Diesel governors everything you Need to know
Fuzz Townshend on why the governor is the guv’nor in oil-burning engines
Diesel engines are a funny old bunch of sticks. They’re blighters to start and then, once they’re up and running, they’re off, uncontrollably, unless a governing device holds them to chosen revolutions per minute (rpm) and fixes a maximum engine rotational speed.
Diesel engines make really good use of the stuff without a constantly varying amount of fuel delivery to their injectors, their rpm rising unstoppably to the point at which crankcase pressure begins to force oil up the bores and into the combustion chambers, where it is eagerly consumed by the engine. Fuel still being injected into the combustion chamber raises rpm even further and consumes until its assured destruction.
Today, we are used to fly-by-wire throttle systems, but the concept isn’t new. Diesel engines used the mechanical version of remotely-controlled throttle operation at the dawn of their usage in road vehicles.
Mechanical governors were used from the outset of the diesel engine and remained standard issue, in some cases right up until the early 1990s. They were based on the concept of centrifugal forces and bobweights and there was no throttle butterfly valve in the air intake.
On pressing the throttle, rather than the pump rack and delivery valves being controlled directly by a linkage, the throttle position determined only the maximum extent to which
’Rather perversely, the pump rack was at near-maximum fuel position at rest’
the unconnected pump rack would be allowed to move. Counteracting this influence were bobweights, components carried over from the age of steam and exactly the same in principle and directly connected to the delivery rack. The faster the pump drive shaft rotation, the further out the bobweights would fly and, via the ‘shortening’ and ‘lengthening’ action of the scissor lever mechanism and its connection to the pump rack, so help to prevent rpm from rocketing.
A balancing ‘anti-hunt’ device was also needed to prevent oscillating revs.
Pneumatic governors were operated by the influence of a throttle butterfly and venturi (narrowing and then reinstating the original diameter of the passage) in the engine air intake manifold.
Rather perversely, the pump rack was at near-maximum fuel position under the influence of a spring acting upon the rubber pump diaphragm when at rest. No pneumatic influence was exerted due to the lack of air speed in the intake manifold. As soon as the engine started, the air passing through the venturi caused a pressure reduction in the connecting tube and thus on the venturi side of the governor’s rubber diaphragm, the latter being directly connected to the pump’s operating rack.
This then pulled the rack back from maximum fuel position, towards idle speed. Due to the position of the pneumatic take-off, engine speed increased when the throttle was opened until harmonic balance was achieved between venturi air pressure, diaphragm spring pressure and atmospheric pressure.
Later diesel engines’ injection pump governing was determined by rotational speed sensors on the crankshaft or camshaft relaying engine speed information to the pump’s control unit and so operating electrical servos that governed rack position.