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Two fundamental methods of improving the brake specific fuel consumption of a spark ignition engine at part load have been investigated. These comprise the reduction of pumping losses by variable intake valve timing and changes of clearance volume to increase the expansion ratio. The resulting Otto-Atkinson cycle engine competes with the Diesel cycle engine. Test bed results from a four cylinder engine are reported which largely confirm previous predictions from thermodynamic cycle simulation. The interactive effects between cylinders on pumping loss and mixture requirements are discussed. These results show that brake specific fuel consumption improvements of up to 13% are obtainable from variable valve closure delay alone at some operating points over the usual range of speed and load, and that an additional variable clearance volume control can increase this to a maximum of 20%.