Saw Fuel Comsumption

[timberwolf]

I did a little experiment today working on trying to figure out just how much saw was cooled by the evaporative effects of the fuel/oil. But as I see the results I think this may be worthy of a tread of it's own.

Hypothesis:

Given that the stocheometric mixture for air/gasoline is 14.5:1 it should be possible to calculate how much fuel would be used knowing the effective compressed volume and the RPM.

Math

44mm piston 33mm stroke and with my exhaust timing = 24.9 CC effective volume compressed by the piston.

X 13,500 RPM = .336 m3 of air
X air density 1.286 kg/m3
= .432 kg of air

divide by 14.5:1 air/fuel ratio
=27.9 grams of fuel or 34.9 ml to run 1 min at 13,500 (just over 2 tablespoons)

Here is what I did to test the theory:

-Saw MS 260, pipe was on it, so pipe stayed on for the test ( I think this was best anyway because it should have reduced fuel losses in the scavenging cycle)
-16"bar .325 square filed 8 pin sprocket
-added 200 ml 0f 40:1 93 octane gas mixed with synthetic/castor based oil
-ran the saw for 1 min in the wood (13,500 average RPM) (16 10" cookies)
-Dumped remaining fuel out into a container and refilled with a graduated syringe to determine how much fuel was used

I found that the saw used 50 ml (40.15 grams) for the 1 min test. (just over 3 tablespoons)

Conciliation:

By taking the theoretical fuel consumption, divided buy the actual I am getting 69.8%. I guess this makes sense as some fuel is unburned, some is lost back out the carb, and also there is all the physics involved with scavenging. If the pipe is working more charge should be packed into the cylinder before being compressed by the piston and hence upping the real life fuel consumption.

Now divided by 13,500 RPM to determine that the saw uses 0.0037 ml (.00297 grams) per stroke. To put this in tangible terms that a little less than 1/10 of a drop per stroke. 1/10 of a drop is not a lot of coolant.

Strange though 1/10th of a drop 13,500 times a min = 3/4 of a gallon per hour in the cut!

Think I will try this again muffler, open header, and with a different saw and see what comes out of it..

 

[timberwolf]

Of course also if the saw was at all rich then it would drive up the fuel consumption too.

 

[Stumper]

Was the saw thoroughly warmed up to operating temperature before you filled it at the beginning of the test? If not then all your measurements were for naught because the fuel mixture expands in the tank as the saw warms up. I hope I'm not raining on your parade-I appreciate what you are trying to do!

 

[timberwolf]

saw was warmed up, but then cooled a bit so that I could fill the gas tank with a known quantity of fuel. Otherwise I would not have a simple way to know how much was used during the warm up/ how much for the test. Though if it is the volumetric thero expansion of the gasoline you are talking about, I could measure the fuel temp going in and coming out and make up for it. However this would only acount for fractions of a percentile.

However now that I think about it more, I should be able to see how much the saw uses at idle per min, then repeat the test with a couple minute warm up at idle and then subtract off the fuel used at idle. or maybe with fast idle to get a bit more heat going.

I guess this is a violation of the chainsaw operators manual, Thou must not fill a hot saw. LOL

 

[Crofter]

Get that saw dyno working and it wold be much easier to get static conditions for a prolonged period. We want temp sensors on head and exhaust please!

 

[Crofter]

Your estimate at the theoretical mixture of 14.5 to one by weight still came out close enough. I think that in a two stroke the mixture will actually be a bit richer than than ideal 14.5 to 1 to control piston temperature. In a 4 stroke the mixture is richer than that for both Idle and full power too. Nice to see you used the trapped volume and not bore times stroke which is about twice as much. Not much of the fuel actually gets to the work does it

 

[ray benson]

stoichiometric mixture - or the ideal air/fuel ratio, at which temperatures are controlled and fuel economy is optimized.Read somewhere that 2 cycle engines make good power at 12 or 13-1. And that low cost engines like Lawnboy- 25% of the fuel is exhausted.

 

[sedanman]

Typical piston ported engine 25% of fuel goes straight out the exhaust, cost has no bearing on this.

 

[Crofter]

It sure would be nice to have electronic fuel injection after exhaust closing and avoid the loss. Don't know how far they are away from having units that would work on a saw. Outboards are using it, but the weight and size of the electronics would not be near the handicap there. It sure has improved the efficiency and dependability of auto engines, but it did take a few years to get the bugs out.

 

[bwalker]

Frank, the y dont need the electronics....yet. Strato charge type sytems like the one used on the husky 575 accomplish the same thing.
FWIW Timberwolf, I know you are having fun fooling around with your saw, but realise to get the kind of data your after would require a dyno that cost around a million dollars.... The run of the mill dyno wouldnt even come close to getting accurate results and when one thinks of the variable that must be controlled its easy to see why.

 

[Crofter]

Frank, the y dont need the electronics....yet.
.

Yet........ but how long will the strato charged meet the epa requirements before 4 strokes with their weight and maintenance handicaps be upon us? Outboards went 4 stroke and hyped them up but they are heavy, expensive, and maintenance is apparently an issue. The EFI seems to be doing a job there now, but this is mostly word from my stihl dealer outboard man so he could be putting his spin on it.

 

[bwalker]

I could go on for hours on why four stroke outbaords suck( used them for 1.5 seasons comercially). Lucky thing that Bombardier almost single handidly saved two strokes in both outbaords and snowmobiles. I believe in a few years time their E tech outboards will be dominating the market and they have already took over the sales lead for snowmobiles.
FWIW the strato sytem redmax uses meets the next round of emmision regs without a CAT. Also of note is the fact that the EPA has relaxed the standards a bit at least for snowmobiles and I assume for OPE also. I also would not be supprised if you see some sort of transfer port injection as found on Ski doo sled motorsor even a DI sytem. This sytem really cuts emmisions and does addmuch in the way of weight.

 

[P_woozel]

Ben what didnt your like about the 4-stroke outboards, I know a couple setnetters in Naknek that use the heck out of theirs but remember they are setnetters I thougfht theye would be a better workskiff motor than the high powerband 2-strokes.

 

[bwalker]

Woozel, They where very heavy and performance suffered. A 25 two stroke will run circles around a 35 four stroke. The added maintenance was a pain as well has having to pull the motor out of the water to change the oil, the fuel consumption was about the same, they cost much more, the mercs we had had reliability issues as well. One of which was if someone hit a rock at high speed it would cause the timing chain to skip. You can guess what happened then...blown motor. We had this happen twice. They where aslo cold blooded in the spring.

 

[bwalker]

Currently we are in the process of switching our fleet over to Yamaha 6,8 hp 2 strokes for our remote lakes and Evinrude e-techs on our big boats on the main lake. The Yami two cycles are very nice motors and are very light. Always a bonus when your humping in a motor, and gas tank in over a mile and a half long portage.

 

[bwalker]

BTW one of the biggest lodges in Canada switched their fleet back to two cycles after a year with four cycles. This lodge is actually featured in Merc ad for the Optimax, di 2 cycle in In Fisherman magazine. The two cycle DI engines actually used less fuel!

 

[glens]

To me a "cycle" denotes the return to a starting point and all that happens in the mean time.  If the engine cycle (intake/compression/combustion/exhaust) requires 2 strokes of the piston it's a 2-stroke-cycle engine and if it requires 4 strokes of the piston it's a 4-stroke-cycle engine.  For the former, there's one cycle of the crankshaft and for the latter, two cycles of the crank.

I know what you guys <i>mean</i>, but how can you use terms like "2 cycle" or "4 cycle" with a straight face?&nbsp; What "cycle" are you referring to?&nbsp; If it's the cycle of the crank you must be talking about two complete engine cycles.&nbsp; Same if you mean piston cycles.&nbsp; Certainly you're not talking about <i>engine</i> cycles, right?

What's so hard about saying 2-stroke or 4-stroke, which are totally unambiguous terms?&nbsp; "2 cycle" and "4 cycle" are terms which mean too much, too little, or nothing concrete at all.

Glen

 

[ray benson]

Doing a search- it seems the terms are interchangeable.Lots of companies use the 2 cycle term.
http://search.yahoo.com/bin/search?p=2 cycle

 

[bwalker]

I think everyone nows what the term cycle refers to, Glens.

 

[timberwolf]

Dispite being a little discouraged by the fact that I don't have access to a million dollar saw dyno and I am not able isolate all the variables, I again ventured out to the garage and did some rechecking and more trials.

First off I wanted to isolate the fuel thermo-expansion question. The gas was going into the saw at -10 C and coming out of the saw at between -7 and -8 C, about 1 to 2 degrees C warmer than it went in so by adjusting by the thermo expansion coefficient of gasoline 950x10^-6 or 0.000950 it adds up to about 2/10 of a percent. Well within the acceptable tolerances for my un accedited saw laboratory, though this may weel be a factor for a longer test duration and is a point to keep in mind if warm fuel from indoors was measured at a colder temp out doors.

Then I wanted to check the idle fuel consumption so I could warm the saw up a bit more before starting the test and not skew the results. So I checked the idle fuel consumption of the 260 for 5 min which used 31 ml of fuel = 6.2 ml per min.

Then I redid the test because I wanted to reestablish a baseline with an idle period before the cut and also check that the 7 degree temperature drop did not change things. The test was done without adjusting the jets and again used very close to 50 ml of fuel after subtracting 12.4 ml of fuel for a 2 min warm-up.

Then I repeated the test with the opened muffler, and found the fuel consumption dropped to 30ml also the number of cookies dropped to 12 inside 1 min.

This all seemed to make sense until I tried the same test with my 066

066 idle =47ml for 5 min or 9.4ml per min
066 with pipe 65 ml for 1 min full out in the wood
066 with opened muffler 67 ml

What the saw used slightly less fuel to cut more wood with the pipe? Though I did run out of cookie dough so I did not get an exact cookie count. But did manage to keep the saw cutting wood throughout the test time.

Now the 066 is close to stock, the 260 is less so and might be blowing a bit of fuel back out the carb.

Should a piped saw be using more fuel?