Poor Man's Tri-Fuel Generator Conversion (repost)

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Here are the details of what I did to convert a 3 1/2 HP gasoline genset to also run on propane and natural gas.

I have a 1971 vintage 1000 watt generator with a frozen engine that my Dad gave me. I went to my local lawnmower repair shop and bought a good used 3 1/2 HP Briggs & Stratton engine for $50. I was lucky to find an exact replacement for the original engine.

This generator will serve as a backup for my Generac 4000XL. I plan to use the 1000 watt generator to run a custom 24 Vdc battery charger capable of 42 amps output to charge my 220 amp-hour 24 Vdc battery bank.

Like many of you, I got propane/NG conversion kit for my main genny, the General 4000XL. For this smaller genny, I decided to fabricate a propane conversion on my own, here are the results.

I have several 20 lb propane tanks, various regulators and such. After getting the 3 1/2 HP Briggs & Stratton engine running correctly on gas, I placed the output hose of a propane tank regulator direcly into the air cleaner input of the engine. This hose has a shutoff valve, which also allows me to vary the amount of propane delivered.

After some experimentation, I was able to regulate the right amount of propane to start the engine, then increase the amount to run it at various power levels. Once I discovered that it wasn't all that difficult to get the engine to run on propane, I drilled a mounting hole in the air cleaner and permanently mounted the propane hose to it, keeping its secondary shutoff valve right next to the air cleaner where it's needed. I ran my generator for over 5 hours with this setup and noticed that it ran cooler than on gasoline, and the exhaust was much, much cleaner.

Next, I wanted to see how things worked on natural gas, which is piped into my home for heating and cooking. I ran a temporary tap from the gas inlet to my hot water heater, outside to the generator. I used a similar approach to direct it into the engine's air cleaner as I did with the propane setup. The damned thing worked great! I had to direct a higher volume of natural gas than I did for the propane gas. It ran a bit cooler than it did on propane due to natural gas having less energy per molocule than propane, as does propane have less than gasoline, as does gasoline have than diesel fuel.

I have seen reports that you loose 10% rated power on an engine going from gasoline to propane and another 10% going from propane to natural gas. Here are the specs from my application:

Gasoline; got 1100 watts total output, 28 oz p/hr usage @ 500 watt load. Propane; got 1000 watts total output, Unable to measure. Natural Gas: got 800 watts total output, 25 cubic feet p/hr usage @ 750 watt load.

My setup for running on natural gas needs some work due to the lack of a good means to connect the hose that I had into the air cleaner and I expect that I could get a little bit better results with a little more effort. I'll be working on improving the method of connecting and regulating both the propane and natural gas lines to my air cleaner.

Anyone who's read this far ought to be asking what I did to cut off the gasoline supply from the engine's gas tank when running on propane or natural gas. I simply closed the air-fuel mixture needle valve on the carburetor. I know that for gasoline operation I have to open it up 2 1/4 turns because that is what "this" engine wants. It is a constant that I discovered while tuning the engine for gasoline usage.

There is an inherent problem with my tri-fuel method. Things work just fine if the engine operates on a fairly constant load. I manually metered the amount of the gas(s) for 300, 500, 750 watts and so on. Each significant change in load does require an adjustment of the amount of gas delivered. Obviously using gasoline presents no such problems because the engine's carburetor is made to deliver the right amount of gasoline through the engine's load range.

A commercially made propane/natural gas conversion kit is much more sophisticated than what I have done and would take care of this problem. As I said, this generator is to be used to charge up my battery bank, and thus, I can depend on a fairly steady load so this arrangement works just fine for my application.

This setup is not perfect as there is no provision for an auto gas shutoff in case the engine stalls, etc. It is strictly a manual operation that would require constant monitoring on your part....but it does work and works well.

-- Buffalo Bob (buffalo_bob42@hotmail.com), December 07, 1999

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