Efficient 12V generator using a leafblower

[maxwell3e10]

I wanted to build a small 12V generator using a leafblower or weedwacker engine. There is a number of videos on youtube about it. But most generators are not very efficient, getting about 10A current or using multi-HP engines.

Here is an efficient generator, it uses a $30 BLDC motor and 1 HP engine to generate over 30A charging current.

[Benta]

And the question is...?

[Marco]

Interesting, but what's the efficiency relative to a real generator?

[maxwell3e10]

I wanted to use a small engine so it can run efficiently at low power. A good point of comparison is Ryobi 900W generator that claims to run for 3 hours at 175 W  output on 1-lb propane bottle.
https://www.ryobitools.com/outdoor/products/details/900-watt-propane-inverter-generator

I am planning to do a similar test with the Lehr engine, see how long it would run on one canister.

[mark03]

Interesting.  What's the application?

[langwadt]

I wanted to use a small engine so it can run efficiently at low power. A good point of comparison is Ryobi 900W generator that claims to run for 3 hours at 175 W  output on 1-lb propane bottle.
https://www.ryobitools.com/outdoor/products/details/900-watt-propane-inverter-generator

I am planning to do a similar test with the Lehr engine, see how long it would run on one canister.

3h*175w ~= 1.9Mj,  1 lb of propane is ~20 Mj

[maxwell3e10]

3h*175w ~= 1.9Mj,  1 lb of propane is ~20 Mj

Small combustion engines have efficiency of about 20% for conversion to mechanical power, or specific fuel consumption of about 400 g/kW*hr for gasoline. Larger combustion engines have efficiencies of around 30%, but only when running at a good fraction of their rated power.
The conversion from mechanical to electrical DC power should in principle be efficient, but when running at low voltage and high current it suffers from resistive and diode rectifier losses. So overall efficiency of 10% is about right. We'll see if using even a smaller engine (25cc vs 40cc in the Ryobi) will improve the efficiency at low power.
Application is small back-up/emergency generator using as little fuel as possible.

[mark03]

Hmmm.  Using this handy table:  https://en.wikipedia.org/wiki/Energy_density  and assuming a battery is 100% efficient vs 10% for the generator, the volumetric energy density (in electrical output terms) of LPG propane is similar to lithium-ion batteries, 25 MJ/L for the propane (before engine and generator losses) and ~ 1-2 MJ/L for the batteries.  And you get to recharge the batteries.  But I suppose the propane is cheaper than the same-size bucket of batteries.

For extended power outages here, I have a pair of deep-cycle marine batteries hooked up to a UPS which keeps my server, wifi, etc. running for days.  Unfortunately, the next priority items are all big power draws which need AC mains:  the refrigerator and the tankless (demand) gas water heater.

[macboy]

And the question is...?

He has no question, and none is needed. This is the "Projects, Designs, and Technical Stuff" forum, and its description on the forum home page is: "Show off your projects or other stuff you are working on. Talk about designs or ideas, ask technical questions, and share technical information. This is the big catch-all thread for anything electronic.".
I for one appreciate when people show off their interesting projects. I find it inspiring.

[maxwell3e10]

For extended power outages here, I have a pair of deep-cycle marine batteries hooked up to a UPS which keeps my server, wifi, etc. running for days.  Unfortunately, the next priority items are all big power draws which need AC mains:  the refrigerator and the tankless (demand) gas water heater.

Having a small generator recharging batteries  and an inverter is a good solution for keeping pulsed loads, like a refrigerator, going for a while. Now with all lights being LED, not that much steady power is needed.

[maxwell3e10]

I measured the running time of the leaf blower generator at about half-load, 15 A and 14 V. It runs on one lb propane canister for 2 hours 46 min. The average power delivered was about 220 W, so a total of 600 Wh or about 2.2 MJ from one canister. It seems to be a little more efficient than the Ryobi inverter generator.

[amyk]

A diesel generator can be more efficient.

[mikerj]

A diesel generator can be more efficient.

Try finding a diesel leaf blower, or any diesel engine of similar size.

[sleemanj]

Never heard of propane powered leaf blowers, or indeed propane powered small engines.  Here in NZ we seem to get leaf blowers in corded, battery, or good old petrol.

[maxwell3e10]

A company called Lehr in US was making propane powered leaf blower, weed wacker and a lawnmower for a few years about 10 years ago. They simply add a pressure regulator and tweak a gasoline-powered engine. In fact, most of the engine they used is the same as Subaru Robin EH025 or Makita MM4 engine. So a gasoline-powered Makita MM4 leaf blower is almost the same and fits the same generator, as shown in the video above.

Probably now no one would make small propane-powered tools anymore as battery operated devices became popular and are more appealing to people who like clean and more environmental tools.

[Zero999]

Indeed any petrol engine can run on a flammable gas. It's even possible to run an engine on wood gas.