Q. about stove design

[Dirtyoldman]

I will try to be coherent as posssible....

I was tinkering with a pepsi stove the other day and noticed some strange things. I had built one stove and while testing it notice that if it was filled more then 1/4 full the outer burner wouldnt light untill the fuel had burned down some so I thought that the inner wall needed to have enough surface area exposed in order for it to heat the fuel to vapor. To test this I built a taller stove and ran a comparison test. The taller stove lit faster and appeared to burn hotter then the short stove and left a smaller amount of residue after burn out so I figured it would boil water faster but when I did a timed boil on each the taller stove took the same amount of time to boil but still burned out faster.

Perhaps this is due to the fact that I was using rubbing alchohol for the test. ( Worst case fuel - 30% water ) I am wondering if the taller stove converted more of the water to vapor and cooled the fire inspite of the higher evaporation rate that the taller stove achieved. This might also be due to my poor windscreen making abilities.

I plan on making a different windscreen that will allow me to test the burners more evenly and check it with ethyl alcohol fairly soon but in the meantime any suggestions?


inner wall height for tall stove was 2"
inner wall heght for short stove was 1.5"
brand used for both coke classic
# of port 16
port size 1/16" drill
inner wall fill none

[Dirtyoldman]

ok after some more testing with different fuels and even stranger results I finally realised the problem. The wind screen reflects heat back to the burner causing it to overheat!

What I really wanted is a burner that would vaporise the fuel at a slower rate so after some thought I made one with the center still intact and holes on the inside of the rim stuffed with fiberglass. The center depression acted as a preheater and with the sceen and pot worked correctly but... still to hot.

fuel,ethyl................. 30ml
time to boil 2 cups... 9 minutes
total burn time........ 12 minutes

so... after some more thought on a lark I stuffed the center of my tall burner with fiberglass...

fuel, ethyl................ 30 ml
time to boil 2 cups... 11 minutes
total burn time........ 20 minutes

This is better but the open air times for the tall burner without fiberglass or windscreen are better yet as it ran for 30 minutes on 30 ml of fuel (boil time unknown) I would consider this arrangement but once I took it out in the windy world it would likely become useless....

[Dirtyoldman]

Well after a couple of bottles of heet I have learned a bit more.

It has become clear to me that the windscreen design Is actually MORE important then the burner itself. I tried oven liners for material but I found it a bit to floppy for my taste. I also tried a coffee can design but It proved rather inefficient. so I made mine from aluminum flashing ala sgt rock. The Problem with the coffee can screen was that it allowed the heat to escape sideways while the new screen forces the heat to travel up the sides of the pot.

I also tried a new burner design similar to the cat stove, I left out the vent holes and instead insulated the outer shell, I also used a fiberglass wick in the center.

All in all Its been a fun experiment, Many thanks to those who not only went before but left a few bread crumbs for the rest of us to follow.

ps. It occured to me that I might be able to cook in my coffee pot and leave the regular pot behind. Worth testing as I LUV my coffee.

[Dirtyoldman]

Has any one tested the differance between fiberglass and perlite?

I tried a new suggestion over my weekend. I took a round altoids can and cut some fiberglass to fit inside.

It boiled 2 cups of water in 4 minutes and ran 12 minutes on 30 ml (1/8 cup) of fuel. Not too shabby a result. I do like the fact that I can put the cover on it and not have fiberglass inside my cooking pot. I also suspect this is the first stove I have made that might be able to boil 3 cups of water.

[Sparky!]

D.O.M. is right about the wind screen... that to me is the lynch pin in the entire process, IMHO. I too noticed that the sides would not light when I filled it like you mentioned. A friend of mine suggested that when I get the fuel into the stove to "swirl" the stove help the fuel into the chamber. I did it on mine and it worked great. YMMV

Sparky

[Dirtyoldman]

Since noone had any experiance to share on the perlite I grabbed a bag from the store and tried it myself.

I didnt notice any real differance in proformance over fiberglass

I did notice that it distributed the fuel better then fiberglass and burned a bit more evenly

It had to be screened to get the bigger grains and tended to grind into dust very easily defeating the original screening.

Overall it seemed to be much harder to deal with then fiberglass so I dropped it.


I found a smaller diameter tin to test against the round altoids and was a bit surprised when it burned faster but didnt boil any better then the larger diameter can. I had figured the more surface area the faster and hotter the burn. I will test this if i can find a can larger then the altoids....

[Dirtyoldman]

I found a can larger then the altoids and it appears I was right...

3" altoids - 2c water 30ml fuel
boil 4 minutes
steam 5 minutes
burn 10 minutes

4" candy tin 2c water 30 ml fuel
boil 6 minutes
steam 8 minutes
burn 16 minutes

30 ml = 1/8 cup
all times rounded to next minute....

[Scamp]

(16) 1/16" ports seems like alot.
I use (14) 1/32" or (24) .024" ports.
I once calculated that only 3 or 4 1/16"
holes would provide the same pressure
(low) as 14 1/32" holes (or 24 .024").

Your stove is probably not pressurized
making it more difficult to light and more
difficult to vaporize fuel. A wick will help
your stove stay lit until the fuel vaporizes
sufficiently to maintain a flame.

Of course the Altoids type stoves are unpressurized. They're simple, safe
stoves.

Thanks for the data on burn times.

Scamp

[Dirtyoldman]

Its the smallest drill I could find...

The tall 2" burner did beautifull by itself. It burned for 30 minutes on 30 ml of fuel with very nice jets...but as soon as I put a pot and windscreen around it boiled off the fuel without even burning it. the lack of a true pressure chamber on the pepsi stoves is a problem with the design and I dont see any easy way to get around it.

Getting the stove lit was definatly never a problem.

[Dirtyoldman]

More on altoids burners...

I got to thinking that instead of trying different size cans for the burner I might change the opening size bycutting a hole in the lid. I didnt want to buy 50 cans of altoids so I made a tin "lid" and started testing.

all tests were for 2c water and 30 ml fuel

1/2 " - got bored after the first hour and didnt get an accurate time. heat output was little more then a birthday candle and too cool for even a simmer.
3/4" - got bored after an hour but did note the output would be usefull for simmering.

1" - after 28 minutes....a low grade boil...

1 1/2" - boil heard at 14 minutes , steam seen at 20, total burn was 38 minutes...

What I seem to have here is a way to customise the exact run times I desire simply by changing fuel amounts and the hole size in the cover.

I did a preliminary test using a 1" hole in an actual lid and got a low grade boil in 40 minutes and it burned for 85 minutes.



This is the setup I have been looking for. Easy to make, hard to break, easy to replace and can be tuned into a very efficient system. I may dig up some lab grade testing and post the results after I get done celebrating...

For those who want to play with it:
altoids round can with fiberglass circles layered inside. Fiberglass should not touch lid when closed. Hole is centered in lid and I cut mine out with a tin snip.

[hawkeye]

I have a question about alcohol stoves, is getting fuel for your alcohol stove easier than getting white gas?

[Dirtyoldman]

since the alcohol used does not have to be drinkable yes.

sources:
gas line anti freeze (heet) - gas station
rubbing alcohol (tends to be sooty) - drug store
everclear (expensive?) - liquor store
Jack daniels (what a waste) - bar
moonshine (id rather drink it and eat my ramen uncooked) - trailside :}

I have even heard you can get it by the gallon from the paint store. (cleaner? thinner?)

The simple fact of the matter is alcohol is used in a large number of products and anything that contains a high concentration of it will work.

I should point out that different stoves might not like some of the sootier types of fuel, I wouldnt try rubbing alcohol in a pressurised one as it would tend to clog the ports.

[Hog On Ice]

look for "shellac thinner" in the paint/hardware store

[Peaks]

I asked the question about availability of alcohol along the trail. It seems that with alcohol stoves becoming more popular, it's no problem apparently to get this fuel along the trail now.