A useful Stirling engine.

G'day all,

Last weekend I was at an event in canberra called The Mont. This is a 24hour endurance mountainbike race, and half of it is done with very expensive LED on head and handlebars. recharging is gerally done a a central point because these events are well out in the sticks.

I have built a partially satisfactory low temp differential stirling.

The idea is to build a small stirling capable of running a generator sufficient to recharge mountainbike lights. How do I start?

The lights will recharge from a lighter socket in the car (some from a USB - and one of my sons is very keen on the idea of beinga ble to recharge his phone while camping, this would also recharge the GPS which would be good).

So the start point is understanding how to build a generator sufficient for that (I have no idea but have recently seen a model using a DVD drive motor, I think that things like regulating the output to protect the device would also be inportant?)

Then the next point is an engine sufficient to drive the gennerator. Ideally, this would be powered from something like a small flame, pperhaps a portable camping gas burner as a maximum, but that is pretty hot.

Can anyone contribute to this please?

I know nothing about the electics/electronics. I know a little bit (not much) more about the engine.

The teenage boys are greatly enthused - it provides a 'really cool' way of being different at a campsite and enabling the electronics on which thier life seems to depend in between bike laps.

Dave

Hi there. You have a worthy project. One of the problems you will bump into is the fact that there are several things which affect the output of your machine.

The first and most important of these is the temperature difference between the ‘hot’ and the ‘cold’ ends of your engine. As you want to make a low temperature engine this temperature difference will also be low and so will be its output per unit volume. Typically this is about 500 Watts per square meter for each degree Kelvin (Wikipedia)

The second factor is the pressure of the system. A difficulty with pressurization is that while it improves the power output, the input heat required also increases proportionately to the increased power. This heat transfer is made increasingly difficult with pressurization is that you have to make the engine more pressure resistant – beef up the structure. The thicker walls of the engine also increase the resistance to heat transfer. You also have to make certain that the working gasses do not leak out of the ‘Hot-Cold’ cycle area. Phillips did this with a ‘roll sock’. Rule of thumb - More pressure the bigger the engine.

The third factor is the working gasses. Advice –stick to Air/Nitrogen, other gasses are either expensive or dangerous.

Don’t forget friction it is always with us!

First I would try to estimate the output you require then the ‘guesstimate’ Temperature Differential and use the figure of say 300W/(M².K) – I have allowed for friction, leakage and all the rest. That should give you a feel for the size of the thing.

Good Luck

Dick.

Edited By Richard Parsons on 02/04/2012 10:00:34

Dave, wecome to Stirling Engines. You won't charge anything with a low temp engine (known a LTD, low temperature differential), these motors will run on hot water, or some even on the heat of your hand. To generate power a high temp motor will produce around 1 watt per CC for a good motor, or as generators of this size are rather inefficient, about .5 watt of electricity per CC. Have a look in my album, there is a motor there with a Ross Yoke that generates 5 watts plus, another (my second motor built in 1994), generates a similar amount. these motors are un pressurised, and have been used as battery chargers. The little one has enough power to opperate a 3 volt radio. Up to now I'v just used various little electric motors as generators. With moderate pressurisation a considerable increase in power can be obtained. An interesting book is available, free on line, it is "Making Stirling Engines" by Andy Ross, there are 68 pages, he even fitted one of his motors to a bicycle, and another as an outboard motor on a boat. Ian S C

All very cool, and thanks.

I knew this would not be a LTD - wrong target as I was after output, not minimising input so at least on the right track there.

I have found Andy Ross's book so will do a bit more reading and research the bike chargers requirements (I have an elec tech mate who was quick to point out the influence of Watt on my power needs. He recommends PV, untill i pointed out that the intent was to charge between night runs)

I'll be back

dave

This is not a wind up but how about the little wind up torch. no batteries, efficient , enviornmentally friendly. Whatever way you go good luck

Bob

Hi there again David.

If you really want go via the Stirling engine route have a look at the Thermo Mechanical Generator or ‘Harwell engine’. These were designed to produce electricity and are used widely on buoys especially those provided by Trinity House.

The engine is very simple. It is probably the simplest of all Stirling engines as it has no rotating or sliding parts/seals. Mind you it is not the most efficient of engines having an input to output ratio of about 10 to 1 (10% efficient)

It has a displacer whose chamber is heated. The displacer is sealed by a flexible seal into the displacer chamber. The displacer is not linked directly to a diaphragm power cylinder. So the TMG is a ‘free piston engine, which is linked to either a moving magnet (or a moving coil) generator. You will see one running here . It is being dismantled in this next clip . It can be hermetically sealed if you want to pressurise it. This model can be pressurised.

The Generator provides AC at about 80 to 100 Hz and the machine shown is giving about 10 volts.

It is a noisy little beggar but what fun!

Hope it helps

Dick

Edited By Richard Parsons on 03/04/2012 13:39:25

Why do I take this stuff on?

Particularly when it has taken over a year to deliver a new chest of drawers (in brown stuff) to my eldest?

Thanks for the replies. I have now read the Ross book. Very helpful to understand the effort and commitment needed to get a good engine. Needs to be supplemented with a practical start point.

He mentions a series of articles in ME for a small version of one of his engines, unclear if this would be sufficient for my task and before my time in model engineering. I plan on looking into that to get started.

I rather suspect that I need to tackle the generator at the same time, any ideas.

As for the wind up, These lights are used at 10-20 kph on a 1m swithchback track through trees, I scared myself silly when I lost my helmet light and had to rely on the handlebars. 900 lumen is now about the standard, and I am not fit enough to wind a torch that fast

Did like the Harwell, but you are right about the noise.

For charging, you could use a 12v battery that could be left on charge its self, using either PV during the day, and or the Stirling Engine. At night when charging the lighting batteries the Stirling Engine could be left on keeping the charge up.

I'm hunting around for bits for the TMG. The problem with those vidios is that I'm on dial up, they take a wee while to load,hours. Ian S C

double posting

Edited By Ian S C on 04/04/2012 15:16:35

Hi Again David Yes there is a lot of the noise from that particulate example of the TMG (Harwell engine) is probably due to the fact that the generator has a moving ‘clapper bar' type magnet. The noise coming from the brass diaphragm which is probable drumming on the Annulus which is also part of the re-generator and the fact it is air cooled. I think with a little bit of a redesign it could be made quieter. This would be true if you used a thermo cycle cooling system and you might get a better output from it. I think it could be fitted on top of a pick nick stove.

The other thing there is an alternator that runs off the bicycle tyres just needs a bridge rectifier chip and you can charge as you go. I do not know about Oz but a Chinkee set costs a very little. in a bike repair shop you could probably get an old one for a 'tube of the amber nectar'

Hmm thinks what do I need to get my hooks on to make one?

Rdgs

Dick

Third thoughts. If you put some AC across the coils you could also use the engine to cool your tubes of the 'Amber Nectar' and warm up your meat pies. The Stirling cycle is reversible. Two uses from one machine. May be if you fitted a bicycle with one of those little alternators you could convince SWAMBO to pedal it.. By the way the you could also use the TMG to drive the bicycle alternator which could then drive a fan. Alternators can also run as a motor if you spin them up to synchronise with the AC frequency.

Regards

Dick

Hi David,

in the days when they usd to have hopt air engine competitions at the Model Engineer exhibition it was only a very small number of engines that could generate more than about a watt. I think the best was about 4.5 watts.

That suggests you will need to make quite a big, well made motor to do teh job you need.

Neil

Andy Ross's 35 cc ALPHA type motor delivers about 44 watts at atmopheric, would be more with pressurisation. Ian S C

That's impressive. I think they are possibly some of the most undeveloped prime movers - dideal for low grade fuel & energy sources.

Neil

Hi Richard,

There is no noise coming from the generator it all comes from the diaphragm which acts as a loudspeaker.

If you look at my Youtube channel you will see that I have made two TMG's one using a moving coil generator which is also very noisey with the other using a moving magnet set up.

The diaphragm do's not touch any parts of the engine I went to great lengths to ensure clearance together with minimum dead space.

The material used is Berylium Copper which is very good for the job but difficult to form due to it's springiness, I found Stainless steel to be not stiff enough and Phosphor Bronze failed due to fatigue after about ten hours running.

The first springs I used were Hacksaw blades they lasted about five mins.

I hope this info wil be of help. Cyril

Hi Stub Mandrel,

In the 1994 competition my 10cc Rhombic Drive engine achieved 11.8 watts

write up was in ME Vol 172 no 3964 . this is not the one on Youtube.

Best wishes, Cyril

Cyril, Is that your TMG Stirling on that Youtube video? Looks like it would be ideal for the purpose mentioned at the start of this thread, just put on the camp fire to charge up your battery.

Lyric Many thanks I still have a bit of Beryllium Copper left . I am now getting queer looks from shopkeepers measuring stainless food containers.

Rdgs

Dick

Cyril, just had a look at no.,3964, congrats, a bit belated but never the less. Pitty no photo of the the winner. The competition needs to be resurected, that and the gnat power one too, these are more than models. It was model Stirling Engine building that led to Dr Don Clucas and others firstly studying (for his Doctrate), at Canterbury Uni, then forming the company Whispertech, to develope, and build the Whispergen. Now theres a thing that would charge your batteries, but you would need a good bank ballance to buy one. Ian S C