Some diesel engines have their speed controlled by a simple butterfly valve
like that in a petrol engine carburettor. The fuel regulation in these
engines is usually by a vacuum device which senses the pressure in the inlet
manifold and regulates the fuel injection accordingally.
In such engine, there is usually enough vacuum available at various time in
the manifold to enable such vacuum operated devices such as a brake booster
to operate due to the increase in vacuum when the throttle butterfly is
closed.
However, the vast majority of modern diesel engines have their speed
controlled via the amount of fuel delivered directly to the engine. In these
engines the throttle cable or linkage is directly coupled to the fuel
injection pump or to the governor in some engines. The speed of these
engines is directly attributble to the amount of fuel suppied to the engine.
In these engines, the inlet manifold is constantly open to the atmosphere
and the differential between atmospheric and manifold pressure is not great
enough to is insufficient to operate such devices as brake boosters. Usually
in these engines, if a vacuum source is required, a vacuum pump is provided
driving usualy off the camshaft or the back of the alternator.
Of course I am only writing about naturally aspirated engines here,
supercharged or turbocharged engines always have a positive pressure in the
inlet manifold so a vacuum is never present. In these engines a vacuum pump
is absolutely essential if a vacuum operated accessory is present.
Regards
Rod Diery
rod@rocketship.com
Kulin, Western Australia
http://www.users.bigpond.com/ROD_DIERY/cmp.html
1942 Rover light armoured car
1942 Ford CMP F-GT No. 8 gun tractor
1942 Chev CMP C15 GS truck
1943 Chev CMP C15A wireless van
1953 Fv1801 Austin Champ
None of them diesel powered but I am an ex-Australian Army marine diesel
engineer.
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