Portable Generator Gasoline Consumption Analysis

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  #1  
Old 04-12-06, 02:02 PM
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Portable Generator Gasoline Consumption Analysis

Hello all,

I am an electrical enthusiast and recently helped my friend pick out a suitable portable generator to power parts of his home during power outages here in Florida (hurricanes). He ended up with a Briggs EXL8000 (8000w steady state, 13500 w surge). I purchased a smaller Coleman 5000w ss 6250w surge in 2004 the day before Hurricane Charlie's eye plowed through 13 miles from my home, knocking my power out for a week...

Anyway, we measured no-load gasoline consumption for the 8000 at 1/2 gal per hour, and the 5000 at 1/3 gal per hour.

Then, I took the manufacturer advertised runtime at half-load, and fuel tank size to calculate fuel consumption per hour at half-load, and I further extrapolated that to what I assume would be a straight line for 0watts steady-state to max steady state watts.

My question is this: Is this really a straight line, or am I completely off base here? I assume for ever watt of power demanded from the gen, a proportional amount of fuel is required.


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Last edited by majakdragon; 04-12-06 at 02:24 PM.
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  #2  
Old 04-12-06, 02:51 PM
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I think you are basically on track. With most generators, the no load rpm is the same as full load rpm, so, that GPH would be a constant (windage loss). Since 1hp = about 750 watts, I'd be interested in the conversion loss of a gas genset myself. A optimum gas engine should burn about 1 gal/hr/11hp. With your measured #'s, and the manufacturers #'s you should be able to graph efficiency vs load for various gensets.
 
  #3  
Old 04-12-06, 03:27 PM
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you are on about right on the track with fuel compusation with gas powered gennys they useally burn 1 gallon per hour per 10 hp but with diesel are much lower on figures [ i will leave this to diffrent matter for other time the same with LP or Natual gaz verison ]

to get the best loading with generator just load up to 80 % max of the name plate the engine will last pretty long time as long you dont overload or leave running unloaded for long peroids of time.

merci , Marc
 
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Old 04-12-06, 05:16 PM
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On my original post, I referenced a link to a Microsoft Excel spreadsheet I have been working on the last few evenings, which calculates all kinds of figures regarding utility power vs. gen power, costs, etc.
Anyway, a moderator edited that link out (gee, thanks. You let me post links before, now all of a sudden no?).

Well what I am understanding you guys as suggesting is that since gen is running 3600RPM, fuel consumption is constant from 0 load to full load. If I am wrong, then forgive me, but that is what I am understanding you are saying. If so, I disagree with that. I believe more fuel is delivered to the engine directly proportional to the wattage it is generating. The gen is stiffer to turn if there is more of a load, right? Stiffer means more force to turn, more force to turn = more fuel to burn.

But back to the spreadsheet...if you would like to download that Excel spreadsheet, feel free to go to my website listed on my profile. Then, just click on: the Hurricane link,
then click on Power Generation. The Excel spreadsheet is about halfway down on the PG page.

Anyway, I originally advised my friend to get the bigger generator mentioned above, but after measuring the "waste" (no load fuel consumption), I now see that during an outage, a balance between average steady-state watts needed and gen capacity should be determined before going out and buying a big fat generator. Of course, you don't want to run your gen at max all the time either I would think...

In all of my observations, it is astonishing that much of the ineffiency is in the gen sitting with no load running at 3600RPM. For example, my friends 8000w steady state uses 1/2 gal per hour no load. Even at the max of 8000w I calculate (based on the straight line theory) that that 1/2 gph isn't even doubled. In fact, it is about 1/4 gal additional for 8000w! What a waste that initial 1/2 gph is. I guess that becomes the convenience factor.

What a real waste of fuel it is if you are running a small load on a large gen...such as 2000w on the 8000w gen.

It would be beneficial to the consumer if manufacturers would readliy provide a chart on their product literature showing gph at 0w, and the line to max steady state load, showing gph all the way. I bet the average homeowner who wants quick portable power doesn't realize the loss with a larger gen.

OK so money for wasted fuel (no load consumption) may not be the primary issue (but then again maybe it is with cost approaching $3/gal again), but having to fill and store 50(!) 5 gallon fuel tanks is truely a bit of a nuisance.

FWIW...interesting observations...
 

Last edited by jasonalden; 04-12-06 at 07:12 PM.
  #5  
Old 04-12-06, 07:25 PM
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I may have misled you. The windage loss is a constant for all loads on the generators you described. To that, you must add the power required for the electrical power, which is variable. On the honda suitcase series, the motor is variable speed, which will help the low output power efficiency.
 
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Old 04-12-06, 11:55 PM
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Thumbs down

On the website:
> did not have to purchase an expensive transfer switch,
> nor pay someone to install it.

Wrong.
A sticker that says, "Caution: Turn off main power first" is not a substitute for an interlock or transfer switch.


> Regarding code, it should be compliant as all I did was I
> added a 240v outlet on its own breaker.

Wrong.

And the male-to-male extension cord is crazy too.

> highly dangerous for you or others if you do not know exactly what you are doing!

No, it is highly dangerous even if you know exactly what you are doing.


So my advice is that you label your website "How not to do it", declare that it is not Code-compliant and can kill people, or better yet, take it down.
 
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Old 04-13-06, 04:19 AM
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Old 04-13-06, 07:17 AM
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Gentlemen, I understand your responses with respect to liability. I am not going to argue that. Alright, feathers down folks.

Perhaps we can get back on subject here...
"Portable Generator Gasoline Consumption Analysis".

I originally placed a link directly to an Excel Spreadsheet, not the Power Generation page on the website. I have now put that Excel link on top of the page so easier to get to.

If anyone cares to expand on the subject "Portable Generator Gasoline Consumption Analysis" that would be great....thanks.
 
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Old 04-13-06, 03:26 PM
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Originally Posted by telecom guy
I may have misled you. The windage loss is a constant for all loads on the generators you described. To that, you must add the power required for the electrical power, which is variable. On the honda suitcase series, the motor is variable speed, which will help the low output power efficiency.
Is this indeed correct? If someone had a 501 cubic inch Cadillac engine and ran it at idle and measured it's efficiency, and then ran it again on another test but this time slowed the engine down ( simulating variable speed), would that still not be a waste due to large engine-parts frictional losses?..the fighting of the compression in massive cylinders?...as opposed to simply having a small engine to do small engine loads and large engine to do large engine loads?

A number of years ago, a company...perhaps Cadillac, came out with an engine that would run on 4 cylinders once it was up to speed and no longer under load. Now, if that large generator has such a feature, then perhaps an oversized generator would be okay?

Sort of like sizing furnaces for houses, also. Even with multiple speed blowers, I think an oversized furnace for the house would not be as efficient as one sized the smallest possible that it needs to be. Just thinking.
 
  #10  
Old 04-13-06, 04:31 PM
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i'm not sure of the question

but then again, reading comprehension was not my best SAT score

My point was that for the posters type of genset, they are constant speed devices. So, the "windage losses" are independant of the load, since the friction is essentially the same for no load vs full load. A smaller generator will (of the same technology) have a smaller "windage loss" than a larger genset.
All this just means you don't want to buy a bigger genset than you need, if you want to maximize efficiency. The OP was trying to measure fuel consumption, and I am suggesting there is a fixed consumption component (no load) and a variable component (due to electrical load).
This is why you don't want to make a habit of using your car as a battery charger, even though it would do the job just fine.

Not so obvious is the question of what happens to efficiency at the higher power levels. Gas engines tend to have better pumping efficiency when at WOT, since there is no loss across the throttle plate. So, for a constant LOAD application, use a small engine at open throttle.
 
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Old 04-14-06, 04:34 AM
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Now that you calculated the no load fuel consumption, and the half load fuel consumption, the rest is somewhat linear. There are some areas of the curve that will be off the curve, as certain designs take advantage of particular hardware, leaving portions of the efficiency curve more efficient (gallons/watt) than others.
 
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Old 04-14-06, 03:09 PM
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Thanks guys for your responses. Yes, it is me, the original poster. They permanently banned me for posting links to my webpage on this whole matter. They can keep banning me all day if they want. I'll try to "comply". Nonetheless, its not that hard "come back".

Anyway, I see that my theory of linearity seems to be right on.
Thanks gents for a great discussion about fuel consumption of a gen.
 
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Old 04-14-06, 05:00 PM
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Originally Posted by telecom guy

Not so obvious is the question of what happens to efficiency at the higher power levels. Gas engines tend to have better pumping efficiency when at WOT, since there is no loss across the throttle plate. So, for a constant LOAD application, use a small engine at open throttle.
I am not challenging here. Could you explain what "loss across the throttle plate" is? I have never heard that terminology before.

All I know about engines power (which then would equate to the "work" you can get out of the engine) is what they call an engines power curve...how engines have more peak horsepower when run at the peak of this curve, which occurs around 5500 (like for a V-8 engine) rpm, and then actually the curve comes back down when you get into the redline zone.

But I suppose generators have a governor on them where at "wide open"...it's not *really* wide open, because the thing would blow up. It probably has a governor? which holds the rpm in that peak power range?

But do tell me about that terminology I refered to before. Maybe I will learn something today or tomorrow.
 
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Old 04-14-06, 05:47 PM
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But I suppose generators have a governor on them where at "wide open"...it's not *really* wide open, because the thing would blow up. It probably has a governor? which holds the rpm in that peak power range?
just about all generators both A.C. and D.C. as well do have a engine governor to keep the speed consant like 4 pole 60 HZ generator have to run at 1800 rpm and 2 pole 60 HZ generators [ common with portable units] run at 3600 rpm but there is other speed but i will not list all of it

the generator governor control the rpm or HZ depending on design reguard of load if no load and full load they are designed to keep the speed constant.

i will give you quick example on gas powered generator [ common portable units ] set the speed pretty steady with out load then put a load on you will notice the speed do drop some until the governor react and open the throttle now if you take the load off from the generator it do the same thing just close the throttle to prevent overspeed.

if the engine dont have governer it hard to keep the speed in right spot all the time.

it work the same thing with cruise control in the car/ truck usage too.

myself i work on so many diffrent kind of generators gasoline , LPG, Natual Gaz, Diesel fuel types and of course turbine too so this is very simple explation expect few newer geneators do have electronic governor on it. it do repsonde very fast with load changes

if need more questions just ask

Merci , Marc
 
  #15  
Old 04-15-06, 08:06 AM
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pumping efficiency

http://courses.washington.edu/me341/oct22v2.htm

there is a little talk here of internal combustion "pumping loss", and why it is higher at partial throttle, compared with WOT.

Very simply, it is easier to move the piston down in the intake stroke when the intake valve is enabling a passage to atmospheric air pressure, rather than a partial vacuum.
 
  #16  
Old 04-15-06, 04:07 PM
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french277v,

Informative. Thanks.

telecom guy,

The last 3 lines of your post make your terminology I was wondering about, now easy to understand. I follow this. Makes sense. Thanks, also.
 
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