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#1
10-20-2008, 01:41 PM
 vivekmari Guest Posts: n/a
MMBtu/hr to gal/hr

Hi
i need to calculate fuel consumption of a diesel generator. all i know is the power rating 750 kw.

the method i follow:
fuel consumtion: kw* (MMBTU/292.87 KW) * (1/139,000 BTU/gallon)

then i stumbled upon this website

dynagen.com/page.aspx?id=96230

where,
for Diesel : 0.080 * kW (genset output) = gal/hr

what is "0.080"? can it be a load factor? if it is it does not account for the heating value of diesel..

thanks
#2
10-21-2008, 03:51 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,536 Rep Power: 19
Re: MMBtu/hr to gal/hr

Quote:
 Originally Posted by vivekmari Hi i need to calculate fuel consumption of a diesel generator. all i know is the power rating 750 kw. the method i follow: fuel consumtion: kw* (MMBTU/292.87 KW) * (1/139,000 BTU/gallon) then i stumbled upon this website dynagen.com/page.aspx?id=96230 where, for Diesel : 0.080 * kW (genset output) = gal/hr what is "0.080"? can it be a load factor? if it is it does not account for the heating value of diesel.. thanks
It is MUCH easier to work entirely in metric and convert at the end. You need to recognize that diesel engines and generators have efficiencies considerably less than one, and efficiency varies between manufacturers, with load, etc. Typical for a good set up might be 30-35%. I will use 33.33....%, (1/3) but you can follow as a proforma and vary efficiency.

1) In an hour, your generator provides 750 kW x 3600 s/h, or 2.7 GJ/h of energy, and needs 2.7/EFF GJ/h of input energy, or 8.1 GJ/h thermal as fuel.

2) Diesel fuel has LHV of 42.79 MJ/kg or 35.8 MJ/L using a table of fuel comparisons from DoE (actual values vary slightly lot to lot). Thus fuel requirement is 8.1 GJ/h x 1L/35.8 MJ = 226.3 L/h (59.8 US gallons/h)

Actual value will depend on actual efficiency, but the above may be within about 10%.

The 2nd source you found roughly agrees (60 gal/h). I believe there are two issues in your first equation:
*It is a theoretical conversion between units and doesn't account for the inefficiency of converting between thermal and mechanical forms of energy (the differnce is given off as heat, requiring a cooling system). Thermodynamics ensures it is no better than the efficiency of the Carnot cycle, and is typically much worse.
*Your value for diesel is an HHV. HHV is meaningless for internal combustion engines as none can tap the energy of condensing the exhaust water vapor and converting to useful motion. HHV "might" be useful with high efficiency furnances, but always use LHV in analyzing engine performance, especially comparing alternative fuels.

If you modify first form to consider 1/3 efficiency and about 7% difference between LHV and HHV, you will also get about 60 gal/h.

Last edited by JohnS; 10-21-2008 at 04:07 AM.
#3
03-25-2009, 06:30 AM
 Unregistered Guest Posts: n/a
Re: MMBtu/hr to gal/hr

#4
03-25-2009, 07:26 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,536 Rep Power: 19
Re: MMBtu/hr to gal/hr

Quote:
Different fuels have different heat contents in BTU/gallon. You need to know the fuel, and this figure to make the conversion. (mmbtu just equals millions of btu.)

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