How to Build a Lawnmower Engine Generator | DIY DC Alternator Generator

I have long wanted to learn how to build a lawnmower generator, because $500 and up price tag kept me from purchasing one new.  After some internet searching and sleepless nights I found a good tutorial for a DIY Lawnmower Generator online at http://www.theepicenter.com/tow082099.html.

What I liked best about the tutorial is that Brian at epicenter has already worked out the kinks and sells the materials.  I have no connection with epicenter, and the only things I have bought from them were for this project, but what dealings I have had with them were fair and honest.  He was also kind enough to let me copy some of his wiring diagrams.

How to Build a Lawnmower Generator

Prepare the Lawnmower Engine

A co-worker gave me an older craftsman lawnmower for this project, it did not run, but with a little bartering, it was repaired.  I ordered the plate, an alternator, wiring harness, pulley, and two belts from the epicenter website (approximately $160 with shipping).

While waiting on the parts, I laid the mower on its side in the bed of my truck (being careful to keep the gas tank side up) and wedged a 2×4 between the truck bed and the mower blade to keep it from spinning while I unscrewed the blade.  I did use some kerosene to loosen the nut (liquid wrench is almost pure kerosene).

I then drove to the local Auto Zone to use their loan a tool program so I could get a pulley puller.  The blade mount pulled off the shaft very easily, so I returned the puller and went home.  I originally tried using a very cheap 5 dollar china made puller but it broke almost immediately.On the way home I stopped by the hardware store to purchase longer bolts and spacers to not only attach the engine to the generator, but also to align the pulley on the mower to the one on the alternator.  It took me a while to find the right bolt/nut/washer combo, but I got 5 bolts, 10 washers, 1 lock nut, and 4 1 inch spacers for about $15.

Build a Frame

Once the plate arrived, I measured it and built a frame of scrap 2×4.  The plate measured 12×24 so I cut two 2 ft lengths of 2×4 and 2 9 inch lengths.  After screwing them together, it made a perfect frame.  Just because I had some, I painted the box with the leftover green paint from the chicken tractor project.

As soon as the paint dried I screwed the plate to the box and attached the alternator.  It fits just like an alternator in a car.  One screw fits in a whole in the plate, while a bolt fits in an adjustment slot in the plate and through the alternator the locks with a nut.  To tighten the belt you loosen the bolt and move the alternator in the adjustment slot.

Add an Pulley and an Alternator

Next I installed the pulley on the shaft.  I set in a 3/16 key-way in the shaft and pushed the pulley onto the shaft.  I used a dead blow hammer to knock it flush.  Be careful and think about what your doing, one side of the pulley has set screws so its longer, and I put that end toward the engine, so once I installed the alternator and the belt, it did not line up, forcing me back to the Auto Zone to get the puller to remove the pulley and put it on the opposite direction.

Once the pulley was installed and tightened, and I installed the engine.  The cut outs for mounting the engine were larger than my bolt heads, so I sandwiched the plate between two large fender washers, the spacers fit on the bolts on the top of the plate raising the entire engine over the frame.  This is because the shaft it much longer than the alternator shaft.

Wire the Alternator

Depending on the type of alternator you use and how it’s regulated, there are different ways of connecting everything.  I used an external switch and my alternator has an internal voltage regulator, so my wiring looks like the first schematic below.

To keep things easy I paid an extra couple bucks for the wiring adapter for the alternator.  You don’t have to use epicenter’s alternator or their harness, but since I would have to either buy a new one or go to the junk yard and remove one on my own I kept it simple and bought theirs.

I used their adapter, some 14 gauge red and white wire, a 50 amp switch, some connectors, and heat shrink to rig up a wiring harness that snaps in to the alternator.  I probably should have used full battery cables to hook the alternator to the battery, but the run is short, the amps are low, and I will be next to the generator while it is running so I can monitor it if it gets hot.

Modify Throttle Cables and Safeties as Necessary

Most lawnmowers come with a safety device that you must hold in order to keep the engine running.  Mine was on the handle of the lawnmower.  I looked at it and decided to keep it functional rather than safety wire it closed.  What I did was wire a washer to the linkage which allows me to pull it tight and loop it over the linkage bracket.  That way if need to stop the engine quickly, I can just pull the washer off the bracket.

In order to use the generator, you must have a battery; this is because the voltage regulator needs to be energized to function.  This generator is really just a souped up battery charger as the alternator’s voltage regulator puts out the exact right voltage for charging car batteries (Imagine that….).

Some other things to consider are that because lawnmowers use light flywheels, they depend on the mass of the blade to idle correctly.  So when choosing a pulley make sure you get a cast iron pulley with a little mass to it.

Adapt as Necessary

You do not have to use store bought parts if you have parts at hand.  I could have gotten by with using a piece of plywood as a base.  If I had drilled a hole for the shaft to sit, I could have used the engine as a template to mark where to drill my mounting holes.  This is a project for using your mind instead of your money to come up with a solution to a problem.  I used more money than needed so I could spend less time considering solutions to problems of mismatched parts.  Lastly, don’t scrimp on the belt quality, and buy more than one.  If you are relying on a generator you made from your dead car then it’s a really BAD day, and you probably aren’t in a position to go to the auto parts store…

I really like this project, it Is one of my favorites I have attempted this far.  Since I cannot leave well enough alone, I plan on taking it a couple steps farther.  The first major upgrade is I plan on making a little switch board to mount the throttle assembly and switch a little better.  Next I plan on converting the carburetor to run on LPG gas from bottles which will make the logistics of fuel storage safer while allowing me longer run times and faster refuels.  I also plan on making a second lawnmower alternator combo, which I modify to make a welder.  This is something 4X4 enthusiasts have done for years.