Jury-rigged generator needs and expectations?

I’ve come across a decent supply of cheap but running lawn mower engines, most in the 5HP range. If I were to hook up an old car/light truck alternator or 2 to one of these engines, besides some car batteries, what else would I need to make a usable generator? What sort of output could I safely expect to get from such a setup? Realistically, I’d probably go for charging a small battery bank and then drawing off of that as opposed to direct draw from the generator.

Anybody got any other ideas that I could do with 5HP mower engines?

You are on the right track here as opposed to an AC generator system. The nice thing about a DC system is any fluctuation in RPM is tolerated by the self regulating alternator. With an 80 amp alternator you are looking at around 1kW power out put from the alternator max. You could go with a little larger alternator but I would stick to an 80 with the 5 hp engine, this way it will over engineered. However, the size of your battery bank will determine your actual power out put as long as it is not a continuous draw. You will need an AC power inverter of course and this is only limited by your budget. To add stability to the system you can install a supper cap to your system. This will absorb any heavy loading and not draw down on your engine. These small systems do not like instability. The car stereo guys can help you with the super cap.

Other than this use your imagination, I cant wait to see your system when you complete it.

Hey BrianH and MattR great consept to builds out from.
General proven practical guidelines are:
Stick wth 12 volt DC.
Takes about 1 horsepower at the driving shaft for every 20 amps an automotive DC alternator will be able to supply as electrical output. THIS IS factoring in belt/alternator/wiring ineffiencies.
A single auto/lawnmower/industial V belt can transfer MAX with good pulley sizing about 5 horsepower of mechanical energy.
At the very best on one of these single cylinder air cooled engines you are going to only get on wood gas ~70% of it’s maximum rated gasoline horsepower.
So 60 amps max at a nominal 12.5 volts would give you 750 watts to put into the batteries.
These are the real, in the world proven, achievable numbers to expect.
Common affordable flooded lead acid batteries are not 100% efficient storage and release devices. And most 12VDC in, to 115VAC out inverters are in the ~70-85% efficincy range.
So figure for every HOUR of engine woodfueled running time you will have ONE HOUR of 500 watts of useable AC power avaolable to use.
Many have found they can get along fine on one of the new Honda or Genrac 1000 watt gasoline engined generator/inverter systems.
And YOU could set up for this on less than half the price of one of these and do this without the gasoline dependence.
An old sitting around to be junked out rotting motor home would give you ALL of the parts you would need!!

Efficency fanantics go nuts over the different systems energy to heat losses when set up DYI like this.
Ha! Ha! Woodgasing you need to pre-condition/dry your fuels chunks for the easiest gasifier reactor/filtering train management . . .
Air cooled lawnmower engines even with downed RPM ( for even MORE HP loss) are irritating noisy hour after hour . . .
So, woven/expanded hanging metal wall-in/basket enclose the engine/alt and belt drive with flat sided chunked woodfuel drying baskets. Size the baskets for one gasifier hopper recharge. Let experince tell you how to swap these FIVE baskets around as the dry. (Hint: ALWAYS put the newest/coldest/wettest nearest the hottest engine exhaust. Final position at the least hot position. You want to dry, not smoky torrify and brown!) Cut down the sound, and use these “wastes” heats to put back into the whole system overall efficiency.
Woodfuel solar energy IN versus usable electrical energy OUT you will BEAT all of the expensive, custom made, direct drive, PM magnet, propritory custom made digital electronic systems in overall energy and dollar cycles efficiency conversion.
Have somthing you built your self so you will understand and be able to service and repair it. And repair it with free, to cheap take of readilly local available to you with easily INTERCHANABLE capable componets. $20. here to the local car hulk hauler gives you the right for DYI take off of the alternator/voltage regulator/brackets/pulleys/wiring harnesses.

Wooden board mount the engine and alt. Make the fuel baskets hanging frame of wood too. Wood will cut down the sounds and viberation retransmission. Give you a good easy flexibilty platform to redrill and swap out to differnt engine and alternator patterns. Too many or wrong placed holes later just get a new base board. Recycle as fuel wood.
Make this a no-weld build up.

Tips: an automotive alternator can for a time be ran with the shaft vertical installed instead of normal horizontal to match the most common lawn mower engines. How long? Depends on the alternator internal bearing design. Beauty of belt drive system when the alternator does fail and lock up . . . belt slippage noise and the hot rubber smell will tell you. Unattended; the belt will friction heat stretch and just be throw off. Engine run untill out of fuel.
Always have to think ahead and make sure failures DO “fail-safe”.

Steve Unruh

Hi Brian, to add on a few little things to Steve’s near perfect startup. When dealing with a small engine add some sort of counterweight to the lawnmower engine to take the place of the missing blade; makes starting and keeping it running at startup much easier. I used 1.5 inches of plywood in 12 inch disks bolted together and above the drive pulley. Some people would also suggest using a much larger alternator then you need and using a resistor to control the field. You will use more fuel then with a fancy electronic regulator but fuel is not our limiting variable here toughness is. This will allow you to charge a larger bank for longer periods then with a maxed out smaller alternator. The larger bank will also smooth out those ripples you get on really small systems. Also check out this site: http://www.microcogen.info/
they will be your best resource. they can be a little intimidating but are a wealth of info.
Best regards, David Baillie

Here is our proto type system, I have had to stop work on this but I will be back at it very soon. This machine has a 220 amp 12 v alternator and puts out 2.4 kW at 12.5 volts. The engine we are using here is a 7 Hp Duromax engine, but I am going to up this to a higher quality 10 hp engine. Either a Briggs or Kohler and now that I have seen Steve’s great advise I may bump it up even more.

Brian great idea adding in the flywheel, wish I had a lathe. Well I guess that will be added to my wish list.

For starting your machine I would recommend getting a starter/generator like they used on the old cars and farm equipment. Pull starting a wood gas engine gets old after a while. The little starters available on most of these little engines (if equipped) will not hold up to prolonged cranking. Its nice to be able to crank the system while you fine tune the valves to get the right air/fuel mixture. The old John Deere lawns tractors used these you might find one used for cheap or you can get new ones made for newer golf carts for just over a 100 bucks. I got this one and it works great for starting these little motors.


I don’t have the field wire hooked up as I don’t plan on using this for charging. Im sure there is some inefficiencies with this starter and maybe Steve could chime in on this. But I think the inefficiency this has is well worth the convenience and ease of starting.

Our system does have an enclosure and the panels on this system are backed with a sound deadening foam. To further eliminate noise we will use a chip dryer system that the exhaust will be piped into.

as a real barebones starter you can remove the starter cord and recoil mechanism and chances are you will have a bolt there . using a 1/2 drill with a deep socket that fits you now have a starter (you pull up when it catches of course). A word of advice would be to duct tape the socket to the drill and round over the top edge of the bolt to allow a bit of a release if you lift off. That is a painful shot to the shin I will not soon forget… :slight_smile:
That sure is a nice looking setup Matt. At that much power why the 12 volts? That there is some mighty heavy cabling or are the batteries and inverter going close?

You have some great ideas, sort of like how they start the drag bikes :slight_smile:

Yeah the unit has room for 4 on board batteries and the inverter will be mounted under there too.

A fellow I know that rebuilds alternators for a living once told me that it takes 7hp for every 100 amps so that’s what i’ve always worked with… as far as an engine goes… bigger would be better in any respect.

Matt, you don’t have the field wire hooked up on the alternator? or am i missing something

Hi Arvid,

Im referring to the the generator/starter I use for starting the unit. I have the single wire alternator for charging and do not need the charging side of the starter/generator. It just free wheels this way.

lol… gottcha… that makes a ton more sense

Hey Guys I agree that for systems developing and fuelS TYPES useage stretching you really DO want contious cranking electric starting.
Yep. I 100% agree MattR the new little SMALL body $100+ permanet magnet starters installed new now WILL die trying to do this. $100 spent would be much better replacing with one of the continous running capable belted motor-generators you linked to. I used to rebuild these Hitatchi’s. Very stout if the later double ball bearing type. The generator/voltage regulator side of it can be tricky to set up reliably.
Look in my picture album here for pictures I put up of my $20 total cost, push lawn mower handle, mower deck pulley, DC electric chainsaw/winch motor hand cobbed up DYI electric starting system to NEVER, EVERY have to hand crank another fellows 6500 watt B&S gen-set ever again. I was six weeks healing up a separated collar bone previously. This electric motor was NOT continuous operations capable so with NO disengaement clutch drive this was set up as a flip off the belt, and set aside system. Got MY DC 'lectric chainsaw motor back later. Left him with the made up head end nutted on engine pulley recommending now HE buy one of these contious run motor-generators and use that instead. He moved on to 3 cylinder Kubota engines for his woodgasing.

BrianH you can get by with hand cranking if you gasifier/mixer system AND fuel type dial-in and STICK with just that with some sweat and patience. Do get some of the special pull core rope and learn how to replace it. Go as long as you can fit and then golf swing torso pull over. This will give you more woodgas sucking intake strokes. Preferably with someone else doing the pulling to leave you to the fuel systems twiddling. Hard to think clearly when you are huff and puff, out of breath.
Dutch Johns Microgasifier PDF give good guidelines for much of this.

Steve Unruh

Thanks all for the great info.

:EDIT: Never mind, I really can’t afford to spend the total of 130$ (80$ plus gas for the truck) to buy a pile of 8 mowers that aren’t going to do me any good for a while.

For anyone else trying to do something similar, here is a DYI instruction set for a simple set-up: www.theepicenter.com/tow082099.html I am guessing by the address, that this was probably written for August 20th, 1999. I’m not sure how applicable their cost figures are any more.

Brian, interesting link, makes me wonder - if a car altenator is designed for a high cranking amp car battery I’m guessing it puts out around 12 amps, I wonder what the long term effect of charging a deep cell battery this way would have on the battery? Haven’t been around too many deep cell batteries but I know my standard charger I use for charging car batteies has a different setting for deep cells.

i’m pretty sure car alternators can put out more amps that that.

Steve G.: Down near the bottom of the first section they added Testing notes:
The largest load we had available during testing drew 39 Amps with the alternator output of 14.4 volts or about 560 Watts. Testing was conducted with an ambient outside temperature of 82 Degrees. We ran the load for 2 hours and the case temperature of the alternator only reached 148 degrees. Also, up above, Matt Ryder stated that the alternators out of econo cars were in the 60-80A at 12.5V range.

I guess I should re-phrase my thoughts. Could a Deep Cycle battery handle the charging rate from a car altenator the way a car battery does? The reason I ask is - as noted before I have a switch ( which I’ve never use ) on my battery charger that specifies 12 amp 2amp and deep cycle. This must be a different amp charge rate however it does not tell me what it is. However I know a ton of people charging deep cycles in their rv’s from there truck system but is that deep cycle getting the perfect charge it needs?. Just wondering if there is a “sweet spot” amperage that deep cycle batteries prefer to be charged at thats different than car batteries. I know car batteries like to be charged all the time and never run down, where a deep cycle can be run way way down suggesting a lower charge amperage to bring it back up. I’m thinking a deep cycle would not prefer a high amperage charge.

That makes a lot more sense now. That does sound like an issue that should be addressed by a knowledgable party.

Steve, in bigger boats where grabbing the battery and home charging isn’t practical, they charge the deep cycle or “house” batteries along with the starting battery. They use a “smart” regulator to achieve this.

If you are talking about a single battery usually sold for a trolling motor then the alternator could charge too fast for the battery’s well being. If you are talking about a deep cycle bank then the problem becomes the alternator may overheat from trying to top up something much larger then it was designed for. Deep cycle batteries like a 3 stage charge cycle ; bulk, absorbtion, and float for longest life and best performance. They do sell electronic regulators that can control an alternator’s output turning it into a three stage charger If that is the goal. In a jury rigged scenario though you could use the alternator unmodified to bulk charge your bank using a simple resistor on the field.to limit amperage output. Bulk charging rates are usually in the c10 range so 10% of the rated capacity in amps of the bank.
David Baillie