RC airplane gasoline engine as an electrical generator

I've never posted on this group, and I am apologize for rushing right it with a specific question.
I have an idea of building a tiny generator for an electric R/C toy
which is supposed to output ~300W of electrical power, with a capacity of, say, 5 Amp-hrs. Is it too crazy of an idea? A portable battery pack of that power/capacity weighs about 20-30 lbs, and I would like to reduce the weight as much as I can. It appears that R/C airplane gasoline engines fall into my desired power/weight category, so I was thinking to hook up such an engine to a generator. I tried to google wattage ratings of RC gasoline engines and did not have much luck. Oh, and a relatively quiet operation would be a plus (say, not more noise than from a scooter/moped).
I am not an RC person, so I apologize in advance for my ignorance.
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snipped-for-privacy@gmail.com wrote:

What? a 300watt capable pack runningat 5A/h.. No way. Not lithium
Ive got a 3.6Ah300W pack and that weighs about 10oz...

It's 750 watts per horepower. probably what you need is a 40 class glo motor coupled to a cooling fan and a 300w class brushless motor (e.g.AXI 2826/12 and 6 silicon diodes to make a three phase rectifier,and you should be able to produce 300w at around 12-16v as long as the fuel is there,.

Forget it. Use teh LIPO battery instead.

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| snipped-for-privacy@gmail.com wrote: ... | > I have an idea of building a tiny generator for an electric R/C toy | > which is supposed to output ~300W of electrical power, with a capacity | > of, say, 5 Amp-hrs. Is it too crazy of an idea? A portable battery | > pack of that power/capacity weighs about 20-30 lbs, | | What? a 300watt capable pack runningat 5A/h.. No way. Not lithium | | Ive got a 3.6Ah300W pack and that weighs about 10oz...
I'll bet he just meant that this would provide 300 watts for 5 hours and accidently stuck in the `amp' unit.
Assuming that your 3600 mAh pack is a 3s (just a guess), that means it could provide 300 watts for about 9 minutes. To make that last 5 hours, you'd need 35 of them, and that would weigh about 22 lbs.
| It's 750 watts per horepower. probably what you need is a 40 class glo | motor coupled to a cooling fan and a 300w class brushless motor (e.g.AXI | 2826/12 and 6 silicon diodes to make a three phase rectifier,and you | should be able to produce 300w at around 12-16v as long as the fuel is | there
Sounds appropriate. You could also use a similarly sized brushed motor and skip the rectifier, but then you'd have brushes to wear out. (What do small, commercially made DC generators usually do? I guess I'd prefer the brushless version just for the reduced maintenance.)
I wonder how long a R/C engine would last being run non-stop before just plain wearing out? They're meant to be small and powerful, not necessarily to last for a long time. Maynard got his to run for around 40 hours non-stop, but he's a God among men ...
(May not be a concern. I'm just rambling ...)
| > Oh, and a relatively quiet operation would be a plus (say, not more | > noise than from a scooter/moped).
A four stroke engine would probably be quieter than a two stroke. It would also cost more, weigh a bit more, but be a little more efficient.
Don't forget that glow fuel is a lot more than gasoline -- around $12/gallon US here. You could use a gasoline engine, but generally they're a lot bigger than 300 watts.
| Forget it. Use teh LIPO battery instead.
Or 35 of them, if I understand what you're asking for better than TNP.
Certainly, a lead acid battery would weigh a lot more. I've got a 100 Ah 12v battery that weighs around 40 lbs. In theory, it could put out 300 watts for around 4 hours -- too heavy, not quite long enough, but a whole lot cheaper than the LiPo solution.
--
Doug McLaren, snipped-for-privacy@frenzied.us Connection reset by peer pressure

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Thank you, Doug, you made many things clear.
There was a typo in my OP - I need 200W instead of 300W, but it's not a huge difference. Need about 2 hrs of continuous power output.
I apologize for the vague specs on what I need. Ideally, I need a 120V AC sine wave at 1.6 amps. Or 24V DC would work, too.
Here is a link to a 16 Ah LiPo battery:
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID !52
It outputs 22V, so for 200W it's ~9Amps, so it technically should last for ~1.5 hrs - is that right?
It weights 3 lbs.
But me making this battery pack work would be equivalent to trying to deactivate a TNT bomb,
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| There was a typo in my OP - I need 200W instead of 300W, but it's not | a huge difference. Need about 2 hrs of continuous power output.
200 W for 2 hrs is a lot easier than 300 W for 5 hours.
| I apologize for the vague specs on what I need. Ideally, I need a 120V | AC sine wave at 1.6 amps. Or 24V DC would work, too.
Ultimately, the form of the electricity doesn't matter _that_ much, because converting isn't too difficult. But if you need AC rather than DC, that does make an engine powering a generator a little more attractive.
| Here is a link to a 16 Ah LiPo battery: | | http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID !52 | | It outputs 22V, so for 200W it's ~9Amps, so it technically should last | for ~1.5 hrs - is that right?
Well, at 22 volts it's time to stop. But it starts at 26 volts fully charged (according to the page, though most R/C chargers would only go up to 25 volts), so the average is around 24 volts, and that works out to 8.3 amps, and so you last 1.92 hours -- very close.
(Perhaps you could go to 7 cells and get the extra power that way? It depends on the load, on how easily it can be adapted to slightly different input voltages.)
Also note that this limits the discharge rate to 6.5 amps, if you buy this pack as-is. (Looks like the individual cells are good to 1C, so they should be able to do 16 A if you make your own harness.)
| It weights 3 lbs.
Impressive. | But me making this battery pack work would be equivalent to trying to | deactivate a TNT bomb,
LiPos aren't _quite_ that dangerous. But yes, you'd have to be careful, you'll need a charger, etc.
Doing 200 watts for 2 hours makes batteries look a lot more attractive for this than 300 watts for 5 hours does.
--
Doug McLaren, snipped-for-privacy@frenzied.us
On the other hand, you have different fingers
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If it figures that close, it will never do it. Inverters are only around 50% efficient. You will need a battery twice as big, I would guess.
--
Jim in NC



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Morgans wrote:

90% plus for larger ones.
You will need a battery twice as big, I would guess.
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What is your definition of larger ones, and in what price range?
I would guess you are talking about the true sine wave inverters. They are indeed more efficient, and cost around three times the price of the modified sine wave inverters, the last time I checked.
He is only looking for 200 watts, so I would guess he is going to be in the market for the less expensive modified sine wave inverters, and the efficiency of them is truly appalling, from my experience.
--
Jim in NC




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Jim, here is an example of a true sine wave inverter:
http://www.powerstream.com/in3s.htm
it claims a 95% efficiency at 1/3 of the power output. Are they serious? Sounds too good to be true. Or is it the efficiency I am looking for (i.e. power lost at the output versus the input)?
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I would be surprised if it is that high, but at that low of a power output, it may be possible.
I would like to see what a 90% output would rate in efficiency. Not better than 50 to 60 percent, I'll bet.
--
Jim in NC



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snipped-for-privacy@gmail.com wrote:

Even driving an audio amplifier fed from a sine wave into a transformer it would be fairly hard to get below 60% efficiency with a class B design.
Mosts switching designs are aroud 90-95% efficient.
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On Sun, 17 Feb 2008 22:57:59 -0800 (PST), snipped-for-privacy@gmail.com wrote:

Not crazy, but probably impractical. To do the math, we need to know how much energy you need to store in the battery. If you specify what voltage you need, we can start calculating.

A 22V 5Ah LiPo battery at weighs less than a tenth of that. Power output of a couple of kilowatts is no problem. You will need a motor and a regulator as well, but they don't weigh much.
Since this is a toy, LiPo may not be safe enough, but NiMH will still be well within your weight limit, and depending on voltage, will most likely be able to provide enough power.

When you include the weight of the generator and the fuel tank, I bet you will come out at a higher weight than electric.

RC combustion engines, like all other combustion engines, are usually rated in horsepower. One kilowatt is about 1.36 horsepower.

If noise is any issue at all, electric is the way to go.
BTW: Most small and medium model engines run on a methanol based fuel. This fuel is much more expensive than gas. You can get gas powered model engines as well, but they are normally larger, normally about 20cc and up.
The combustion engine has only two advantages left, compared to electric power these days:
1: Recharge time. Filling the tank takes a minute or two, while charging the battery takes at least half an hour.
2: Cost. High-performance batteries are quite expensive, and they do wear out.
--
RoRo


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On Sun, 17 Feb 2008 22:57:59 -0800, runcyclexcski wrote:

As mentioned elsewhere, you haven't specified a voltage, and hence don't have enough information to know how much energy is to be stored.
To figure out wattage, multiply horsepower by 700 (it's really 730 some- odd, but 700 will get you well within the ballpark). The gasoline weed- whacker engines that get converted to RC use generate over a horsepower, if I'm not mistaken. Then figure that your generator is going to impose an energy conversion tax of between 5 and 30%, depending on the generator.
You probably _don't_ want a weed-whacker engine that's been converted, though -- an RC engine doesn't need fan and shrouds for cooling, but a stationary generator set certainly does. So what you really want to start with is probably an unconverted engine from a weed-whacker or chain saw.
Figure you'll need some way of transmitting power from the engine to the generator -- a belt is probably a good way, if the side loading doesn't trash the bearings on either the engine or the generator. Then figure out how you're going to gear things so the generator spins fast enough and the motor at a good speed. Finally (heh heh heh) figure out how to regulate the speed of the engine and what comes out of the generator so you can do something useful with it.
You could make this all smaller with an RC glow-plug engine, but you'll find that the fuel will be less available, the engine will be harder (possibly much harder) to start, and you'll have to rig the blower and shrouds to adequately cool the engine.
--
Tim Wescott
Control systems and communications consulting
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Thank you, all, for the great responses.
Ideally, I would want an AC 120V output wave and 1.6 Amps, respectively. But if a sine AC wave is not an option, 24V DC and 8 Amps should would work.
While waiting for the responses, I did some reading in LiPos. 5000 mAh batteries indeed should do the trick. But I am a total dummy in electronics, so safely integrating the battery (and generating a 120V sine wave?) without shorting it and blowing everything up would be a challenge. In addition, I found numerous LiPos blowing up videos on UTube (even with mechanical stress), and all those were 2-5 times less powerful than the 5000 mAh battery that I will likely need. Just how mature is the LiPo technology?
But all around it looks like my generator idea still loses compared to the LiPo idea. So, I am totally for the LiPo idea, as long as I can do it safely.
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You would be money ahead to get a commericial inverter, and if you want 300 watts, get a 500 constant, (and some other higher number, whatever they say) peak watts. They are a square (modified) sine wave, but they do OK for nearly anything you want to run.

As long as you don't physically damage them, and don't push them too hard, in amps draw and in charging rate, and keep them cool, they are pretty darn safe. Violating one of the three above can get you in trouble.

The generator is cooler, though! <g>
--
Jim in NC



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snipped-for-privacy@gmail.com wrote:

Find a video of the explosion and fire you can get from a quart of gasoline, and ask yourself which is safer.
--

Tim Wescott
Wescott Design Services
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snipped-for-privacy@gmail.com wrote:

LIPOS are pretty good, and an be seriesed and parallelled pretty safely
Best deal on high capacity I have found yet is www.maxamps.com
Check them out. They do 10Ah stuff..
as long as you don't run them flat, short them or charge them beyond their rated voltage, ort leave them in te sun too miuch, they are safe. Its really the fact that we perate them without any safety circuits to get the most power out that means that carelessness can cause accidents.
I would say you should investigate a small UPS or caravan type inverter if 120VAC is what you want, and run from 12v (3 or 4 series cells) at about 20Ah
The generator option is amusing, but you will have to DIY it a lot more.
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Thank you guys, you are the best.
Yes, a quart of gasoline blowing up would make a pretty good fireball, too, wouldn't it?
I did not realize that RC airplane engines are not meant to be run long. I guess I would need a toy semi-truck analog, if you know what I mean - low RPMs. But then the size and the eight go up - and I am in the 4-stroke category.
At any rate - looks like the LiPos are the way to go. Instead of do the wiring myself I could indeed get an inverter. I still need to figure out a way to limit the discharge rate. E.g. the 12 Ah LiPo battery I found (btw it's $500) is rated at 20V and 6A max discharge. That gives me 120W, and it's already at max output (safety?). OK, I could put two of these in series, I guess... but I still need to understand how to make a circuit to limit the discharge rate. Would an AC inverter have such a circuit pre-installed?
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| I did not realize that RC airplane engines are not meant to be run | long.
Well, that's just supposition on my part, but certainly getting one to run for hours might be tricky, and I don't know how long it would last.
The analogy would be a 3.5 liter V8 Indy car engine making 650 horsepower -- compare that to a 3.6 liter engine in your truck that puts out 250 hp. But the Indy car engine only needs to last one race ...
I guess I would need a toy semi-truck analog, if you know what I | mean - low RPMs. But then the size and the eight go up - and I am in | the 4-stroke category.
For R/C engines, two and four stroke engines are a little different than the rest of the world.
For R/C engine, 2 and 4 stroke means just that -- 2 and 4 strokes. It has nothing to do with the lubrication system, for example, and most R/C engines, 2 or 4 stroke, have the oil mixed in with the fuel.
It's not like the small engine world, where `four stroke' means not only four strokes but also a closed lubrication system. Most R/C engines will create a slime of oil at the exhaust.
You can get a big 2 stroke R/C engine easily enough, much bigger than 200 watts. 200 watts is perhaps on the small side ...
| I still need to figure out a way to limit the discharge rate.
Your load should do that for you. You certainly don't want to hit any load limiter on an inverter, because then it just shuts off.
| E.g. the 12 Ah LiPo battery I found (btw it's $500) is rated at 20V | and 6A max discharge.
If that's the link you gave, the 6A limit came from the way the cells were put together. Individually, they can do 16A. So you just have to wire them up yourself.
There are other battery options, and you can have places make you a custom pack set up exactly the way you want it. But it'll cost you ...
--
Doug McLaren, snipped-for-privacy@frenzied.us
`Mate, this parrot wouldn't VOOM if you put four million volts through
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Doug McLaren wrote:

If you want relaible running, get a old but good lawn tractor..should be a 10-30hp engine and an alternator in that ;-)
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