On the alt.energy.homepower group today this topic arose. I started a reply, ended up researching things a bit more, doing some math and decided that a blog entry was necessary. I learned a few things in the process.
First off, what we’re talking about. The Listeroid engine, here built into a very nice genset by Rocketboy 
Yep, a very nice OLD technology engine that’s as reliable as a box of rocks.
Next, a definition. Brake Specific Fuel Consumption or BSFC. This is the standard way of measuring an engine’s fuel efficiency and is expressed in units of lbs/HP-Hr. or pounds of fuel consumed per horsepower-hour produced. (You folks who are shackled to the SI system of measure will just have to figure out the conversions for yourselves.) This is an energy-in, energy-out calculation with a whole bunch of normalizing thrown in to make it easy to wrap your arms around. BSFC varies from about 1 for low efficiency engines to about 0.25 for the largest and most efficient.
>>Listeroids come in 6hp to 24hp, but I wouldn’t call the 6hp “small”. They
>>work great on everything from veggie to motor oil and tranny fluid.
>This link http://www.kk.org/cooltools/archives/001011.php 
>cites a claim of .3 gallons per hour @8,000 watts.
I have a serious problem with that claim. The accepted expression of fuel economy is pounds of fuel per horsepower-hour. Pounds can be converted to gallons for a specific fuel, of course. I’d consider 0.5 lbs/hp-hour to be a very good figure for a small engine.
Brake specific fuel consumption (BSFC) generally gets better with engine size. The very largest engine, the Sultzer, is also the most fuel efficient. See the Table below.
|Engine Description||Disp, Cu In||HP||RPM||BSFC|
|Sultzer marine, max power||1,300,000||90,000||100||0.278|
|Sultzer Marine, max econ.||1,300,000||53,244||90||0.260|
|Stultzer Marine, overspeed||1,300,000||100,000||101.5||0.280 est|
|MEP-531A 2 KW genny (yanmar)||12.9||4.2||3600||0.480|
|Lister 12/2||175||12||650||0.714 to 1.0 *|
All the numbers are taken from the manufacturer’s specifications and normalized to English units.
(Damn, do you have any idea how lot it took to rustle up those numbers? Engine makers are kinda sparse with BSFC data.)
Of particular interest is this graph of BSFC vs RPM. This is taken from the data sheet for the Cat C18 engine. It’s remarkable to see the BSFC drop to its lowest (best) point at very high speed. Exactly counter “conventional wisdom.
* Let’s take a closer look at the Lister. It has a rep for good economy but I’m not so sure. Here  a user of the 6/1 has measured 0.125 gal/kw-h. Let’s do some normalizing. Using 6.2 lbs/gal for diesel fuel, that is
0.125 * 6.2 lbs = 0.763 lbs/kw-hr.
One KW is 1.34hp so converting kw-hr to hp-hr
0.763 lbs/kw-h * 1.34 hp/kw = 1.02 lbs/hp-hr
The BSFC was measured on a 6/1 but it should be the same on a 12/2 since a 12/2 is really just 2 6/1s on the same crankshaft.
This is figured at the generator terminals (presumably). If we assume the generator is about 70% efficient then the engine BSFC comes to
1.02 * .70 = 0.714.
I think that number may be a bit high, based on perhaps some errors in measuring the original 0.125 gal/kw-h value. Problem is, after several hours at the Altar of Google, I can’t find anything else to base a calculation on.
Looking at the situation from the gallons-per-hour angle, that 4.2hp, 2kw army generator uses 0.333 gal/hr. Straight scaling from the army generator to a 12/2 is:
0.333 * 12/4.2 = 0.943 GPH.
A gallon per hour seems about right for a 12 hp engine and is quite similar to my 10KW high speed diesel running at maybe 3/4 load. Accounting for the low speed and water cooling, I might buy 0.8 GPH but not much lower, at least not without a turbocharger.
It’s interesting to note that with these small engines, there’s not a major economy advantage between diesel vs gas. From the manual of my QuietPack (photo above) 5.5kw generator, the full load fuel consumption is 0.97 gal/hr. Scaling from the army diesel generator,
0.333 GPH * 5.5/2 kw = 0.907 gal/hr.
Close enough to the same. My QuietPack’s engine is a Vanguard 27 HP engine being run at only 2200 RPM which, from the published power curve, is only about 11 HP. I doubt that Generac did anything to optimize the engine for this lower speed (cam and cylinder head part numbers are the same as for the regular 3600 RPM engine) so I bet the BSFC could be made a LOT better with some optimization.
I wish someone would actually measure a Listeroid’s BSFC sometime. Google didn’t turn up anything useful. Google hit the Lister engine forum where some people were slinging some pretty non-credible numbers around. Someone needs to do some actual measuring. (my numbers above are all from manufacturers’ spec sheets.) I’d like to have some more data to work with before I took that rather poor BSFC number “to the bank”.
While I’m at it, is there a Listeroid mailing list (NOT forum) somewhere? I’d like to get on it.