# Specific Fuel Calculations

Discussion in 'Diesel Engines' started by Woodsail, Jan 30, 2013.

1. Joined: Jan 2013
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### WoodsailNew Member

Hi All,
I'm sitting a maritime engineering oral exam and I've been prewarned of some fuel calculations questions.
A work mate gave me some questions that he got asked, but one of them is of some concern to me.
his question: Calculate fuel quantity used in litres for a trip given
a; specific fuel coefficient
b: bhp/grams/hour
c: relative density of fuel.
I'm not that sure that this is a valid question, I think more information is required IE: Vessel displacement, trip duration or distance/ speed.

The only fuel coefficent data in the Tafe text relates to

Fuel consumption per day = displacement ⅔ X speed³ fuel coefficient

bhp/grams/hour is STR8 forward , convert bhp to KW as with the relative density of fuel ( mass ) to fresh water( volume ).

I would appreciate if anyone has or knows about the above formular

Regards

Woodsail

2. Joined: Oct 2008
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Location: Japan

Hi woodsail
welcome to the forum.

To work out the amount of fuel consumption is pretty straight forward, with a few givens.

If you use the grams per kW/hour, gkWh, and divide this by the density of oil, 850 kg/m^3, and then multiply by the power of the engine at the rating, or rpm, under question, you shall arrive at the fuel consumption in litres per hour.

Then knowing the amount of fuel (or working backwards), divide the amount of fuel by the fuel consumption you just calculated gives you the amount of hours you have to burn.

So:

Fuel consumption is simply = (gkWh x MCR)/850 = litre/hour

3. Joined: Jan 2013
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### WoodsailNew Member

Thanks for getting back to me, it was a great help. I have another question ( if I may )
Being asked to find the volume of a cylindrical tank ( pi x R2 x L) if it's standing up !!!!! and all text & questions are based on it being upright . I have never seen a fuel tank on a vessel standing up !!!!!!!!!!!!!!. So to find the volume of a part filled cylindrical tank ( horizontal ) would not be the same as if it was vertical ?????( vertical is easy to work out ). I would love to throw that question back at the examiners !!!!!!!
cheers
Woodsail

4. Joined: Oct 2008
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A tank, is a tank, is a tank. Its height, length and breadth (or diameter) is irrelevant to its orientation and location. Turning a box upside does not alter its dimensions, only its orientation and "point of view" for you, the observer.

No, the calculation for a horizontal tank (if cylindrical) would not be the same as vertical.

But the procedure remains the same, find the area, multiply by the length, if constant section.

5. ### Number4Previous Member

Hi Woodsail,
You should tell your examiners that they should be using g/kw/hr or lbs/bhp/hr but that it is not cricket to be mixing them up.
Personaly, I would convert everything into metric, work out the equation, then convert into whatever antiquated unit is required. Working half and half is a nightmare.
Aircraft have been known to fall out of the sky because of the confusion of units US gals, Imp gals, litres, lbs, and kg.

100 kw x 0.22 kg/kw/hr = 22 kg (per hour) x 10 hours = 220 kg (mass of fuel used)/ 0.85kg/litre (density of fuel) = 258.8 litres

If you really want to show off, ask them "What temperature is the fuel?" and refer to your density/temperature chart.

To calculate the fuel in your horizontal cylinder tank, you just need to know the side area the fuel is occupying and the length of the fuel tank.
http://en.wikipedia.org/wiki/List_of_centroids
Good luck with the exam