalence ratio on the emissions of nitric oxide.