Maintenance of a CNG bus is higher than the diesel bus because you got the usual maintenance cost of the diesel bus plus the cost of maintaining the (probably) $20000 CNG tank system rather than a simple $100 diesel tank. However, the annual fuel bill for a CNG bus is likely only 2/3 of the diesel bus. That could save about $7000 per year per bus. Plenty to pay for increased maintenance and the extra cost of buying a CNG bus. A diesel city bus going 5mpg and doing 35000 miles per year will spend ((35000/5)*3.2)= $22400 worth of diesel per year when diesel cost $3.2.

CNG/LNG will be used far more in the future for city busses and long-haul heavy duty trucks. Also international shipping could turn to LNG instead of bunker oil. Globally such a transition could take nearly 10 million barrels per day out of the 86 mbd global market. It will happen because it makes increasingly economic sense as oil prices goes up (expect $120 per barrel and $4 gasoline) and natural gas prices stay the same as a result of an emerging global shale gas industry.

This is great, every time I see a CNG bus in operation I smile. They are quiet and clean, no smoke billowing out as they roar to pull away from the curb.

I truly believe that people like to hear these stories. It represents progress where things are getting better, problems are being solved. Your tax dollars at work is the old phrase, we could use more examples like this.

It may be true that CNG reduced PM emissions by 80% in early bus engines, but there's no evidence that that is currently still true.

Based on 2011 CARB certified emissions, a comparison of the exhaust emissions from the CNG and diesel versions of the Cummins 8.9 liter urban bus engines shows that the diesel version is lower in all criteria emissions except NOx (0.20 g/bhp-hr NOx for the diesel version; 0.13 g/bhp-hr NOx for the CNG version). PM emissions are 0.000 g/bhp-hr for the diesel version; 0.002 g/bhp-hr for the CNG version.

http://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2011/cummins_ub_a0210538_8d9_0d20-0d01_ng.pdf (CNG version)

http://www.arb.ca.gov/msprog/onroad/cert/mdehdehdv/2011/cummins_ub_a0210546_8d9_0d29-0d01.pdf (diesel version)

Diesels have been cleaned up to the point where they are actually cleaner than other ICE technology.

I hope you guys are wrong.

CNG seems so clean (and I suppose they are much, much cleaner than OLD Diesels).

But even if you are right – we don’t have to base our beliefs on reality.

Reality is overrated. Perception is what we work with.

Seriously – are the “10 to 15% less to operate” Diesels really cleaner too?

I am sure their original cost is much less.

There are enough positive sides to CNG/CSG engines to justify an extensive conversion program of the 5M large city and urban buses used in USA.

Since the local supply of NG and SG is good for 100+ years, could the 10+M large trucks and many thousand locomotives be converted to CNG/CSG?

It could be an easy way to reduce crude imports, create local jobs and reduce air pollution.

Our city diesel buses (and those of most other cities) certainly do not run clean. They never did and will not run cleaner in 10 years. So, an upgrade to cleaner CNG/CSG would be beneficial for the environment.

Secondly, since they are huge crude oil eater, it would have an immediate effect of crude import.

With xx million out of work, such a program would help to create more jobs and to get out of the current speculators and right wings made recession.

The low emissions achieved from CNG does not require the high EGR and larger capacity engines, + or particulate filter, + or NOx traps that the clean diesel equivalents are using in some combination or seperately to achieve the emissions target.

If CNG buses were to utilize any of the above 'tricks, then we could expect them to way outperform the diesels.

To be fair apples cannot be compared with oranges in this case.

Again I can confidently state that any engine, petrol or diesel running eother LPG or NG or H2 fuels will last ~ 2X as many kilometers because of the benifical friendliness of these gaseous fuels to the oil lubrication system.

That is a big economic benifit to double the oil contaminant induced wear in an engine.

The extension to base lubricating oil life can be 10X but only about2X for the oil conditioners ie friction modifiers/ viscosity modifiers etc etc.

There are other well publicised objections to NG relating to extraction methods including shale recovery, fracking and associated water and aquifer contamination issues that need consideration. That is a nother story though.

"Every thing has downsides"
Just depends how far the investigation is ready or willing to go. They will of course be investigated and reported and the suits and shiny bums will get involved and we will all benefit from rigourous scrutiny.
It just makes it a little harder to pick the winners as they can change on a daily basis.
We can say however.
Investors beware!

Valid point about incorporating additional emission controls on CNG similar to clean diesel, but on the other hand, that would increase the initial cost of the CNG bus and likely decrease the efficiency which is already 25% less as well.

The only point I've tried to make is that from an emissions perspective, California knows, and has known since at least 2001, that they could have achieved at least as much of an emissions benefit with clean diesel while avoiding the additional operating costs involved with CNG buses (10%-15% higher according to LA Metro themselves). Not disputing there are other benefits of CNG.