first time question, and not much of an electrical guy. so please be patient.

My dad however, is an electrical engineer, who loves little goofy gadgets as most engineers do. For Christmas I want to get him a stirling engine, something similar to this; http://www.amazon.com/gp/product/B00DS1NLXQ/ref=s9_simh_gw_p21_d0_i5?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-3&pf_rd_r=0MB6C0MFFEPQB5NAPB9P&pf_rd_t=101&pf_rd_p=1688200422&pf_rd_i=507846 ,

however I want to use the flywheel mounted pulley to run a small generator to charge a cell phone. I know its not practical but he would enjoy it.

my question is, is there any other options besides the bike generators devices available that can help keep this package "small". the charger is rated 5v @ 2.1 amps.

another thing I wonder about is the 5volt constant, if so I cannot know for sure if the engine is capable of holding a steady rpm being an engine that rely on temperature changes to work. is there a way to use capacitors to store the power so that the charger always gets what it needs.

wow, I guess in my head this was a lot easier. if anyone can help push me in the right direction that would be great. even if the best option is to come up with a different gift idea haha.

thanks in advance guys, and take care.

  • 1
    \$\begingroup\$ A Stirling engine of that type and size has no prospect of providing 10W of power, sadly. Actual output will be far far below 1 Watt. The one shown runs an LED but a good modern LED will give a creditable glimmer on 1 mA so at 1 MA PLED ~= 1 mA x 2 to 3 V = 2 t 3 mW (0.002 - 0.003 W). Even several times this level will not do much to charge a phone. If you are serious about running a few Watts of alternator or generator then a larger than that steam is more practical - and more costly. It has the advantage too of being able to be run on compressed air when desired. As a guide .... \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:15
  • \$\begingroup\$ Abs max Carnot efficiency = = (Thot-Tcold) / Thot in degrees absolute. eg add 273C to degrees C. So if Thot = 220 C and Tcold = 20C (copious water cooling) Th-Tc = 200 and Th = 273 + 220 = 493 K saay 500K. So carnot efficiency = (Th-Tc)/500 = 40%. In practice when Th/TC is 3:1 real women get 50% + of Carnot and at 2:1 real ment get approahing 50%. At Th/Tc = 500/300 = 1.67:1 uber mechen may get 20% of Carnot and small models may get 5% - 10% or less or much less. So (500-300)/500 x 5% to 10% = 2% to 4% (or less). So thermal flow through engine needs to be 10W/0.04 ... = 250W-500W thermal.... \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:19
  • \$\begingroup\$ Thermal power abs max = Revs/second x volume x delta-T x Joule litre of air x Compression ratio wrt atmospheric. This is easily summarised by saying that if your small engine does not make a shrill shriek of higher pitch than a small gas turbine (say 100,000 RPM) and explode all over the room after 0.2 s of operation then it will not do the job. Alas. | You can short circuit my claims by laying hands on a model as shown and measuring generator Vopen circuit and I short circuit. Pmax is < than Voc x Isc by a factor of probably 2 to 4. Results are liable to be 10's of mW at best I think. Sadly. \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:26
  • \$\begingroup\$ MA - mA, mechen -> menchen, 2 t3 -> 2 to 3, carnot -> Carnot, saay-> say, ment -> men, app... whatever. \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:29
  • \$\begingroup\$ Alternator: For the alternator in Bruce Abbot's answer you can use any permanent magnet brushless dc motor with direct access to coils OR a stepper motor such as found in Olde 5.25" floppy drivs (remember them) and SOME olde hard drives. These need to rotate properly when removed. SM's from 3.5"drives tend to not work as the end bearing is attached to the drive body and taking out the SM debearings it. Any small PMDC motor that you can feel magnetic "cogging" from may do. \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:34

The first question is, can the engine produce the amount of power required?

5V @ 2.1 Amps = 10.5 Watts. We know your sterling engine can light a small LED, but that is probably only a fraction of a Watt. A typical bike dynamo produces around 3~4W at 20kph, but needs 10~12W of mechanical power to do it. So your sterling engine would have to produce about 20W to get sufficient power from a bike dynamo.

I don't know how much power that engine can develop, but I doubt it is even close to 20W. However the phone charger might still work with a more limited supply, so what kind of generator is required to get any power at 5V?

The engine is supposed to run at 1300~1500rpm. If we assume the lower figure under load then the generator needs to produce 1V for every 260rpm (260rpm/Volt). However you could use a belt drive with different sized pulleys to increase generator rpm and then the rpm/V would be higher, eg. at 1:4 it would be 1040rpm/V. This is similar to the Kv of small brushless motors that are used in model aircraft.

You could use a small brushless outrunner motor like this as an alternator, coupled to the engine via a 1:5 pulley set (20% extra rpm to account for electrical losses). The motor wires would be connected to a 3 phase rectifier (using Schottky diodes for low voltage drop) with a large electrolytic capacitor across the output to keep the voltage smooth. To regulate the output voltage you could use a 5V LDO linear regulator. The circuit might look something like this:-

enter image description here


If your charger can put out 5 volts at 2.1 amperes, that's:

$$\ P=\ {I}{E}=\ {2.1A}\times {5V}\ =\ {10.5}\ watts $$

Assuming that the charger's running at about 80% efficiency means that you'll need to put about 14 watts into it, and if that's coming from a mechanical generator with an efficiency of 80%, the stirling engine will have to supply the generator with about 18 watts which, at 746 watts per horsepower, comes out to about 1/24 horsepower.

More particularly, to answer your question you'll need about a 14 watt generator.

  • \$\begingroup\$ answer mostly good in principle but I think you've gone astray on generator sizing. A generator is usually specified based on its output, so about 11 Watts, but even if using input it's 10.5/0.80= < 14 Watts. This is mech input to generator. FWIW a SE of this style would be immensely lucky to be 5% efficie ntthermally so thermal input needs to be about 14W/.05 ~~~= 280 W! :-( \$\endgroup\$ – Russell McMahon Nov 6 '14 at 11:10
  • \$\begingroup\$ @RussellMcMahon: You're right, I messed up on the output of the generator. If the output from the charger is about 11 watts and it's running at 80%, then its input from the generator has to be about 14 watts. Then, if the generator is running 80%, its mechanical input from the SE has to be about 18 watts. I read that practical SEs have been built with 50% efficiencies, so I put that in there, but I don't know much about about SEs, so I'll defer to you on that one. :) \$\endgroup\$ – EM Fields Nov 6 '14 at 12:07

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