# Fuel Consumption



## Keltic Star (Jan 21, 2006)

Engineers Help!

I'm in the process of selecting engines (1200 bhp range) for a series of 20m fast Cat ferries. The sales engineer of the favoured brand is a smart a--ed young know it all who's trying to outsmart this old deck ape despite the fact that over the years I've probably ordered more engines than he has yet taken orders for.

When I asked him for fuel consumption figures at various load conditions he warned me to be careful in interpreting the "specific" vs the "absolute" values. I well understand that the former is expressed in g/wKh and the latter is in lts/hr. Is there something else important that I am missing or is he pulling my plonker? 

Could one of you kind engineers steer me in the right direction because I'm too old and stubborn to give in and ask the little p---k for help.

Cheers
Bob(Jester)


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## Satanic Mechanic (Feb 23, 2009)

Edited in an attempt to clarify as I didn't read the post right first time!!!

Ah this is always a favourite, he's not being naughty as such but there is no reason he can't give both figures, nor for that matter is it difficult to work 

The specific consumption tells you how efficient the engine is using a set quality of fuel and this varies with load so for instance (just using any old figures) an engine doing 1000kw at 10g/kW.hr will use 10000g of fuel an hour however if the same engine at 500kW uses 15g/kW.hr it will use 7500g of the same fuel an hour, so while it is using less fuel it is using it much less efficiently. Also of course poorer quality fuel will alter the specific consumption

In answer to your question though, any figure will do as they both show consumption, the one to watch is what the calorific value of the datum fuel is as it can be "optimistic" and if you are comparing engines make sure they are using the same datum. Personally I always get the the figures In Specific, take the datum fuel calorific value and use - kilo or megaJoules/kW.hr as this removes the fuel quality factor so since we now often change fuel types it's an easier figure for me.


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## steamer659 (Mar 18, 2009)

I am a great proponent of the slower speed diesels- driving generators. Something in the 720 rpm range or below, constant speed driving a high voltage electric propulsion set... I've found from personal experience that the slower, constant speed engines have longer maintenance intervals, but of course in a cat hull, the size limitations would dictate....


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## Derek Roger (Feb 19, 2005)

In addition to SM comment Bob make sure the specific consumption is given in 
grams / Brake Horse power / Hr for the particular fuel your client will be using and not some convoluted evaluation which suits their purpose . Test bed results being the best with all the parameters known .
When comparing engines I always had to do my own calculations to be able to compare apples with apples .
When consulting I always had trouble getting the information I wanted from suppliers ; CAT in particular .

Asking for a guarantee with a penalty usualy opens up the door for true figures .


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## Duncan112 (Dec 28, 2006)

As SM says the calorific value is important, manufacturers have a habit of swapping between higher and lower CV to suit their purposes. About 5 years ago I was doing a feasibility study into a gas CHP plant for Khazikstan and it proved impossible to directly compare the stated performance figures for the four contenders, such were the vagaries of the way they polished their own figures. (Plant never got built as the political risk was deemed too high by the owner) FWIW I thought the 10 year costs of Wartsila engines once guaranteed availability was taken into account outweighed short term fuel benefits, the owner wanted CATs because of their lower initial cost.


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## Keltic Star (Jan 21, 2006)

Gentlemen:

Thank you all for your replies which are greatly appreciated. I can understand the difference with the fuel quality available and I will try to get something nailed down considering the boats will be using the same bunkering facility for the first seven years of their life. 

Derek, I like your idea of asking for a guarantee, particularly as the client is very serious about fuel consumption and potential loss. $100,000 dollar fuel monitoring systems are to be installed on all the boats. 

Cheers


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## berbex (Feb 17, 2013)

On the question of guarantees, it works but tread with caution.

If you mention it at the start you are likely to find it included in the bid as an extra price or maybe hidden. you may then have paid for guarantees whether or not you invoke the clause.


The guarantee figure accepted may be so low, again like other penalty clauses, that the supplier would get his profit margin irrispective of whether you are satisfied or not.

Good luck, it worked well for me once, but not twice with the same supplier.


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## Satanic Mechanic (Feb 23, 2009)

Beat me to it on guarantees! Guarantee figures liable to be pessimistic with get out clauses on fouling etc. Soooooo try this :
1. Get yer units sorted out , up to yourself what you're comfy with but rally tryout to have a mixture of imperial and SI that's furlongs per fortnight territory. Personally I go for kW, kJ, kg 

2. Select specified powers for given speed - do they have a sea and fouling allowance already if not put one in ( I usually use 19 % allowance - 15 for sea, 4 for fouling over years) 

3. Get the specific consumption in mass for each figure

4. Get the specification for the datum fuel

4. Convert specific mass to specific energy

5. Get the specification for the actual fuel

6. Use the lower calorific value to work out the mass of fuel and the density to workout the volume

These figures are a good place to start for actual consumptions


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## chadburn (Jun 2, 2008)

Keltic Star said:


> Gentlemen:
> 
> Thank you all for your replies which are greatly appreciated. I can understand the difference with the fuel quality available and I will try to get something nailed down considering the boats will be using the same bunkering facility for the first seven years of their life.
> 
> ...


What happened to those superbly made polished Fuel Sample Can's with the brass Esso type screw caps, a credit to any Tinsmith. Get a sample of the Fuel they are actually using during the test and have it checked out independently(Thumb)


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## berbex (Feb 17, 2013)

chadburn said:


> What happened to those superbly made polished Fuel Sample Can's with the brass Esso type screw caps, a credit to any Tinsmith. Get a sample of the Fuel they are actually using during the test and have it checked out independently(Thumb)


You could say "dumped overboard".

There are two important issues with fuel.

1.----What are the chosen units happy with. And what is the change in maintenance and other costs when fuel quality changes? Some engines may be more finicky than others.

2.----Fuel specs have changed considerably in these last years. What fuel quality has the customer in mind, and what can he get on the market?

View 1 and 2 together in choosing a prime mover.

Once lately I heard something that shocked me. A supplier agent said that when his filters get choked its ok in an emergency to bypass filter. Its ok to him but you would not know it if you get a load of **** too. that is also something you have to be prepared for.

Keltic star quote "smart a--ed young know it all who's trying to outsmart this old deck ape ". It is not where he's been to school but what he studied. Find out if it was engineering or drama.


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## Satanic Mechanic (Feb 23, 2009)

chadburn said:


> What happened to those superbly made polished Fuel Sample Can's with the brass Esso type screw caps, a credit to any Tinsmith. Get a sample of the Fuel they are actually using during the test and have it checked out independently(Thumb)


You don't usually go to the confirmation tests. They will use a fuel and reference it to ISO 8217(?)

Complete minefield this since consumption is such a big issue the vendors are introducing fiddle factors that a Greek bunker barge would be proud of. For me take it all the way back to energy and calculate from there


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## Derek Roger (Feb 19, 2005)

Satanic Mechanic said:


> You don't usually go to the confirmation tests. They will use a fuel and reference it to ISO 8217(?)
> 
> Complete minefield this since consumption is such a big issue the vendors are introducing fiddle factors that a Greek bunker barge would be proud of. For me take it all the way back to energy and calculate from there


Brocklebanks kept the samples but did not test unless we had a problem with the bunkers .
Excessive vanadium content was a problem with some bunkers which caused havoc with exhaust valves and some turbo chargers in medium speed installations .


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## Keltic Star (Jan 21, 2006)

Satanic Mechanic said:


> Beat me to it on guarantees! Guarantee figures liable to be pessimistic with get out clauses on fouling etc. Soooooo try this :
> 1. Get yer units sorted out , up to yourself what you're comfy with but rally tryout to have a mixture of imperial and SI that's furlongs per fortnight territory. Personally I go for kW, kJ, kg
> 
> 2. Select specified powers for given speed - do they have a sea and fouling allowance already if not put one in ( I usually use 19 % allowance - 15 for sea, 4 for fouling over years)
> ...


SM, thanks for the mine of information. Agree centipedes is the way to go although I still have to pull out a ruler to see how many feet a metre looks like.

For the Technical Building Specification, we think we can safely go as far as your Stage 4 and then put in a legal disclaimer for Stage 5 as the actual fuel spec. is not available to us at the design and pre-build stage and therefore beyond of our control. 

Thanks


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## Keltic Star (Jan 21, 2006)

berbex said:


> On the question of guarantees, it works but tread with caution.
> 
> If you mention it at the start you are likely to find it included in the bid as an extra price or maybe hidden. you may then have paid for guarantees whether or not you invoke the clause.
> 
> ...


Agreed, I never ask for concessions or guarantees until the hand is poised to sign the contract! I managed to get a pile of undisclosed shipbuilding subsides that way once .


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## Keltic Star (Jan 21, 2006)

chadburn said:


> What happened to those superbly made polished Fuel Sample Can's with the brass Esso type screw caps, a credit to any Tinsmith. Get a sample of the Fuel they are actually using during the test and have it checked out independently(Thumb)


Whilst very important considering the cost, the engines fuel consumption is only part of the concern, of bigger importance is to ensure no one is nicking it out of the fuel tanks. Apparently the practice is rife worldwide these days.

In addition to the fuel monitoring system, there's another electronic system specified to make sure no one has nicked the ship either.


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## berbex (Feb 17, 2013)

Keltic Star said:


> Whilst very important considering the cost, the engines fuel consumption is only part of the concern, of bigger importance is to ensure no one is nicking it out of the fuel tanks. Apparently the practice is rife worldwide these days.
> 
> In addition to the fuel monitoring system, there's another electronic system specified to make sure no one has nicked the ship either.


No, the practice of nicking, lube oil especially, has been going on for long years. Leave a loophole in the system and you've asked for it.

Once I signed for a heavy issue of lube, a very good quality, that i thought that particular engine ought to be drunk. It failed that same night, cam shaft bearing. Its sump was bone dry, so were others. I suddenly had 2 major problems. The repairs was one. Firing half the crew as it turned out was more costlier and problematic. I was new to the site with a very steep learning curve.

Apparently turbine lube foamed in car engines, but the high grade diesel engine oil was a different story.


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## Satanic Mechanic (Feb 23, 2009)

Satanic Mechanic said:


> Beat me to it on guarantees! Guarantee figures liable to be pessimistic with get out clauses on fouling etc. Soooooo try this :
> 1. Get yer units sorted out , up to yourself what you're comfy with but rally tryout to have a mixture of imperial and SI - that's furlongs per fortnight territory. Personally I go for kW, kJ, kg
> 
> 2. Select specified powers for given speed - do they have a sea and fouling allowance already if not put one in ( I usually use 19 % allowance - 15 for sea, 4 for fouling over 5 years)
> ...


KS - mine of information maybe - but not good at numbering using an iphone- see above for more accurate post.

You are of correct the builders and buyers have two different points of view. As a builder I would go with your suggestion and state in the specification something along the lines of 

" fuel consumption at Normal Continuous Rating with a 15 % sea margin allowing for sea states up to WMO sea state 4 and a 4% percent margin allowing for 5 year surface deterioration exclusive of fouling of wetted hull areas shall be xxxxMJ/shaft kW.hr"

Obviously fettle the above for own purposes(You could use engine kWs as well but remember to put in a transmission allowance)


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## Derek Roger (Feb 19, 2005)

Satanic Mechanic said:


> KS - mine of information maybe - but not good at numbering using an iphone- see above for more accurate post.
> 
> You are of correct the builders and buyers have two different points of view. As a builder I would go with your suggestion and state in the specification something along the lines of
> 
> ...


On technical issues I tend to agree with you but on this one I think you have lost the plot .
The question is not on a guarentee the vessel s performance ; only that of the engines performance re fuel consumption .

The engine does not care about sea states or hull fouling ; even if the hull is tied up to the dock it is all irrelevant .
The issue is what is the engines specific fuel consumption in gm / BHP /Hr .
This to be provided by the engine manufacturer for the full range of operating loads with a graph .
The information to be based on a specific fuel with known calorific value .
Then one can compare apples with apples in selecting an engine .

Seems pretty simple . Derek


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## Satanic Mechanic (Feb 23, 2009)

It was more of a continuation of KBs last post

Depends if speed is an issue which it almost always is, therefore you give the rated speed at NCR which includes an allowance for sea state and commonly fouling these days, such is the criticality of the consumption. I don't know of a single builder in the world that does not weight the NCR (MCR in the case of diesel electric ) by 15- 21 % when giving the rated speed and power . It's all part of the calculation so if the owner says the vessel s using too much fuel your first question is - what were the conditions

Now the initial question was on the engine performance, I am suggesting the safest way to give this is in specific energy ensuring that it is calculated at the correctly weighted power.


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## chadburn (Jun 2, 2008)

The Test/Demonstration engine may well have been blueprinted.


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## berbex (Feb 17, 2013)

I don't mean to flog this horse, but there is some more to it. There are other factors that are important but are not given in the brief above.

What is the setup like---from engine to propellor? Is the engine being driven at constant revs (at optimum speed/lowest bsfc) or at variable speed and load. These may vary substantially between engine types. Look at the family of engine curves of bsfc against speed for various power/throttle(read injector fuel pump) settings.

What may appear to be a better engine than another will change if that engine finds itself working off it best performance point while the lesser may fit at its most economical.

If certain parameters are already fixed, like engine revs, then you are more restricted.

What bsfc figures you have to use (for any engine) is dictated by where the operating point is/sits on the curves.

Even when you are comparing apples to apples, there still are many varieties of apples. What makes a good dessert may not make a good mince pie.


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