Page two.
Battery powered, portable, turbo stove.

March 2007 - Mach-5.

Be sure to read page one first.

While trying to get a stove to work without a fan - I drilled nine -10 mm holes in the bottom of a flask and used larger the holes than normal in the throat – that was Mack-three. It didn't work well without a fan but I later found it would almost work with a 40mm PC fan. I was still using the wooden base shown on page one. This base is getting very sad looking. Under normal use the wood does not char but when you do things like let molten metal run into it, things get ugly. It was only a temporary test rig and it was time to make something more practical.
Mack-four is shown here. The end cap from flask was removed from the flask, pop riveted in place and a large hole made through both layers of metal. Another hole was made in the side and the fan puttied in place.
The new flask has nine 10 mm holes through the bottom as before.

Because there is less restricting the air flow now - the little fan is perfectly adequate. It works just as well as the earlier models which use more powerful fans.

At the end of the session I poked an aluminium tube into the coals. I expect it to take a while to melt but it didn't. The tube just kept on disappearing. Some molten melt ran into the base but most didn't and I had to soak the flask in caustic soda to clean it out. The inside of these stoves appear to be capable of reaching around 1000 deg C (a guess). They can melt brass but can't reach the 1083 deg needed to melt copper – I plan to change that soon.

All the stoves so far are based on the same model flask. These are one litre flasks. There is a fair range of stainless steel flasks plus other double walled vessels to choose from. There are cups (250ml) , food warmers (400 ml), 500ml to 1.8 litre flasks, ice buckets and so on.
Mack-five was intended to be a small, light, self-contained stove for camping and possibly backpacking. I bought three different 500ml vacuum flasks because the size does matter. I was originally planning on making something like Mack-4 but with everything able to be packed inside the tin for storage and transportation. I just didn't have a tin big enough, so I had to use what I had and think “inside the box” - so to speak.

The tin used is inverted so the hinged lid is on the bottom. This makes it easier to fix the flask in place and easier to install the fan. The fan is placed at the bottom to try to keep it away from the heat as much as possible. The flask end-cap was removed by drilling a small hole and popping it off with compressed air (water also works). The cap isn't used so it doesn't matter too much if it is destroyed. Five 10 mm holes were drilled through the two layers of metal. Sixteen - 3 mm holes were drilled in the throat. It is easier to add or enlarge holes than remove or narrow them so I started with fewer smaller holes than may be needed. It turned out my initial set of holes worked as is.

The 3 mm holes can be seen here. The large hole for the top of the flask to protrude and for the fan were made with a nibbling tool then enlarged using a grind stone in a drill press. It was just trial and error till the flask had enough clearance for airflow between the bottom of the flask and the lid of the tin.
The fan was screwed into place inside the tin with the supplied grate fitted to protect the fan blades.
The flask is held in place by two pop rivets at the bottom. These are visible in some of the photos including this one (if you click it to enlarge it).

For the test burn, the fan was powered from a 12V plug pack. The stove worked as well as any of the others. The small size of the combustion chamber doesn't seem to be a problem. The flame intensity seems the same as before but obviously it isn't going to burn as long on a single fill of fuel (wood).
The top of the tin got quite hot, the bottom was a lot cooler but still hotter than I would like it to be. The fan survived but I don't like it getting too hot.
On another test I insulated the inside above the fan with rock wool. This seemed to make the matter worse – I think the insulation stopped the air circulation inside the tin causing to the heat up even more and burn the paint.

The battery pack and speed controller.

Ideally one would use hi-tech batteries. Lithium polymer (and LiIon) have good energy density (about 4 times better than NiCad) but are expensive, must not be run flat or over charged and can exploded if mistreated. Nickel metal hydride would be my first choice except for the cost. They have about twice the energy density of NiCad. I would have liked to use 10 AA sized NiMH cells to make a pack which can be stored inside the flask. This would have left room inside the tin to store a pot stand of some sort.

I took the cheapest option – Nickel Cadmium (NiCad).. The cheapest way I know to (easily) make a 12V NiCd pack is the buy a cheap cordless drill and gut it. K-Mart near me, sell a Chinese made drill for au$19. For this I got 10 - 1.2 AH cells – a charger and yet another spare drill body (which may be gutted for other parts one day).

The cells had to be re-arranged so they would fit inside the tin. A thin layer of fibre-glass made the assembly rigid. The glass was molded around a plastic bottle which had the same diameter as the flask (75mm). Something like the loo-fan controller I made in 1998 should work as a controller but would be overkill.
I wanted to keep it simple enough so most people could build it. The simplest speed controller would be a potentiometer is series with the fan. A 50 ohm 3 Watt wire wound pot should do the trick but I haven't tried it. Rather than waste power heating up a (variable) resistor I decided to turn it into light instead.
I used a 6 position rotary switch and four sets of LEDs to drop the battery voltage to the fan. This gives me 5 speed settings including full on plus an off position.
For people who understand electronics this is trivial. Others should get help or stick to just using a potentiometer.

At full power my fan is drawing 100mA - so one charge should give at least 12 hours usage. On the lowest setting the current is halved so battery life would double. It would be extended even more by using a efficient (switched mode) controller.

This is how it fits inside. The socket on the bottom of the battery pack (right in the photo) is for charging the battery. The fan has been moved to the outside to distance it from the hot flask. If I removed the insulation then the space beside the fan (where the matchbox is) could be used to stow a pot stand.
As shown here the stove weighs 940 grams. This is slightly lighter than my pump-up shellite stove full of fuel. For a five day hike on Hinchinbrook I took this plus an extra 600 ml of fuel – so if you can collect fuel (which you can't in most parks here) then this turbo stove makes sense. Even if you can't collect wood there is usually (other people's) rubbish to burn.

For longer hikes you'd want either better batteries or something like a small solar panel to keep it charged. The battery could also be a useful emergency power source.

A human powered version would not be out of the question but it is not something I intend to pursue at the moment.

When I've done reef trips waste disposal is a problem. Typically we go for two weeks and most rubbish is brought back with us. We're not supposed to bury food scraps on the island. The rules regarding dumping it in the sea or burying it on the reef flat change over time. I personally dry my scraps (expect fish scraps) in a mesh bag and bring them back to the mainland. A turbo stove is quite capable of incinerating stuff like this – even damp tea bags burn once the fire is hot enough.

One problem with wood fires when camping is finding dry wood in wet weather. I haven't done the experiment yet but I'm confident that these stoves will burn wet wood once they have heated up enough. If you keep some charcoal from a previous fire you can probable make more dry wood and charcoal for next time, I will try it some time.

A little later – Oops - I've just melted the fan.
I just gave the battery pack it's first run. In view of my wet-wood comment above I wanted to see if I could ignite charcoal using a plastic milk bottle top as a starter – I could. When the coals where definitely alight I added twigs and sticks. So I thought I'd boil some water. I stood the billy on two bricks, this gave maybe 40 mm clearance. The flame was a bit large and spread out and around the billy. Smoke from hot paint came through the holes in the bricks.

I turned the fan all the way down and the flame tamed down nicely. The metal near the fan seemed hotter than ever and the fan started to make noise and stopped. The top of the flask was glowing a dull red but the flask which was visible through the fan was glowing bright orange. Remember the flask is double walled – imagine how hot the inner wall must have been.

I'll look at the damage tomorrow and maybe try putting a heat shield between the fan and the glowing metal. I bought spare fans in case something like this happened. A mack-4 style stove wouldn't have this problem.

Next day, Shield up.

I used the flask end cap as a heat shield. I ran stove for around five hours. So far so good.

More later.........

Feel free to use the information here to build stoves – even if it is for profit.
Make sure you read both pages and follow up the links.

Fire is dangerous.

I do not recommend using them indoors without ventilation.

Be sure you understand what carbon-monoxide poisoning is.

People still die doing stupid things like running combustion engines indoors – be very careful.
But have fun,