A production, direct injection diesel engine was modified to incorporate BP Oil's novel variable compression ratio (VCR) engine concept. The operation of the VCR mechanism was demonstrated and the performance and emissions potential of the concept evaluated.The VCR prototype achieved twice the specific power output of the baseline turbocharged, aftercooled engine. Its exhaust smoke emissions at high load were consistent with conventional direct injection (DI) diesel engines using equivalent fuel injection equipment. Light load particulate emissions and high load fuel consumption indicated a need for further development of the combustion system.Unexpectedly low HC emissions were achieved considering the large crevice volumes in the combustion chamber of the prototype engine. Likewise, exceptionally low NOx emissions were demonstrated. It is believed that the low HC and NOx emissions could be inherent features of the unique combustion system configuration.Further development of this novel engine concept is recommended using a prototype engine incorporating recent advances in diesel engine technology.