Diesel consumption

[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 ?

[Panope]

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

Steve

[Wotname]

^^ what Panope said

Consider running the engine in neutral at 2,900 rpm - the fuel burn would be minimal.
The more load, the more fuel is burnt and a dirty bottom increases the load quite a bit.

The throttle setting determines the RPM and the governor will keep pouring in the fuel until that RPM is reached (obviously within reason).

[Recy]

Set 2900 rpm on the engine when when you are moored and see what fuel consumption you will have then.

[a64pilot]

Usually due to prop slippage fuel consumption is pretty flat regardless of boat speed, on displacement hulls of our speeds. Not perfectly flat of course, but good enough.
However when a bottom is fouled, the prop usually is too, and a fouled prop does increase drag substantially and will drive fuel consumption up.

Try this, see what your max RPM is tied or the dock and underway, it shouldn’t be much different, but see what your max RPM is with a badly fouled prop is.

To really get a handle on engine fuel consumption vs power output you need a torque meter and an exhaust pyrometer.
That would be silly and a waste of money on engines out size of course.

Having said that, it’s surprising how accurate a common rail motor’s computer is if you can tap into the data, they don’t measure fuel flow, but knowing RPM, fuel temp, air temp and pressure, fuel pressure and fuel injection pulse width they can determine fuel flow very precisely.

However of course for us mere mortals, determining fuel flow from a few fill ups and adding in a fudge factor that your comfortable with is plenty good enough

[Captain Bill]

You seem to be confusing the way diesel engines work with the way gasoline engines work. The "speed control" of a gasoline engine controls the amount of fuel delivered to an engine whereas the "speed control" of a Diesel sets the RPM. In an automobile that is heavily loaded one pushes down on the accelerator more to reach the desired speed (RPM). In a gas powered boat one advances the throttle lever more to reach the desired speed on a heavily loaded boat. The speed lever on a diesel on the other hand simply sets the RPM that the governor will try to maintain. The governor will automatically deliver as much fuel as is needed to maintain that RPM up to the limits of the fuel delivery system. Hence you set the RPM at 2900, but because of the dirty bottom the governor had to deliver a lot more fuel to reach that RPM.

[a64pilot]

Originally Posted by Captain Bill

You seem to be confusing the way diesel engines work with the way gasoline engines work. The "speed control" of a gasoline engine controls the amount of fuel delivered to an engine whereas the "speed control" of a Diesel sets the RPM. In an automobile that is heavily loaded one pushes down on the accelerator more to reach the desired speed (RPM). In a gas powered boat one advances the throttle lever more to reach the desired speed on a heavily loaded boat. The speed lever on a diesel on the other hand simply sets the RPM that the governor will try to maintain. The governor will automatically deliver as much fuel as is needed to maintain that RPM up to the limits of the fuel delivery system. Hence you set the RPM at 2900, but because of the dirty bottom the governor had to deliver a lot more fuel to reach that RPM.

So you set the throttle on a farm tractor and drop the plow the engine will maintain RPM, or your mowing and get into heavy grass the engine doesn’t slow down?
Or Diesel trucks don’t need cruise control, just a throttle lock and the governor will add fuel to Climb the hill?

No, that is not the way a Diesel works, the reason that a generator always has an external governor to control the high pressure fuel pump is because they don’t work that way.
The governor in a common Diesel has actually very little authority over fuel control, yes there is some, but not much, even an alternator will drop engine RPM, you have to add throttle to get it to recover.

[OldMan]

Originally Posted by a64pilot

So you set the throttle on a farm tractor and drop the plow the engine will maintain RPM, or your mowing and get into heavy grass the engine doesn’t slow down?
Or Diesel trucks don’t need cruise control, just a throttle lock and the governor will add fuel to Climb the hill?

No, that is not the way a Diesel works, the reason that a generator always has an external governor to control the high pressure fuel pump is because they don’t work that way.
The governor in a common Diesel has actually very little authority over fuel control, yes there is some, but not much, even an alternator will drop engine RPM, you have to add throttle to get it to recover.

Captain Bill is correct wrt to how the OP's 3GM30 governor works. This is not a common rail diesel. The throttle cable adjusts the governor not the fuel rack on the high pressure pump. The fuel rack is controlled by the governor.

Take a look at the fuel rack on the high pressure pump with the engine stopped, it's in the full fuel delivery position. When the engine starts, the governor backs it off to whatever speed is asked for by the throttle position.

From the service manual on that engine:

[OldMan]

Originally Posted by a64pilot

So you set the throttle on a farm tractor and drop the plow the engine will maintain RPM, or your mowing and get into heavy grass the engine doesn’t slow down?
Or Diesel trucks don’t need cruise control, just a throttle lock and the governor will add fuel to Climb the hill?

No, that is not the way a Diesel works, the reason that a generator always has an external governor to control the high pressure fuel pump is because they don’t work that way.
The governor in a common Diesel has actually very little authority over fuel control, yes there is some, but not much, even an alternator will drop engine RPM, you have to add throttle to get it to recover.

FWIW, my diesel generator has no 'external governor', the idle speed set screw happens to be set to 1800 rpm, the internal weighted spring governor maintains rpm through all loads (+/- 5%).

[Captain Bill]

Originally Posted by a64pilot

So you set the throttle on a farm tractor and drop the plow the engine will maintain RPM, or your mowing and get into heavy grass the engine doesn’t slow down?
Or Diesel trucks don’t need cruise control, just a throttle lock and the governor will add fuel to Climb the hill?

No, that is not the way a Diesel works, the reason that a generator always has an external governor to control the high pressure fuel pump is because they don’t work that way.
The governor in a common Diesel has actually very little authority over fuel control, yes there is some, but not much, even an alternator will drop engine RPM, you have to add throttle to get it to recover.

Actually that is what I'm saying, but you neglect to note the "limits of the fuel system". A truck climbing a hill exceeds the limits of the fuel system, which is why they have gears. ICEs have a limited RPM range in which they provide optimal torque and speed (HP). In a boat the transmission is used to change optimal engine speed range to optimal propeller speed range. The governor of traditional diesel engines, but not modern common rail diesels, basically moves the fuel rack to maintain the requested RPM. If the load exceeds what the engine can deliver on that RPM the governor will move the fuel rack to full flow in an attempt to deliver that RPM. Of course it gets a bit more complicated than that depending on the RPM selected and where that RPM sits on the engine torque curve versus load. But basically as pertains to the OPs question the engine burns more fuel to maintain 2900 RPM, because the governor is moving the fuel rack to a higher flow position to maintain the 2900 RPM he has set.

[a64pilot]

All I can respond to is you guys must have never operated many Diesels, when you hit a load, it takes throttle to get through it, the governor is of limited use to maintain RPM.
Ever driven a Diesel car or truck? You have to add throttle to pull a hill, one that doesn’t require a downshift.
Even the alternator when it comes on line will sag the RPM.

No one is talking about common rail.
Common rails are pure electronic and will respond however they are programmed to do.

[a64pilot]

Originally Posted by OldMan

Captain Bill is correct wrt to how the OP's 3GM30 governor works. This is not a common rail diesel. The throttle cable adjusts the governor not the fuel rack on the high pressure pump. The fuel rack is controlled by the governor.

Take a look at the fuel rack on the high pressure pump with the engine stopped, it's in the full fuel delivery position. When the engine starts, the governor backs it off to whatever speed is asked for by the throttle position.

From the service manual on that engine:

Read what you have posted, it states the governor prevents over revving, an RPM limiter. It governs speed and has limited throttle authority. It won’t for instance add full throttle when A load is applied, it will add up to the limit of the authority it has.

[OldMan]

Originally Posted by a64pilot

Read what you have posted, it states the governor prevents over revving, an RPM limiter. It governs speed and has limited throttle authority. It won’t for instance add full throttle when A load is applied, it will add up to the limit of the authority it has.

You are mixing the functions of throttle and governor. The throttle is simply a request for a particular RPM. The governor does everything it can to deliver that request. The governor has full authority (and the only authority) over fuel delivery, from full shutoff to 100% of (high pressure) pump output capability.

You are correct in your examples when the load is more than the engine can produce at a particular RPM, the only way to match the load is ask for more RPM.

But, if the load is within the capability of the engine at the RPM, the governor will adjust the fuel delivered to match the load. That is how a fixed RPM generator maintains 1800rpm from 100W load to 8000W load, as an example.

This is the referenced governor from the OP (1 cyl model vs 3 cyl model). Note the throttle cable attaches to the regulator arm in the upper left. The governor lever 2, lower middle, attaches to the HP pump. The throttle never touches the pump, only the governor. Since there is no control over air flow into the engine, fuel delivery is the only speed control.

[OldMan]

To get this back to the OP's question.....


A prop is happiest when every revolution it moves thru the water a distance equal to it's pitch. E.g., a 16 x 12 prop wants to move 12" for every revolution. If it moves less than 12", it takes more power as it's trying push more water than what it's actually moving. When a hull bottom is clean, the prop may move the boat 8" for every revolution, when the hull is dirty maybe only 6" (or 4"). It takes more power as the prop gets further from it's ideal of 12" per revolution.


The OP's engine was still operating within it's capability, albeit, a higher fuel burn due to higher load from the prop only moving 6" per revolution vs 8", as an example.

[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.