FAQ - Efficiency - technical details

A Review of Stove Weights, Fuel Weights and Fuel Efficiencies
Send comments and corrections to the maintainer Roger Caffin

People argue at great length about which stove is best, or lightest, or whatever. Rarely do they put the argument into it's proper context: the full weight in your pack for the trip you are doing. For that matter, some people get very excited about whether the fuel for the stove for the trip will cost $1 or $2 - when they have spent $40 (2005: make that $80!) on petrol for the car to get to the start of the trip. Strange, and misguided imho. So, let us explore weights and efficiencies and costs. This is a technical page with lots of tables of numbers and few pictures. Sorry about that.

Among some walkers in America there is a great fascination with alcohol stoves at present. Granted, the Pepsi Stove is extremely light (10 grams!), but this is not the full story. You also have to look at the weight of the fuel used and a whole lot of other factors if you want to be serious about minimising your total pack weight. You should also consider how easy it is to cook dinner: you're there for fun after all. This brings into play the amount of cooking you do and the energy efficiency of the fuel. If you are just doing a one night trip from home we can assume there are no problems in organising your fuel, but on longer trips you must consider how you are going to refuel. In some countries you must even consider what fuels are available. (In Nepal for instance you have kero, kero and kero.) So you end up having to do a balancing act between all these factors:

• Weight of stove
• Weight of fuel for trip
• Weight of fuel container
• Incremental refuelling weight

I cannot predict how much fuel you will use: I can only tell you how much I use cooking for two people (my wife and myself). In this context, our typical daily menu is like this:

• Breakfast is cold - usually muesli.
• Morning tea includes a cup of tea or coffee, plus biscuits etc.
• Lunch is cold: biscuits, jam, honey, cheese, etc.
• Afternoon tea usually just a snack (chocolate, peanuts, dried fruit) to keep us going until dinner.
• Dinner is a real meal of several courses: instant noodle soup, stew and rice (or pasta), cocoa and fruitcake.

Your meals will differ, but all things scale to some extent and my figures are the only ones I have, so let's assume my figures are representative. You will note that we do not do any extended "cooking": most of our food is freeze-dried and requires only a minute of boiling, plus some Dutch-oven^* treatment. We don't do any frying either. (Makes washing up very easy.) Regional variations will happen of course: in France we had to eat an awful lot of fresh French bread and local Brie cheese, washed down with coffee. My wife had tea and croissants for breakfast. Life was so hard ...

* Dutch Oven: a neat trick for cooking your food. The name derives from a Dutch practice I believe. Bring the food to the boil for about 15-30 seconds, with a lid on the pot. Then turn off the stove and keep the pot hot (lid ON) with some sort of insulation around it. I sometimes use my hat, but a shiny pot out of the wind actually loses very little heat by radiation. Leave for about 10-15 minutes. The food will probably cook almost as well as if it was boiling, but you use no fuel in this time and have no risk of burning the dinner either. This works well with most foods (pasta, dehy, etc), although some forms of rice can be a bit stubborn. You might be surprised just how much water gets absorbed in the process.

I have listed typical fuel usage here for two people. See under Tricks for how to reduce this even further. The figures given here are reasonably conservative. I emphasise that the gas consumption figure is based on a lot of nights and a lot of trips, including 6 and 8 week trips across the Pyrenees.

You can immediately see that what fuel you use will have a significant influence on how much fuel you need to carry. Why are there these differences? It is partly because the energy you can get from each fuel depends very much on the molecular structure of that fuel. This is shown in the next section. More explanation of the physics is given in the Technical Notes under the table.

However, there is more to it than just the efficiency of the fuel. When you use a petrol or kero stove you have to prime it. This takes time and fuel. Sometimes you also have to let it warm up - this too takes fuel. There is also a very strong temptation to just leave it running even while you are not actually using it, and this uses yet more fuel. Many walkers have commented on this.

The figures given above are typical values for cooking for two people in a fairly relaxed manner. However, if you want to go lightweight, there are ways of reducing the fuel weight. I have listed some of these on the Stoves - Technical details page. There are of course other ways of reducing fuel consumption for the ultra-lightweight enthusiast: use a miniature metho stove, abandon the stove completely and use a wood fire, or eat cold food. The latter is an unpleasant thought.

Safety Note: Never use genuine petrol in a stove. The refinery adds a number of quite nasty things to it to make cars run better: these are not good for your health. Things like lead (brain damage), benzene (serious carcinogen) and other more esoteric things. The term 'petrol' is used here solely as a generic. Use fuels such as Shellite, Coleman Fuel and other similar fuels sold by stove manufacturers. In addition, some of the additives gum up the stove and block the jet.

Stove Operation Times

Another question often asked is how is how fast a stove works: how fast can a stove boil a litre of water? This seems to be a favourite machismo "feature" of some of the stove manufacturers. Well, fair question, but you shouldn't place much emphasis on it. You might want a reasonable speed when making a cup of tea, but 30 seconds here or there simply don't matter. Enjoy the view. Of equal importance is the time it takes to set the stove up and get it going, and whether you can turn it down to a simmer nicely. Most of the figures here come from the manufacturers or vendors, so treat them all with caution anyhow.

Most of the times are pretty rough: should we measure the Set-up time in summer or when your hands are frozen? Lighting time reflects the amount of priming needed: practice makes perfect of course. I was going to include "Tank time" or how long a "tank" lasts, but this really only concerns the metho stoves, which have very small tanks. Anyhow, how do you measure it? Do you have the stove on roar or simmer? Do you have a small or large tank or gas cartridge attached? If you think I have a figure wrong, please tell me.

Fuel	Stove	Set-up time	Lighting time	Boiling time
Metho	Trangia	2 min	1 min	9 min *
Metho	Home brew (big!)	1.5 min	1 min	3.5 min *
Metho	Home brew (small)	1.5 min	0.5 - 1 min	15+ min *
Shellite	Coleman Feather 442	2 min	0.5 min	4 min
Shellite	Coleman Peak Apex	2 min	0.5 min	3.5 min
Shellite	MSR Whisperlite	2 min	0.5 min	3.5 min
Shellite	MSR XGK	2 min	0.5 min	3.5 min
Shellite	Optimus 8R	1 min	1 min	10 min
Shellite	Primus Multifuel	2 min	3 min	3.5 min
Shellite	Primus Varifuel	2 min	3 min	3.5 min
Kero	MSR Whisperlite	2 min	3 min	3.5 min
Kero	MSR XGK	2 min	3 min	3.5 min
Kero	Coleman Peak Apex	2 min	3 min	3.5 min
Kero	Primus Multifuel	2 min	3 min	3.5 min
Kero	Primus Varifuel	2 min	3 min	3.5 min
Gas	Snow Peak Titanium	1 min	4 sec	4 min
Gas	Snow Peak GS-200D	1 min	4 sec	5 min
Gas	Coleman Pocket	1 min	4 sec	3.5 min
Gas	Coleman Backpack	1 min	5 sec	3.5 min
Gas	Coleman Max Extreme	1 min	6 sec	3.5 min
Gas	Camping Gaz Twister	1 min	4 sec	3.5 min
Gas	Camping Gaz Twister Piezo	1 min	1 sec	3.5 min
Gas	MSR Pocket Rocket	1 min	4 sec	3.5 min
Gas	MSR Rapidfire	1 min	4 sec	3.5 min
Gas	Primus Yellowstone	1 min	4 sec	3.5 min
Gas	Primus Multifuel	1 min	4 sec	3.5 min
Gas	Primus Titanium	1 min	4 sec	3.5 min
Gas	Trekker (typ)	1 min	4 sec	8 min **
Gas	Trekker modified	1 min	4 sec	3 min **
Gas	Caribee (typ)	1 min	4 sec	3.5 min

* The metho stoves have a very little tank which does need refilling frequently. The big problem is that you usually have to let the stove burn out and cool right down before you do refill it. This time is not included. Some home made versions are larger, heat faster but eat metho faster, while some are very light but deathly slow.
** The Trekker gas stoves have small holes in the burner. These seriously limit the gas flow so it boils water more slowly; conversely it can simmer at an extremely low level, which is useful. As an experiment, I modified a Trekker burner by drilling all the holes slightly larger (needs a mill to do it properly) and this made it run even faster than an expensive bushwalking stove. This completely voided any warranty of course.

• All these fuels are 'hydrocarbons': molecules consisting of lots of carbon atoms with other bits dangling off the sides. Some of the dangly bits are hydrogen atoms, while some are mini-hydrocarbon molecules themselves. The simple hydrocarbons are usually chains of carbon atoms as illustrated in the picture to the right. The green balls represent carbon atoms; the red balls are hydrogen atoms; the blue and yellow lines are the bonds between the atoms. The picture repesents butane, with 4 carbon atoms. The possibilities become immense as the number of carbon atoms goes up. Some hydrocarbons like benzene (a carcinogen in petrol) are actually rings of carbon atoms (with dangly bits). Anyhow, each hydrocarbon has a different number of carbon atoms and a different boiling point (as given in the table above). In general, as the number of carbon atoms goes up, so does the density and the boiling point.
• Petrol (Shellite, Coleman Fuel and similar proprietary fuels), kerosene and diesel are mixtures of a large range of hydrocarbons: a different fraction from the oil refinery cracking tower which converts the crude oil into useful fuels. In general the petrol fraction is the more volatile fraction of these three: it consists of shorter chains than the other two. 'Heating oil' is even less volatile than any of these. All of these molecules are longer than the propane and butane chains: propane has only 3 carbon atoms, while butane has 4. The Isobutane sold by MSR has 4 carbon atoms, but only 3 are in a chain while the 4th is attached at the middle, and effectively sticks out the side. But note very carefully: propane and butane are essentially the same sorts of hydrocarbon chain as found in petrol and kero. Petrol even contains butane. Under the right conditions, they are all liquid hydrocarbon fuels.
• Petrol is so dangerous because it has molecules which are short enough to be quite volatile, but not so short they will immediately vaporise and waft away like propane. These volatiles are there to help start your car in cold weather and make the engine run better. Some of them are carcinogenic though. Kerosene is so much safer because it has no really volatile components and no additives. But of course, a kero stove needs more priming to get going.
• There is no such thing as a single fixed boiling point for mixtures such as petrol, kerosene or the butane/propane mix: the mixture 'boils', but the boiling point changes as the components boil at different rates. Why? because the liquids are only mixed, not dissolved in each other. This distinction has engendered a lot of debate on the web at times: for more details see the special page on mixtures. What this means is that you can boil off most of the propane in a cartridge in very cold snow and leave a lot of butane sitting there, useless.
• A lot of the fuels have very similar figures for energy per mass (or grams to boil 1 litre), because what really counts is the number of carbon atoms in each molecule rather than the size of the molecule. This is because most of the energy comes from the oxidising of the carbon atoms.
• There was some belief that gas was very inefficient, but this is not so. The idea may have arisen because the density of liquefied gas is noticeably lower than petrol or kero, but what counts on your back is mass, not volume. Gas has a greater amount of energy per mass than petrol or kero: see the figures.
• An increase in the length of the hydrocarbon molecule cause the liquid to have a higher density and boiling point. But this does not make the vapour more viscous in terms which matter to us. The reason you are told to change the jet on a liquid fuel stove when switching from petrol to kerosene is because the molecules contain more hydrogen and carbon, and need a different fuel/air mix to burn properly. You could get a similar effect by changing the size of the air holes on quiet burners, but that is much harder to do in practice. It is discussed further under Jets.
• Methylated spirits ('metho', mainly ethyl alcohol, CH₃-CH₂-OH) has poor performance because it has one oxygen atom to two carbon atoms. This increases the mass of the molecules or the density without increasing the fuel value. In America you can also buy methyl alcohol CH₃-OH. This is quite toxic, which is one reason the government adds it to ethyl alcohol to make 'methylated sprits' in Australia. It is meant to stop you drinking (cheap) ethyl alcohol which has not had Excise Duty paid on it. Anyhow, it has a fuel value even lower than metho. However, in many of the lighweight Pepsi stoves so much of the flame heat is wasted that the efficiency for both alcohols is similar - and low.
  However, Peter Surna wrote that "I use a stove for morning milo and to heat water for freeze dried meals for two people. 20ml for the first, 40ml for the latter will cover it in a wide range of temperatures and wind conditions. The thing with these stoves is you do have to know how much fuel you will need and pick the right meals. There's no doubt about that." The idea is that for short trips you have a very light stove and not a lot of fuel: ulralightweight. He has a point.
• I am not sure what is in the various solid fuel tablets, although Hexamine is hexamethylenetetramine. I hope this helps.
• In theory it takes about 0.38MJ to heat 1 litre of water from 10C to boiling, or 1.8kW to do this in 3.5 minutes. Real stoves need double this energy to allow for heating inefficiency. At full power this efficiency will drop as so much heat goes up the side of the pot and is wasted. This will also vary significantly depending on wind speed, wind shield, pot shape and size, initial water temperature, and use of a lid. Watching the pot is also rumoured to slow it down :-)

For those who have never met the Pepsi Stove, a few words of explanation. You take two aluminium soft drink cans, cut them up and stick them together to make a sort of copy of the Trangia burner. The unit is 'closed' except for the filling hole (which may or may not get closed) and the jets. The metho gets hot and the vapour pours out the jets, as with a Trangia. Obviously you don't count the cost of the soft drink cans, but the metal-loaded epoxy used to glue the bits together is expensive. The result is still very cheap and light, but you need some sort of pot support around it, and I haven't counted the weight of that. The unit holds just so much metho, burns it all up, and then has to cool down before you can refuel it. If you put too much fuel in, you have to let it burn away. If you don't put in quite enough, you have to wait until the stove cools before you can refuel and finish cooking dinner. Once they get going some of them pour out the heat, far more than the little beercan cooking pots can absorb in most cases. Simmer? Forget it. They have a fanatical following in America among people who have never looked at the weight of the fuel, and who do all their cooking by pouring boiling water over ... something.

Now let's consider the requirements for two weeks and see what happens. If you are planning even longer trips, just scale from here. I have cut out two of the combinations as they don't add anything and increased the size of the liquid fuel bottle. I have also assumed the larger gas cartridges as they are more weight efficient (see above). The 450 g cartridge does raise the height of the cooking pot, making the assembly less stable, but we had no trouble in the Pyrenees with the combination as long as we used a small plywood stove base. We found the larger gas cartridge lasted 2 weeks just nicely; we used up three in the 6 weeks.

In addition, I have reduced the weight of fuel required per day slightly and by various amounts. These reductions are based on actual experience with very long trips: one gets a little more efficient with time.

• Kerosene: from 700 to 650 gm: not much reduction as priming is complex. The Coleman Peak 1 Apex 22 oz bottle has been assumed, but I suspect (strongly) that an extra Nalgene bottle at 130 grams would be needed.
• Petrol: reduced from 700 to 600: priming is easy. Again, I suspect (strongly) that an extra Nalgene bottle at 130 grams would be needed.
• Gas: reduced from 560 to 450 grams because that was the amount we actually used over a 6 week trip recently. The bottle weight is for a Primus 4350 g cartridge.
• A factor in fuel use for a liquid fuel stove is the hassle of lighting the stove: one tends to leave a liquid fuel stove running for short periods between courses because of this. Lighting a gas stove is so easy, so it gets turned off immediately, increasing efficiency. If you have one of the piezo stoves, it's just magic.
• Metho: reduced from 1120 to 1000 grams mainly to match. Larger bottles have been assumed: a 1 litre Sigg bottle weighs 150 gm but an extra bottle would be needed as well: 50 gm has been allowed for this.
• Esbit tablets, which have a 'trade secret' formula but are probably the same as the common Hexamine tablets, cost about $5.20 for a packet of six tablets, each tablet weighing 14 grams. Not included is the weight of the plastic blister packs, since I don't know it. There is also the ADI brand available in Australia. I imagine the weights and efficiency would be very similar.