not sure is this right place to post ? or correct forum ???? generator fuel consumpti

[engineer707]

I have a Cummings 250 HP emergency generator for my building and I need to find out the fuel consumption for the year. I perform monthly test and run the generator for 30 mins each month at idle speed and take readings. It says I have a fuel rate of 1.90 GPH and a fuel consumption of 5 gallons and total fuel consumption of 5 gallons. How do I figure out my monthly and yearly usage?

Do I divide the 1.90 GPH in half since I only run for 30 mins and then multiply that by 12 months ? or do I multiply 1.90 feul rate by 12 months or do I multiply the 5 gallons by 12months ? any help or direction would be great

 

[GoldDigger]

How many months have you run the generator. It sounds like 5gallons is the total over that many months.
1.90GPH would be multiplied by 1/2 and then by 12.
The 5 gallon figure would be consistent with five months usage so far.
It sounds like you are not running the generator under load for your 1/2 hour test or I would expect a higher fuel consumption rate.
This is of little or no use in predicting fuel consumption during an outage. Running at idle would be OK for a gasoline engine, but potentially harmful for a diesel.

 

[drcampbell]

... Running at idle would be ... potentially harmful for a Diesel [engine].

Modern Diesel engines -- those with electronic, common-rail and/or high-pressure fuel injection -- generally do not have the idling problems of antique engines. (engines with a mechanical injector pump) Of course, 30 minutes idling isn't a terribly rigorous or comprehensive test.

"Diesel" is a proper adjective and gets capitalized.

 

[Barbqranch]

The problem with running a Diesel engine at very low power for extended time is called "wet stacking". I don't know if modern engines are immune or not. The cause is not getting the engine and exhaust up to operating temperature for a sufficient period. In the short term, it may not cause much trouble, but I understand it can cause permanent damage.

I have read that NFPA has standards regarding this.

 

[Ingenieur]

Sounds like 2 numbers
rate 1.9 gal/hour
total 5 gal

add up the hours run for the year H
H/year x 1.9 gal/hr = gals/year
divide by 12 for monthly

total gals / time period in months = gal/month
5 gal / 6 months = 0.83 gal month
very close to 1.9 gal/hr /1/2 hr = 0.95 gal

 

[drcampbell]

The problem ... is called "wet stacking". ...

Diesel engines control speed & power by metering fuel, not by throttling air. So there's an abundance of air flowing through the engine at all times. At idle, the air:fuel ratio can be 100:1 or more, and all that excess air keeps the cylinder and exhaust temperatures down.

In older engines, the lower injection pressures made for larger fuel droplets. At the (relatively) low idling cylinder temperatures, there isn't enough time for each droplet to evaporate and burn. (remember that there's only 16 milliseconds for ignition & expansion events to happen at 1800 rev/min) Unburned fuel, including unburned fractions such as soot & tar, deposits itself in the exhaust system. (and on surfaces near the exhaust outlet) Much like a chimney fire, all that fuel accumulated in the exhaust system will eventually ignite, which is why the NFPA took an interest. If this is an older engine, wet stacking is a very likely outcome of running the engine only at idle.

The higher injection pressure of modern Diesel engines (~30,000 lb/in² vs. 1-2000 lb/in² created by a mechanical IP) makes for much smaller droplets which combust much more completely.


As for the original question, I'd say the answer is "Not nearly enough.". Diesel fuel has a limited shelf life and it's important to avoid storing it for more than two years or so.

 

[JFletcher]

I have a Cummings 250 HP emergency generator for my building and I need to find out the fuel consumption for the year. I perform monthly test and run the generator for 30 mins each month at idle speed and take readings. It says I have a fuel rate of 1.90 GPH and a fuel consumption of 5 gallons and total fuel consumption of 5 gallons. How do I figure out my monthly and yearly usage?

Do I divide the 1.90 GPH in half since I only run for 30 mins and then multiply that by 12 months ? or do I multiply 1.90 feul rate by 12 months or do I multiply the 5 gallons by 12months ? any help or direction would be great

Does the unit have a Run Time Meter? If the genset has run loaded it will affect your readings.

That fuel use rate sounds like it's at idle, unloaded. How are you taking your fuel readings? Mfg spec? Flowmeter? Sticking a tank/tank level?

If you just run it at idle 30min a month, that's 6 hours/year @ 1.90GPH = 11.4gallons/year, or about a gallon a month (11.4/12).

This would be an undetectable amount thru almost any means of measurement other than run time x mfg spec.

 

[infinity]

Of course, 30 minutes idling isn't a terribly rigorous or comprehensive test.

I agree running with some sort of load would be better. An idle test will only tell you that it will start and idle.

 

[Ingenieur]

I agree running with some sort of load would be better. An idle test will only tell you that it will start and idle.

Not good for the dpm filter and catalyst either
if a ceramic filter hot is better
same for cat
results in fouling and loss of eff

 

[Smart $]

...
"Diesel" is a proper adjective and gets capitalized.

While some grammarians may disagree, that is about as absurd as requiring capitalization of watt, tesla, and newton units of measure.