The effect of combustion time and combustion variability on performance and emissions was investigated with a single cylinder CFR spark ignition engine using gaseous fuel (propane). A system was developed which allowed the collection of the products of combustion of individual engine cycles. This system consisted of an alectrically operated gas sampling valve drawing gas from immediately behind the exhaust valve, and an electronic discriminator/valve driver system. Engine cycles were selected for sampling according to the time for flame propagation to a point in the combustion chamber diametrically opposite the spark site, this being a strong indicator of total combustion time. Flame arrival was detected with an ionisation probe. A Gas Chromatograph and a modified Chemiluminiscent NOX analyser were used for gas analysis.For any particular engine operating point, flame propagation time varies randomly from cycle to cycle (cycle by cycle variability). Thus, by using the system described, it was possible to study the way in which total combustion time and combustion phasing affect combustion product formation and the effect of combustion variability on resultant bulk exhaust emissions.Experiments performed with varying spark timing were used to predict the effect of combustion time on NOX and efficiency for optimum power combustion phasing. Reduction in NOX, together with an increase in efficiency is predicted as a consequence of careful selection of combustion time.A simple model was constructed to correlate (semi-empirically) the measured NOX values at MBT spark against an effective combustion temperature and mass fraction of oxygen. The effective combustion temperature was taken to be a function of combustion time and equivalence ratio. This simple model yielded good prediction of measured NOX values.Graphs of CO, HC, NOX, A/F against combustion time are presented for a number of engine operating points.The cycle-by-cycle variations of HC, CO NOX and combustion time are presented in non-dimensional form (coefficient of variation) against equivalence ratio. NOX is found to be extremely sensitive to combustion time. CO and HC are much less sensitive but do show significant variations under certain operating conditions, indicating that control of combustion variability would be advantageous in reducing emissions as well as improving power and thermal efficiency.