Diesel engine warm-up

[nb Innisfree]

True of a petrol engine certainly but our modern diesel car is capable of maintaining a steady engine speed until such time as the load is too much for it and it is time to select a lower gear. As I said earlier, modern automotive diesel engines have complex electronic controls that manage the engine and it is possible to stall the engine (rather than kick out clouds of black smoke) if too much load is applied at low speed.

 

Modern electronic common rail diesels behave exactly the same as previous mechanical diesels the only difference in there being more efficient fuel management preventing overfuelling and therefore black smoke. If a diesal car climbs a slope and has enough power to do that without changing gear then the vehicle WILL slow if accellerator is left at same setting. An ordinary non electronic diesel car WILL stall if overloaded, exactly as an electronic one.

 

Tell me, if you were driving a diesel car on a level road and you encountered an upward slope, without altering accellerator pedal what would happen to the car's speed? What would happen, in the same circumstances, if you encountered a downward slope?

 

If I have an hour or so to spare this evening (after fixing the washing machine!) I'll see whether it's viable to scan and post a couple of pages on basic mechanical governor design. MP should find it interesting, even if noone else does

 

Just to muddy the waters a bit, CAV used to make (I expect others did too, if it was any good) a 'minimum and maximum' governor for motor vehicle engines. This controlled the idling speed and limited the top speed, at other times the fuel rack was contolled directly by the accelerator pedal.

Maybe this style became widespread for vehicle engines, I really don't know, but it's really only suitable for road vehicles. Industrial and tractor engines, which form the basis of most canal boat engines now, need more of a 'proper' governor.

 

Tim

 

All diesel engines top speed are governed. Fuel rack in modern canal boat and car engine is controlled directly from pedal/lever. Tractors and generators are controlled indirectly.

Edited July 6, 2009 by nb Innisfree

[MoominPapa]

If I have an hour or so to spare this evening (after fixing the washing machine!) I'll see whether it's viable to scan and post a couple of pages on basic mechanical governor design. MP should find it interesting, even if noone else does

 

Oh, yes please!

 

MP.

[Timleech]

All diesel engines top speed are governed. Fuel rack in modern canal boat and car engine is controlled directly from pedal/lever. Tractors and generators are controlled indirectly.

 

That's where we fundamentally disagree

 

Tim

[nb Innisfree]

That's where we fundamentally disagree

 

Tim

 

Yeah I know

[Naughty Cal]

So what is the proper method of warming a diesel engine?

 

We just fire up and go. The magnetic clutch wont let the supercharger kick in until the engine is warm so we cant kill it by overworking it whilst cold.

[Proper Job]

Oh, yes please!

 

MP.

I like a nice bit of engine porn myself too

 

So what is the proper method of warming a diesel engine?

 

We just fire up and go. The magnetic clutch wont let the supercharger kick in until the engine is warm so we cant kill it by overworking it whilst cold.

I'm not sure the average NB engine is a sophisticated as yours

 

Magnetic clutches and superchargers! Although some of them appear to have them fitted when they go past

[Timleech]

I just did a quick test on a Vetus/Mitsubishi, 4cyl, 3 years old, one of the commonest nb engines around.

It has no instrumentation at present, and it's not driving an alternator.

Two simple tests. First run the engine up to a marked setting on the lever, out of gear, observed the engine speed by ear (maybe 2000 rpm).

Then go up to the same setting, but in gear. The engine rpms were very much in the same ball park.

 

Next, uncouple morse linkage from gearbox. Run engine in gear up to a 'fast cruising speed', then pull out of gear without touching the speed control. There was a significant rise in rpm, but not a dramatic one. I reckon that if the lever were controlling the fuel pump directly, the amount of fuel being delivered to achieve that speed in gear would have taken the 'light' engine up to maximum revs. It didn't come within a mile of doing that.

 

I'm convinced by those tests, if I wasn't already sure of it, that that engine is governed throughout its speed range.

 

Tim

[Naughty Cal]

I'm not sure the average NB engine is a sophisticated as yours

 

All the more reason not to knacker it. Dont fancy shelling out 15k for a new one

[nb Innisfree]

I just did a quick test on a Vetus/Mitsubishi, 4cyl, 3 years old, one of the commonest nb engines around.

It has no instrumentation at present, and it's not driving an alternator.

Two simple tests. First run the engine up to a marked setting on the lever, out of gear, observed the engine speed by ear (maybe 2000 rpm).

Then go up to the same setting, but in gear. The engine rpms were very much in the same ball park.

 

Next, uncouple morse linkage from gearbox. Run engine in gear up to a 'fast cruising speed', then pull out of gear without touching the speed control. There was a significant rise in rpm, but not a dramatic one. I reckon that if the lever were controlling the fuel pump directly, the amount of fuel being delivered to achieve that speed in gear would have taken the 'light' engine up to maximum revs. It didn't come within a mile of doing that.

 

I'm convinced by those tests, if I wasn't already sure of it, that that engine is governed throughout its speed range.

 

Tim

 

Was the engine in a boat and was it connected to a prop?

What rpm was 'fast cruising speed'

 

Lever controls fuel pump but is overidden by governor, so engine is, as you say, governed throughout it's speed range. If, with car or boat, you open up accellerator/lever quickly to say half revs setting (1500rpm?), governor, a second or so later, will take engine up to 1500 rpm, consistent with that setting. If load is then applied rpm will drop, amount depending on load. The only way to get engine back up to 1500rpm would be to increase lever setting, if, as you say, that isn't the case then when you started to drive up a hill the car wouldn't slow down but would carry on at the same speed without the accellerator being touched which any driver will tell you this doesn't happen. The same happens, in reverse, when going downhill, load decreases and rpm increases, gov doesn't decrease rpm unless accellerator is reduced.

 

When going uphill you have to put your foot down to maintain speed.

 

When going downhill you have to reduce accellerator to prevent car speeding up.

 

In other words engine speed has to be adjusted manually for varying loads, it isn't automatic.

[Timleech]

Was the engine in a boat and was it connected to a prop?

What rpm was 'fast cruising speed'

 

Lever controls fuel pump but is overidden by governor, so engine is, as you say, governed throughout it's speed range. If, with car or boat, you open up accellerator/lever quickly to say half revs setting (1500rpm?), governor, a second or so later, will take engine up to 1500 rpm, consistent with that setting. If load is then applied rpm will drop, amount depending on load. The only way to get engine back up to 1500rpm would be to increase lever setting, if, as you say, that isn't the case then when you started to drive up a hill the car wouldn't slow down but would carry on at the same speed without the accellerator being touched which any driver will tell you this doesn't happen. The same happens, in reverse, when going downhill, load decreases and rpm increases, gov doesn't decrease rpm unless accellerator is reduced.

 

When going uphill you have to put your foot down to maintain speed.

 

When going downhill you have to reduce accellerator to prevent car speeding up.

 

In other words engine speed has to be adjusted manually for varying loads, it isn't automatic.

 

Yes it was in a boat, not a car, driving a prop. Typical modern nb installation except that all the electrics are defunct.

Therein lies the difference, it's a BOAT ENGINE in a boat, not a car!!

 

Tim

Edited July 6, 2009 by Timleech

[Albion]

I've decided to dig out my Bosch Automotive Handbook for chapter and verse on this argument and it gives the following explanations for diesel governors:-

(I was going to scan the two pages and post them but the book is a small pocket-sized book and won't scan sufficiently clearly to be able to read on-screen so I've had to somewhat precis the info without affecting the meaning.)

 

Speed governing

The main function of the governor is to limit the maximum engine speed. Depending on type the governor's functions may include maintaining specific, constant engine speeds such as idle or other speeds in the range between idle and max speed. The governor can also adjust full load delivery in accordance with engine speed (adaption), boost or atmospheric pressure and it can be used to meter the extra fuel required for starting. The governor adapts the delivery quantity required for these conditions by making corresponding adjustments to the position of the control rack.

 

Mechanical (flyweight) governors

Flyweights, which act against the force of the governor springs, are connected to the control rack. At a constant speed the forces are in equilibrium withe the control rack positioned for the delivery corresponding to the engine's output at the respective point on the engine's performance curve. A drop in engine speed, due to load for example, results in a corresponding reduction in centrifugal force and the governor springs move the flyweights, and with them the control rack, in the direction for increased fuel delivery quantity until equilibrium is restored.

 

Variable speed governors

Maintains a virtually constant speed in accordance with the position of the control lever. Applications: Commercial vehicles with auxiliary power take off, construction machinery, agricultural tractors, marine and stationary applications.

 

Minimum-maximum-speed governors

Effective only at idle and maximum revs. Torque between these two extremes is determined exclusively by the accelerator pedal. Applications: For road vehicles.

 

Combination governors

Are a synthesis of the two governor types described above. Depending on the specific application active control can be in the upper or lower engine-speed range.

 

The book then goes on to talk about speed droop of mechanical governors and says that variable speed governors on small high speed engines generally achieve a consistency of 6 - 10% at the top end of the speed range. After these descriptions the book goes on to talk about the various add-on devices that modify the performance of the governors for things like altitude, manifold pressure, temperature etc.

 

I hope this helps the argument and, as Bosch has a certain reputation within the field of diesel fuelling and governing, I believe that we should take their word on the matter perhaps.

Roger

Edited July 6, 2009 by Albion

[NB Alnwick]

My favourite book on the subject is an early edition of 'Diesel Engine Maintenance' by D. I. Hartley-Smith. It pre-dates modern electronics and the popular use of diesel engines in private cars but its basic description is extremely relevant so I make no excuses for repeating the following description of how a diesel (compression ignition) engine differs from other internal combustion engines.

 

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.

 

Note that the rack is controlled by the governor and is not directly connected to an accelerator pedal or speed regulator lever.

 

Edited to say that I am not disagreeing with 'Albion' or Bosch - I just like the simplicity of the above explanation. Our Kelvin has a very precise hydraulic governor made almost entirely out of gunmetal by Kelvin themselves and it maintains extremely accurate engine speed down to about 140 rpm - irrespective of load - but the speed does rise gradually (by a very small amount) when the engine has been started from cold and until it is thoroughly warmed up.

Edited July 6, 2009 by NB Alnwick

[Guest]

What about stopping an engine after a hard slog? Do I just switch it off, or do I let it idle for a couple of minutes and then switch it off?

[nb Innisfree]

What about stopping an engine after a hard slog? Do I just switch it off, or do I let it idle for a couple of minutes and then switch it off?

 

Personally I let it idle for a few mins.

 

Occasionly I give it a quick blip up to max revs to stretch the conrods.

 

 

Yes it was in a boat, not a car, driving a prop. Typical modern nb installation except that all the electrics are defunct.

Therein lies the difference, it's a BOAT ENGINE in a boat, not a car!!

 

Tim

This discussion (not an argument) won't be resolved here, nor in the pub, but it will be better conducted in the latter.

[churchward]

What about stopping an engine after a hard slog? Do I just switch it off, or do I let it idle for a couple of minutes and then switch it off?

There is something to be said for letting an engine return to a better temperature if it has been getting hot but in reality since the engine won't be moving if you switch it off a rise in temperature won't do any harm. Depending on how the skin tank and pipework is setup there can still be some coolant flow anyway by thermo siphon assuming any thermostat fitted gets hot enough to open.

[Naughty Cal]

We have to let ours idle to allow the turbo & superchargers time to settle down again. Gives us the opputunity to check the gauges and pressures are correct before turning it off.

[churchward]

We have to let ours idle to allow the turbo & superchargers time to settle down again. Gives us the opputunity to check the gauges and pressures are correct before turning it off.

That is true, if the super charger or turbo has any oil feed from the engine system or other pumped supply then allowing them to slow down after high rpm with the engine idling is a good idea. Otherwise you cut off the oil supply when they are spinning very fast.

[Timleech]

Oh, yes please!

 

MP.

 

OK, you asked for it!

 

From "Modern High-speed Oil Engines" 1949/1950

 

Link1

 

Link2

 

Link3

 

Link4

 

Tim

[MoominPapa]

OK, you asked for it!

 

From "Modern High-speed Oil Engines" 1949/1950

 

Link1

 

Link2

 

Link3

 

Link4

 

Tim

 

Tim, your link1 was broken: I've fixed is in the quote above.

 

Thanks for scanning those. They seem to confirm most of the conclusions we've come to. I must admit I'd not thought about the fact that the force from bob-weights varies with their position as well as rotational speed: the larger the radius of rotation the bigger the force. Makes sense once you think about it.

 

MP.