View Full Version : penny stove modifications

2008-01-12, 16:49
I've currently been working with the penny alcohol stove and find it to be reliable and efficient. I wanted to make it more efficient, and found a way to do so. here's the facts.

used the i heineken and 3 pop can design.

original boil time 6 minutes 15 seconds (2/3 ounce of fuel, 2 cups water, stainless steel 5 piece mess kit pot with lid, using issue esbit stove and sgt rock windscreen)

added perlite on inside:

new boil time with same equipment 6 minutes and 30 seconds.

boil time has increased but i was able to achieve rolling boil with 1/2 ounce of fuel instead of 2/3.

also, with 2/3 ounce of fuel total burn time without simmer ring 8 minutes 30 seconds.

now with perlite 2/3 ounce of fuel total burn time without simmer ring 10 minutes.

with simmer ring original total burn time 20 minutes, 47 seconds 2/3 ounce of fuel.

with simmer ring and perlite using 2/3 ounce of fuel total burn time 25 minutes, 14 seconds.

now my stoves aren't perfect, and i couldn't get close to the results of the original penny stove, but i can still boil two cups of water with 1/2 ounce of fuel, it just takes a little longer, and as many have said, "if you're in a hurry, why are you walking?"

2008-01-13, 00:24
I've made and used a lot of stoves of the original Pepsi can design.
Pretty much like this design (www.pcthiker.com/pages/gear/pepsiGstoveinstruct.shtml), with a few simple modifications.
- Once I found a design I liked and also enjoyed making I stuck with it.

I haven't ever clinically tested my stoves, so I can't quote any figures, but one of my modifications is the addition of insulation between the walls of the stove and I think it makes for a better stove. It appears to me that it makes for a smother burning and more economical stove.

I don't put in a lot of insulation, and I've actually even substituted cotton, so it's not acting as insulation as much as a wick. Also perhaps it's helping the stove reach pressure sooner by limiting the amount of air needing heating inside the walls and helping regulate the temperature needed for that better.

I don't know, just know I like it better built this way.

2008-01-13, 13:30
yeah i used to do open flame stoves with cotton or fiberglass wicking, then i started making the penny stove. i'm not very good at making stoves but i wanted to increase the efficiency. i figured if insulation or wicking works in an open flame it might work in a pressure stove.

2008-01-16, 00:31
You guys ever consider that the insulation (cotton perlite or fiberglass) is just acting as a wicking material to aid in the vaporization of the alcohol?

It adds to the effective surface area for vaporization and, therefore speeds the burn. A given amount of fuel will release the same amount of energy regardless of how you burn it - if other factors remain constant.

What matters is how effectively you transfer that energy to your pot to cook with.

Entropy is governing your actions - try to understand it.


2008-01-16, 00:54
Wow, Yuppie Redneck you sure know everything. I will just ask you for the answer the next time I have a question.

2008-01-16, 17:14
yuppie is full of knowledge (chuckle, chuckle). by the way, for those of you who don't know, entropy is a term used by scientists to explain energy that is unavailable during a thermodynamic process, or heat loss in this instance. in nuclear physics we explained as the extra peanut butter left over in a jar that you can't eat...

2008-01-17, 06:25
Entropy isn't just a way to describe temperature equilibrium and the inevitable heat death of the universe, it relates to the order, randomness, and chaos in all things.

From a paper by James Clerk Maxwell (a really smart dead guy):

"It follows from this that the idea of dissipation of energy depends on the extent of our knowledge. Available energy is energy which we can direct into any desired channel. Dissipated energy is energy which we cannot lay hold of and direct at pleasure, such as the energy of the confused agitation of molecules which we call heat. Now, confusion, like the correlative term order, is not a property of material things in themselves, but only in relation to the mind which perceives them. A memorandum-book does not, provided it is neatly written, appear confused to an illiterate person, or to the owner who understands it thoroughly, but to any other person able to read it appears to be inextricably confused. Similarly the notion of dissipated energy could not occur to a being who could not turn any of the energies of nature to his own account, or to one who could trace the motion of every molecule and seize it at the right moment. It is only to a being in the intermediate stage, who can lay hold of some forms of energy while others elude his grasp, that energy appears to be passing inevitably from the available to the dissipated state.”

I hope that clears up any confusion.

2008-01-17, 15:42
yuppie is full of knowledge (chuckle, chuckle). by the way, for those of you who don't know, entropy is a term used by scientists to explain energy that is unavailable during a thermodynamic process, or heat loss in this instance. in nuclear physics we explained as the extra peanut butter left over in a jar that you can't eat...

JR is right, but to clarify (I hope):

I was actually referring to entropy as the second law of thermodynamics - not as a philosophy. I mean, think about it.... A discussion about stoves, burning things, energy output....... A little grounding in actual scientific fact seems in order :)

Laws of Thermodynamics | Back to Top
Energy exists in many forms, such as heat, light, chemical energy, and electrical energy. Energy is the ability to bring about change or to do work. Thermodynamics is the study of energy.

First Law of Thermodynamics: Energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another. The First Law of Thermodynamics (Conservation) states that energy is always conserved, it cannot be created or destroyed. In essence, energy can be converted from one form into another.

The Second Law of Thermodynamics states that "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." This is also commonly referred to as entropy. A watchspring-driven watch will run until the potential energy in the spring is converted, and not again until energy is reapplied to the spring to rewind it. A car that has run out of gas will not run again until you walk 10 miles to a gas station and refuel the car. Once the potential energy locked in carbohydrates is converted into kinetic energy (energy in use or motion), the organism will get no more until energy is input again. In the process of energy transfer, some energy will dissipate as heat. Entropy is a measure of disorder: cells are NOT disordered and so have low entropy. The flow of energy maintains order and life. Entropy wins when organisms cease to take in energy and die.

If I were to explain entropy to a kid, I would say this: Entropy is the energy lost when you convert one form of energy to another. As it applies to our stoves, entropy is the heat that doesn't get absorbed by the food in our pot.

We all know that the canister stoves put out about twice as much energy per wt of fuel than alcohol. By reducing the heat lost to the environment (entropy), we are able to heat our food effectively. BUT, the energy in a given amount of alcohol (providing it's at the same state, temperature, elevation, and pressure) is going to be constant (First Law of Thermodynamics). That being the case, boil times, time at cooking temperature, whether the stove heats our food properly with a given amount of fuel - all depends on how much heat is lost when we convert the stored energy of our fuel into heat.

It's getting a bit wordy - so let me highlight it. With alcohol stoves what matters is your windscreen and pot combination. As far as energy released - burning alcohol in a paper cup releases just the same amount of energy as the same amount of alcohol in an expensive or high tech stove.

No matter the fuel, the faster you boil - the more fuel you use in a given time. Consider a car, the faster you go, the more fuel you burn. (It's a flawed analogy, I know, but it helps explain the second law.)

They call me a 'Renaissance Redneck' On account of I kin reed. :o

2008-01-18, 03:57
I was just goofing. I'll try to not confuse things any further.

“Every mathematician knows it is impossible to understand an elementary course in thermodynamics.”
V Arnold
“You should call it entropy...... nobody knows what entropy really is, so in a debate you will always have the advantage.”
John Von Neumann

2008-01-18, 10:13
Sgt Rock's discussion of his researches into his own stove workings illustrates the practical limits of stove design quite well. He first calculates the available energy in the fuel, and then the amount necessary to raise the water in the pot to boiling. This gives him the smallest amount of fuel that could possibly heat the water. Stove efficiency is then given as a ratio of the actual fuel needed to the least amount possible. Efficiency cannot be greater than 100% (First law), and in practice, cannot even be 100% (Second law). He begins by shooting for 50%, and exceeds this by quite a bit, but the theory tells him that there is a limit to the gains he can make by stove design.

There is a lot more than this that can be derived from the thermodynamics of heat processes regarding stoves. If the fuel were to be burned in a closed space, with no escape, and the heat had nowhere to go but into the water, then the stove efficiency would be given by the ratio of the heat capacities of the burnt fuel/oxidizer/container plus water to that of the water alone. That gives the absolute best stove efficiency possible, and gives the practical information that the lightest stove/pot system will be the best, if the heat transfer is perfect. One could fiddle with the materials and their heat capacities, but as a practical matter, that is probably not worth the candle. Without calculation, the practical limit is probably around 80-90%.

The difference between that and the performance of any stove design is mostly due to heat loss, or inefficiency in heat transfer from the flame to the pot. Some is lost heating the stove and pot support to a much higher temperature than is needed. With the very low mass of the pepsi-can stoves, this is already minimized. A low mass pot will help. Most of the rest is lost in poor coupling of the flame/hot gas of the burnt fuel and the pot. The factors at work here are containment and rate of heat transfer vs. exhaust/flame dwell time. The measure is how much heat is wasted by a high exhaust temperature and by heating the surroundings rather than the pot. Very cold surroundings will have some effect, which can be minimized by a good reflector/insulator surrounding the system. Then the better the heat transfer to the pot, the better. The pot should have good radiative absorption, good heat conduction, and good contact with the gas stream from the flame. So, a blackened pot of copper or aluminum, and the most surface area in contact with the hot gas stream. That explains the remarkable Kelly Kettle system, and also calls for the flattest pot (biggest bottom) made of the highest heat conductivity metal practical, which is pure Aluminum or Copper. It also calls for the localization of the flame near the centre of the pot bottom. It also explains the question of burner rate vs efficiency. The higher the rate, the less time available to cool the exhaust stream against the pot surface, so the lower th transfer efficiency. There is also a question of the volume of exhaust flow vs the area of contact. So there will be an optimum burn rate, and therefore burn time, for any given fuel/stove/pot/reflector system. There will also be an optimum geometry of pot to flame (eg., pot support height). So getting the stove to burn fuel faster is not the way to optimize efficiency. There is an optimal rate at which fuel should be burnt. This will be different for all other variables, such as ambient temperature, pot, support and reflector design and so forth.

The other heat loss mechanism is cooling from the water itself, which is minimized by a lid and pot insulation in the cool zone of the system (away from the flame). The best practical means for this is a highly reflective wind-screen that encloses the whole pot and stove. The lid should also be reflective, and a bottom reflector might help also.

The inclusion of wicking materials and insulation in the stove itself are going to affect the burn rate, and therefore may affect efficiency by the mechanisms explained above. But simply more wick is not necessarily better, nor is a wick the only way to go. The size of burner holes and their geometry may have an equivalent effect. Thus goes for the air holes as much as for the flame ports. And mixing efficiency will affect the flame length, which affects all the other flame channel considerations, like pot position and bottom size, etc. So there is a complex balancing act.

However, get the fundamentals right, and the fiddly details will matter much less. That is why Rock's Ion Stove works well: He concentrated on the factors which have the greatest effect. And instead of wicking, he flattens the stove bottom, to increase the free area of the fuel.

Mostly: Wide-bottom Aluminum pot, preferably black bottom with an insulating (bright metal) lid, all as light as possible. A heat sink bustle around the pot might help if the burn rate is too high for the pot geometry. I think MSR used to make one. Support the pot above flame where the burn is essentially complete, so the flame is not quenched against the cold pot, and the exhaust gas temperature is highest at the centre of the pot bottom. Surround with a bright metal reflector/wind screen with a sufficient gap so that the wind screen itself does not cool the exhaust stream (ie., stays cool throughout the burn). Burn slow enough that the exhaust stream is well cooled by the time it leaves the pot. Make sure the wind screen/reflector is clean and bright, and possibly use a bottom reflector as well. This will also serve to protect the ground and make the system more stable. Which brings up those other considerations: ease and safety of use. The stove must light easily, burn consistently and be stable on its supports, be easy to set up and fuel, and be light and easy to pack.

If simmering is a requirement, this puts a premium on flame stability and fuel capacity as well as air flow control. The most efficient method of flame control is by control of the air inflow. In other words, try to put the damper in the air side rather than on the flame side. But the limit on simmering will come from flame instability owing to poor burner geometry, so in the end, a flame damper may also be required. This will affect fuel efficiency. Some sort of wicking may help here.

Final note: If you want a general cook stove rather than an efficient water boiler, then flame control and pot versatility will be more important than fuel efficiency alone. Fuel capacity and the ability to stop the flame will also be useful. But such a stove is not an ultralight pack stove.

SGT Rock
2008-01-18, 10:44
Great post GGS2.

One thing I want to add - where you talk about the pot with black outside, shiny top, transfer sink, etc. What I decided when doing all this was my other goal besides efficiency of fuel was efficiency on weight and bulk. As a backpacker the goal is to make the entire weight and bulk more efficient - fuel efficiency for most only enhances these characteristics.

So when you look at a system like the JetBoil which had done a lot to add stuff to the system to increase fuel efficiency, they add in weight. So in the end, a less fuel efficient stove like a Coleman F1 Ultralight actually ends up being more weight efficient when using standard pots.

Again - that was a fun post to read man. I needed that. Thanks.

2008-01-18, 14:52
thanks to ggs2 for responding so thoroughly, and thanks again to all of you for even reading my post. i love this site, keep up the good work rock, be safe this weekend.

2008-01-18, 15:49
Gee i new all that just do not how to say it. but one thing how do you lite these stoves with less flame to get more out of it match is a wast lighter might be good flint steel 2 out of 3 strikes fero rod. I got it let the wife do it she can get the wood stoves going better then me. :angel:

2008-01-18, 19:20
... how do you lite these stoves with less flame to get more out of it ...

I use a lighter, but I don't depend on it. If you don't have some last resort lighting system, you're playing with(out) fire. That would be ok if you have the outdoor skills to make fire by friction, in the rain, eg. Old joke: Making fire by rubbing two sticks together is easy if you make sure one of them is a match.

2008-01-18, 19:45
Again - that was a fun post to read man. I needed that. Thanks.

Funny thing about trying to write a post like that is that after you send it, you can always find small errors and improvements. They will have to wait for the book, I guess. But all the qualifications you mention are valid and important. They illustrate the difference between a lab curiosity and a practical stove.

I should also like to mention that there are numerous small inefficiencies which are time dependent, and which mean that the stove should probably run a little faster than otherwise might be optimal. And one should not get efficiency mad, but efficiency simply means less fuel needs to be carried, and therefore lower pack weight. So a little stove weight can be added if it reduces the fuel load.

For me, I am more interested in not carrying fuel at all. I would like to have an efficient, low mass, solid cellulosic fuel burning system. What's that mean? A wood/paper/grass burner that is almost as light as an alcohol stove. The Kelly Kettle is a prototype, but it can be improved.

I would also like to suggest that for short trips of a few days, for overweight people like me, the most efficient system is to leave the stove, and even the food bag, at home. Not an option for zero body fat people, but it works for me. Wouldn't do for a long hike, though. The fuel I burn is fat, and the fuel efficiency is quite high, even though I've never measured it. I have to carry too much extra, though. I've done up to two weeks that way, and lost weight, even while refueling like a thru at town stops.