CNG for Cars: Q & A

Questions from California, and my answers:

We've been seeing the promotion of compressed natural gas for cars for a while, but what REALLY made my ears perk up was the idea of filling up your CNG vehicle from a fitting on your household gas line. That would be awesome! No more gas stations!

So, what do you know about:

1. Cost of CNG engines, or converting to them ( I hear conversion costs $2K - $4K);

A: Big problem with CNG is you need a pressure tank. They are big, round, heavy and expensive. Liquid fuel tanks can be shaped to fit the available space. With CNG tanks, you need to shape the car to accommodate them. Of course you can just put them in the trunk, but it impacts your trunk space.

2. Performance of CNG vehicles vs. gasoline vehicles;

A: Performance should be comparable.

3. Can you really use household gas?

A: Problem with household gas is it would have to compressed, which consumes energy (power for the compressor), and dissipates energy (heat) from compressing the gas. Unless you can tap into the high pressure line on the other side of the pressure regulator, which is also upstream from the meter. Can be dangerous, and probably illegal, unless you get a special high pressure meter and have the gas company install it.

4. Cost of installing a refueling fitting to your household line;

A: If you are taking gas from your stove fitting and doing it yourself, then the biggest expense would be the compressor. Actually, the biggest expense would be the insurance when they found out what you were doing.

5. Running cost per mile of CNG vs. gasoline -- any cheaper?

A: Gasoline is a commodity item and the price follows the market. Natural gas is sold in larger quantities (a years supply for a city as opposed to weeks supply of gasoline for a car), but it also follows the price of oil. Many industrial processes can use either one, and they use such vast quantities that a small difference in price can cause them to change suppliers. Witness natural gas fueled gas turbines used for generating electrical power.

There is also the tax advantage. Natural gas isn't subject to highway taxes. Of course, neither is gasoline or diesel for off-highway use. Used to be highway taxes were a big deal. Something like half the price of a gallon of gasoline. But since the price of gas has shot up so much, they are not quite so significant. Highway taxes are at a fixed rate, so much money per gallon, not a percentage.

One of the big questions in my mind has always been the safety of the pressure tanks. I think this video clears up that question pretty well. As a plus, it's pretty entertaining, in a Mythbusters kind of way. Found on CNG Utah.


Safety Video CNG