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Discussion in 'Firewall Forward / Props / Fuel system' started by TiPi, Oct 4, 2019.
Direct drive should be hand prop-able. Redriven will make it a lot harder.
There is a pull cord starter option for these engines. Probably the lightest option after hand propping. Fairly easy to adapt to alternative flywheels, too.
I have hand propped my Honda GX670 several dozen times. Yes, it takes a good flip. Just difficult enough that someone with less skills might have difficulty. The compression release has no effect that I can determine. (feels like full compression when hand flipping)
I may look into a recoil starter. I didn't think about the speed requirements to get one of these started by hand, it may be too fast to be practical. Not sure if I'll use a redrive; I'm trying to avoid it if possible. Looks like a light-weight starter and battery may be the simplest option though.
Not sure if this has been covered here (I didn't see it, but that's no proof), but does anyone have a specific fuel consumption (SFC) value for any of the common industrial V-twins in their original, unmodified, form? I imagine that these engines are more alike than different in the respects that greatly influence SFC, so a general value is a good starting point. SFC for the "converted" engine will mostly depend on the conversion and what's done to the engine, so I'm most interested in the original SFC if the number is available. If not, a full-power fuel consumption rate, along with the power produced at that point, would be equally useful.
I don't think it has been covered here yet. The only testing I could find was a study done by the California version of the EPA (CARB). The numbers there made a rich running Rotax look thrifty. 0.9 Gal/Hp/hr was pretty average for industrial engines from 100CC up to the one 600cc twin (Kawasaki)they tested. We should be able to do better?
Edit: Gal should be pounds - brain fade, not a typo
Honda is pretty forthcoming with technical info. This page indicates that their GX670 engine (24HP) consumes 265 grams per HP per hour (but it doesn't say what throttle setting that's at). 1 US gallon of gasoline weighs 2835 grams. So, 20 HP for an hour would be 5300 grams or 1.87 gallons. (also, 265 grams = 0.58 lbs, so the SFC is 0.58 lbs/hr/HP).
Edited to add: It's specifically not what you asked, but TiPi did provide some fuel consumption specs for the >converted< 810cc engine at a couple of output levels, according to the POH of the German SD-1. That's here: https://www.homebuiltairplanes.com/forums/threads/briggs-vanguard-conversions.31620/page-62#post-497144.
Roughly: 25 HP = 5.9 l/hr. For gasoline, 1.65 lbs/liter, so 9.735 lbs/25 HP = .39 lbs/hp/hr. Hmmm.
For a small air-cooled carburetted engine, I would have guessed it would be something like .45 - 0.50 lb/HP/hour, but I suppose it could be significantly better with careful leaning.
Well, I should have known. LOL
Thank you both. 0.9 lbs/hp/hr sounds not unsurprising for the very small, 100cc-500cc motors. I believe I've mentioned that I'm interested in the Predator 670cc, which has exactly zero fuel consumption information in its promotional material or user manual. I do struggle with one of these engines doing 0.38 lbs/hp/hr, but then I can't actually refute someone's claim. It's not entirely out of the realm of possibility, I suppose.
It sounds like I should presume the "usual" 0.5-0.55 lb/hp/hr figure for now. I've been planning on calling the Harbor Freight product information line to get a consumption figure, and I'll report back once I manage to get through (and they give me an actual answer).
Tis my plan as well. 0.55 matches what I've subconsciously figured from years of using these things. The 0.9 figure may have been the result of a 'profile' test to simulate typical owner use patterns.
My 12hp (rated) mower uses about a gallon an hour running in grass tall enough to drag the motor down below the governor speed if I go too fast - except for a few turns in already mowed areas.
10 Hp average at 1 g/h = .6 #/hp/hr. It runs rich at 7000 ft density altitude.
0.38 seems suspiciously optimistic to me.
Loncin V-twin graph:
Industrials used to run very rich to cut down on cooling requirements. The EPA has ended that tradition. I've seen lower figures quoted for other Chinese engines.
So, if we take their graph showing a 3600 RPM/17 HP SFC of 375 grams/kwh, that would be 280 grams/hp/hr or .62 lb/HP/hr. That's pretty dismal, but very close to the Honda estimate of .58.
Things are more efficient by backing off the power just a little. At 3000 RPM/16 HP the SFC is .46 lbs/HP/hr. Now we're talking.
I guess the practical import of these numbers is the aircraft range/flying hour per tank. From an "aggregate cost of flying" perspective, it hardly makes any difference if we burn an extra 1/2 gallon per hour (of MOGAS yet!) in the grand scheme of things.
Exactly. Or, from my perspective in designing an airplane, how big a tank do I need to attain the specified mission range?
It sounds like 0.55-0.60 might be a more conservative design point for now, until I can get more direct information from the manufacturer. As you say, at these consumption rates, the "excess" fuel, if any, would be relatively small if the "real" value is closer to 0.50.
If you find out who makes the HF engine, ask them directly! It's probably quite similar to the Loncin, though. Loncin do make some engines for HF.
These are the numbers from Honda for the newer GX series (630/660/690), note that the fuel burn is at "rated power". Add another 10% for the full power SFC
Translated to SFC looks like that:
There wouldn't be large differences between brands provided they are fitted with a dual-barrel carby (simpler engines with single-barrel would most likely run a bit richer).
Outstanding. Thank you.
I have found that sfc varies greatly with a particular engine in regards to power output demanded I.E. The Pratt and Whitney R-2800 would be .45lbs per hp hr or more in climb power 1350hp, around .42lbs per hp hr in loaded cruise at 1200hp, we could get around .38lbs per hp hr when running between 600hp and 800hp.
It just makes sense that you can lean more (with limitations) at reduced power. If the bang ain't as big it can be hotter.
I don't see why these industrial engines can not be run around .33lbs per hp hr at around 50% rated power or .25hp per ci. Expecting them to run that lean at 75% power or more is not likely however.
Engines actualy run hottest very close to stochiometric. Go leaner, they cool off again, but not as much as on the rich side. If your CHTs are OK, you can safely lean until misfires spoil the fun. So, you may be able to run at full throttle, very lean and be near 75% power. Getting below 0.4lbs/hp.hr would impress me greatly.
At 75% power or .375hp per ci, 0.4lbs/hp per hr seems reasonable. I am sure it is also reasonable to get 1/3lb per hp per hr in a small engine at .25hp per ci.
I suppose we are all guessing, but I'd be very surprised. Air cooled SI engines generally get less efficient as they get smaller, just due to the greater amount of surface area (which increases heat loss, ring friction, etc) relative to the displacement. The Lycosaurs can see BSFCs down to .38 lb/hp/hr when running LOP, but I don't think we'll be seeing that in these smaller air-cooled engines, even with aggressive leaning.
And, again, for some of us, what might be achievable in operation is less important than having a solid baseline around which to design an airplane. I suppose it might be possible, with aggressive leaning and reduced power settings, to get these industrial V-twin engines down to an SFC of 0.4 or so. But I can't (or at least shouldn't) design an airplane around that presumption. Aircraft design is a process of making sure that what you design can achieve the desired specifications and requirements, not making it "the best it might possibly be." I can't always guarantee best-possible-performance, but I should be able to guarantee performance that meets the requirements under normal use.
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