Steam and methylated spirit consumption is a big subject and so my first task was to break this down into some simple numbers and ideas. This page is all about explaining those ideas and first approximations. All of these calculations are targeted at the design of my miniature traction engine and in particular the boiler and engine.
Methylated spirit or denatured alcohol is used in a number of camping stoves and so there are a lot of online articles discussing the amount of fuel you should carry for a given number of days.
On a recent test of our EkoFuel bioethanol fuel in a Trangia alcohol burner system we used approx 22 to 23g of bio-ethanol to heat 1litre of water to 100°C from 21°C. The theoretical amount of bio-ethanol required was ((100°C – 21°C) x 4184J/kg.K (water)) / 29700J/g (ethanol) =11.13g.https://ekofuel.org/blog/category/comparisons/
This gives us a couple of useful figures, a good typical test case for the amount of fuel needed to heat water over a given temperature range. Also, if you divide the theoretical amount of fuel needed by the actual amount used => 11.13g / 23 = 48%
48% efficiency for a boiler appears to be a really good figure, but we will use this until we get a better figure to use. Once the boiler is up and running I will try and instrument it and get some hard data.
This gets us to boiling water, to convert this into steam we need to add more energy. This energy is known as latent heat of vaporisation and for water this has a value of 2,260,000J/kg.
If we now calculate the amount of energy required to heat 1g of water 80°C (20°C to 100°C) and then energy to convert to steam at 100°C.
80 x 4.184J/g.K + 2260J/g = 2595 Joules
The amount of fuel required theoretically to achieve this, given heat released by burning ethanol is 29700J/g:
2595J / 29700J/g = 0.087g fuel / g of steam
this is theoretical, the efficiency of the boiler will reduce this. As I don’t have a value for this yet I’m going to use that of a trangia at 48%
0.087 / 0.48 => 0.181g ethanol / g of steam
This is a really useful number to use going forwards.
At 1 atmosphere 1g of water => 880cc of saturated steam at 100°C
The engine design for my traction engine is 12mm bore and 12mm stroke. Volume of working fluid consumed per revolution:
π x 0.62 (bore in cm) x 1.2 (stroke in cm) x 2 (double acting) = 2.7cc / rev
at 300rpm x 2.7cc => 810cc/min
At the end of the stroke the steam will still be under pressure. I’ve assumed the steam will still be at 1.5 atmospheres
810 / 880 x 1.5 = 1.38g water per minute => 0.26g fuel / minute
This feels like a fairly simple walk through of the steam and methylated spirit consumption.
Having approximated the engine’s steam consumption at a given speed and having calculated the consumption of fuel for the boiler I should now be able to calculate the overall efficiency….