My company truck (yrs ago) ran on CH4 (methane, CNG). We had a fueling station right at the gas plant, and we just filled up all the ops trucks there.
Price: cheap like dirt. methane is plentiful, readily available, and easily handled.
changes to motor: with gassers, there are some issues, mostly timing, ignition (we ended up using an induction-type of plugs), etc. Only very slight power loss, as I recall.
Problems: need to highly compress the stuff... where propane compresses at 15 psi, this stuff was at like 600 psi... in a composite-epoxy tank. If the tank ever ruptured, you'd be at the centre of a pressure explosion (not a flame explosion), but they were a LOT safer than people gave them credit for. The tank was big, though, and that was an issue.
Problem: range. My 3/4 ton was good for about 250 miles, then I had to refill. Considering my daily drive was 185 miles, that was OK, but still, it was an issue,because there aren't many CNG fuelling stations around. You need to plan carefully.
Problem: you couldn't (at least at that time) run a duel-fuel system... it was either gas, or converted to CNG.
The local school division also ran their buses on CNG. All the upsides, and the buses had room to store the big tanks underneath. They quit doing so because the hockey moms worried about the big gas fireball explosion that would result from an accident. Bah. I've BEEN IN a CNG explosion... in a tin compressor building that had a gas leak and went boom. Took out the windows and blew panels off the roof and walls.
While three of us were inside, wearing SCOTT packs and trying to shut things off.
Scared the hell outta me, but overall, being IN them isn't bad, being on the EDGE of them sucks. Methane won't burn except in the right proportion with oxygen, which you only find at the outside of the blast.
Anyway, you get the picture... CNG can be a viable fuel for gassers.
Diesels? Never seen one converted to run CNG, but I have seen them on LNG (much more compressed!)
I also know if you drive a Kenworth onto a wellsite when some stoopid engineer is busy opening up the well to atmosphere, you go for an uncontrolled thrill-ride across the prairie, unable to shut the truck off, so I know that a diesel WILL burn methane! (these days, all diesel vehicles onsite must have airflow cut-off devices...)
Overall, the big issue is re-fueling, Oz. Same as with a propane vehicle. With a CNG vehicle, if you don't plan well, go to strange places, or just can't find a filling stn, you're calling a tow truck.
Links of interest:
Idaho National Laboratories
http://www.inl.gov/lng/fueling.shtml
New Energy and Fuel
http://newenergyandfuel.com/http:/newenergyandfuel/com/2008/08/27/methane-aka-natural-gas-in-your-tank/
