This is the SI Thermal efficiency equation that I use to compare stoves.
Efficiency(%) =( (mws(Tws-Twe)Cw+(mws-mwe)He)/(mfs-mfe)Hf)100
mws = Mass of water at start (kg)
mwe = Mass of water at end (kg)
Tws = Temperature of water start (ÂșC)
Twe = Temperature of water end (ÂșC)
mfs = Mass of fuel at start
mfe = Mass of fuel at end
He = Specific heat of evaporation water J/kg = 2256 kJ/kg
Cw = Specific heat capacity of water J/kg.K = 4186 J/kg
Hf = Specific heating value of fuel J/kg
Ethanol = 26.8 MJ/kg, Propane = 46.3 MJ/kg Butane 45.6 MJ/kg
The results from this equation only take into account the amount of heating energy in the fuel not the mass of fuel and the problem with this is explained in an example below.
example: to raise the temperature from 20ÂșC to 98.4ÂșC (approx boiling point at 600m) of 500ml water with my Pocket Rocket canister stove, I use 8 g of propane/butane mix fuel with 370400 J of heating energy available with 3g evaporation eff = 46.1%,
Then if I use my Trangia alcohol stove to boil 500ml it would use about 13.8 g of fuel with 369840 J of heating energy available and 3 g evaporation eff = 46.2 %.
To use the above figures in the equation above, they give the same thermal efficiency result but the Trangia used 1.725times more mass of fuel than the Pocket Rocket.
I am wondering if any BPL forums users could help me improve this thermal efficiency equation or come up with a better way of comparing thermal efficiency. One thought that I have is to use g/l/80ÂșC instead of a percentage.
BTW the most efficient stove pot combination that I have tested is the JetBoil with the 1.5l pot, I heated 0.5l water from 22ÂșC to 95ÂșC in 2m 30s with only using 4g fuel. These figures give a thermal efficiency of 84.2%
Tony
