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Diesel consumption when in neutral


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So I thought this would be a fairly easy thing to Google-Fu - turns out my fu isn't.

If I am running the engine at 1800rpm in neutral am I using less, or the same amount of fuel that I would be if the prop is engaged?

If the answer is less then it must stand that I use more fuel

When the alternator is outputting 100amps; than

When the alternator is outputting 5 amps

Which is really getting to the root of my question, which is, does my alternator use up available kW if the engine is running anyway, or is the kW already there and just disappearing as heat/sound.

I would think it does, but the reason I am asking is because I do not notice a decrease in engine revs when the alternator kicks in at 100amps and the engine is already doing 1800rpm.

I would expect to notice 1.2kW on a 20kW engine.

 

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Our off the grid diesel generator at 1500rpm uses around 0.9 litres/hr unloaded and about an extra 0.3 l/hr for every KW we draw of it - i'd expect a similar thing to happen to your consumption .no free lunches sadly applies to the humble volvo or in our case an LD20 also.

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6 hours ago, CarpeDiem said:

So I thought this would be a fairly easy thing to Google-Fu - turns out my fu isn't.

If I am running the engine at 1800rpm in neutral am I using less, or the same amount of fuel that I would be if the prop is engaged?

It is not an easy question to accurately answer. It's like, how long is my piece of string and is it longer if I change the colour from blue to red, but what if the Blue has a green shade.
The ruff rule of thumb is, that between no load and full load at the same RPM, there should be a difference of 20% in fuel used. This is because the Fuel is injected at a set dose per cylinder each time the Piston gets to the top of the bang stroke. As the load increases, the RPM will fall away and the governor opens to add a little extra fuel to give more bang and hopefully maintain the same RPM. The Fuel dose to the cylinder is a factor of RPM. The RPM is a factor of Hp of the engine into a Load.
Is that helpful?

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conservation of energy is the rule - you cannot make something out of nothing.

you will use more fuel running at 1500 than at idle, and you will use more fuel running at 1500 with any load than at 1500 with no load.

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I can't comment on your fuel consumption question, but definitely, if you are drawing 100 amps of your alternator when underway, you have less power available for propulsion. If you have the alternator engaged, but less draw on it, i.e. batteries full, or fridge turned off, you will have more power for propulsion.

Probably only noticeable if you are trying to confirm max speed in flat water with a clean bum. Normally there are so many other variables you would never notice, like chop, wind direction etc.

When we repowered we went to a fractionally smaller engine, and I was very keen to see if we could still hit the target hull speed at max revs. Turns out we can, but the whole system is sensitive to a fouled bum. Additionally, we only went for the standard 40 A alternator, as that is all we need, leaves more power for propulsion, and we have a reasonable solar set up.

My understanding of fuel consumption is, within the range you are ever going to be able to measure, it is basically dependent on revs. Although as Wheels says, there are some variances. Certainly for planning fuel consumption and how much to take, the main dictator is revs.

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Only real way to know is measure consumption 

We did this sailing back from Europe 

Started running motor at 2400rpm at 8kts and burning over 10l/h In the Med

Then crossing the Atlantic we ran at 2000 doing 7kts and burning 5.6l/h

By the time we got to the pacific we were down to 1500 doing 6kts and burning 2.6l/h

We would record engine hours and Ltrs every time we fill up.

Tanks hold 500lts, 

So at 8kts can motor for 400nm but at 6kts we get to Fiji or 1150nm huge difference 

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yup - specific fuel curves are usually available for marine and industrial engines.  Our old Yanmar QM is sitting in hte middle of its curve at the moment.  As Jon notes, increased rpm make a large difference for a not-so-large speed gain.  This is the nature of IC engines and physics.

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The basic answer to your initial question is that the power is already there, it is wasted as heat and noise. On an efficient system, only about 35% of energy (diesel) is converted to propulsion. The rest is heat and noise.

On your alternator question, the engine will have available / spare power right up to max revs, if the prop etc is sized correctly for the boat. The prop power curve curves in a different direction to the engine power curve. When you get to max revs, the prop power is matching the engine power. If you go past this point, i.e. fouled bum, then the prop needs more power than the engine can give, and the engine is 'held', labours and blows black smoke etc.

Anywhere below / lower revs than that point, the engine with have spare Hp to run the alternator without you noticing.

I'll see if I can find an example prop / engine curve.

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The attached doc shows the engine power and fuel consumption curve for a Beta 35. Note that fuel consumption increases exponentially with revs, but it is on a fairly low scale axis, so it doesn't look dramatic. This doesn't have a prop power curve on it. I'll have a hunt around for one.

Beta 35 brochure.pdf

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A very close rule of thumb is that for every 25A of power generated requires 1Hp from the engine.
The reason I say rule of thumb is becuase a certain amount of energy is lost due to friction of the pullies/belt/s.
So based on that rule of thumb, a 100A ALT output is going to require 4Hp from the engine.

In saying the above, ALT installations of 85-100A is about the max when driven with a single belt. Above that current rating, it should be a twin belt system. but twin V belts have a lot of friction. This is due to that the V belt is driven by contact with side of belt and side of pulley. Significant increases in effieciency can be gained by changing to a flat ribbed belt system called a Serpentine Belt.

Diesels are completely unlike Petrol engines. In a petrol engine, both Fuel and air need to be controlled so as the engine does not run too rich or too lean. Being able to control both means a far greater range of power can be delivered. The rule of thumb is as about 80% more fuel is consumed between no load and full power. Although much more in high performance engines, but that is a whole different and very complex subject
In a diesel, power is directly proportional to the Fuel delivered. The air sucked in is always the same per volume of cylinder. Thus the volume of air into the engine intake varies only with RPM.
A diesel can still suffer from too rich or too lean, but not in the same way a Petrol does. I won't go into that here. So lets say the engine is operating at 1500RPM out of gear. When it is placed into gear(not considering that this is not good practice at 1500) the engine will now have an increase in load. Lets say the Alternator is switched on as well. As I said in the other post, the engine would normally slow down under the load. But for a Marine Engine, the Governor is such that it tries to maintain the same throttle setting of 1500RPM no matter what the load is. So the fuel rack opens and allows a greater amount of fuel to be delivered to the injector. The difference between no load fuel dose and full load dose is that 20% of extra fuel. Anymore than than causes black smoke due to not being able to get enough air into the Cylinder.
The addition of say a turbo allows more air to enter the cylinder. Thus more fuel can be burned and hence a turbo can increase Hp by about 50%. As long as 50% more fuel is delivered. Because in the end, the power directly comes from the volume of fuel that can be poked into the engine. Because Quantity of fuel delivered is proportional to RPM, the max power of a Diesel is usually around the 75-80% of max RPM. Above that RPM and the cylinder is struggling to get enough air to be efficient, so the power curve starts to drop again.
Modern high pressure rails with electronic injection timing has gone a long way to improving the thumb and so the figures do very between old and new engine technology.            
          Because Diesel is denser than Petrol, there is a lot more energy per CC of Diesel than Petrol. Coupled with the 20% increase of consumption against 80% increase for Petrol, you can see why Diesel is the fuel of choice in the Commercial/Trucking world.
 

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5 hours ago, Jon said:

Only real way to know is measure consumption 

We did this sailing back from Europe 

Started running motor at 2400rpm at 8kts and burning over 10l/h In the Med

Then crossing the Atlantic we ran at 2000 doing 7kts and burning 5.6l/h

By the time we got to the pacific we were down to 1500 doing 6kts and burning 2.6l/h

We would record engine hours and Ltrs every time we fill up.

Tanks hold 500lts, 

So at 8kts can motor for 400nm but at 6kts we get to Fiji or 1150nm huge difference 

That's a big and / or thirsty engine to be burning 10lph - what is it a 75-100hp unit?

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Volvo D3i 110hp Common rail diesel 

Full noise is 3000rpm at 10kts burning god only knows as I’ve only ever opened if up for short bursts

no real advantage in going over 2400 as not much more speed just a hole behind the boat

 

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26 minutes ago, Fogg said:

I’m guessing the difference between 2400 and 3000 rpm is about extra 40-50hp and extra 10lph but only 0.5kts boatspeed.

Pretty much but actually another 1.5 kts

so first 6kts are at .43l per nm

last 1.5 kts are at over 6l per nm

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Did a fishing and dive trip to the Three Kings two years ago, great fishing weather and we motored or motorsailed the whole way at 8+ kts

There was five of us and we were away for ten days, it took $500 to refuel on arriving back at Westhaven 

So $100 each or $10 each per day, cheap game fishing / dive trip

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When a big Launch would go buy, I used to scoff at how much fuel he would likely be using and how little I would be. Then one day I went for a ride on a mates big launch. We covered the ground at 27kts and went way out to D'Urville where I showed him around. Then all the way back to the Sounds again. I calculated in my head the amount of fuel he used to what I would have used. Except he did the trip in some 2hrs and I would have taken 8 hrs. The total Fuel used was exactly the same. That was a surprise.

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1 hour ago, wheels said:

When a big Launch would go buy, I used to scoff at how much fuel he would likely be using and how little I would be. Then one day I went for a ride on a mates big launch. We covered the ground at 27kts and went way out to D'Urville where I showed him around. Then all the way back to the Sounds again. I calculated in my head the amount of fuel he used to what I would have used. Except he did the trip in some 2hrs and I would have taken 8 hrs. The total Fuel used was exactly the same. That was a surprise.

 

I don't know how big your mates launch is, but a 13.5m/45' launch will typically use around 130-150lph to achieve 27kts (an older Riv 48 is an eye watering 230lph!) referencing Len Gilbert/John Menzies test data, whilst I guess your "old girl" would have used 10-12lph. So on a liters/NM basis the launch is using about 3x what you would have used. 

 

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It was a long time back and the exact figures would be a bit fuzzy now. I may have the 27kts part wrong.
His was a Bayliner 3388.
Mine has the Perkins 6.354 engine. Usual consumption was around 10lt/hr if I remained conservative with speed. But that figure would climb fast if I pushed it.

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In fact on thinking about it, the other boats speed may have been 24kts. That would make sense with time. I remember he was about 4 times faster than me. So I would be around 6kts, thus 24kts for him.
Looking up the specs, those boats had twin cummins 150Hp engines. Fuel consumption is 7.5Gal/hr. US gall is 3.8lts, so 7.5 x 3.8 x 2 = 57ltrs per hr x 2hrs = 114ltrs
My boat 10ltrs an hr for 8 to 10hrs =80 to 100ltrs.
Hmmmm, maybe I didn't work that out in my head perfectly, but 100 to 114 is not a hell of a lot of difference and as I said, the numbers are fuzzy now and he may not have been pushing 24kts

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