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Old 03-08-2008, 08:37 PM   #1
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runs on propane?

so what is the deal with running an engine on propane i'm interested but have many conflicting thins. can it only run on gas engines. what about my diesel, i heard it can be used supplementary on hills and such, like nitro for a gasser? are there buses that run on just propane and or another gas?
thanks
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Old 03-08-2008, 10:20 PM   #2
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Re: runs on propane?

Ah propane...

When you hear about people with propane powered buses they are referring to spark ignition ("gas") engines. There are advantages and disadvantages to the fuel.

Advantages:

1. Occasional tax benefits
2. Single tank for heat, stove, engine, etc
3. Clean burning
4. High (~105) octane
5. Less moving parts than a carb
6. No varnishing


Disadvantages:

1. Fuel availability (much harder to find at midnight than gas)
2. Cold starting performance can suffer in severe conditions
3. Fewer BTUs per gallon
4. Typically buses have less power on propane. This is an engine design issue. More on that later.
5. Fuel costs are typically higher nowadays whereas it used to be a benefit
6. Though it performs better than a carb, propane will never surpass fuel injection.

Most buses you find with propane were initially built to be powered by gas. This means relatively low compression and more conservative timing. Propane will power these buses, but because of its lower BTU rating it will generate less power and lower mileage. The lower cost of the propane fuel will offset the cost to a degree, but does nothing for the power. Aside from really advancing the timing there is little you can do to get around this on an existing engine.

However, if you were building from scratch you can take advantage of propane's higher octane rating to regain some power and mileage. Will it necessarily make it advantageous? That's hard to say, but it will mitigate many of the losses. Running higher compression (say 12:1 or even 13:1) will help a lot. Running a cam with slightly more lift, relatively short duration, and a narrow lobe separation angle will build higher cylinder pressures for the higher octane and deliver higher torque, even if the curve isn't as flat. Obviously there is much more to it such as port design, intake, etc, but that is a basic overview.

The mixers used are almost exclusively Impco. The 300A is what you would typically find in a "dual fuel" setup which can run on either gas or propane while the venerable 425 is what most propane only buses will run. Both are readily available.

As far as diesel goes it certainly is a power gainer as proven by the guys bombing their diesels for bragging rights. However, it usually comes as a later resort after turning up the fuel and boost. There has been a lot of talk about it on the board here but to be quite honest, I'm not going to put the money into it or risk my longblock to gain a few percent of a mile per gallon or a few minutes on a long trip. Do some google searching and you will find lots of information if you are interested.
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Old 03-12-2008, 08:41 PM   #3
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Re: runs on propane?

Great list of the pros and cons. One extra "con" for propane - hard to refuel when you run out of gas. Yes, you CAN rig up something with a barbecue tank.... but it is not as easy as just getting a gas can and hitch hiking back to your rig.

One more on the pro AND con side I can add from personal experience. Propane is not taxed as a road fuel. You save some money at the pump. However.... propane is not taxed as a road fuel so (at least in Washington state) you may have to pay extra to license your rig.

In Washington, the fee is based on the vehicle's gross weight..... so my bus that sees perhaps 500 miles a year costs almost an extra 200 dollars. Ouch!

I will change over to gasoline so I can license it that way - although I may keep the propane mixer (Impco 425, like you said) on top of the carb so that in an emergency my gummed up carb won't be an issue - just open the propane tank and go.

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Old 03-12-2008, 11:33 PM   #4
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Re: runs on propane?

Quote:
Originally Posted by bender477
why in the world would you want to do this ? propane is more expensive per gallon than gasoline and it burns less efficiently
Actually, propane is LESS expensive that gasoline in a lot of places. It used to be MUCH less expensive, but the gap is narrowing. Right now the price difference is about the same as the difference in BTU's in a gallon, so the cost per BTU of energy is about the same.

Propane can run efficiently - it is already a gaseous fuel and so mixes well. It has a VERY high octane - so if you modify your motor to run with higher compression it will be much more efficient, and the "miles per dollar" will be about the same, gasoline vs. propane. It also does VERY well with turbocharging - you don't really have that much limit on your boost.

I have to admit - the idea of my carb getting all gummy from gasoline while my bus sits for a year and then not having it able to start up and bug out (the bus is not only my home - it is our "escape pod" in case of a major wildfire) is enough for me to keep the LPG setup.

The reason the school system in Oregon changed to propane in 1983 for their school buses was that the exhaust was much less toxic. Think - there are all these little kiddies wandering around about exhaust pipe high.
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Old 03-16-2008, 02:24 AM   #5
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Re: runs on propane?


Quote:
do a woodburning steam powered bus
Naw. Do a wood burning SPARK IGNITION bus.
They are for real. Were used in Europe during WW2, due to gasoline shortages.

http://en.wikipedia.org/wiki/Wood_Gas_Generator

Sorry -- I just had to be a smart aleck again.
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Old 03-21-2008, 12:39 PM   #6
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Re: runs on propane?

what is required to make a gas engine run on propane?
I've given up the supplemental power idea but it seems like an excellent way to run my generator, no?
thanks
again
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Old 03-21-2008, 01:10 PM   #7
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Re: runs on propane?

To make a propane engine run on gas you need something that will take liquid propane and ensure it is gaseous (in a bus for the big engine this is with a device that takes heat from the radiator). In something smaller with less fuel flow you just use a tank that feeds gas and not liquid, like a barbeque bottle or (some) forklift tanks and most RV tanks.

You run this into a "mixer" - which is a propane version of a carburetor - in that it mixes propane and air in the right proportions.

The timing on a propane powered engine should be more advanced than a gas engine - the propane needs to start burning in the cylinder sooner. I have my bus running an extra 14 degrees advanced over the gasoline specs.

The one issue you may run into is the valve seats tend to get a bit "cooked" if they are not hardened. Running propane you will not be cooling the valve seat with a charge of fine mist gasoline liquid coming from the carb.... you might want to look up (go use google) how much damage some of the old "valve in block" briggs and stratton engines could get. I did not find anything in a quick search for generators, but I know it is an issue. You may also want to allow your exhaust to be lower pressure (replace the stock lousy exhaust with a flex metal pipe into a standard car exhaust - much quieter and lower back pressure) so it won't heat up so much.

On the plus side.... no gas gum in your genny. I'm thinking of converting mine prior to next winter.
On the negative side.... hard to siphon 5 gallons of LPG out of the tractor!
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Old 03-21-2008, 03:52 PM   #8
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Re: runs on propane?

Quote:
On the negative side.... hard to siphon 5 gallons of LPG out of the tractor!
oh so true, and god help you if you let it run dry going down the road.

my original bus is propane powered, sitting in a storage yard and no propane in it...no one wants to deliver unless I buy 100 gallons at a time.
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Old 03-21-2008, 04:00 PM   #9
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Re: runs on propane?

You CAN rig up a deal where you use a 5 gallon bbq bottle (turned upside down) to put fuel back into the big tank. One issue, of course, is that the "modern" valves have some kind of anti liquid device to prevent overfilling, that may or may not cause a problem.

I found this trick by Googling.... you might find it a way to get just enough into the big tank to get to a gas station that sells propane. I haven't rigged up the hose required to do this for my LPG powered bus.... but I don't drive it much, either right now.
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Old 03-23-2008, 09:56 PM   #10
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Re: runs on propane?

upside down huh..I have the hose setup *S* not enough power to get more than a fast walk, and no way up a hill...i'll try that the next time I get to play with her though.
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