Increasingly stringent diesel engine emissions legislation is creating the need for precisely controlled engine timing. Closed loop timing is possible based upon a suitable feedback sensor. Sensing the fuel injection event is convenient, but fails to take into account the variable ignition delay period. By optically sensing the start of combustion (S.O.C.) a closed loop system based upon the final event can be achieved. The system configuration, and some problems encountered during the design and development process are discussed.