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Shift to 3-way fuels could be messy

Some time ago I read in the Monitor about a dual-fuel system for automobiles, using both natural gas and gasoline. Now that the cost of gasoline is well over propane/gasoline, but what about making it a three-way conversion while natural gas is so much cheaper than oil? Col. M. Morgan Albuquerque, N.M.

Typically, what people have done with natural gas is to try to carry it in compressed bottles. "The problem with that approach," says Dr. Craig Marks of the General Motors Research Laboratories, "is the very high weight of the high-pressure bottles in order to carry a substantial amount of fuel."

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Some cars in the past -- mainly fleet vehicles -- were converted to natural gas, to be sure. But going to a three-way conversion has got to be highly complex, and you need to find the hardware somewhere and require the space in the car -- under the hood and in the trunk -- to put everything.

The mixing-valve arrangements for natural gas and propane are similar but not identical, Dr. Marks notes. Further, the commercial equipment that you might find on the market would be hard put to do the job.You might have to go to customized hardware, and that could be very hard on the pocketbook.

Propane liquefies when you put it under a little bit of pressure. Natural gas, however, remains in a gaseous state unless you refrigerate it to almost absolute zero.

"I remember when we were working with hydrogen and oxygen in fuel-cell research, the rule of thumb was that you could store a pound of hydrogen in a 100-pound bottle," says Dr. Marks. "Natural gas has a density that is 16 times the density of hydrogen, so I guess you'd get 16 pounds of it in a 100-pound bottle."

Remember, there is an additional hazard imposed by a three-way installation. You have some increase in risk by using propane, to be sure, but the hazard multiplies with natural gas. If there is a leak in the system and the gas collects, such as in your garage, you have an explosive possibility on your hands.

Propane, of course, is a quite common arrangement, and a lot of people have been making such conversions over the years. Recreational-vehicle manufacturers , such as Winnebago, now are providing a propane conversion option in at least some of their RV fleets.

To sum up, if you try to go to a three-way system, you may end up with a mess on your hands.