6/1 vs 20/2 fuel consumption......scale?

[DKMC]

Does fuel consumption between engines "scale"?
IOW, does a 6/1 (or a 6/10) running at say.....2000 Watts use about the same ammount of fuel per hour
as a 20/2 running at 2000 watts?
I understand a 2 cyl might use a bit more fuel, and that there are additional frictional losses,
but curious if part loading of a larger engine can be economical.
I want to run economically at light loads, but still have plenty of reserve power available.

I guess another possible solution would be 2 single cylinder engines coupled with a clutch that
could be engaged to 'bring in' the second engine?

[Ronmar]

I would also say 2 generators.  The first KW costs the most to create.  Each successive KW up to the load limit cost less and less in terms of fuel.  IE: doubling the load from 1 to 2 KW does not double the fuel consumption.  My experience with my 6/1, and the fuel consumption graph and research done with Mobile Bob thru load testing bears this out.  Partial loading is less efficient.  Here is my 6/1's consumption graph at 350' above sea level.  There are of course a few factors that also effect efficiency such as engine temp.  I found while running these tests that the 6/1 is more efficient when ran hot, than when cooler.

[rcavictim]

I would also say 2 generators.  The first KW costs the most to create.  Each successive KW up to the load limit cost less and less in terms of fuel.  IE: doubling the load from 1 to 2 KW does not double the fuel consumption.  My experience with my 6/1, and the fuel consumption graph and research done with Mobile Bob thru load testing bears this out.  Partial loading is less efficient.  Here is my 6/1's consumption graph at 350' above sea level.  There are of course a few factors that also effect efficiency such as engine temp.  I found while running these tests that the 6/1 is more efficient when ran hot, than when cooler.

Ron,

That looks like really good data.  Very carefully measured.  Nice work!

[Derb]

Hi Fellas. This is one of the great mechanical givens - a small engine run with a good load efficiently uses less fuel than a large engine with the same load running unloaded. Cheers.

[DKMC]

So the $64 question is.....how much worse on fuel?
Enough that it is worth fussing with another set, trying to split loads, or
coupling 2 singles with a clutch?

It would be an interesting Rube Goldberg study to couple 2 singles with some sort of
air clutch so the second engine could be cut in under motion.
I just wonder if that would or could work feasibly ??
I wonder it 2 singles would play nice together, or if they'd need to be 'aligned' crank wise to
keep from tearing each other to bits?

dk

[quinnf]

Ron,

Seeing a chart like warms the dark recesses of my cold and clammy heart.  That's impressive data, and not all that easy to collect.  Now, how much black smoke were you getting at Pmax?  Seems you might expect to see an inflection in the curve as you get near to overloading the engine.  Maybe you didn't take it that far.  

[Edit:  I see the 0.5 and 3.5 kW were estimates (extrapolations).  So it's likely that at 3.5 kW with all the smoke (unburned fuel) the consumption wouldn't show a corresponding increase in power output, at least not to the same degree.]

Quinn

[LowGear]

Nice to know information when coupled with honestly taken data.  Thanks,

Casey

[WStayton]

Hi Guys!

  I have a procedural question here . . .

  How was the fuel consumption measured to FOUR decimal places?  0.0001 gallons is something like half of a half of a drop - you would have to have an analytical balance to measure that!!!

  I get kinda nervous when I see numbers that seem too carefully measured to be true!!!

  Most of the guys who are running a Lister(oid) would do well to measure the fuel consumption to +/- 0.001 gallons on anything like a repeatable basis - Is the methodology for this test spelled out somewhere that I can look at?  I really am curious how that precision of fuel consumption was measured.

  Presumeably the time that elapsed during the run was measured with a stop watch, and how you press the start/stop button can make a differance of .1 second, just to start with, so unless there are forty runs for each data point, its all still sort of a WAG to about the second decimal place.

  Not trying to throw rocks here, but the differances in BSFC (yea, I calculated them, so the data are in a form that makes more sense to me!)  are VERY small and the temperature is specified as 60 - 70 F that would make enough differance in the fuel density to wipe out half of the differance in BSFC that is portrayed.

  So what REALLY happened?  Is this one datum for each point on the graph?  And how were the values of gallons to FOUR decimal places derived?  And how a long a time period is represented for each datum?  One minute, ten minutes, thirty minutes, an hour???  Inquireing minds, and nosy ones, like mine, want to know!  <grin>

  Again, I not trying to throw rocks for the sake of throwing rocks, I really am curious about the methodology!!!

   Edify me, please!

Regardz,

Wayne Stayton

Regardz,

Wayne Stayton

[DKMC]

Yea, so....
A 10/1 at 5kw load burns xx ounces of  fuel an hour.......(or a 6/1 and a 12/2 as their more common)

And a 20/2 at 5kw load burns xx + how many more 'ounces'? per hour?
0.6oz more, 6oz ounces, 16oz??
Any guesses?

Anybody ever compare this?
We have (questionable?) fuel data from the 6/1 engine, but not any 2 cylinder data.

[rcavictim]

Wayne,

Here's how it's done.  You measure it with a micrometer, mark it with a grease pencil and then cut it with an axe. 

[LowGear]

I get wrongly reported significant numbers when I forget to round off the calculator or spreadsheet.  I'm still completely comfortable with the study.  No one claimed it was brain science or rocket surgery that I read.

Casey

[admin]

having fought the testing battle this is what i finally got down to using to get accurate and most importanly
repeatable results.

i measure fuel consumption in grams weight and then calculate it to gallons for the sake of discussion here and elsewhere.

i gave up and the stop watch method, however it is acceptably close enough if the test is long enough in duration to dilute
any start/stop issues.  i used 15 minute runs when testing "only" after a full load run and only then after the engine temperature
was at full operating temp and stabilized.

i ended up using a GE digital kw/hr meter, one of the residential units
what i found was a way of setting the meter to calibration mode wherein it can measure down to watt/hrs with a +/- 2 watt/hrs resolution.

using both the kw/meter as described and a good electronic gram scale where i could measure everything in real time, i then became very easy to get accurate and repeatable numbers.

i could then even measure and calculate exactly how much fuel (btu's) it took to cover base loads such as belt drives, auxilliary water pumps,
field currents, windage and all sorts of other stuff, right down to some pretty fine numbers.

before the use of the gram scale and the GE meterhead i was left to try and do all these things, like taking all sorts of measurements, do the calculations, over and over again and it just wasn't much fun, was not at all accurate or repeatable.

personally i won't ever setup to do this sort of testing again without both an electronic gram scale and the GE meterhead.

and yes i have 3 of those heads,  set aside for when i get back to doing this sort of thing again.

now as for the consumption curve bending over as the engine max's out in power...

with the s195 changfa i never saw this sort of expected bend in the curve, at least not the typical sharp drop seen with other engine's.

the engine would pull up to 6kwatts into a resistive load, no problem with the graph trending up linearly
right through 7kwatts, then up to just over 8kwatts with significant smoke, then the head gasket would let lose.

the head gskt was the limiting factor keeping the fuel curve almost a straight line.

8kwatts is not at all bad for an engine rated at 12hp continuous or 13.2 for one hour.

upgrading to a better head gskt allowed me to operate at the 8kwatt load, however i could not push past this
as that is all my st7.5 head can muster.

best economy for the changfa S195 as tested was right at 10kw/hrs per US gallon of pump diesel fuel, that attained at
full bore output of 8kwatts into  a resistive load. 

iirc Bill Rogers reported near 11 kw/hrs per US gallon of diesel fuel with his 1115 changfa operated at full load.

as for significant digits, here is my take

the digits are only significant if they can be used to extrapolate or predict results in testing under differing conditions.

any good testing must be repeatable

Ronmar's testing was very good, accurate enough and the results were predicted with astonishing accuracy at the time.

fwiw

bob g

[Ronmar]

Wayne, it was indeed stopwatch and flow method.  The numbers on the graph are the raw data, please feel free to round them off to whatever gives you a warm and fuzzy:)

I setup a burret type rig with a container up high, and a thin line feeding down to a T in the fuel line feeding the filter.  The bypass line from the injector was also fed back to this point so all fuel entering the engine was consumed by the engine.  Immediately ahead of this T was a valve to the tank that was secured when the test was started.  I would run the engine get it up to temp, put some fuel into the test rig and close the fuel to the main tank.  When the fuel in the test rig burnt down to my measuring mark, the watch was started, and a measured volume of fuel was added to the test rig.  When this measured fuel was consumed down to the mark, the watch was stopped.  I went thru this routine MANY times, as I discovered after doing individual runs, I did some successive runs and got different data.  Initially I was not warming the engine throughly enough, so the end of a series of runs was giving me better numbers than the previous single runs at those same loads.  Fuel was measured with a syringe, as I didn't have access to a good scale.  The load was purely resistive and energy determined with amp and volt meters.  It would have been nice to have a recording energy meter like Bob describes, but I was using good quality (fluke) test equipment all in current cal status.  I took timed data rounds and averaged these to determine the energy produced.  Sure would have been easier with a device that did that for me:) The test runs were a pretty busy live by the clock process...

It was the best I could do with the tools I had available, and I like to think I am a reasonably through man.  At any rate, I am very comfortable with the numbers, and was able to get excellent repeatability.  If I wasn't I would never have made them public.  When I did successive runs at the same load, I could predict to within a fraction of a second when the fuel would reach the timing mark.  At the time Bob was working on a  formula to calculate this very thing.  One of the things that really brought it all together was when I gave him my initial few readings, he was able to predict with the formula to within a very close tollerance my last I think it was 2 readings?

Bob, where did we discuss this?  It was way before SOMRAD, was it on Georges old forum, or the LEF?        

Oh, and RCA, I used chalk, not a grease pencil:)

[cujet]

I see no reason that chart can't be extrapolated into a 12/2 twin. Simply double the consumption numbers and the loads. The various fuel consumption specs don't vary much between Listeroid designs. Like they do when you compare to air cooled variants, or high speed diesels.

In my case, I have a 20/2 twin. I configured it with a high speed and low speed pulley. However, I've found that it's efficient enough to leave on "high speed" without excess consumption with my daily moderate loads. TV, computers, AC on low, etc.

[mbryner]

I see no reason that chart can't be extrapolated into a 12/2 twin. Simply double the consumption numbers and the loads.

Not that I'm any expert, but you wouldn't have 2 belts, 2 gen heads, 2 field coil losses, etc., so I'm guessing just a little less than double fuel use.   Since we were talking about 4 significant figures and cutting w/ axes....