Positive Air Shut-Off
It is the valve fitted to the intake manifold which, when commanded, closes the air path completely and stops the engine. In cases of diesel engine runaway where fuel shut-off does not help, it is the only proven solution that can stop the engine. It is installed so that it is triggered automatically when overspeed is detected.
The operating principle is direct: diesel needs air to burn; when the air path is closed, combustion stops whatever the fuel source is. The valve typically consists of a spring-loaded closing disc and a latch mechanism holding it; when the latch is released the disc closes under spring force. In other words it is a construction that falls to the safe side.
The points to watch during installation are these:
- The valve is fitted between the air filter and the turbo or manifold, in the path through which leaking vapour also passes.
- Triggering must come from an independent chain that detects overspeed.
- After it has operated it is reset by hand; this makes investigating the event mandatory.
This equipment is part of the engine's safety chain and is independent of the electronic throttle control kit. No air shut-off or overspeed function is declared in the GS740 and GS740-3P documentation; in installations that carry a runaway risk this protection must be planned separately.
Context in which this term is used: Electronic speed and throttle control on a diesel engine