Diesel engine warm-up

[NB Alnwick]

Perhaps there is some confusion with terminology. Many people use the term 'throttle' incorrectly. Strictly speaking, a 'throttle' is a device for restricting or adjusting the supply of air or gas to the engine.

On a traditional petrol engine this will be connected to a foot pedal (also known as the 'accelerator pedal') or a hand lever.

 

A diesel engine doesn't usually have such a device so the lever used to adjust the engine speed on a diesel should properly be referred to as a 'speed regulator' or just 'regulator'.

[Timleech]

Yes as I said a diesel is an unthrottled i.c. engine i.e, no throttle, part of the reason why petrol engines can close the economy gap with diesell on a fast run, the petrol flat out is unthrottled. If you have a fixed throtte setting on diesel and the load increases then rpm will decrease, or vica versa, same as petrol. Having said all that I have worked on a throttled diesel, a tractor. We had removed butterfly assembly and then turned the engine to adjust valves, having no throttle it went to full revs, big panic scrabbling for the fuel pump stop lever!

 

 

CAV and other pneumatic diesel engine governors were common at one time, they used a butterfly valve in the air intake to control airflow through an adjacent venturi. They were fine until the diaphragm at the pump end developed a leak, whereupon the engine would 'default' to full fuel.

 

The term 'throttle' as often (mis)used to describe the speed control of whatever sort, as in 'full throttle' = handle right down, flat out, foot to the floor etc., rather than in its strict sense of a flow restrictor.

 

Tim

[Timleech]

It consists of a pair is centrifugal weights pulling on a spring and the other end of the spring is held in a position which corresponds to the speed regulator setting. The fuel flow is controlled by the position of the weights which depends only on the speed of the engine and the position of the speed regulator.

 

Assume that the engine is at a stable speed and load: now increase the load. The speed will drop and the spring will pull the fuel rack in the direction of more fuel flow. The engine will speed up, but it can't ever get back to exactly the same speed with the new load, because the fuel flow depends only on speed and regulator setting. If the engine reached the original speed it would be running with the original fuel flow: impossible with a heavier load.

 

 

MP.

 

That's the bog-standard mechanical governor arrangement.

I don't understand your last sentence?? Heavier load requires greater fuel flow, that's what the governor does, there's no reason AFAIK why the governor can't bring the speed back up provided it's well designed and the load is within the limitations of the engine.

(Admittedly I'm not an expert in governor design)

 

Tim

[oggie]

Not read all the replies on this topic but thought I'd add that I recently read through the the owners manual which came with my Isuzu 42hp engine. Which states engine to be started and ran at one third throttle ie: 1000 rpm for a full 10 minutes to warm the engine before putting it under load.

I try and adhere to this whenever possible but the engine does come under partial load when the Adverc management system kicks in after 5 minutes of running.

 

Oggie

[NB Alnwick]

That's the bog-standard mechanical governor arrangement.

I don't understand your last sentence?? Heavier load requires greater fuel flow, that's what the governor does, there's no reason AFAIK why the governor can't bring the speed back up provided it's well designed and the load is within the limitations of the engine.

(Admittedly I'm not an expert in governor design)

 

Tim

 

Older pneumatic governors or those with centrifugal weights are often unreliable at very slow speeds - especially when the linkages a badly worn - you only have to listen to an old bus or truck engine, hunting up and down when idling, to hear the effect.

 

Older marine engines designed for accurate rpm at given regulator settings often have hydraulic governors which were more expensive but in terms of accuracy and reliability are almost as good as modern electronic/computer controlled systems.

[MoominPapa]

That's the bog-standard mechanical governor arrangement.

I don't understand your last sentence?? Heavier load requires greater fuel flow, that's what the governor does, there's no reason AFAIK why the governor can't bring the speed back up provided it's well designed and the load is within the limitations of the engine.

(Admittedly I'm not an expert in governor design)

 

Tim

 

The spinning govenor weights pull on the spring and extend it until the force of the spring equals the force from the weights. Those two forces are the only things controlling the position of the fuel rack. Therefore the fuel flow depends only on the engine speed and the speed control position: without moving the speed control the fuel flow depends only on engine speed and vice versa: for a given engine speed the fuel flow is fixed. Without moving the speed control it's impossible for the engine to be running at 1000rpm and light load, and then to continue running at 1000rpm with heavy load. To increase fuel flow with heavy load, the engine must have slowed down is bit (since engine speed is the only variable which affects fuel flow.) The speed range from unloaded to flat out is not very large (say 100 rpm) but it must exist.

 

I'm not sure I explained that any better the second time?

 

MP.

[Albion]

The spinning govenor weights pull on the spring and extend it until the force of the spring equals the force from the weights. Those two forces are the only things controlling the position of the fuel rack. Therefore the fuel flow depends only on the engine speed and the speed control position: without moving the speed control the fuel flow depends only on engine speed and vice versa: for a given engine speed the fuel flow is fixed. Without moving the speed control it's impossible for the engine to be running at 1000rpm and light load, and then to continue running at 1000rpm with heavy load. To increase fuel flow with heavy load, the engine must have slowed down is bit (since engine speed is the only variable which affects fuel flow.) The speed range from unloaded to flat out is not very large (say 100 rpm) but it must exist.

 

I'm not sure I explained that any better the second time?

 

MP.

I think what you're trying to describe is hysteresis which exists in all simple mechanical feedback systems. Whether it is of any real significance in the real world is another matter of course.

Roger

[Timleech]

The spinning govenor weights pull on the spring and extend it until the force of the spring equals the force from the weights. Those two forces are the only things controlling the position of the fuel rack. Therefore the fuel flow depends only on the engine speed and the speed control position: without moving the speed control the fuel flow depends only on engine speed and vice versa: for a given engine speed the fuel flow is fixed. Without moving the speed control it's impossible for the engine to be running at 1000rpm and light load, and then to continue running at 1000rpm with heavy load. To increase fuel flow with heavy load, the engine must have slowed down is bit (since engine speed is the only variable which affects fuel flow.) The speed range from unloaded to flat out is not very large (say 100 rpm) but it must exist.

 

I'm not sure I explained that any better the second time?

 

MP.

 

No, if the speed drops the weights come in and more fuel is supplied; if the speed rises they go out and less fuel is supplied. They adjust the fuel supply until the selected speed is reached, the speed being selected by the tension or pressure (depends on design) of the spring. If it worked as you described the governor would be a waste of space, surely?

Yes there will be some speed variation with load, how much depends on the quality of the governor design and its condition. Some of the Lister manuals tell you what the expected speed tolerances are, usually only a few percent IIRC.

 

Tim

[MoominPapa]

No, if the speed drops the weights come in and more fuel is supplied; if the speed rises they go out and less fuel is supplied. They adjust the fuel supply until the selected speed is reached, the speed being selected by the tension or pressure (depends on design) of the spring. If it worked as you described the governor would be a waste of space, surely?

Yes there will be some speed variation with load, how much depends on the quality of the governor design and its condition. Some of the Lister manuals tell you what the expected speed tolerances are, usually only a few percent IIRC.

 

Tim

My argument is that there has to be some variation with load, because it's only because of the slowing of the engine that extra fuel is supplied: if the engine returned to its original speed it would also have returned to the original fuel flow. This isn't an imperfection, it's an essential feature of a simple spring and weight governor. My experience with the FR2 (eg by putting the box into neutral when the engine is working hard) is that the band is about 100rpm. I guess the width of the band depends on the rate of the spring: the lower the spring rate that smaller the band.

 

MP.

 

 

 

I think what you're trying to describe is hysteresis which exists in all simple mechanical feedback systems. Whether it is of any real significance in the real world is another matter of course.

Roger

Hysteresis is an effect which depend on the direction of change, it corresponds to friction in the pivots and linkages in this case. That's not what I'm talking about: I'm talking about equilibirum speeds, ignoring friction.

 

MP.

Edited July 4, 2009 by MoominPapa

[Timleech]

My argument is that there has to be some variation with load, because it's only because of the slowing of the engine that extra fuel is supplied: if the engine returned to its original speed it would also have returned to the original fuel flow. This isn't an imperfection, it's an essential feature of a simple spring and weight governor. My experience with the FR2 (eg by putting the box into neutral when the engine is working hard) is that the band is about 100rpm. I guess the width of the band depends on the rate of the spring: the lower the spring rate that smaller the band.

 

MP.

 

 

I don't understand why you insist that speed = fuel flow? More load requires more fuel for a given speed, that's what the governor does. In simple terms, 1500 rpm developing 3 bhp (maybe alternator and general windage etc) will take one-sixth of the fuel per pump stroke of that needed for 18 bhp @ 1500rpm. I'm well aware there are 'fixed costs' which make it much more complex, but that doesn't really matter here.

 

Tim

[NB Alnwick]

I don't understand why you insist that speed = fuel flow? More load requires more fuel for a given speed, that's what the governor does. In simple terms, 1500 rpm developing 3 bhp (maybe alternator and general windage etc) will take one-sixth of the fuel per pump stroke of that needed for 18 bhp @ 1500rpm. I'm well aware there are 'fixed costs' which make it much more complex, but that doesn't really matter here.

 

Tim

 

Unless, of course, the governor and its linkages are badly worn then the engine speed tends to hunt up and down - which is why older engines (like those that used to be used on fairground generator sets) often do this.

[Timleech]

Unless, of course, the governor and its linkages are badly worn then the engine speed tends to hunt up and down - which is why older engines (like those that used to be used on fairground generator sets) often do this.

 

Yes, I'm working on a National at the moment, they are notorious for this and it's a particularly bad example,

but aren't we talking about what the governor is supposed to do

 

Tim

[MoominPapa]

I don't understand why you insist that speed = fuel flow?

Because with a governor which is just balancing centrifugal weights against a spring, the only thing which affects fuel flow is engine speed.

 

You agree that fuel flow depends on the position of the fuel racks? The position of the fuel racks depends only of the amount of compression of the spring? The amount of compression of the spring is determined by how hard the weights are pushing on it. The only thing that controls how hard the weights are pushing is the speed they are spinning.

More load requires more fuel for a given speed, that's what the governor does. In simple terms, 1500 rpm developing 3 bhp (maybe alternator and general windage etc) will take one-sixth of the fuel per pump stroke of that needed for 18 bhp @ 1500rpm.

 

So explain to me how the governor can increase the fuel flow six times when the speed control hasn't moved and the engine speed hasn't changed. Those are the only two inputs to the system!

I'm well aware there are 'fixed costs' which make it much more complex, but that doesn't really matter here.

Agreed.

 

MP

[RLWP]

So explain to me how the governor can increase the fuel flow six times when the speed control hasn't moved and the engine speed hasn't changed. Those are the only two inputs to the system!

 

MP

 

Load on the engine. If the load increases (plastic bags around the prop for instance) the governor tries to hold a constant speed by increasing the fuel flow.

 

Richard

 

I guess that you knew that...

[alan_fincher]

A diesel engine doesn't usually have such a device so the lever used to adjust the engine speed on a diesel should properly be referred to as a 'speed regulator' or just 'regulator'.

Understood,

 

But FWIW, something like the original operator's handbook for a BMC diesel refers throughout to the thing that controls engine speed as the "throttle".

 

Check that the gear lever is in the neutral position, fully open the throttle, and operate the starter switch.

 

It's not hard to understand why many other people do the same.

 

Note also that repeated advice on here not to open the "throttle" when starting, that's what the instructions say to do.

 

Followed by....

 

Release the starter switch, and close the throttle as soon as the engine is running.

[NB Alnwick]

Understood,

 

But FWIW, something like the original operator's handbook for a BMC diesel refers throughout to the thing that controls engine speed as the "throttle".

 

 

 

It's not hard to understand why many other people do the same.

 

Note also that repeated advice on here not to open the "throttle" when starting, that's what the instructions say to do.

 

Followed by....

 

Obviously they do not mean 'throttle' in the correct sense. It is a shame that they do not use the correct terminology but so many words in the English language seem to have taken on meanings that have little relationship to what was intended.

 

Personally I'd like to 'throttle' these aberrations

[Timleech]

Because with a governor which is just balancing centrifugal weights against a spring, the only thing which affects fuel flow is engine speed.

 

You agree that fuel flow depends on the position of the fuel racks? The position of the fuel racks depends only of the amount of compression of the spring? The amount of compression of the spring is determined by how hard the weights are pushing on it. The only thing that controls how hard the weights are pushing is the speed they are spinning.

 

 

So explain to me how the governor can increase the fuel flow six times when the speed control hasn't moved and the engine speed hasn't changed. Those are the only two inputs to the system!

 

Agreed.

 

MP

 

No, the position of the fuel racks depends upon the interaction between governor weights and spring, which will vary with load.

 

Tim

[nb Innisfree]

I think we have to differentiate between types of governor, I can't speak for trad engines but all modern canal boat engines don't have speed governer but do have a fuel gov. If load is applied rpm will drop depending on load and gov will prevent rpm going above or below that lower rpm, it won't maintain original rpm, in effect it maintains rpm set by 'throttle' setting. If a speed gov is fitted as well as (not instead of) a fuel gov, as on a tractor, then this will maintain rpm at a certain level irrespective of load. In effect it controls, up or down the, 'throttle' setting

 

All road vehicles have a fuel governor but not a speed governor, the same as modern canal boat engines

Edited July 4, 2009 by nb Innisfree

[NB Alnwick]

I think we have to differentiate between types of governor, I can't speak for trad engines but all modern canal boat engines don't have speed governer but do have a fuel gov. If load is applied rpm will drop depending on load and gov will prevent rpm going above or below that lower rpm, it won't maintain original rpm, in effect it maintains rpm set by 'throttle' setting. If a speed gov is fitted as well as (not instead of) a fuel gov, as on a tractor, then this will maintain rpm at a certain level irrespective of load. In effect it controls, up or down the, 'throttle' setting

 

All road vehicles have a fuel governor but not a speed governor, the same as modern canal boat engines

 

There really is no difference - the governor always controls the fuel oil supply to the injectors (via the 'rack' on the injection pump) in order to maintain the engine rpm at a speed that coincides precisely with the regulator (often erroneously called 'throttle') position. The speed of the engine is varied only by moving the regulator - adding or removing load should not make any difference to the engine speed.

 

This applies to road vehicles, marine engines and agricultural engines irrespectively.

 

I have not seen an engine fitted with separate governors - all the governors that I have seen have upper and lower speed limits (often adjustable) but within that range the speed is maintained to coincide with the regulator.

Edited July 4, 2009 by NB Alnwick

[NB Alnwick]

To clarify matters, the following is a model answer from my copy of Diesel Engine Maintenance.

 

Each cylinder has a fuel injector designed to meter and inject fuel into the cylinder at the proper

instant. To accomplish this function, the injectors are actuated by the engine's camshaft. The

camshaft provides the timing and pumping action used by the injector to inject the fuel. The

injectors meter the amount of fuel injected into the cylinder on each stroke. The amount of fuel

to be injected by each injector is set by a mechanical linkage called the fuel rack. The fuel rack

position is controlled by the engine's governor. The governor determines the amount of fuel

required to maintain the desired engine speed and adjusts the amount to be injected by adjusting

the position of the fuel rack.

[nb Innisfree]

There really is no difference - the governor always controls the fuel oil supply to the injectors (via the 'rack' on the injection pump) in order to maintain the engine rpm at a speed that coincides precisely with the regulator (often erroneously called 'throttle') position. The speed of the engine is varied only by moving the regulator - adding or removing load should not make any difference to the engine speed.

 

This applies to road vehicles, marine engines and agricultural engines irrespectively.

 

I have not seen an engine fitted with separate governors - all the governors that I have seen have upper and lower speed limits (often adjustable) but within that range the speed is maintained to coincide with the regulator.

 

You say that adding or removing load will not alter engine speed on road or marine (sorry don't know how to quote yet) but if you go from a level road and start to climb a hill engine speed will drop, it will not maintain engine, and therefore, vehicle speed. To maintain engine/vehicle speed you will have to press on the accellerator pedal and therefore increase speed regulator. This fuel governor is standard on all diesel engines, to maintain engine rpm at a given level then an extra dimension in the gov. has to be introduced which will override speed regulator setting hence 'two' regulators, tho both may be incorporated in one so it would appear not to be. Agricultural vehicles require a speed gov. completely different to a road or marine application, if tractor is driven on the road then the speed gov. becomes a bit of a liability and needs an override to disable speed gov. and use fuel gov. only. Can't speak for modern tractors but if an old tractor is road drive and brakes are applied with out declutching governor will increase fuel flow which will try to increase engine speed therefore fighting against the brakes, a bit scary until you get used to it and declutch!

 

 

 

To clarify matters, the following is a model answer from my copy of Diesel Engine Maintenance.

 

Yes that is correct, but if load is applied speed will drop to a lower level but governor will maintain a revised fuel supply consistent with that new lower speed, it will not increase fuel supply to maintain previous rpm at a higher load.

[Timleech]

You say that adding or removing load will not alter engine speed on road or marine (sorry don't know how to quote yet) but if you go from a level road and start to climb a hill engine speed will drop, it will not maintain engine, and therefore, vehicle speed. To maintain engine/vehicle speed you will have to press on the accellerator pedal and therefore increase speed regulator. This fuel governor is standard on all diesel engines, to maintain engine rpm at a given level then an extra dimension in the gov. has to be introduced which will override speed regulator setting hence 'two' regulators, tho both may be incorporated in one so it would appear not to be. Agricultural vehicles require a speed gov. completely different to a road or marine application, if tractor is driven on the road then the speed gov. becomes a bit of a liability and needs an override to disable speed gov. and use fuel gov. only. Can't speak for modern tractors but if an old tractor is road drive and brakes are applied with out declutching governor will increase fuel flow which will try to increase engine speed therefore fighting against the brakes, a bit scary until you get used to it and declutch!

 

I don't pretend to know anything about modern vehicle diesel governors, (aren't they 'part of the computer' now anyway?), and certainly precise speed control isn't important for them, but marine and stationary governors are essentially 'constant variable speed' devices. I'm not clear how that has to be 'two devices in one' as you seem to be saying, the simple bob weight type does a fair enough job by itself for most purposes. More critical jobs might need something fancier.

I have a mini-digger with a 3-cyl Mitsubishi, exactly the same engine as sold by Vetus and others for boats, that keeps the set speed regardless of load.

With your tractor diving analogy, surely you would slow the engine/ 'throttle' setting before applying the brakes? I know that if I am driving my car (VW diesel) on tickover, eg in a traffic queue, and touch the brakes the 'governor' will do it's damndest to keep the speed up. Can't say I've tried pressing accelerator and brakes at the same time!

 

Tim

 

Yes that is correct, but if load is applied speed will drop to a lower level but governor will maintain a revised fuel supply consistent with that new lower speed, it will not increase fuel supply to maintain previous rpm at a higher load.

 

That will surely depend upon how 'good' the governor is?

 

Edited to add that 'I knew it was there somewhere'.

From the Lister ST range workshop manual, this is an engine with a fairly straightforward bob-weight mechanical governor and which was until recently common in canal boats with plenty still around - Paraphrasing - For the fixed speed governor, speed variation between no load & full load, 4.5%. For Variable speed versions, which essentially have an external governor spring instead of a pair of internal springs, 8% speed variation between no load & full load at maximum speed.

 

 

Tim

Edited July 4, 2009 by Timleech

[nb Innisfree]

I don't pretend to know anything about modern vehicle diesel governors, (aren't they 'part of the computer' now anyway?), and certainly precise speed control isn't important for them, but marine and stationary governors are essentially 'constant variable speed' devices. I'm not clear how that has to be 'two devices in one' as you seem to be saying, the simple bob weight type does a fair enough job by itself for most purposes. More critical jobs might need something fancier.

I have a mini-digger with a 3-cyl Mitsubishi, exactly the same engine as sold by Vetus and others for boats, that keeps the set speed regardless of load.

With your tractor diving analogy, surely you would slow the engine/ 'throttle' setting before applying the brakes? I know that if I am driving my car (VW diesel) on tickover, eg in a traffic queue, and touch the brakes the 'governor' will do it's damndest to keep the speed up. Can't say I've tried pressing accelerator and brakes at the same time!

 

Tim

 

 

 

That will surely depend upon how 'good' the governor is?

 

Tim

 

Forget computers in a modern diesel the principle is still the same.

A diesel car in 1st gear and tickover will try via gov to maintain fuel flow consistent with tickover, if you try to go up a steep hill it will eventually stall without pressing accellerator.

 

Yes I would slow engine throttle before applying brakes on tractor, but if emergency/unexpected braking is needed then thrre may not be enough time to reach for throttle lever so declutching is needed. Older tractors don't have a foot accellerator because they have speed a gov. which have to be set by hand.

 

Generators have a speed gov. maintaining steady revs despite varying loads as does a lawnmower.

 

If a marine engine had a speed gov. then if propellor become clogged up at say 1500rpm cruising speed then gov would try to maintain 1500rpm and would eventually reach full throttle, black smoke galore untill blockage became so bad it overrode engine and eventually stalled it.

 

A boat/car engine has a gov which maintains fuel flow for a given throttle setting, a tractor has a gov which maintains rpm for a given throttle setting

 

Phew I think I have covered it, just going to get my tin hat

[Timleech]

Forget computers in a modern diesel the principle is still the same.

A diesel car in 1st gear and tickover will try via gov to maintain fuel flow consistent with tickover, if you try to go up a steep hill it will eventually stall without pressing accellerator.

 

Yes I would slow engine throttle before applying brakes on tractor, but if emergency/unexpected braking is needed then thrre may not be enough time to reach for throttle lever so declutching is needed. Older tractors don't have a foot accellerator because they have speed a gov. which have to be set by hand.

 

Generators have a speed gov. maintaining steady revs despite varying loads as does a lawnmower.

 

If a marine engine had a speed gov. then if propellor become clogged up at say 1500rpm cruising speed then gov would try to maintain 1500rpm and would eventually reach full throttle, black smoke galore untill blockage became so bad it overrode engine and eventually stalled it.

 

A boat/car engine has a gov which maintains fuel flow for a given throttle setting, a tractor has a gov which maintains rpm for a given throttle setting

 

Phew I think I have covered it, just going to get my tin hat

 

What makes you think that doesn't happen? Except of course that the black smoke shouldn't happen as the fuel should be limited to just below the 'black smoke' point

 

You may be right about modern car engines, not my area of expertise, but I'm sure you are wrong about (most) boat diesels as seen on inland waterways. Maybe those based on modern car engines are as you describe, but they are very much in the minority.

 

I suspect the 'tractor overrun' thing has as much to do with the hand throttle as any governor arrangements, you would find the same thing in a car with a brick on the accelerator pedal

Tim

Edited July 4, 2009 by Timleech

[NB Alnwick]

Forget computers in a modern diesel the principle is still the same.

A diesel car in 1st gear and tickover will try via gov to maintain fuel flow consistent with tickover, if you try to go up a steep hill it will eventually stall without pressing accellerator.

 

Yes I would slow engine throttle before applying brakes on tractor, but if emergency/unexpected braking is needed then thrre may not be enough time to reach for throttle lever so declutching is needed. Older tractors don't have a foot accellerator because they have speed a gov. which have to be set by hand.

 

Generators have a speed gov. maintaining steady revs despite varying loads as does a lawnmower.

 

If a marine engine had a speed gov. then if propellor become clogged up at say 1500rpm cruising speed then gov would try to maintain 1500rpm and would eventually reach full throttle, black smoke galore untill blockage became so bad it overrode engine and eventually stalled it.

 

A boat/car engine has a gov which maintains fuel flow for a given throttle setting, a tractor has a gov which maintains rpm for a given throttle setting

 

Phew I think I have covered it, just going to get my tin hat

 

I am not sure where you are getting mixed up but in simple terms, there is only one basic type of governor. It governs the speed by regulating the fuel supplied to the cylinder for any given speed/power requirement. The speed can be varied where the governor is connected to an accelerator pedal (as in a car) or a hand speed regulator (as on a boat or certain agricultural engines) - on constant speed industrial engines there may be an internal adjustment to set and maintain the correct speed.

 

On modern engines, an electronic or computer controlled device acts as the governor but it still does exactly the same job.

 

You can stall a diesel engine if the load is significantly increased at low rpm but the governor will try to keep it going and this is exactly what happens when I get something nasty wrapped round the prop - my engine doesn't slow down but the normally quiet exhaust suddenly starts to bark and the engine gets very hot very quickly!