The Otto-Atkinson cycle engine operates on the Otto cycle at full load, for good power density, and on the Atkinson cycle at part loads, for reduced fuel consumption. The Atkinson cycle has a larger expansion ratio than compression ratio. This is achieved by increasing the geometric compression ratio and employing late intake valve closing.A modified 1.6L I-4 engine was tested with retarded, but fixed, valve timings and increased, but fixed, geometric compression ratios to simulate the Otto-Atkinson cycle. The engine demonstrated 15% improvement in BSFC relative to the standard spark-ignition engine with 10% EGR at 1500 RPM, 2.62 Bar BMEP. Tests also revealed 50% reductions in BSNOx and BSCO emissions but a 60% increase in BSHC.A variable compression piston and a coaxial camshaft were designed and fabricated for the Otto-Atkinson cycle engine. Tests demonstrated the piston to be functional under motoring conditions, although the response was slower than required.