very small/light petrol/LPG generator?

That's achievable with a UPS, unless weight or recharging time is an issue.

OK, so a lead-filled UPS may not be the best answer then ;-)

Energy densities according to WIkipedia : Li.Ion 0.7MJ/kg Lead Acid 0.1MJ/kg Petrol & LPG 49MJ/kg

How about a thermal battery?

John

My APC SmartUPS 700 wouldn't quite make that, maybe 45 mins. Its small 6 x

Maybe one of the 300W car invertors jobbies? They are small and light apart from the lead acid power source. But medium car battery will have ample capacity for 1hr@250W (20A/Hr ish). A 40 or 50AHr battery won't be fully discharged, car batteries don't take particulary kindly to deep discharge.

It does depends a bit on what you want to power and how fussy it is about the waveform. The cheap stuff can have an "odd" waveform.

Plus point a UPS/Invertor doesn't produce any nasty exhaust fumes.

but the latter requires very heavy conversion equipment. Its a classic case of an OP that doesnt include the necessary info

NT

around the 200-500w mark?

may have better

option but really don't

Well I know a lot about the above, and yes, its ceratiunly dable that way..abck to back asomething like a 3-10cc RC engine with a 300-500W brushless motor and a 3 phase bridge, and you would get something in the low tens of volts at suitable power..

for fuel - we need 1

An hour of fuel for say a .40 cu in 4 stroke would be about 300mls I'd say. Best to get a petrol engine tho. model fuel is horrendously pricey. Small strimmer type thing.

Thats a thought. A lithium battery for 250watt hours would probably weigh less than a motor and generator.

two of these would work

Have you looked at the power to weight ratio of rechargeable batteries? A D cell weighs at 66g and can have 6WHr, ie 1.2V and 5Ah. If you have 50 in series - parallel with diode protection between banks, the solution would only weigh 3.5Kg or so. Am I going wrong somewhere?

Lead acid equivalent would be 3 or so times the weight.

And lithium polymer about half the weight.

With A123 cells in between.

option but really don't

A motorcycle alternator might be a suitable electrical end.

for fuel - we need 1

How lightweight? Can you park up a vehicle nearby (which if you are using the vehicle to get there is 'free' weight)?

How much can a fit man on a bicycle generate?

Owain

I discounted lithium on cost - but not sure what the cost would be. Rechargeable D cells if bought in bulk would be relatively cheap - few £ each.

about 50w.

I'd heard about 80W, but I don't know if that's meant to be continuously (e.g. a horsepower is an approx measure of what a horse can maintain for most of a day). I can generate a horsepower belting up the stairs, but I can't do it all day long.

I tried out a measuring bike someone had in their living room, and even though I was dressed up in suit, tie, and jacket (seeing a customer), I could generate 250W quite easily, but certainly couldn't keep that up for too long (especially in the suit and without any cooling breeze).

I would love to have a meter on my (real) bike which told me how many watts I was generating. I think I saw one once, but it was very expensive (IIRC, it used the chain speed and used some way to estimate the force on the chain, to work it out). Must admit I haven't looked for a few years now.

the average "in-shape" cyclist can produce about 3 watts/kg for more than an hour (e.g., around 200 watts for a 70 kg rider), with top amateurs producing 5 watts/kg and elite athletes achieving 6 watts/kg for similar lengths of time. Elite track sprint cyclists are able to attain an instantaneous maximum output of around 2,000 watts, or in excess of 25 watts/kg; elite road cyclists may produce 1,600 to 1,700 watts as an instantaneous maximum in their burst to the finish line at the end of a five-hour long road race.

quote from Wikipedia