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Cycle-by-cycle pressure data have been recorded for a spark ignition engine operating over a wide range of steady state and perturbed running condition. The data base has been analysed to derive mass fraction burnt, pressure development and work mean effective pressure characteristics for individual cycles. Cross-correlation coefficients have been calculated to identify predominant relationships. The effect of combustion phasing on cross-correlation coefficients is particularly significant and three regimes of behaviour have been identified. These are associated with early, optimal and late cases. The cross-correlations between parameters derived from cycle-by-cycle data do not uniformly reflect trends seen between cycle-averaged values of these. Auto-correlation results have been examined for interactions between successive cycles with less success, although, again combustion phasing can have a significant influence on the strength of auto-correlation coefficients.

Variable Valve Timing (VVT) has been demonstrated by other authors as a means of improving engine performance and idle quality. The possibility of improving fuel economy as well with VVT is investigated here. The effect of VVT on the mechanical efficiency of the engine is a crucial design factor affecting fuel economy. A number of concepts are evaluated with experimental and computer modelling results. Among these, Late Intake Valve Closing (LIVC) is shown to be a practical concept applicable to engines with two intake valves per cylinder where the intake valves can be phased relative to each other to extend the total intake opening period.