A programme of work is being undertaken to improve the performance of a spark-ignited natural gas engine, that has been converted from a diesel engine. The aim of this work is to reduce the fuel consumption and NOx emissions. All experimental data and predictions refer to full throttle operation at 1500 rpm.The work to be reported here will include baseline tests that have been used to calibrate a two-zone combustion model. Particularly important are the predictions of the NOx emissions. The simulation has then been used to predict the effects of using: a higher compression ratio, and a faster burn combustion system.The design philosophy of the resulting fast burn combustion system is discussed, and some preliminary results are presented. There will be a discussion of the ignition parameters that affect the lean burn operation, and the effect of the spark plug gap position is discussed in the context of results from a phenomenological model of turbulent combustion. Finally, comparisons are made between the baseline performance, and the measured and predicted performance of the fast burn combustion system. Brake specific NOx emissions of about 1 g/kWh can be achieved, but at the expense of reduced output and a lower brake efficiency. Under lean burn conditions the faster high compression ratio combustion system has increased the brake output by ll%, with a corresponding improvement in the brake efficiency.