A four-stroke, spark-ignition engine is described that seeks to achieve high expansion ratio and low throttling losses at light load, whilst retaining good knock resistance at full load operation and without the need for expensive mechanical changes to the engine. The engine does, however, incorporate a second inlet (transfer) valve and associated transfer port linked to the intake port. The timing of the transfer valve is different from that of the main inlet valve. Load modulation is achieved by control of the gas outflow from the transfer port.A computer model of the engine is first validated against measured data from a conventional engine. Comparisons are made of incylinder pressure at part load conditions, total air flowrate through the engine and intake port air velocities as a function of crank angle position.The computer model is then used to compare the performance of the new cycle with that of a conventional engine and so calculate possible thermodynamic gains at part load conditions and assess the potential for improved knock resistance at full power conditions.At light load conditions significant gains are achieved. These are sensitive to the selection of transfer valve closure point and this relationship is analysed. The full load power of the cycle is shown to be sensitive to the size of transfer volume used. The relationship between transfer port volume, full load power and end of compression temperature is therefore investigated.