bmwm750

So, I've recently become interested in CNG conversions. When you look at the cost per therm for gas (about $.84 per therm in my neck of the woods), multiplied by the widely accepted 1.15 GGE multiplier (Gallon Gas Equivalent), plus you electric cost (about 800W or another $.15 per GGE) to power the compressor to fill your tanks in your garage, that's just a tick over $1/gallon, plus . Now, you may use 10% more fuel on CNG, but, a bit over $1/gal vs $4/gal, it's a pretty substantial savings.

A CNG compressor is a few thousand, so you amortize that over a nice long period.

The real question is, what does it cost to convert a car like ours over, and what exactly are the legal issues that come up?

I know that there are tax credits for buying a new CNG car, and I imagine there's some way to apply with a conversion, but really, all I'm after is a cheaper alternative to burning the crap that we call gasoline at the pump these days. As an added bonus, CNG burns FAR cleaner, so it's better for the environment, and your engine.

Heck, if you could find a way to do this, a V10 S8 sounds REALLY attractive, when you can fuel it for $1/gallon!!!!!

jakematic

There are "legal" concerns in doing this if the EPA hasn't approved the conversion kit.
Thankfully you are not in California which makes it even more difficult to do.

Have a look at this for some high level Q&A http://www.cngprices.com/cng_conversion.php

mishar

It is pretty expensive conversion and it is just matter of time when converted vehicles will be charged additional taxes (ones we are paying included in the gas price). Same will happen with electrical vehicles once they become common.

jakematic

Good point Misha.
Eventually, in my opinion, there will be a tax per mile driven imposed to recoup "revenue" lost to higher efficiency vehicles and generally declining fuel purchase volumes.

bmwm750

This wouldn't surprise me one bit. However, in the meantime, not withstanding the legal implications, if you "stay under the radar" and convert your own car and re-fill in your garage, I am very doubtful that any of the powers that be would be aware of you existence.

Bangerla

I would look at some of the european websites for conversions. The conversions are much more popular in Europe than they are in the US. You may be able to find someone who has already put together a kit for the A8. CNG tanks are quite large, and you will lose most of the trunk space. The LPG conversions are a bit more common, and they have tanks that would fit in the spare tire area.

mishar

I agree if everything goes well. Just check first what are fines if they found that you are doing it. In some other jurisdictions they are sized to discourage "under the radar" conversions.

aTOMic

Have you considered LPG? There was a thread over on A8parts.com a while back, some really energetic individual converted his (D2) A8 from CNG to LPG and got +30% more efficiency, although I don't know how he measured it. I say he is energetic because he converted the car to manual trans too! (Here's the thread)

bmwm750

I know LPG is quite popular in the UK and parts of Europe. I understand it to be higher energy density than CNG, thus greater power and efficiency.

In the US, however, I don't think Propane is all that cheap compared to Natural Gas?

mishar

Propane has lower energy density than methane (main ingredient of natural gas) per weight. Propane is good because it liquefies under pressure so it requires smaller tank. Interesting is energy/$ density and natural gas is there even better. Before taxes.