Diesel governor...can anyone explain...

[Neal]

what it does? I think it's something to do with matching output to load at a given level of rpm..is that right?

And why does a diesel engine have one, but a petrol engine doesn't?

Just trying to fill one of the many gaps in my mechanical knowledge.

Thanks.

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[Rowana]

This is my understanding, but then I'm not an engineer . . .

The petrol engine is controlled by the vacuum advance/retard which ensures that the spark plug is fired at the optimum point in the cycle depending upon the load on the engine as indicated by the vacuum in the inlet manifold.

The governor on a diesel engine does a similar job by reducing the fuel input to the injectors by altering the effective stroke of the fuel pump under no load. As the load increases, the governor adjusts the fuel pump to send more fuel to the injectors as required.

As I've said, that's my understanding, but if I'm wrong, I'm sure someone with much more knowledge than I will correct me !

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[Heckler]

near enough but the analogy with advance and retard is wrong, sorry,
governors attempt to keep the speed of the engine constant as the load increases. in my diesel laguna it was very strange to ease the clutch out in 1st and then second etc and see the car speed increaseuntil in 5th she would do 30 mph.
it was also dangerous in as mutch as if you let the clutch out and then braked with the clutch out the car would surge forward as the governor attempted to keep the speed of the engine constant.
the injector pump consists of (very simplistically!!) a piston with a variable stroke pumping fuel at hi pressure to the injectors, it has a rack and pinion that changes the stroke of the piston to allow more or less fuel to be injected per stroke, the rack which changes the stroke of the piston is connected to the governor, when the engine is stopped the rack is in the fully open position holding the piston on full stroke, as the engine starts the governor closes the rack and stroke of the piston to correspond to what the throttle is set to . ie the throttle controls the governor which then controls the speed of the engine, the throttle does not directly "throttle".
on most diesel engines the throttle does not have a butterfly in the intake to "throttle" the speed of the engine like a petrol engine does.
they just have a wide open air intake and the amount of diesel injected controls the speed of the engine, hence the noise you hear from that sort of diesel is very "throaty" because you hear the sound of air going in.
some engines do have butterflies which control a diaphragm which then controls the governor.
most diesels do not have advance and retard because it is not necessary.
stu



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[clio]

A lot of diesel engines do have advance or retard, it was controlled on the old engines via the helix on the fuel pump plunger, constant beginning, veriable ending =fuel injected with the piston in the same position, end of injection veries with speed, veriable beginning, veriable ending=injection veries with speed at both ends of the cycle,veriable beginning, constant ending engine speed controls the start of injection, this is the main reason for a governor on a diesel engine. The electronic controlled engines, have fuel injection controlled by the engine computerwhich also controls the speed of theengine

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[PeteMcK]

In fact, petrol engine do have governors if they're being used in anything from lawnmowers (my 2-stroke Flymo has one controlled by a vane in the flow from the cooling fan) to tractors (e.g. the old petrol-paraffin Grey Fergie). Any engine which has variable power demand where you want to maintain, automatically, constant rpm, has, or should have, a governor.
Diesel cars don't really need an all-speed governor but that's what diesel injection pump technology comes with.


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[Birdseye]

i always thought that one of the main reasons for a governor was to limit the engine speed. fuel to a petrol engine is controlled by the carburettor intake butterfly, assuming no fuel injection. the fuel supply to a diesel, on the other hand, is a function of engine speed - the faster the engine goes, the faster the injector pump pumps, the more fuel the engine gets and so on. so without a governor, the diesel engine would speed up until it blew up - assuming no other constraints.

certainly diesel engines are governed top a maximum speed unlike most car petrol engines (bikes are often rev limited too). thats why you have the hair raising experience ona diesel car mot of having the mechanic put his foot to the floor and maintain the engine at max speed

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[Heckler]

petermck took the explanation a bit further and makes it a bit more understandeable, the helix increases or decreases the stroke of the piston as i explained altho i called it the rack because a rack does move a pin in the helix, to increase or decrease the amount of fuel and consequently the power and speed of the engine. i hasten to add that is the general principal and loads of engines work using different mechanical methods to achieve the same ends.
stu

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[spannerman]

This is a good discussion you are all half right, but Sonsy_lass comes closest. What no one seems to have realised is that diesel engine speed is related to the the amount of fuel injected and nothing to do with pump speed. A diesel engine is the ultimate 'lean burn' engine with fuel ratios of 25 to one and above. The engine will run as fast as it can for a given amount of fuel as there is no restriction in the inlet limiting air supply, so when you press the throttle you are applying tension to a spring connected to the rack in the pump this determines the point at which the spill port on the pump opens, terminating the injection duration. the longer the pump is allowed to inject the more fuel and more power therefore RPM is developed. A diesel will run as fast as it can until it reaches a point it can't develop anymore power for a given amount of air/fuel in the cylinder this results in clean smokeless combustion. The governor is what controls this tendency to run away with the RPM's and maintains a balance between what the driver demands and what the engine is inclined to do. You have all heard a large truck idling where the RPM rythmically goes up and down, this is the governor in action, when the engine gets a certain amount of fuel it will accelerate automatically, then the governor senses the increased pump RPM and moves the rack and cutting back the amount injected, then when it senses the RPM falling it moves the rack the other wayincreasing the amount of feul. this happens continually with out the driver being aware of it, operating the throttle just sets the rack to a desired RPM band which the governor maintains.


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