Gear

I am in a bit of a dilemna. Basically, I am always pro gas everytime. Its, clean, easy to use etc etc.

However, I want to know which is the most fuel efficient for the weight.

 The liquid fuel would be KEROSENE v Bhutane/propane. So for example, if I carried 350 ml of kerosene. Would I expect it to last the same amount of time as that of say a medium sized one costing 3 pound 50pence Primus Bhutane/propane gas cannister? Weight also about 350 grms.

I know from experience, that on average I can get around 3 days usage from a medium sized Primus or Coleman gas cannister. But for the liquid fuels for the same weight...I have no idea.

Any information would be greatly appreciated because I am planning a trip where I need to carry all the fuel and dehydrated food for around 8-10 days. The temperature NOT dropping below 0 degrees. However, it would be at high altitude over 3500m-5000m.

If anyone knows about this issue. Any information would be greatly appreciated. Thanks.

Not sure of the anwser but have you investigated the stove/pot combinations that have heat exchangers like Jetboil or MSR Reactor or Primus Eta power? Your gas will last considerably longer then. For liquid fuels you will need to take into account the weight of the fuel bottles/pump and the generally heavier stove too.

Quite a lot on efficacy of various stove types over time on the web.

This is as good a place to start as any. 

Point of info on the altitude... a talk I went to a couple of years ago about a ski tour over the Karakorum showed that gas appears to work just fine.  I was initially quite surprised they'd used gas, most expeditions going for petrol/kerosene/parafin presumably as it's much easier to get hold of in that part of the world, but if you can get the gas there then the typical advantages shoud return.

Pete. 

3 days for a medium sized isn't much. I can get 7-9 days out of a single medium canister (trip with 2 people so should be even longer when used on a solo trip). I don't use any special heat exchangers but I always use a windscreen and heatreflector. I don't use the stove on full power; 1/3 or 1/2 is good enough, takes a bit more time but can save a lof of gas.

If gas is available, it's my clear choice.

 Gas at altitude: it's perfectly logic why gasstoves also work at altitude. The stoves functions based on the difference between vapour pressure inside the canister and atmospheric pressure. Since at altitude, the atmospheric pressure drop, the pressure difference will increase. Well, more or less; there is of course the lower temperature  which has a negative influence.

It's much more about the efficiency of burning and where the heat goes than the relative densities and enthalpies of combustion of fuels. Thus, Jeboil, heat-exchanger stoves, as mentioned above, will outdo any inefficient stove, regardless what it's burning: 

ΔH(combustion (propane)) = -2220.1 kJ mol-1

ΔH(combustion (butane)) -2881.9 kJ mol-1

These numbers basically say how much energy is given off from burning them. You get more energy (larger negative number) from burning butane. 

The densities of butane and propane are quite similar (both are turned into liquids in the cylinders anyway), but butane has more 'heat power' for its weight. Propane and butane are mixed in gas cylinders so they operate at lower temperatures (as butane can become difficult to light).

Kerosene and petrol are more mixture-based with all sorts of stuff in them, and as such exact values of the energy released when you burn them are hard to get. However, I would guess both would have exceptionally high combustion heats. However, I've no idea how you'd get hold of energy-dense kerosene, but if you've found a way then that might be the one to go for...

(Apologies for the chemistry! ) 

IMO, and with at least a few years of multi fuel, meths and gas stoves I would say kerosene is the most efficient in terms of weight, cost(far, far cheaper), fuel volume etc. 

I would imagine this is for the same reason deisel cars are more efficient than petrol etc...

For 1-2 day solo trips I use gas, as it is so convenient.  Kerosene is the way I go if away for more than a 2-3 days or with the wife as we like our brews and cooking...

Some useful fuel figures here.  Apologies for the use of some hideous units, like grams per gallon for density...  which will be US gallons, too...

The liquid petroleum fuels all have lower heating value figures of around 43MJ/kg.  Since these cover the fuels naphtha to petrol to diesel, I'd say you wouldn't go far wrong to assume that paraffin/kerosene is about the same.

Propane and butane have LHVs of about 46MJ/kg.

So there's really very little in it, I'd say, assuming you can get a stove that burns the fuel efficiently (and they should).

You DO need to make sure you compare the same units, though; keep everything in volume terms, or weight terms.  You've tried to compare 350ml of kerosene against 350g of butane/propane.  This link mentions this, and confirms my guess that LHVs (energy/mass) of liquid petroleum fuels are about the same.  Liquid densities here.  Kerosene = 817kg/m3 = 0.817kg/l.  So 350ml would weigh 0.817*350 = 286g.

The Lower Heating Value is the one you want to use, as it assumes the water vapour is lost, and so the associated heat of vaporisation is also lost.  It you had a perfect condensing boiler, you'd use the Higher Heat Value, since you recover the heat of vaporisation.

I've been trying to work out the energies for meths (ethanol/methanol mix), from the chemical bond energies, and it's interesting to see how much variation there is in quoted figures, for HVs, and for bond energies and bond dissociation energies.  Bloody chemists...

Note how ethanol and methanol have LHVs about half that of the other fuels.

I might be able to sort out that meths calculation, CP (you must be an engineer (can do most of the maths but jealous of chemists )) once I've done this exam...

The bond enthalpies vary between data because some use species-specific data whereas other sources use mean bond enthalpies, drawn over many compounds. As has been concluded, the macroscopic diifferences between fuels outweigh the small molecular differences, so 10 KJ mol-1 ain't gonna make that much difference.

IMO the chemical energy of the fuel is dwarfed by the efficiency of thermal transfer to the pot (as evidenced by the jetboil's much greater efficiency).  I use a low heat output (which generally requires the control of a gas stove) to minimise heat loss to the atmosphere (I'd guess 80%+ of the heat from my whisperlite on full power goes to heating up the world, vs negligible ambient heating effect from my f1 lite gas stove). This doesn't apply in very cold (-20C & worse)/windy environments where the pan is losing much of its heat back the the atmosphere and a low heat output simply brings the pan to a low equilibrium temperature.

When backpackinglight.com did a bunch of stove tests some years back they used the stoves on c.25% power which they found was optimum. Similarly, the ion stove (mini coke can stove) is more efficient than a normal coke can stove because of its low heat output.  By judicious use of a low flame, a good windshield and the right pot/stove combo, you should be able to get 28 0.9L boils from a single 225ml canister.

My suggestions to minimise fuel weight would be to worry about how much to cook, what to cook, and how to cook (windshield, low flame etc.). This will have a much greater impact than the chemical energy of the fuel (tho' maybe if you were debating butane vs. watered down meths it would start to matter!).

> The bond enthalpies vary between data because some use species-specific data

I understand those differences (although it's a long time ago, I did get an A in A level chemistry...).  But there's variation between sources even accounting for the molecular context of the bonds (AND the individual bond dissociation energies, e.g. plucking individual H atoms from a methane molecule).  It's also why I mentioned both bond energies (an averaged value) and bond dissociation energies (the unique, context-specific values).

You're right; I'm an engineer, but I'm not jealous of chemists; after all, it's really just a branch of applied physics.  A sloppy branch, it would seem...

I'll agree that a few kJ/mol between friends isn't going to make a lot of difference; I just wasn't expecting to see the variation in the figures that I'm seeing.

> IMO the chemical energy of the fuel is dwarfed by the efficiency of thermal transfer to the pot

Absolutely.  That's why I'm looking at the combustion energies I should expect from a given fuel, so I can calculate the efficiency of my burner/pan combinations (cf the Meths Stove Q&A thread...).

I think I'm up to about 53% efficient at the moment, if my calculations are right...  I tried the JetBoil pan last night, and, whilst it did speed the boiling a bit, didn't improve it as much as I expected.  But then, the pan base was higher above the burner than in my other tests, so I need to sort that out before I dod more comparative burns...

What is the effect of the kerosene/deisel/petrol viscosity differences have on burner performance?

BTW, we switched to kerosene to melt stuff at work from diesel over a year ago because it was so much cheaper. Had to change the pumps and improve the fit of the delivery pipes to stop leakage of kerosene too. Not completely stopped, so might get a multi-fuel stove to take advantage. Anyone in the NW England with a bucket let me know.

I bought a Primus gas stove last year, it was the ordinary micron one but had the piezo ignition, think it is called the express now, but not quite the same. Can you buy the eta express pan and windshield separately? Want the eta express but don't want to buy the stove.

Can people recommend how much gas to take for 9/10 day trip? I will be using stove to make brews for breakfast and in the evening plus enough water for up to two standard sized reiter expedition dehydrated main meals each day. Typical usage I guess, but need to know what to take. I'm new to trips and up to now just done weekenders (plus some longer weekends) and I just take the small primus gas cans (new ones if the old ones seem a bit empty).

I really have now idea and want to minimize weight. I also have the AGG cookset that I will be taking the pot and cosy from (leaving the can stove, measuring cup, primer tray, heat reflector and windshield behind). I guess the meths would not be the most weight efficient option. BTW I find my primus gas stove without shield is quick enough to boil with low power, I turn it on about 1/4 turn which is just neough to light it plus a bit more. I would say 1/4 power. I still find it quicker than my old go gas thermotech with heat shield on full.

I think the eta express is a better option than jetboil due to its wider pan. I prefer wider pans. I guess you might get a heat spot in the primus eta express, which is less efficient, but I prefer it for the good efficiency and the fact that the lower arrangement should make it more stable.

Has anyone compared the efficienct of these newer heat exchanger type of pan/stove kits? I would think these have more effect on fuel efficiency for longer trips than the fuel type. Why don't mulit fuel stove manufacturers come up with heat exchangers for their stoves? Would they just soot up?

What other technology s out there to use? I mean heat exchanger technology is not new. Just like cyclonic filtration was not new when Dyson first introduced them to household dust extraction systems. There must be some more heat transmission technology out there. Any fuel and power engineers out there with ideas? Forget about the chemists as it would take an engineer to develop the ideas. Isn't that right CP?

I've compared the heat exchanger stoves - Jetboil PCS, Primus EtaPower & EtaExpress and MSR Reactor - with various standard canister stove/pan combinations and they are roughly twice as efficient. However none of them is particularly light so this doesn't necessarily translate into lighter weight in the pack.

The EtaExpress pan is available separately from the stove.