Field Expedient Engineering

This new page will be a home for widgets, gadgets, and other devices, made from as much reclaimed or repurposed hardware as possible. This first installment is about how you can build your own gas powered generator.

This design is capable of powering DC equipment directly, as well as AC equipment with the use of an inverter. With a sufficiently sized inverter, you could use it to keep your refrigerator running during a power outage, heat food in a microwave, or run a small window mounted air conditioner. Oh, and don’t forget lights.. It is equally useful at home during a power failure as it is for off grid RV camping or to power a small cabin.

There are four basic components to this system, an engine, an alternator, a battery, and an inverter.

The engine can be anything of reasonable size with at least three horsepower. While you could use a small diesel fueled by waste vegetable oil or even a steam engine that could burn biomass, in this how to we will focus on gasoline engines. Small four stroke engines, as commonly found on lawn mowers, rototillers, and pressure washers, are perfect for this project. Look for a 3 to 7 horsepower rating, any bigger is overkill and will burn more fuel. A used mower from a garage sale or craigslist is probably the best bang for your buck. I took a slightly more expensive approach and used a 6.5 horsepower Honda clone purchased new at Harbor Freight Tools for $90.

The alternator is a standard car alternator. They can be had for about $20 from pick-a-part type wreckers. Delco units, such as the 10si and 12si commonly found in GMC vehicles, are great for this application because they have the voltage regulator built in. An alternator actually produces 3-phase AC at voltages many times higher than the battery, which is regulated and rectified to battery charging voltage. If you use an alternator without an internal regulator, you must use an inline regulator or you will damage your system, possibly even causing a battery to rupture.

Batteries, batteries. For something that seems so simple, they aren’t. Look for a separate battery article soon. For this project, pretty much any 12v car battery will work if you intend to keep the engine on whenever you want power. However, if you want power for small things 24 hours a day, get better batteries. Group 29 or 31 marine batteries are much better than car batteries, and cost under $100 at Walmart. Two six-volt golf cart batteries wired in series is even better. If you are using this in an RV, you can connect it directly to your house batteries. For a more portable stand alone unit, you can mount the batteries and inverter right by the engine and alternator. If you take this approach, consider using some kind of platform with wheels. It will be heavy! Now, what happened to that lawnmower chassis..

To power normal household appliances, you need an inverter. Inverters come in a wide variety of sizes. A 150 watt inverter is big enough to charge most electronics or run a laptop. A home refrigerator usually takes 300-400 watts to run, but has a large spike of over 1000 watts to start. To run a fridge during a power failure, select an inverter in the neighborhood of 1500 watts. A 2300 watt inverter can power several power tools at the same time, a small AC welder, or an air compressor. Keep in mind, the larger inverters can discharge the battery faster than the alternator can charge it. This is fine for short duration loads, such as 1200 watts for 3 minutes for a microwave. Hook up a 1000 watt halogen work light, and the generator won’t keep up. If you have those kind of power demands, you probably already own a commercial generator. If you really want to use this design for continuous higher power applications, a 6.5-7 hp engine can run two alternators, wired in parallel for twice the output. Always use heavy gauge wire for inverters, and keep DC wire runs as short as possible.

As far as the actual assembly goes, get creative. A pulley around 4 inches on the engine seems ideal, with the pulley on the alternator that is used in the car. All it takes is a frame or platform sturdy enough to hold all of the components, and some device for tensioning the belt. I bolted my alternator and horizontal shaft engine to a wooden disc that used to be part of a large spool. A drilled piece of steel bar stock from the engine to the alternator tensions the belt. If you use a lawnmower engine, you could make something up with steel plate and the mower frame. 2×8′s or similar are an option for horizontal shaft engines. The whole thing could be mounted on a furniture mover or other sturdy cart, or even the salvaged lower frame of a shopping cart. Just make sure the materials will withstand the heat and vibration of the engine, and the weight of the entire system.

Once you have a frame which holds the engine and alternator pulleys in alignment, mount the pulleys, install the belt, and tension it, It’s time for wiring. Delco alternators have either a one wire or three wire arrangment. In both cases, the body of the alternator is ground, this goes to the negative (-) terminal on your battery. The treaded post on the back, marked “BAT”, goes to the positive terminal of your battery. Use good heavy wire for this, 4 gauge is preferable. (Tip for scroungers- if you have acquired a large quantity of lighter wire, you could double or triple up each run instead of buying new stuff.) If you use a one wire alternator, that’s all for the wiring. If you have a three wire unit, there will be two more terminals on the back. One is to excite the field, the other is a remote voltage sense input. Connect the field excite terminal to a switch, a 12v bulb, and then to the positive terminal of the battery, all in a line. Turn the switch on to run, and off again after shutting down the engine. If for some reason the alternator is not producing enough power, the light will come on. Connect the second wire, the sense input, directly to the positive battery terminal. These connections can be done with much lighter wire, 16 gauge is sufficient. You now have the alternator wired to the battery. If all you want to power are 12v DC car type items, you can wire up some cigarette lighter style plugs and call it a day. If you want 120v 60hz household style power, get ready to install an inverter. This is just as simple as wiring the one-wire alternator. Select an inverter of appropriate size, and wire it to your battery with heavy wire. Auto parts stores and retailers like Walmart sell battery cables that already have a connector for post-style batteries on one end and the other end ready to connect to the bolt style input on larger inverters. Smaller inverters may come with a cigarette lighter type connector which can be used or eliminated. Don’t use them on inverters over 120 watts, or electrical losses and melted connectors are likely. Follow the guidelines in the inverter’s manual for wire sizing. For a larger inverter, 1500 watts plus, you may need to buy 0 or 00 gauge wire and crimp ends from an auto parts store to assemble your own high current wiring. Once the system is assembled and the inverter is hooked up, its time for a test. Start the engine and inspect the belt for tension. If you have a multimeter, you can measure the voltage at the battery. If you don’t, its a worthwhile tool to add to your kit. Basic ones cost as little as $5 and are indispensable if you are doing any work with electricity. At this point, you should be turning gasoline into mechanical energy, and that into electrical energy. Switch on your inverter, plug in your fridge, and have a beer. They’re going to stay cold during this power outage…

Other hints and tips:

This concept is absolutely scale-able. In an emergency, even a wrecked car with a running engine could be used to run as many alternators as you could scrounge up and belt on.

Engine pulleys will likely be the most difficult component to locate. In an emergency scenario, you could make one from plywood and coat it in epoxy. Think hand drill turned crude lathe. Just don’t expect it to last as long as a metal one.

This generator, because it uses batteries, is more efficient than a contractor style generator for running small loads 24/7. However, if you need to power large AC loads for extended periods, a conventional generator will serve you better.

Stay on top of maintenance! Small engines are usually used periodically, not continuously. If you can’t make it to the auto parts store because of zombies, an oil change of cleanish looking crankcase oil from a car every 100 hours is better than leaving the same stuff in there. Don’t forget to check spark plugs and air filters. Monitor your engine hours! A cheap stopwatch and pen/paper tied to the rig is a great idea for extended use.

Small utility engines tolerate low quality or old gasoline quite well, but storing them without a fuel stabilizer like sta-bil is asking for trouble. Gasoline tends to gunk up the tiny passages inside carburetors if left alone. If you have a generator of any sort, haul it out every month or so and fire it up to blow out the proverbial cobwebs.

Remember, this is field expedient engineering. If you have $$$$’s to buy a Honda or Yamaha inverter generator, they will give you more electricity with less fuel and higher reliability. However, if you don’t own a generator and need power in a pinch, you could scavenge most of these parts from a typical suburban household. With a dead car and running lawnmower as donors, you could probably build this generator, DC lighting, a radio, fan, evaporative cooler, and more.

Thanks for reading and check back here for more expediently engineered solutions, handy whether you are about to go camping, want to stay as comfortable as possible through a natural or man made disaster, and especially if you’ve fought off enough zombies to read this site.







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