What governs diesel engine revs at max. load? Governor or props?

Montemar

Montemar

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I would be grateful for clarification of what ultimately controls the maximum engine revs on a diesel engine.
If a boat is under propped and not achieving its best speed will the engines go to their maximum governed speed and no higher or do they keep on going up until damage occurs?
If a boat is over propped then the engine would only rev up to the point its power output balances that needed for the propeller to work which would be below the maximum specified revs for the engine.
This is all do do with the right props. for the engine.
It seems the best way to identify the correct props. is to slightly over-prop. then go back a little in diameter or pitch to get the best match.
:confused:
 
Broadly correct, though the governor will generally stop the thing over revving and damage occurring, if underpropped

I say "broadly" because props have lots of tweakable parameters ie pitch, diameter, blade area ratio, # of blades, cupping, etc
 
When manufacturers state a rpm range at WOT it is to be taken seriously.

The warranty on a new engine is void if the engine is not propped to reach the range.

To answer the OP: Prop it right and the governor won't be an issue
 
omega2 said:
If you are over propped there will be a lot of black smoke, if under there will be a lot of revs then a bang.!!
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Sorry I disagree with the last part of your statement

If a diesel engine is under propped at full throttle the governor will prevent the engine revving past it maximun.

The same will happen if you open the throttle with the engine out of gear so there is no load on the engine.

A petrol could over rev if the fuel system allows unlimited fuel/air as there is generally no governor on a petrol engine
 
The governor will prevent over revving...

On a petrol engine with any form of electronic control (from electronic ignition to full EFI) the ignition and / or the injectors are modulated at max revs to prevent any further increase.

An old carb induction points ignition petrol engine can over rev but usually not to the point of destruction. Valve bounce and limitations in the ignition should prevent it, but you still risk major damage if the situation develops.
 
Rogershaw said:
Sorry I disagree with the last part of your statement

If a diesel engine is under propped at full throttle the governor will prevent the engine revving past it maximun.

The same will happen if you open the throttle with the engine out of gear so there is no load on the engine.

A petrol could over rev if the fuel system allows unlimited fuel/air as there is generally no governor on a petrol engine
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OK then a little bang eventually
 
omega2 said:
OK then a little bang eventually
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No, no bang at all as the governor on a diesel engine will prevent the engine revving past it maximum continuous rev's.

On a diesel engine the throttle adjusts the speed for the governor and it the governor that controls the speed of the engine until the governor reaches its maximum position then the speed cannot increase. If you load the engine with say by over propping the speed will not increase to the maximum set and the governor will cause and oversupply of fuel trying to get the engine to maximum revs and as a result unburnt fuel will exhaust out the engine as black smoke.
 
Rogershaw said:
No, no bang at all as the governor on a diesel engine will prevent the engine revving past it maximum continuous rev's.

On a diesel engine the throttle adjusts the speed for the governor and it the governor that controls the speed of the engine until the governor reaches its maximum position then the speed cannot increase. If you load the engine with say by over propping the speed will not increase to the maximum set and the governor will cause and oversupply of fuel trying to get the engine to maximum revs and as a result unburnt fuel will exhaust out the engine as black smoke.
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And the excess unburnt fuel will "wash " the cylinder bores ,loosing lubrication -leading to scoring -leading to excess premature ring wear , leading to excess oil consumption -eventually ( assuming its been ing topped up -the oil ) -to a runaway engine -as the now diesel fuel diluted sump oil enters the cylinder from the leaking rings -to be compressed .
 
Portofino said:
And the excess unburnt fuel will "wash " the cylinder bores ,loosing lubrication -leading to scoring -leading to excess premature ring wear , leading to excess oil consumption -eventually ( assuming its been ing topped up -the oil ) -to a runaway engine -as the now diesel fuel diluted sump oil enters the cylinder from the leaking rings -to be compressed .
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+1
 
Being polite there are some common misconceptions being aired in this thread.

If the injectors are in good order and the injector pump and governor are set correctly you will never have unburnt diesel in the cylinders.

Now discussing black smoke. This is the result of incomplete combustion. It is basically carbon but there are other particulates involved. An indepth explanation fills books, thick and abstruse ones. A 'properly adjusted diesel never smokes other than a trace on acceleration.

But you can get extra power from a diesel by overfueling it, 30 to 40% extra. I used to do this regularly as a demonstration to students with a Ford 6D on a dyno at Loghborough. However we had to check that we had some wind and that the locals did not have their washing out. We produced serious smoke.

Now what prop to fit Simple theory says if you want maximum economy you run WOT but fit a prop to hold the revs down to peak torque. If you want maximum speed fit a prop that gives you the revs at WOT that corresponds with peak brake horsepower. NB this is often a bit less than the max revs allowed by the governor.

If you have a displacement hull the prop guys will give you something between the two points with economy in mind you may tend to wards propping to achieve just a bit above peak torque, this is like running a car in overdrive.

I drive a raggie with a Perkins 154. It has a max rpm somewhere around 3000. Running at WOT in flat water and no headwind I might see 2100 but I achieve hull speed at about 1700 - 1800. Yes I am overpropped but running near peak torque so I use less fuel.

Prop choice for semi displacement and planing hulls is much more complicated and a lot depends on the likely operating conditions. A bit of a 'black art ' I am told.
 
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LittleSister said:
So, apart from not achieving quite the maximum potential speed for the boat and engine power, is there any downside to being a little under-propped?
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Certainly not, all engine manufacturers actually insist on underpropping in their installation guidelines.......Mandatory requirement in order that when vessel becomes weeded engine is not subject to damaging engine overload condition.

Obvious from posts here many contributors here still do not get it! Too many gasoline girls out there....

A diesel engine governor is akin to cruise control on a car, you set the revs and governor provides the correct amount of fuel to hold the set revs DEPENDENT on engine load (propeller demand).

Mechanical marine engines have governor droop in the region of 10% we have good governor stability at set engine speeds. Governor ‘droop’ is the point at which the engine reduces the fueling after rated engine speed is reached. If we have for example a 300 hp engine at a rated engine speed of 3,000 rpm. Assuming 10% governor droop fueling is gradually tapered off after 3,000 rpm out to 3,300 rpm cut off point (high idle).

Good practice is to size propeller to in order a vessel in clean condition with half tanks will demand no more than say 270 hp at 3,000 rpm and allow engine to actually go out on to the ‘droop’ slightly at WOT. As vessel becomes weeded propeller will demand more power up to the maximum output available, 300hp.

If a diesel engine is not allowed to reach rated engine a dangerous condition develops, for example if propeller demand is so great that our engine can only make say 2,800 rpm the governor is attempting to respond to demand by providing fueling to make 300hp however we must remember that unlike a gasoline engine a diesel engine requires excess air, if our 300 hp engine is attempting to make maximum power at a lower rpm the AFR (Air to fuel ratio) goes all to heck with a direct result of elevated exhaust gas temperatures, the diesel engine killer.

If our 300hp engine is propped excessively ‘light’, say to demand say 200hp at 3,000 rpm the governor will only provide fueling in order to respond to the lower demand, just why people think this will lead to excess fueling I have absolutely no clue.

Another myth is running an engine at high idle will lead to it going bang. If we again consider our mythical engine with high idle of 3,300, properly designed should be way off two major destructive reciprocating engine conditions.

Every engine design has a ‘no follow’ speed, simply put this is the critical speed at which the valve gear stops following the camshaft profile, components previously held in check by valve spring tension suddenly become loose which can lead to bits ending up in the wrong place, for example push tube can jump out of the tappet and drop down into the cam gallery, the cam says get out of here and the whole lot tries to exit to atmosphere through the side of the block.

The second condition is crankshaft torsional activity; a six cylinder engine will have two periods of torsional activity in its operating range, but there is a third one lurking out there beyond the normal operating range, the ‘potentially destructive torsional crankshaft speed’. Operating an engine on the dyno around this speed can be exciting as it is possible to get an engine though this critical speed, but on most occasions crank will simply fracture across the web, tin hat time again.
 
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Many yachts are mildly over-propped to give better slow speed manoeuvrability, so bearing in mind that they almost never open up past 70% throttle is the caution you gave against over propping still a concern?
 
Colvic Watson said:
Many yachts are mildly over-propped to give better slow speed manoeuvrability, so bearing in mind that they almost never open up past 70% throttle is the caution you gave against over propping still a concern?
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I assume you allude to the fact that many yachts have poor 'bite' when going astern.

Provided the owner is aware that he is over propped and never uses WOT to say punch a tide, but what happens when vessel is skippered by somebody not aware that giving the motor the beans is damaging it?

Yes I have come across the situation when working on a Grand Banks re-power when owner was obsessed with cruising economy and claimed he would never use WOT. We had to re-prop to get manufacturer sign off.

However the absolute no no is on planing vessels.
 
Latestarter1 said:
If a diesel engine is not allowed to reach rated engine a dangerous condition develops, for example if propeller demand is so great that our engine can only make say 2,800 rpm the governor is attempting to respond to demand by providing fueling to make 300hp however we must remember that unlike a gasoline engine a diesel engine requires excess air, if our 300 hp engine is attempting to make maximum power at a lower rpm the AFR (Air to fuel ratio) goes all to heck with a direct result of elevated exhaust gas temperatures, the diesel engine killer.
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Thanks for your comprehensive reply.
This condition sounds like the one Portofino and omega2 referred to with excess fuel washing the bores and presumably lots of black smoke also previously alluded to.
Basically some underpropping is preferable to overpropping as a simplistic statement.
Propping so max revs and max speed are reached together sounds good as it then allows cruising at 90% of max revs. and acceptance that a dirty hull and props will slow the boat rather than clogging on flat out to compensate.
 
Quite correct regarding under propping however our friends are incorrect in their assumptions, there is no loaded condition which a diesel engine governor will have excess fuel washing the bores, diesel combustion requires excess air to control cylinder temperature, fueling at lower than designed rpm results in skyrocketing exhaust gas temperature, exhaust valve temperature quickly reaches a a point materials start to become plastic and valve closing velocity normally acceptable becomes destructive above normal operating temperatures lead to potential failure of friction weld where stem joins the mushroom, or valve seat is being hammered when it is at the temperature at which it is shrunk in, at that point failure is just a whisker away.........
 
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