Lawnmower + Alternator =

Lawnmower powered alternator

Building a lawnmower engine-powered, automotive alternator generator set.

Exectutive Summary:
Inspired by an article in QST, and my interest in power generation in general, I built the above pictured lawnmower engine-powered alternator genset.  I determined somewhere around halfway through construction that it's not worth it, at least in terms of effort, cost and amount of potential energy possible from this unit.  But once having gone that far, I wanted to complete the unit since I had no portable source of emergency power generation, I had a bunch of money tied up in parts, and following through to completion would also serve to prove unequivocally that it wasn't worth it.  :-)

All Photos Clickable for a Larger View

Donor Mower
GM Alternator
The mower that made the
ultimate sacrifice.
Orphan GM alternator that has sat
idle for nearly 10 years. 

It all started with the acquisition of a defunct lawnmower.  I already had a GM alternator languishing in the garage, an orphan of having turned my S-10 pickup into an electric vehicle.  So I had most of what I needed already, or so I thought.  After it was all said and done I ended up with about $150 tied up in steel, welding supplies, wiring, hardware and a marine battery.  Not to mention the number of hours it took to cut and weld the steel, fit the pieces and complete it to it's finished state.  If that hasn't dissuaded you by now, read on...

Getting from here to there...
With the acquisition of the mower, it first had to be determined why it had been junked.  The compression was good, and it was only about 4 years old.  What could possibly go wrong with a Briggs & Stratton engine that you couldn't fix?  Well, long story short the rubber priming bulb was disintegrating and shedding gooey blobs through the various passages of the injection-molded plastic carburetor.  Plastic carburetor?  Things sure have changed since I last owned a gas lawnmower (my current one is electric...).  No amount of carburetor cleaner or compressed air could dislodge the gooey particles.  A visit to the Briggs & Stratton web page showed the entire replacement carburetor cost $19, including a new primer bulb.  That took care of the mower, after R&R'ing the carb it started right up and ran perfectly. 

12V Pup
NS8O Vert Genset
This is the 12V Pup as outlined in the
June 1999 issue of QST.
This photo of a 12V genset built by NS8O
served as my inspiration to build my own
genset as an 'over-under' rather than

Now, to couple the engine to the alternator.  There are two ways to do this.  One is to use a belt and pulleys per the QST article, the other is to use what is called a spider coupler.  The advantage using pulleys is by changing their diameters, you can multiply the RPM from the engine to the alternator.  Alternators work best around 5000-6000 RPM, way faster than any lawnmower engine turns.  The downside is that you have to mess with belts, belt adjustment, and engine pulleys aren't compatible with the automotive belt style alternator pulley.  If mounted per the QST article, the engine spins the alternator in the wrong direction.  That doesn't affect it's power output, but it does affect the cooling fan.  You can buy bidirectional alternator fans, but that's just one more thing to mess with.  After careful consideration I decided to go with direct drive using a spider coupler, choosing mechanical advantages over output power.
Spider Coupler
Initial Frame
The spider coupler as provided by
The EpiCenter.  The engine side is
standard 7/8" with 3/16" key.  The
alternator side is custom drilled and
tapped to fit the shaft and threads
of GM 10 and 12 series alternators. 
The humble beginnings of
the frame.

I thought it'd be easier to transport and use a genset that was a bit taller than the QST version which is long and flat.  Once the physical configuration was decided upon, I set about fabricating the frame.  I'm not fond of the nut and bolt method used in the QST article so welding was my method of choice.  I had some steel on hand, and decided I would focus on sturdiness before weight.  Well, I got my wish.  I used 1-1/2" square, 1/8" thick stock.  With the dimensions I ended up with this frame could easily support a two-ton automobile, much less a 20lb engine and 8lb alternator.  But it's no more work to use this material, so that's what I ended up with.  Once the engine, alternator and battery placement were determined, it was a relatively basic exercise to cut and weld the pieces together.  The only critical parameter was keeping the motor shaft true to the alternator shaft, but even that is mitigated somewhat by use of the flexible spider coupler.  The coupler itself was already drilled and tapped for a GM alternator shaft.  This made mating the engine and alternator quick and foolproof, compared to having to use pulleys with sheaves,  taper-lock couplers and belts, which require some method of tensioning adjustment.

Completed Frame
Unit Weight
Completed frame after painting.
Color: Chevrolet Orange (since the
alternator came out of a Chevy). 
With gas, oil, and battery the
Lawnernator tipped the scale
at 115lbs! 

How it works
After reviewing GM alternator specifications at various RPM's, as well as lawnmower engine specifications, I estimate the Lawnernator's output to be somewhere around 300-400W @14VDC.  Not enough to do much more than run a few lights and radios by itself, but with the deep cycle battery attached as a load leveler, it is capable of several times that amount during peaks.  Another nice feature of the deep cycle battery is you don't need to run the engine all the time, just enough to keep the battery charged.  Operating the unit in this manner is actually very efficient, since the alternator will be at full load while the engine is running, instead of just partial load were the unit running at less than peak load.  By connecting a 14VDC to 120VAC inverter, your peak power and AC current quality is limited to the what the inverter is capable of, which can be considered additional flexibility over an AC-only genset.

Genset Running
Genset Running
If you make lawnmower sounds, this
photo could be a video.  The evidence
that it's running is the DVM is showing
In a pinch,  the Lawnernator can be
run off of propane.  No special
fittings, just stick the hose in the carb
and fiddle with the gas flow until
it starts and runs under load.

Useful Links:

Lawnmower powered alternator sites and articles:

NS8O's Interesting Generator Pages:
Alternator and Genset Parts Links:

I didn't see it worth the effort to document the frame and details of the parts assemblies, since anyone building such a unit (and not heeding my advice) likely wouldn't have the same components I started out with anyway.  And, it certainly wouldn't be worth it to start out from scratch by buying an engine and alternator, even used.  When you can buy a 120V, 1 to 2kW generators at many discount tool and auto parts stores for less than $250 it doesn't make a lot of sense to spend even half that for a <500W unit that only puts out 14V.  But, it did provide a fun project that can serve a useful purpose should the need for temporary 12V power arise, certainly enough for most Field Day operations.

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