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Old 02-13-2017, 08:21 AM   #1
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Engine question

Was reading some engine book. Got confused about the function of the governors, hopefully find some help here.

If I keep an engine at a constant speed, say 2000rpm. I understand the boat speed would vary widely base on factors like weight, current and wind etc. But why would the load on engine change? It's not like the water is sometimes getting more dense? Is it the current?
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Old 02-13-2017, 08:34 AM   #2
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Wind, waves, current, a clean boat bottom vs a bottom with growth are all things that can impact the load on a boat engine at a given speed.
I'm sure there are lots more...
Bruce
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Old 02-13-2017, 09:09 AM   #3
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A diesel with mechanical injection and no governor is inherently unstable. If you set fuel rate control to a fixed setting, it will either rev very high, or slow down and stall. Something must modulate fuel rate to keep rpm in desired band. Thus the governor.

Opposite a gasoline engine with throttle plate, those are inherently stable rpm wise and have no such need.
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Old 02-13-2017, 09:36 AM   #4
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3 things are in play with any diesel. Speed (rpm), load and fuel.
The engine fuel pump delivers a measured qty of fuel which engine burns to produce power...if the load drops with constant fuel supplied the speed rises and vice versa.
If the fuel is reduced with constant load then the speed will fall and vice versa.
The speed control lever tells the governor what speed you want. The governor's function is to adjust the amount of fuel to maintain a constant (desired) speed with variations in load.
In the case of an engine driving a boat the load comes from the propeller which is in turn loaded by the boat's resistance to move which is in turn influenced by several factors. For any particular boat these would mainly be the boat weight, hull and propeller condition and environmental forces.
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Old 02-13-2017, 09:54 AM   #5
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Many years ago when running big diesel AC gensets, the purpose of the governor became apparent. An AC genset by design must run at constant RPM or motors and electrics downstream will get fried.

The answer to this constant genset RPM vs varying load is the governor, it adjusts fuel flow to meet load requirements as electrical needs fluctuate over a very wide range. Governors from a diesel genset built 80 years ago are a marvel to behold vs what are in use in this century.

A former TF member, RicB, I think of as the governor man. He has seen them all and did a good job of explaining same.
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Old 02-13-2017, 10:26 AM   #6
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Once you get a marine diesel in gear and under load, you don't really need a governor. You could set the fuel racks at a fixed setting and prop load will keep rpms fairly steady.

Come back to idle and in and out of gear and you then need a governor.

Also need a governor to prevent overspeed at no or light load.

AC generators, especially utility grade machines, need a very accurate governor to keep 60 or 50Hz. And to accommodate parallel operation and/or grid sync'ing. Whole nutha can-o-worms there.

You learn the importance of a diesel governor when you start a Detroit Diesel after repair with governor disconnected and manual control of fuel rack. It will NOT hold a steady rpm no matter what you do.
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Old 02-13-2017, 11:30 AM   #7
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I appreciate the folks that understand these things trying to explain it to folks like me who haven't a clue... Can those who know tell me if my understanding is closer to the truth now?

My simple mind thinks that a boat's engine that is running at 4 knots at 1500rpm will be under almost twice the load when running at 8 knots at 1500 rpm. As the OP mentioned, the density of water doesn't change.

So imagine I am idling through my harbor's no wake zone at 800 rpm at just under 5 knots. I reach the mouth of the harbor and suddenly increase the throttle to 1500 rpm. I don't instantly hit my 7.2 knots cruising speed. It takes time for the boat to accelerate and settle into its new trim. The amount of work being done while the boat is accelerating is greater than after it has achieved its cruising speed. I don't touch the throttle and the rpms don't seem to change. Is it a governor that is controlling fuel flow to keep that rpm constant under the changing load conditions?

The same would apply if I turn into a 20 knot headwind. My speed drops and the rpm stays constant. The engine has to do more work to keep that rpm constant yet I have not touched the throttle. Is it a governor that is regulating fuel flow to make that happen?

I never thought about this stuff before. I have always thought that the throttle controlled the fuel being supplied to the engine and rpm was simply a consequence of that. The fact that the actual behavior of the boat didn't match that assumption never dawned on me.
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Old 02-13-2017, 11:41 AM   #8
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Yes, governor is in control of engine fuel rate. You set "throttle" to 1500, and as load changes the governor will increase and decrease fuel rate to keep rpm near constant.

If you want to see it in action, get going at normal cruise and simply pull gear into neutral. Engine will go up say 50-100rpm and just sit there. Fuel racks will go from where ever they are (say 30% fuel) and drop it down to like 5%.

Also, the term "throttle" is a bit of a misnomer in diesel speak. It is really a governor speed control lever. The term "throttle" applies to steam engines and gasoline engines. Even though most people still call it throttle, including me unless around other engine engineers.
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Old 02-13-2017, 11:44 AM   #9
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Dave, you are correct in your analysis above. Most of us come from a gas engine automobile experience base. Governed diesels in boats act differently, so it takes some thought to wrap our minds around it.

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Old 02-13-2017, 12:00 PM   #10
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As I understand it, the governor controls the flow of fuel to the engine, and as such it controls the speed of the engine (Not the speed of the boat). With a mechanically controlled engine, as the load increases (wind and waves pick up), the engine will loose RPM becasue it is working harder with the same amount of fuel. The fuel available controls the work output. With an electronically controlled diesel, it operates differently. With my Cummins I set the RPM and away I go. If the wind and waves increase, I notice my fule burn increasing. The engine maintains the same previously set RPM, and the computer increases the fuel flow to accommodate the additional work the engine is performing as it maintains the set RPMs under the increased load.
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Old 02-13-2017, 12:45 PM   #11
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In both computer and mechanically controlled engines, the governors operate similarly. They will increase or decrease fuel rate to maintain rpm. The big difference is the computer controlled ones can very accurately hold rpm where the mechanical ones are a little sloppy, called "droop".

Set rpm on a mechanical engine, and go from full load to no load and rpm will vary 50-200rpm. On a computer engine it might vary 0-50 rpm. Depends on the system.

Another complicating thing is a "road load" governor. These are meant for highway vehicles to simulate how a gasoline engine behaves. Push a little on the accelerator, and you want a little increase in fuel. If using a variable speed governor like marine or industrial engines, push a little on the pedal and it goes to 100% fuel, makes for weird driving.

If anyone is using Detroit 8.2's, you know about road load governors. One of the few marine engines that use this type. Works ok in trucks, but does weird stuff on boats.
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Old 02-13-2017, 12:50 PM   #12
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Thanks everybody who replied! Good info good reading. Excellent forum!
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Old 02-13-2017, 02:23 PM   #13
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Thanks everybody who replied! Good info good reading. Excellent forum!
+1

I learned something basic and useful this morning about engines I have been using for 25 years.
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Old 02-13-2017, 06:58 PM   #14
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So imagine I am idling through my harbor's no wake zone at 800 rpm at just under 5 knots. I reach the mouth of the harbor and suddenly increase the throttle to 1500 rpm. I don't instantly hit my 7.2 knots cruising speed. It takes time for the boat to accelerate and settle into its new trim. The amount of work being done while the boat is accelerating is greater than after it has achieved its cruising speed. I don't touch the throttle and the rpms don't seem to change. Is it a governor that is controlling fuel flow to keep that rpm constant under the changing load conditions?

Actually, this is a common misconception. As a boat goes faster - drag forces keep increasing (not counting planing type hulls). The amount of work being done while the boat is accelerating is less than that once it has reached its cruising speed. If it wasn't - the boat wouldn't accelerate. The boat keeps accelerating until the engine output equals the "drag" of the boat moving forward - at which point the boat stops accelerating.

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Old 02-13-2017, 08:02 PM   #15
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Actually, this is a common misconception. As a boat goes faster - drag forces keep increasing (not counting planing type hulls). The amount of work being done while the boat is accelerating is less than that once it has reached its cruising speed. If it wasn't - the boat wouldn't accelerate. The boat keeps accelerating until the engine output equals the "drag" of the boat moving forward - at which point the boat stops accelerating.

Ken
Great point on the increasing drag. Doesn't drag tend to increase with the square of velocity, or is that in another context? I still have to thing about it a bit.

It seems to me that if the boat is moving faster through the water then there is going to be less resistance to the prop turning, since we are dealing with a fixed pitch prop here. The productive effect of the prop is to push water aft. If the water is already moving that direction then it seems to me it wouldn't have to work as hard to push it at a given rpm. I may be oversimplifying it but I imagine a kayak. If I use the same stroke length and cadence, it is harder to do from a standstill than it would be once the kayak is at speed.

But I don't know. I'm just an eye guy.
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Old 02-13-2017, 08:14 PM   #16
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I've operated under the assumption that the governor controls engine revolutions, not directly the speed of the boat or fuel consumption.
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Old 02-13-2017, 08:20 PM   #17
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The way I look at it is this - when the throttle is advanced to a certain point and left there, the engine starts speeding up as does the boat. The boat keeps speeding up until the engine's output matches the force required to move the boat at that speed. As long as there is "extra" force available, the boat moves faster. And as the engine(and boat) go faster the engine is developing more horsepower. There's never any "letup" or backing off on the force the engine is putting out unless the throttle is pulled back - with a commensurate reduction in speed.

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Old 02-13-2017, 08:43 PM   #18
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The way I look at it is this - when the throttle is advanced to a certain point and left there, the engine starts speeding up as does the boat. The boat keeps speeding up until the engine's output matches the force required to move the boat at that speed. As long as there is "extra" force available, the boat moves faster. And as the engine(and boat) go faster the engine is developing more horsepower. There's never any "letup" or backing off on the force the engine is putting out unless the throttle is pulled back - with a commensurate reduction in speed.

Ken
That makes sense if the the amount of energy expended was constant. However, in this case the rpms are constant and the amount of fuel (energy) is being regulated (changed) to keep that rpm constant.

To be honest, it probably doesn't any appreciable difference for our types of boats. This discussion has been helpful to me by pointing out that for the most part my "throttle" simply is setting the rpm and the ECU is handling the amount of fuel supplied.
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Old 02-13-2017, 09:16 PM   #19
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The way I look at it is this - when the throttle is advanced to a certain point and left there, the engine starts speeding up as does the boat. The boat keeps speeding up until the engine's output matches the force required to move the boat at that speed. As long as there is "extra" force available, the boat moves faster. And as the engine(and boat) go faster the engine is developing more horsepower. There's never any "letup" or backing off on the force the engine is putting out unless the throttle is pulled back - with a commensurate reduction in speed.

Ken
Mehhhhhh....not really. No one has really mentioned how a mechanical governor actually works. When you operate the "throttle", you are setting spring pressure. That spring pressure modulates fuel going into the engine. That spring pressure is also regulated by flyweights. The faster the engine "attempts" to turn(ie low load), the more the flyweights move outward and adjust spring pressure and therefore fuel flow. So like Ski said, if you are under load and pull it out of gear, the engine will "attempt" to rev...the flyweights move outward due to centrifugal force to maintain spring pressure...and therefore maintain RPM.

Now the opposite is true as well. As the engine "attempts" to bog down when loaded up, the flyweights draw inward(due to spring pressure) therefore maintaining RPM but increasing fuel flow and therefore EGT/pyro temps.

Kchace, you are setting spring pressure and therefore RPM. So the engine seeks RPM....not equilibrium as you are suggesting. If it always found equilibrium then overfueling(black smoke/overload) would never occur. In your example, the engine will seek the RPM related to the spring pressure that is set, and the boat will do whatever with the power delivered. It may find equilibrium. It may not. It will try to find that RPM and fuel will be metered to achieve that. If the load is too great, the governor will keep dumping fuel and you will have black smoke and high EGTs....and eventually engine damage.

My explanation likely has many errors in it. Diesel governors have always fascinated me so I have done some reading. I am learning but still have more to go. Believe it or not, diesel governors and aircraft prop governors are similar. Their overall goal is different(although it is about RPM control), but the mechanics are the same. Flyweights governing a fluid. Oil pressure in an airplane constant speed prop. And fuel flow in a diesel governor. But the "control ;ever" sets spring pressure. And flyweights keep that spring pressure constant....

So in your example....I have a planing boat. I push the power levers up to a certain position/RPM. I look at the tachs...and guess what, they are right where I set them. Let's say 2200. The boat is still getting up and out of the hole. The RPMs are 2200. The boat is still accelerating. And once it is up and on plane and the equilibrium you mention, is met, the RPMs are still at 2200...ish. A gasoline engine would be bogged down coming out of the hole with a set power level position. As the boat comes out of the hole, RPM will increase dramatically as the boat comes out and reaches equilibrium. IOW, RPM as set at the beginning of the power up will be very different than the RPMs reached once the boat planes out.

As it relates to constant speed gensets...they don't have "throttle" levers. The spring pressure is set. So it maintains RPM regardless of load. Fuel consumption will vary based on load. RPM WILL NOT!!!

Ski????
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Old 02-13-2017, 09:20 PM   #20
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Many years ago when running big diesel AC gensets, the purpose of the governor became apparent. An AC genset by design must run at constant RPM or motors and electrics downstream will get fried.

The answer to this constant genset RPM vs varying load is the governor, it adjusts fuel flow to meet load requirements as electrical needs fluctuate over a very wide range. Governors from a diesel genset built 80 years ago are a marvel to behold vs what are in use in this century.

A former TF member, RicB, I think of as the governor man. He has seen them all and did a good job of explaining same.
Yes, those old school governors were amazing for their time. Heck, everyhing made back then considering the tools and equipment they had was quite an engineering feat.
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