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A phenomenological model of turbulent combustion has been developed and validated against data from wide ranging tests on a Ricardo E6 engine. Most tests used iso-octane, with a range of air fuel ratios and ignition timings, for tests at full throttle (with and without knock) and at part throttle. Some full throttle tests were also conducted with methanol and toluene. The engine performance was characterised by mean and coefficient of variation (CoV) of: the peak pressure, the maximum rate of pressure rise, the i.m.e.p., the burn rate and flame speed measurements. The results have been used to argue that the cycle-by-cycle variations in combustion should be characterised by the CoV of i.m.e.p. in preference to the CoV of the maximum cylinder pressure. Evidence is also presented to support the observation that the cycle-by-cycle variations in combustion are lower when the early combustion is more rapid.