The D200 is a spring loaded poppet valve designed to automatically stop an engine by closing down the air intake should excessive overspeeding occur.
Three basic build options are available. The D200 has standard springs, the D200S strong springs and the D200SX extra strong springs. The initial selection of the springs is determined by the rating of the engine to which the valve is to be fitted.
Versions of this valve are also available with the addition of various combinations of manual, oil and air pressure operated shutdown options.
The closing force on the valve is provided by the intake air flow passing through. As the air flow increases, the closing force builds up. This is resisted by the valve springs, the pre-load on which is adjustable such that at a given airflow the resulting force overcomes the spring resistance and causes the valve to close. Once closed the valve will not reset to the open condition until the engine stops.
This type of valve may be fitted to either naturally aspirated or turbocharged engines. It should be noted however that for a given valve setting the repeatability of the actual shut down speed has a greater scatter in the case of a turbocharged engine. However, unless for special reasons a precisely repeatable shutdown speed is required, adequate protection from excessive overspeed and potential resulting damage is still achieved.