Gasoline vs LP Fueled Generator

[Chris Olson]

We recently replaced our Generac EcoGen with a Honda EM4000SX standby generator. The new Honda has an electronically controlled engine and Honda's iAVR system that allows the generator to put out 25% over rated power for up to 10 seconds while still maintaining voltage and frequency +/- 1%.

I'm always interested in fuel consumption of various generators and fuel cost/kWh. So I did a fuel consumption test on the new generator. Environmentally conscious folks might be interested in the numbers I got, as well as folks trying to decide if they should buy an LP fueled generator for off-grid standby power.

First the EcoGen - It had an energy input of 109,600 BTU/hr, or approximately 1.3 gal/hr @ 5.5 kWh output at our elevation. That yields an overall efficiency of 17.1%. The cost of LPG around these parts is $1.90/gallon (current pricing), giving a fuel cost/kWh of 44.9 cents.

I did the fuel consumption test on the Honda by powering some 240 volt water heater elements in parallel @ 120 volt, which drew 31.6 amps. I selected the water heater elements as the load because they remain at a constant draw and give more accurate numbers. The Honda requires 50,980 BTU/hr input, which is .44 gal/hr @ a continuous output of 3.8 kW. That yields an overall efficiency of 25.8%. The cost of regular unleaded gasoline, once I get the road tax back on it at the end of the year on our income taxes, is 3.64/gallon (present prices), giving a fuel cost/kWh of 42.1 cents.

The EcoGen is touted by Generac as being "eco friendly" with "clean burning" LP gas. Well, this turns out to be bogus. One gallon of LP gas produces 12.669 lbs of CO2 so the EcoGen emits 2.99 lbs of CO2/kWh output.

One gallon of gasoline produces 20.350 lbs of CO2/gallon. So the gasoline fueled Honda emits 2.36 lbs of CO2/kWh - 79% of the CO2 emission of the EcoGen.

There were a number of different considerations that caused me to go with gasoline instead of diesel. The main one being cold weather starting - a diesel simply requires too much energy input to start at -30F. We also downsized our generator. Our inverters have the capability to share loads with the generator, so for auto Load Start Amps we don't need a large capacity genset. The generator is required to deliver only what the inverters can't supply to meet heavy loads. I had trouble keeping the 6 kW EcoGen at full load where it runs the most efficient, and that made the cost/kWh even worse than the full load numbers described above.

The Honda is not quite as efficient as good diesel gensets, but with Honda's new iGX engine it comes really darn close. And finally, since ULSD was mandated the cost/BTU of gasoline is actually less than diesel fuel when the road tax is refunded. The bad part about gasoline is that we have to pay the road tax up front when we purchase the fuel, and don't get it back until we file our income taxes at the end of the year. But the main thing is that we do get it back, and don't have to the pay the road tax for off-road use.

I put some photos of our generator project on my web albums:
Picasa Web Albums - Chris Olson
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Chris

[Rob Beckers]

Chris, nice generator!
I've always thought the main reason the off-grid people avoided gasoline is because it doesn't store well. It starts to change (octane number going down and other nastiness) as soon as it leaves the refinery. I use fuel conditioner for the little gasoline I use for the forklift and mower, but even so they say to use it within 6 months. Do you have a high enough turnover of gasoline that this time limit is not a problem?

-RoB-

[Chris Olson]

Yes, I'm putting Sta-bil in it so it stays fresh. They claim Sta-bil will keep it fresh up to a year. The most I'll keep on hand is what it is the generator and a 5 gallon can.

The generator will probably go all summer on two tanks of gas. But in the winter months I suspect it will be getting fueled once a week. So I think it will be fine. We're not "preppers" here, or anything like that, so I don't really see the sense to storing a two-year supply of fuel.
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Chris

[Ralph Day]

Hi Chris
Nice setup. Did you put in an exhaust fan that will draw out any gasoline fumes before the genset fires up? Just a thought that if you had a leak, a shed full of fumes and then a startup, the possibility of explosion is there.

If the genset comes on automatically, not just by remote, having a fan invoked before the gen start cycle could save trouble.

Ral;h

[Peter Klaassen]

Hi Chris,
I am not sure about your area but there are probably mice that would also enjoy the cosy shelter you've built for your generator. We have a Yamaha generator that became a home for mice. It was in the corner of our wood shed and the mice moved into the heat shielding space around the exhaust for a time during the summer until the generator was started. It had been idle over the sunny summer days. Luckily there was no damage, but just FYI, a metal screen on the bottom might be a good idea. Your research into efficiency is good to know.
Pete

[Chris Olson]

We haven't had a lot of problems with rodents in our other generators because the cats keep track of the critters. We did have one striped gopher awhile back and one of the cats staked out its hole for a half day until the gopher came out and the cat caught it.

On the fumes issue I'm undecided. I've never seen a purge fan on any of the enclosed diesel, LP/Natural Gas or gasoline gensets in RV's and so on. The Honda has a non-vented fuel cap with an evaporative control system on it that captures fumes off the tank and burns them in the engine, so no fumes even come off the tank. The generator actually runs cooler in the enclosure than it does standing in free air because it has a constant airflow over the entire unit.

The bigger issue will be keeping it warm enough in cold weather for the air cooled engine. I installed a DuctStat in the enclosure to turn the ventilation fan off if the temp inside the enclosure drops below 70 degrees F while the generator is operating. If it gets too much cold air, air cooled engines tend to over-cool which reduces their thermal efficiency. Air-cooled engines like to run hot with the cylinder head temp at 300-350 degrees - almost double the temp that liquid-cooled engines run at.

I'm also undecided about what to use for pre-heat. I was going to put a silicon pad heater on the engine block right under the cylinder for cold weather starting. But examination of the block revealed that there's no suitable place to put one. So my plan B is to use a 125 watt heat lamp in there that comes on at around zero F, controlled by a thermostat and that shines near the base of the engine. Those infrared heat lamps heat objects really good and air-cooled engines respond to external heating really well compared to liquid-cooled designs because it takes a lot of energy to heat water. Heat lamps are also pretty safe compared to liquid-fueled heating methods. So I think that's what I'm going to use.

A heat lamp could use up to 2.5 kWh/day to keep the engine warm in really cold weather. But the primary use of our generator is for load start and not battery charging. So it has to be able to start within 6 seconds of when a heavy load comes on, gets a 15 second warmup and then goes to full rated load to reduce the load on the inverters before they kick out due to overload. That requires keeping the engine at a reasonable starting temp. And air-cooled gasoline engines are famous for being able to take that kind of abuse, which would kill a diesel powered unit in short order. But it will still require some form of pre-heat in extreme cold so the engine responds right away.
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Chris

[Chris Olson]

I think I'm going to make one final enhancement to my new generator installation - an automatic auto-throttle controller. When the generator starts it needs to come up to rated speed right away so the inverters can sync with it, and it has proper freq and voltage. But for load start it is typically running at full rated load, the load drops off so the inverter is done with it, and it just disconnects it and runs it for 2 minutes for cool-down, then shuts it off.

I would like the engine to idle at 2,200 rpm for the two minute cool down so it doesn't flood the cylinder when the spark is suddenly shut off running at rated speed.

I think I came up with a way - I installed a volt meter by the gen controller so I can monitor the voltage of the starting battery for the generator. I notice when it starts that the voltage is initially below 13.0 volts. But after it runs for 5 minutes or so the voltage gets up to 13.8 volts. The auto-throttle switch is just a simple SPST on/off. It is open for auto-throttle off and closed for auto-throttle on. A little ice cube relay can handle that part. I found an old Coleman air dump load controller board in a box of junk I had. I am going to set that up to monitor the genset battery voltage, and when it gets to 13.2 volts activate the "dump", which will engage the little ice cube relay and enable the auto-throttle.

Just another use for some old stuff I had laying around :)
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Chris

[Ralph Day]

So Chris, is this going to be a new protocol in the "how to run the place without Chris around" manual? If I ever get around to compiling mine it will be the size of an Outback manual. Yours will probably be the size of the old Encyclopedia Britannica!:eek:

Ralph

[Rob Beckers]

Not to hijack this thread, you're the two guys that probably have an answer to this one: Where can I buy a decent hydrometer to measure SG of battery acid? None of my suppliers lists these things any more these days, and I don't like selling expensive deep-cycle batteries to customers without selling them a hydrometer at the same time (and teach them how to take care of their expensive batteries so they actually last the expected years), especially since those things are cheap anyway...

-RoB-

[Chris Olson]

Ralph, I got that automatic auto-throttle control working very nicely. I made a movie that shows how it works.

Honda Generator Auto-Throttle Control - YouTube

Yes, our new system is pretty much automatic, and I've gone to great pains to make it that way. I got better things to do than monitor a RE power system all the time. We have our system to where we can basically turn on anything we want, anytime we want, and don't have to worry about scheduling loads or whether or not we have the power to run it. And my wife pushes it right to the max - she'll turn on her 240 volt induction range to cook something, then run down in the basement to the laundry room and pull wet clothes out of the washer and put them in the dryer (240 volt electric), then throw another load in the washer. The inverters can't run the range and clothes dryer by them selves for very long. But with the Load Start set properly so the generator comes online and helps out, then the combination of the inverters and generator can run it all.

Xantrex still makes the only inverter that can do that. Neither Outback nor Magnum has one with generator load support yet. Reportedly, Magnum Energy is coming out with one called the MSH4024RE - it's their version of the legendary Trace/Xantrex SW4024.

But it makes it very nice for us - especially my wife. We spent too many years waiting until the sun was shining and wind blowing to even wash a load of clothes.

RoB -
This is the one I got:
Amazon.com: OTC 4619 Professional Battery Hydrometer: Automotive

It is very accurate, and has a mercury thermometer in it for doing the temperature correction.
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Chris

[Ralph Day]

I'm sure I have bought mine from:

http://ontariobattery.com/

but I can't see hydrometers listed. Just call them and ask. Mine has little glass nipples on the float. These contact the tube sides with little surface area and hence less friction for truer readings. About $50. Mind you, I've checked them against Candian Tire ones and the readings are always the same (when you get the cheep's float to float properly).

Ralph

[Jerry Smith]

I use a propane powered welder/generator when I work or weld in food plants. I like them, here are my reasons-

1. If you spill gasoline you have a mess and an odor that lingers, If you leak propane the smell disperses pretty quickly.
2. Your oil and spark plug will last longer with propane.
3. Since propane is under pressure, you shouldnt have much trouble starting the engine in different weather or altitude changes.
4. propane gives off Carbon Dioxide and gasoline gives off Carbon Monoxide.
5.Propane engine isnt as efficient as Gasoline, not much of a difference but over time it will be a big difference.
There are a few other differences, but dont get me wrong, A Gasoline powered Honda generator will last a long time with regular maintenance.

[Chris Olson]

3. Since propane is under pressure, you shouldnt have much trouble starting the engine in different weather or altitude changes.

A couple corrections. LPG is only under pressure at tropical temperatures. At 40 below zero like it gets here there is no pressure in the tank and you can pull the valve out of the tank and look in there and the stuff just sits there and doesn't do anything. Spent way too many hours building charcoal fire under the LP tank in cold weather to try to get enough pressure to even get the engine to cough on it.


4. propane gives off Carbon Dioxide and gasoline gives off Carbon Monoxide.


That is a common misconception. They both emit CO and CO2. LPG fueled engines emit less CO than gasoline engines, but they are not as low on CO emissions as a diesel is.