LPG/LNG Alternative Fuel-gctid364031

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Hey guys,

I often wonder why there are no LNG conversions in marine applications for small boats. A standard 4 cycle gasoline engine can be converted to run off propane or NG with a cheap bolt on kit for a carburetor and a pressure regulator. In fact there are kits that allow you to run from all 3 fuel sources, gasoline, propane, and NG.

The fuel storage in modern molded composite LNG tanks is safer than gasoline and has indefinite storage life. Obviously lack of refueling infrastructure is a challenge, however given the low cost of natural gas vs gasoline in terms of BTU per dollar should be attractive enough to look in to this for recreational boats.

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I see a couple of problems with the conversion; you hit the biggest on the head- fuel availability. Not many land stations available to refuel, and I can't think of any on the water. Next would be the efficiency of LPG vs gas- can the decreased BTU per dollar offset the lower efficiency of LPG in a 4 cycle engine?

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Other logistical issues aside, you would have a serious safety perception problem to overcome. I know it's not entirely valid, but some people have an unnaturally high fear of CNG.

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Propane is very viable, cheap and low pressure, but to be safe you really need a bit of a locker for the tanks with venting high and low but with shade from direct sunlight. This means you have a fairly large boat and the tank can't be in the bottom center of it. If I was going to do something like this I would probably utilize a pair of tanks in an offshore style bracket and still have the full gas tank as a reserve.

Then you'll be stuck with using it on and I/O with closed cooling, because propane requires the warm coolant to convert the LPG to gaseous form or it will ice up and stop working.

The problem with NG is that the pressure is so great it takes alot of power to compress it to 3500 psi, and with compressor maintenance and pro-rated rebuild cost you're back up to around $3 per gas gallon equiv not to mention the safety issues of a 3500 psi system.

[BLCarl Frequent Contributor Find all posts View Profile Close BLCarl]

I spent a few years working with a large fleet of propane mid size trucks. Mostly 429 Fords and some BigBlock GM. Cost wasn't the main issue for them at the time and they put tons of money into the system.

My shop and our competitors all had problems with exhaust valve/seat/guide life. There were enough engines that Fleet managers were willing to expiriment and document everything in a effort to improve the situation. As I was moving on from that position they had come to the conclusion that It was more cost effective to change to larger heavier Diesel trucks. I still see their trucks on the road today ,All big diesels.

I see the new generation of city busses and trucks using the gasious fuel and wonder how or if have overcome the issue. (maybe they just live with it)

In a large fleet situation a company can afford the infastructer to make something cost effective. I sometimes wonder if it's really cost efective or just political and the costs are just passed on to the consumer or the taxpayer. (Us)

[johnrupp Seasoned Member Find all posts View Profile Close johnrupp]

LEHR just introduced a LP powered outboard. It is available in 2.5 and 5 HP. You can check it out at www.golehr.com.

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Recently I bought a PG/NG carburetor kit for my Generac 7.2KW house generator. The installation took less than 20 minutes and another 20 minutes for mixture adjustment on the pressure regulator.

When installed, the 14HP gasoline engine ran much better from my BBQ LPG tank than gasoline. It sounded more consistent and I observed less auto-throttle movements. I have not tried running it from the domestic NG connection, as I have not yet plumbed the pipe to the generator shack outside.

This got me thinking. It appears propane has effective octane rating of 104 which means you can have higher compression ratio and more power from same engine size compared to gasoline. Also since fuel is in a gas form at room temperature, there is no need to atomize fuel. A PG/NG carb is a simple tube with venturi slots on the sides that inject fuel gas in to the air stream. It just seem like NG or PG are superior fuels to gasoline even for existing engines.

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GrindKore wrote:
Recently I bought a PG/NG carburetor kit for my Generac 7.2KW house generator. The installation took less than 20 minutes and another 20 minutes for mixture adjustment on the pressure regulator.

When installed, the 14HP gasoline engine ran much better from my BBQ LPG tank than gasoline. It sounded more consistent and I observed less auto-throttle movements. I have not tried running it from the domestic NG connection, as I have not yet plumbed the pipe to the generator shack outside.

This got me thinking. It appears propane has effective octane rating of 104 which means you can have higher compression ratio and more power from same engine size compared to gasoline. Also since fuel is in a gas form at room temperature, there is no need to atomize fuel. A PG/NG carb is a simple tube with venturi slots on the sides that inject fuel gas in to the air stream. It just seem like NG or PG are superior fuels to gasoline even for existing engines.

Propane is liquid below a certain temperature or above a certain pressure. If you've ever had a forklift or BBQ ice up the converter/regulator and stop working in cold weather that is why. Still, not a hard thing to overcome.

Hardened valve seats are a must, bronze guides also a good thing as gasses will run "drier" than liquids obviously...maybe leave the guide seals off of the exhuast valves so they see a little motor oil. Due to the octane and lower BTU's, 11:1 compression is rumored as optimum.

Typically a gas engine will lose power on propane, but if you raise the compression and ramp up the timing(or turbocharge), you can pretty much get it all back. The torque may actually increase, but due to the gaseous form of the propane it takes up more space in the intake tract and top end HP is usually a little less. I've done alot of research on this, and there is no reason for it not to be more mainstream except a corrupt government that doesn't want you to have it.

[BLCarl Frequent Contributor Find all posts View Profile Close BLCarl]

rkcarguy wrote:
Propane is liquid below a certain temperature or above a certain pressure. If you've ever had a forklift or BBQ ice up the converter/regulator and stop working in cold weather that is why. Still, not a hard thing to overcome.

Hardened valve seats are a must, bronze guides also a good thing as gasses will run "drier" than liquids obviously...maybe leave the guide seals off of the exhuast valves so they see a little motor oil. Due to the octane and lower BTU's, 11:1 compression is rumored as optimum.

Typically a gas engine will lose power on propane, but if you raise the compression and ramp up the timing(or turbocharge), you can pretty much get it all back. The torque may actually increase, but due to the gaseous form of the propane it takes up more space in the intake tract and top end HP is usually a little less. I've done alot of research on this, and there is no reason for it not to be more mainstream except a corrupt government that doesn't want you to have it.

The gasious fuels appear to climb up between the valve guide and stem and ignite the oil and burn the guides. Bronze guides don't tolerate the intence heat and fail quickly. The exhaust temp is higher than gasoline for some reason the contact area of stellite valves and seats tend to spall or groove in until the valves suck into the port. Some propane forklift engines also suffer from this problem. The fleet that I was involved with came into the yard at night with the exhaust manifolds glowing dull red. Engines that are not designed from the ground up to run on this gas may have early failure.