A range of small spring loaded poppet valves designed to automatically stop an engine by closing down the air intake should excessive overspeeding occur.
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 a spring, the pre-load of which is adjustable such that at a given air flow 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 automatic shut down speed has a greater scatter in the case of a turbocharged engine. However, unless for special reasons a precisely repeatable shut down speed is required, adequate protection from excessive overspeed and potential resulting damage is still achieved.
Types D34BF, D34BFS, D35BF, D35BFS, D39BF and D39BFS all include an integral engine air cleaner. Types D29, D30 and D41 are not available with an integral air cleaner.
A manual shut down option is available for all types.