Just ran my generator for 105.5 hours straight here in Galveston. I filled up the tank 7 miles from campsite anticipating we might lose power for a day or so on Saturday. Power went out 1:30 am Sunday. Power came on this morning and that also meant the diesel pump was working at the convenience store. I used 44.3 gallons and drove 28 miles, two trips seeking to refuel. Estimate my generator consumption to be ~ 41 gallons so .4 gph.
I do not have Aqua Hot so this is all generator. Load was two rooftop heat strips, 200 watts in the bay and frequent use of electric ceramic heater (1500) watts plus lights etc.
That's an excellent bit of real world data for the Forum. Since the generator engine runs at constant RPM, i wonder how much the load actually affects the fuel consumption?
Many Foretravel coaches have 10Kw generators with bigger diesel engines. The generally quoted "average" fuel consumption number during past Forum discussions seems to be about 0.5 GPH, so you are real close with your 8Kw unit..
Chart below from PowerTech Owners Manual. We coach owners must normally run our units at around 50% load?
I used to figure .5 a gallon an hour on my 8kw. It was .75 - 1 gph on my 12.5. I ran my 50kw generator for 70 hours here at home and put in 64 gallons so even the big ones are similar.
Basic Thermodynamic: (Last lecture I attended was in 1968, so cut me some slack, please)
Energy content of a gallon of diesel fuel: 139000 BTU
1 KWhr = 3412 BTU
used .4 Gal/hr = 34759 BTU / 3412 = 10.2 KWhr
Peak Thermal Efficiency of diesel engines (amount of mechanical work generated vs heat, noise, vibration) approximately 20 % - 40 % for small diesel engines. (I pick 25% because generator is in partial load)
Electricity consumed: 2550 Watts per Hr on average.
I bet you all wanted to know this.
Regards
Klaus
I did not proofread my last post, I did not catch the mistake I made in simple multiplication. Turns out you all have to cut me slack with elementary school math.
Chris M lang sent me (tactfully) a PM to point it out, I assume he did not want to embarrass me on the forum.
139,000 BTU/gal multiplied by .4 gal equals 55,600 BTU and not 34,759, as I said.
I assumed a Peak Thermal Efficiency of 40% for small diesel engines ,which is a reasonable number supported by literature.
I settled on 25 % Thermal Efficiency for partial load.
That results in a corrected average Electricity consumed number of (55,600 / 3412) x .25 = 4074 Watts per hour.
If I had paid more attention to the numbers in the picture of Chuck's post which were published by Kubota, I would have noticed that there is a factor of 4 between Fuel Consumption at 100% load (usually peak) and quarter load. (.8 gal/hr versus .2 gal/hr). That indicates that in the load range from 25% to 100%, Thermal Efficiency and Fuel Consumption are linear, which is certainly not true for many internal combustion engines.
It also points to an average Electricity consumed of 4000 Watts per Hr.
As a result of this discussion I will order a digital power meter and start monitoring my coaches electrical power consumption.
Regards
Klaus
You guys are making my head hurt.
I used to be horrible at math,, now I've worked up to mediocre. 🙂
And I feel like I've "found my people" ;) (my last thermo class was either in 1982 or 1983)
I do excel in philosophy but there is not much call for it..
A diesel (or gas) generator will use more fuel as the load goes up. If it is operating no load at 1800 rpm, it only has to consume enough fuel to balance the loss from the water pump, fan, engine loss from the rings, oil pump, etc, etc. As the load increases, the amount of BTUs needed to overcome the parasitic load plus the power generated will increase. With a 2 or 4 cycle engine, the lowest efficiency will be at the lowest amount of power generated and as the load increases, the parasitic loads will be a smaller percentage of the total amount of power generated. 80 percent of maximum output is generally the most efficient use of fuel.
With a 2 cycle like the famous Detroit 2-71 "Deuce" 20Kw generator, the generator takes advantage of the high torque at low RPM to operate at 1200 rpm. This is also where the necessary Roots blower is efficient.
Here is a fuel consumption chart for diesel generators: https://www.generatorsource.com/Diesel_Fuel_Consumption.aspx
Pierce
Disagree with reply # 8
Philosophy is the study of the fundamental nature of knowledge, reality, and existence.
As such it is the basis of understanding of all human interactions, whether you formally study it or acquire it through experience.
Empires and people's fates rise and fall, and philosophy can help to understand the reasons.
Thermodynamics will do not help you in this.
It will make you understand thermal combustion cycles, but compared to why a mob of people suddenly storms the Capitol, that knowledge is quite insignificant.
But you certainly need it for specifying and selecting a diesel generator.
On the other hand, when your coach generator suddenly quits and you need practical machinery knowledge to fix it, and you can have that without ever having heard of the three laws of Thermodynamics (Hint : First Law "Energy can neither be created nor destroyed")
Regards
Klaus
Fuel consumption on my RV generator is 7 gallons for a 24 hour period. Luckily I did not need to use it during the recent weather but I slept better at night knowing it was available to use if we lost power here in Missouri.
Formally studying philosophy is less painful than acquiring the same knowledge through experience.
Thermodynamics- philosophy- You guys are all above my pay grade!!!!
I just go by the Kenny Rogers song ""You got to know when to "holdm" and when to "foldm" when to walk away and when to run
Chris
A coupe of decades ago when I was recuperating in what they call the "Progressive Care Unit" one of the therapists gave me a pamphlet to study titled "Conservation of Energy," lessons on how to work with what I had left. Yeah, I set the alarms off.
But acquiring the same knowledge through experience is probably a lot more fun.....
For years we also used 0.4 gal / hour of 8kw gen run. A year ago we refined it down to 0.35 gal / hour.