Richard, actually I think you're probably quite brilliant as you twigged to something few of us ever consider.
Take a look here for a primer on the topic of BSFC.
https://en.m.wikipedia.org/wiki/Brak...el_consumption
Notice the discussion regarding power per gram of fuel. Then later on, the discussion comparing BTUs of various fuels.
In both cases, the fulcrum around which all computations pivot is a constant weight of fuel consumed, not volume.
Probably the easiest image to visualize is that of the volume occupied by a single gallon of water. Compare the volume of a gallon of water at 20 C, to that of a gallon at 100 C. Clearly, volume increases with temperature. However, were the volume held to a constant with increasing temperatures, the actual amount of water would decrease.
Think of completely filling a teapot with room temperature water, then heat to a boil, then cool to room temperature. Now examine the amount of water remaining.
What if that was not water, but a volume of fuel. Same thing happens. Less fuel in the same volume, yet we always buy fuel by the gallon.
Ordinarily not much of a problem for road vehicles. Aircraft operating maximum range missions with warm fuel, bigger problem.
I think we can all agree this isn't a double digit effect, but it is something to remember.
Take care,
-Matt
Quote:
Originally Posted by Dr4Film
My Monaco came with a 128 gallon diesel fuel tank. The Aqua-Hot and Generator pickups in the tank are supposed to be around the quarter tank mark. Through trial/error I determined that the Aqua-Hot and generator both quit after I have used about 82 gallons of a full tank. I determined that similar to you when one day in Alaska they both quit and I headed off to the pump. So for my tank it is more like 1/3rd of a tank is the critical point. Unfortunately the fuel gauge is not that accurate or reliable so if I am stopping for the night I make sure that the gauge is more than half full.
One day while heading to Florida for the winter I "pushed" the envelope too far while on the road and figured out exactly at what level my Cummins engine quits.  Luckily I was able to coast directly into a diesel pump and proceeded to pump 101 gallons of fuel. Therefore there are about 28 gallons of fuel which are not usable due to the shape of the tank bottom.
Actually, this has me totally confused.
I do not profess to be a genius but I've been told that I have an average IQ with not much book smarts. However, I do have a lot of common sense and your statement in regards to "we pay by the gallon but burn by the pound" does not compute with me.
Can you explain in layman's terms?
My Cummins engine burns so many gallons per mile, my Aqua-Hot burns so many gallons per hour and my generator burns so many gallons per hour.
I have never seen any quantitative parameter with regards to any of these where they burn by the pound. Plus I don't think it matters much to the weight of the coach.
Dr4Film ----- Richard
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