Generator from a gas engine?

[DigiTan]

My parents recently junked on of those garden weed eaters with the small rotary motor, and I thought it would interesting to build it into an emergency generator or even a energy source for a high-powered bot.

I heard this is rarely as simple as coupling a motor to the driveshaft, so I'm doing as much pre-research as possible. Are there any good online guides or schematics for these types of homebrew generators? I'm thinking of DC output if it's for a robot/UGV, and AC if I want a generator.

In the spirit of K.I.S.S., I'm Keeping It Simple for now...

 

[williB]

if you want a simple way to do it , yes you can just connect a DC motor to the weed eater output..

 

[DigiTan]

That's actually sort of the level I'm going for, but I want to know how it would adjust to load changes and things like that.

 

[Nigel Goodwin]

That's actually sort of the level I'm going for, but I want to know how it would adjust to load changes and things like that.

It wouldn't! - you need a regulator on it, which is a mechanical device that increases the throttle as the load increases. These commonly work off the pressure of the exhaust gases.

Presumably you could design an electronic version as well, to try and keep the revs constant?.

 

[audioguru]

Those thingy's are so noisy that you don't need an electric motor as a generator. Just mount a small speaker on it and use the speaker as a high-output microphone! :lol:

 

[stevez]

I'd check to see what RPM and HP you can expect then see if that matches the alternator or generator. A concern that I have is the longevity. Most of those motors are fairly lightweight and the normal operation is intermittent so engine cooling is simpler. In service powering a generator or alternator the load requirements might be more demanding. I know some have taken lawnmower engines and coupled automotive alternators as portable power sources - with some success. The flexible shaft would seem to be an advantage in facilitating the coupling of the motor.

 

[moody07747]

if you want a simple way to do it , yes you can just connect a DC motor to the weed eater output..

yea but it will be hard to start it and keep it going

weed eater engines run at really high RPM but they have little torqu

i tried to put one on my bike once but it would not even push me...the engine would just stall

 

[williB]

you could start it with the motor itself , and once it starts , just let it charge the battery..

 

[Oznog]

You could use it with a battery and inverter, or to charge the battery on a UPS.

A car alternator can do that job, some are quite strong. They do need to be run fairly fast so it might need a belt drive with a step-up.

 

[DigiTan]

Okay, I'll look into these regulators some more. I'm betting on measuring the load changes electrically to maybe get the quickest response time. That might also give me a chance to integrate the output power and maybe have some kind of "time remaining" display. I'm sure I'll probably gear the RPM down with a belt or something.

This talk about alternators got me thinking...do riding mowers have alternators built-in? Many of them have headlights.

Finally, say I wanted to start the engine electrically. Could the alternator do this as long as I isolate it from the load during start-up? I can get relays and motor-start capacitors sort of cheap.

 

[williB]

What i would do is get a DC motor , hook it right to the shaft output of the weed eater..
connect the Plus and minus to a battery through a switch .
throw the switch , start the weed eater , give it gas and weed eater is now charging your battery , i suppose a diode or two would help when you want to charge the battery , without the motor drawing power from the battery..EDIT:: when the weed eater slows down..
but that is all you need ..

 

[Russlk]

The automotive alternator is an AC machine with diode rectifiers, so it will not run as a motor. The old fashion automotive generator, on the other hand, will run as a motor. All you would have to do is connect the battery, the motor starts and begins charging the battery. You might use a flexable coupling, available at the hardware store, or belt drive by putting a pully on the weed whacker. You would need to experiment with pully ratios.

The riding lawnmower does have a battery charging generator, but it won't run as a motor and has low power output.

 

[Dean Huster]

I doubt that the typical (especially the el cheapo) line trimmer engine is capable of turning an alternator/generator with no load, let alone having any kind of load at all and driving the alternator at speed. When you consider how the engine drags just whacking weeds with flexible line .... well ..... ever notice how a line trimmer engine looks just like a model airplane engine -- just a little bit bigger?

Dean

 

[DigiTan]

Alright. It sounds like --if I'm to get anywhere--I need to be in search of a DC motor (maybe with a low holding torque), the belts/gearing, and probably an an motor to work as a regulator. If the engine is really this weak, about much electrical power do you guys think I could draw before it stalls. I think this is an 8 HP model. For now, I'd be satisfied 1 or 2KW.

Also what size motor should I look for? I can get deer feeder or treadmill motors pretty readily. Are these overkill or not enough?

 

[Russlk]

8 horspower weed whacker? No way, 1/8 HP maybe. It would probably generate 100 watts.

 

[bryan1]

Hi Digitan,
The weed-eater motor will more than likely be a small 2 stroke motor and as Moody said it will be reving at 7,000 to 10,000 rpm unloaded. To expect any torque out of the motor will be a waste of time, you might be better going for a motor off a conventional lawn mower. I did a small project a couples of months ago using an aircon pump off a ford motor vehicle set up as an aircompressor. The lawn mower engine powers the pump fine and I can get around 90 psi back pressure in the line. Now if you hook up a car alternator to the motor make sure you gear it up to around 3,000 rpm mininum and you should get a reasonable output. There's plenty of info around on this so a search should provide some more infomation.

Cheers Bryan

 

[Nigel Goodwin]

You mentioned HP (Horse Power), not BHP (Brake Horse Power), generally 1Hp is roughly 125cc of engine, your 'weed wacker' is probably only 25cc (0.2 HP).

I noticed there was some criticism of two stroke power!, a two stroke engine puts out considerably MORE power than a similar sized four stroke engine - simply because it fires twice as often, and uses more fuel (and also happens to run far more slowly).

 

[williB]

I found a real nice 24V -2.5 A DC motor in an old copy machine, which turns real easy , no load..
it would be perfect for your weed wacker..

 

[biac]

generator

I'm also making a small gas powered unit,I am using a 12v dc starter/generator from a golf cart. turned by a 1.5 hp 4cycle honda tiller engine.

New these cost about $150 -250 ,But you if you can find a used one that mabey just needs brushes,these are a sweet item as they can crankup your gas engine,then switch to charge mode.

I am making this for my friends rv,which has its own inverter 12vdc - 120vac,it's just to keep the deep cycle battery full.

 

[williB]

see .. see...
that's the way to do it..
biac you gotta give more details of your project..