Three position diesel engine intake air throttling system

An intake air throttling system for a diesel engine comprising an air intake passage. The system includes a throttling valve mounted in the air intake passage so as to selectively throttle air flow through it. This throttling valve can be positioned to at least a first position in which it substantially fully closes the air intake passage, a second position in which it partly throttles the air intake passage to a substantial extent but does not fully close it, and a third position in which it does not throttle the air intake passage to any substantial extent. The throttling valve is controlled by a control system, which positions it to the first position when the engine is being stopped, which positions it to the second position when the engine is being idled, and which positions it to the third position when the engine is being operated in non idling load bearing operational condition. Thereby, when the engine is being stopped, the cutoff of intake air reduces stopping vibration considerably, while when the engine is being idled the partial but substantial throttling of intake air reduces idling vibration and reduces wandering of engine idling speed; and both these effects are accomplished by the use of one throttling valve.