Coug, I'm tired so hope I get my words right...ya can't examine kw load for fuel usage comparisons. Since we're comparing a nearly identical 2-cylinder to a 4-cylinder...at idle 2 vs 4 of the same size pistons will use the proportionate amount of fuel, same holds true for at rpm...1800 rpm is 1800 rpm so the CFM the engines displace is exactly double, given the average fuel use since the pistons and injectors are approx the same. The only fluctuation would be a surge draw or constant high load draw.
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That isn't exactly how it works.
The 4 cylinder engine isn't going to be using the same amount of fuel in each cylinder as the 2 cylinder engine for identical overall power output with everything else being equal. The 4 cylinders are producing half the power output each of the 2 cylinders.
The general rule of thumb (at least for propane generators) is that it takes 16k btus of fuel to produce 1kw of electricity. There is still some minor fuel used just to keep an engine running even with no load, but you aren't talking a half a gallon per hour for those 2 extra cylinders.
To borrow real world numbers from another thread:
https://www.steelsoldiers.com/threads/mep-802a-fuel-usage-report.172820/
Post number 6 in this thread lists these results from one member's testing
Results for 802A
.25 gph at 600 watts
.28 gph at 1000 watts
.35 gph at 2000 watts
.45 gph at 3500 watts
.60 gph at 5000 watts
Results for 803A
.45 gph at 2000 watts
.50 gph at 3500 watts
.60 gph at 4500 watts
.90 gph at 9000 watts
So while the 802 is more fuel efficient (due to less rotating mass to keep spun up) the overall results aren't actually all that different when the 802 is under full load and the 803 is running half load (500 watts of power for the same fuel used, numbers underlined above).
Or if you want to look at this from another angle, I'll start with what you said. The 2 cylinder engine will use .5 gallons per hour for 5kw of load. The 4 cylinder has twice as many cylinders, so therefore it must burn twice as much fuel for the same 5kw of load because it is spinning at the same speed.
Then if the 4 cylinder engine is using 1 gallon of fuel to power 5 kw of load, but still only uses 1 gallon of fuel to make 10kw of load as stated in the manual, where is all that additional power coming from to power the extra 5kw of load you just added to it?
On the other hand, does that mean it will still use the same amount of fuel when there is no load applied to the engine? Will the 802 use the exact same amount of fuel at no load as it does at full load? (pretty sure looking at the numbers above that this isn't true)
The CFM of air entering the engine might be identical at 1800 rpms, proportional per number of cylinders, but that does not mean that the engine will be injecting an identical amount of fuel.
Diesel engines don't care much about the amount of air going into them as long as it is enough. Unlike a gasoline or other spark ignited engine, you don't have to achieve the correct ratio of fuel to air in order for a diesel to work. A diesel engine injects the amount of fuel it needs to maintain it's speed under load, and that is it. If you inject more fuel than it needs to maintain speed, even under varying load conditions, all you get is extra heat and soot.
