Diesel consumption

[OldMan]

Originally Posted by a64pilot

I’ll try one last time and then I’ll quit, yes the governor schedules fuel, but it doesn’t have full authority, the closer to set point of the RPM the less authority it has, if you have a large load applied like a hill driving a truck for example you need to add throttle to reset that RPM setting and give the governor more authority, I am NOT saying increase speed or RPM but to maintain it, anyone who has actually driven a truck will recognize what I’m saying.
In order for it to operate smoothly, as the flywheel weights raise due to set point RPM approaching fuel is trimmed back slowly it loses its authority or it would be a full on full off thing and at least the engine would hunt in RPM, probably pretty severely.
For this reason as a load is applied if it’s a decent load throttle has to be added to maintain speed, even the load of an alternator we have all heard the RPM sag and return as it cycles on and off if temp protected.

The throttle and the governor work together to schedule fuel.

If that were true, fixed speed diesel generators would not work. Yes, when my genset is loaded up, it sounds different, but the difference is higher fuel burn causing a more throaty sounding exhaust. RPM is maintained, not by moving the throttle, but the governor increasing fuel delivery.


Not sure about your truck example, but on the OP's engine, the governor has the ability to set maximum fuel delivery, meaning the maximum the pump can produce, at any RPM. Take the inspection plate off and see where the fuel rack is when the engine is off, it'll be in full output position. Why? Because the idle speed set screw on the throttle is set for ~800rpm and the engine isn't turning that speed.

[a64pilot]

I said I’d quit but I guess I can’t.
Your right, it’s a full throttle because there is no speed, crank it up and watch, it slowly retards fuel until the engine is at set RPM, but it does not have full authority to add full throttle until the RPM is well below set point. It does not hold full throttle until RPM set point is reached and then suddenly pull it back.
Back to the truck, yes if you don’t add fuel the governor will likely add full fuel, but your speed may have dropped from 1800 RPM to 1000 before it does, and at that low and RPM it can’t provide enough power to both pull the hill and accelerate, so you as the operator as you begin to pull the hill You add throttle before the governor has drooped enough to be able to add full throttle, and you make it over the hill, where you the operator reduce throttle to keep from going too fast down hill, because if you leave the throttle set you will be going way to fast.

Most bigger trucks have throttle locks to operate the PTO’s and what I’m saying is very easy to demonstrate.
Same with a farm tractor or even a Diesel Lawnmower.

[OldMan]

Originally Posted by a64pilot

I said I’d quit but I guess I can’t.
Your right, it’s a full throttle because there is no speed, crank it up and watch, it slowly retards fuel until the engine is at set RPM, but it does not have full authority to add full throttle until the RPM is well below set point. It does not hold full throttle until RPM set point is reached and then suddenly pull it back.
Back to the truck, yes if you don’t add fuel the governor will likely add full fuel, but your speed may have dropped from 1800 RPM to 1000 before it does, and at that low and RPM it can’t provide enough power to both pull the hill and accelerate, so you as the operator as you begin to pull the hill You add throttle before the governor has drooped enough to be able to add full throttle, and you make it over the hill, where you the operator reduce throttle to keep from going too fast down hill, because if you leave the throttle set you will be going way to fast.

Most bigger trucks have throttle locks to operate the PTO’s and what I’m saying is very easy to demonstrate.
Same with a farm tractor or even a Diesel Lawnmower.

I think I see the disconnect.



Remember, the governor doesn't control the throttle, it controls fuel delivery.



The amount/quantity of fuel delivered, controlled by the governor, is also limited by the RPM. The high pressure pump delivers more fuel at a higher RPM as it pumping faster.



Your truck slowing down up a hill is a function of the engine not capable of producing the power required to maintain speed at the set RPM.


The OP doesn't have that problem, he stated the engine was still running @ 2900RPM, i.e., the engine did not slow down, but the boat was going slower and burning more fuel.

[a64pilot]

No the disconnect was in this thread that due to propellor slippage a decrease in boat speed will cause you to burn more fuel, but not much more, you just slow down. You lose MPG due to speed loss, but burn rate doesn’t increase by much as the prop just slips.
However a dirty prop to turn it the same RPM will increase fuel burn significantly due to it being significantly harder to turn the prop due to drag.

As I said earlier to prove this try to see how much max RPM is reduced by tieing your boat to a dock and go to full throttle with a clean prop, then try to motor with a barnacle encrusted prop.
The dirty prop will reduce RPM more.

However a Diesel governor is variable in authority, you have to have a significant reduction in RPM for it to have full authority and then as the engine increases in RPM. The authority decreases so the engine has a steady droop.
It does not drop 10 RPM and then command full throttle until set point and then back way off.

That is why the truck will drop in RPM as the hill is begun to be climbed, but will accelerate if the throttle is depressed.
The reason a boat drops little RPM when going to heavy wind and seas is more to do with the propellor slip than additional fuel added by the governor.
I think his primary reason for increased fuel consumption was his prop was also dirty along with the bottom, not the bottom being dirty
However we aren’t going to agree and that’s fine.

[a64pilot]

Originally Posted by Panope

.....because the engine's governor must supply more fuel to maintain the same engine speed (due to higher propeller load).

Steve

This is it in a nut shell, but the prop load is higher due to the prop having a higher drag, not the hull

[Wotname]

Interesting discussion re the governor and throttle - I'm enjoying it!

Back to OP and slightly more theoretical look at the question.

Originally Posted by carlager

We have a yanmar 3gm30f.

At 2.900 rpm the speed is 6,5 knots and diesel consumption is 2l/hour.

The bottom has not yet been antifouled and is very dirty.

At 2.900 rpm the speed is 5,2 knots and diesel consumption is 2.8l/ hour.

It is obvious that the speed will decrease with a dirty bottom, but why does the diesel consumption increase ?

Because you need more power (as others have also posted) to overcome the extra drag and power is a function energy and time. In this instance of a boat engine, it becomes a function of RPM (time) and fuel (energy). In your example, your RPM (time) has not changed therefore in order to deliver more power, the fuel (energy) has to be increased.

Originally Posted by a64pilot

This is it in a nut shell, but the prop load is higher due to the prop having a higher drag, not the hull

I guess this is situation dependant but I agree that as a generalisation a dirty prop will have a greater effect than a dirty hull if only because a fouled prop prevents it from doing it's designed job - converting torque to motion.

[Panope]

A data point:

My engine will achieve 3750 rpm with a clean bottom/propeller and zero wind. Boat running free at about 7.5 knots.

My engine will achieve 3300 rpm when tied to a dock (propeller clean).

Steve

[Wotname]

Originally Posted by Panope

A data point:

My engine will achieve 3750 rpm with a clean bottom/propeller and zero wind. Boat running free at about 7.5 knots.

My engine will achieve 3300 rpm when tied to a dock (propeller clean).

Steve

Yes that makes sense and is close to what I would expect if the prop is correctly matched to the engine/gearbox/hull shape etc. IIRC, I used to get 3600 underway at WOT but only 3400 tethered to the dock (2GM20 & 31' boat).

For other readers, the prop should be designed to be most efficient when the water is flowing onto the blades is at hull speed, not stationery (as when tied to the dock).

[skipperpete]

Don't Quit a64pilot, you have a clear mind re diesel engines and power output.
The word "throttle" is accepted by us all as the term for speed control but is a petrol engine carburettor term and is a variable restriction to control airflow.
There are two distinct types of governor on diesel engines. Variable speed type as on most of our propulsion engines and
Constant speed as on our generators. Neither works very well out of its own specific role.

[OldMan]

Originally Posted by skipperpete

Don't Quit a64pilot, you have a clear mind re diesel engines and power output.
The word "throttle" is accepted by us all as the term for speed control but is a petrol engine carburettor term and is a variable restriction to control airflow.
There are two distinct types of governor on diesel engines. Variable speed type as on most of our propulsion engines and
Constant speed as on our generators. Neither works very well out of its own specific role.

Weights and springs used for both. Look at the OP's 3GM30 and a 3TNE74 (common for generator use), they both have the same governor setup. No reason either couldn't be used for the same application.


Both these governors can operate the rack on the pump from full closed to full open, all based on the engine rpm set by the throttle and actual.

[a64pilot]

Originally Posted by OldMan

Both these governors can operate the rack on the pump from full closed to full open, all based on the engine rpm set by the throttle and actual.

Yes they can, but to get full throttle or the opposite no throttle the RPM has to vary significantly from set point. For example you may set 2000 RPM no load and then apply a load and the motor will sag to 1800 RPM and stay there because that is where the governors ability to add fuel and the load match, to recover to 2000 RPM, your going to have to add throttle, which would be maybe say 2200 RPM under no load.

Due to the way the flywheel weights operate, meaning that they slowly open up as speed is increased the governor’s authority closely follows those weights, it doesn’t get full authority until they are fully in and doesn’t lose authority until they are fully open.
It’s variable it’s not an all on or all off system.

Anyone that has ever operated a farm tractor will attest to this, you set the RPM to the PTO point, 540 RPM If memory serves, but that goes back decades, engage the PTO to drive a mower or hay bailer or whatever and then increase the throttle to recover the RPM.

[OldMan]

Originally Posted by a64pilot

Yes they can, but to get full throttle or the opposite no throttle the RPM has to vary significantly from set point. For example you may set 2000 RPM no load and then apply a load and the motor will sag to 1800 RPM and stay there because that is where the governors ability to add fuel and the load match, to recover to 2000 RPM, your going to have to add throttle, which would be maybe say 2200 RPM under no load.

On my genset, it'll vary ~4%, 1875 RPM at no load and 1800 RPM at full load, but certainly not 10%. This is across a range of ~2-3HP at low load to ~14HP at high load. Corresponding fuel delivery will be close to linear with load, i.e. ~200g-hp-h. So, the governor has the capability to increase fuel delivery by 4-5 times in order to maintain RPM within 1800-1875 range.

Originally Posted by a64pilot

Due to the way the flywheel weights operate, meaning that they slowly open up as speed is increased the governor’s authority closely follows those weights, it doesn’t get full authority until they are fully in and doesn’t lose authority until they are fully open.
It’s variable it’s not an all on or all off system.

Anyone that has ever operated a farm tractor will attest to this, you set the RPM to the PTO point, 540 RPM If memory serves, but that goes back decades, engage the PTO to drive a mower or hay bailer or whatever and then increase the throttle to recover the RPM.

You are correct in that a 10% variation in RPM due to loading won't result in the governor slamming to full fuel output. But if the load is within the engine power output capability at 540 RPM, the governor should be able to maintain the RPM (within the design constraints of the mechanical governor, usually 4-5%).

[a64pilot]

Originally Posted by OldMan

On my genset, it'll vary ~4%, 1875 RPM at no load and 1800 RPM at full load, but certainly not 10%. This is across a range of ~2-3HP at low load to ~14HP at high load. Corresponding fuel delivery will be close to linear with load, i.e. ~200g-hp-h. So, the governor has the capability to increase fuel delivery by 4-5 times in order to maintain RPM within 1800-1875 range.





You are correct in that a 10% variation in RPM due to loading won't result in the governor slamming to full fuel output. But if the load is within the engine power output capability at 540 RPM, the governor should be able to maintain the RPM (within the design constraints of the mechanical governor, usually 4-5%).

Yes, but you do see or agree that the authority of the governor is variable, and it pretty much has to be or the engine will hunt in RPM, it takes a gradual easing off of fuel as set point is approached to be a smooth steady RPM.
My little generator has to be set at 62 hz no load to run at 58 hz under significant load, a larger, better generator will do better, and I think some of the newer ones are electronically controlled via a linear actuator controlling the fuel control arm. Those due to software can have full authority at set point RPM. Software can dampen response.

However I believe that a generators governor is set tighter than a regular motor, it can be as it only runs at one set RPM, and has to be to be to prevent excessive droop in RPM.

Whatever its worth I have worked extensively with governors in turbine powered helicopters, they need both droop compensation and an anticipatory system, they can’t wait until a droop in RPM occurs, they have to be able to anticipate it and add fuel before the droop occurs.
A lot of this is done with software now of course, but a lot is still mechanical.

Old helicopters had a “correlation” device that added a set amount of throttle as the collective was increased and the pilot trimmed it with a hand throttle, then about the time of the Huey It was all mechanical and due to the Huey’s excessive inertia rotor system it worked, but then later as blades became much, much lighter the inertia was gone and it took electronics with mechanical back ups to step up and maintain set RPM.
Even the Huey had a mechanical “droop compensator” that had to be adjusted, it added fuel with the collective, and the governor then trimmed the droop compensator.

But helicopter or lawnmower the concept is the same, maintain set RPM.

I don’t know if it’s 5% or 10% but an engine that is designed to run at widely varying RPM the governor is “looser” than one set to run at one RPM, reason is throttle response varies with RPM of course and to prevent hunting it has to be looser.
If you have ever experienced hunting, it’s annoying. Sometimes you will hear a DD with the rack out of adjustment hunting, it’s that rump, rump rump at idle.
At speed it’s an annoying variance in RPM, manifests as a surge, some very long period, some short period, but both are annoying.

[skipperpete]

Thanks A64pilot for explaining the two governor type operations, especially "droop" which is actually an important feature on 3 phase gensets that are run in Parallel, it allows a bit of wiggle room when they synchronize.

[Wotname]

Originally Posted by a64pilot

....

Anyone that has ever operated a farm tractor will attest to this, you set the RPM to the PTO point, 540 RPM If memory serves, but that goes back decades, engage the PTO to drive a mower or hay bailer or whatever and then increase the throttle to recover the RPM.

True!