Natural Gas Buses – No Better Than Diesel Buses?

In Spring 2003, Max Nofziger ran for mayor of Austin, and part of his transportation platform was to replace the city’s diesel buses with ones that run on natural gas. Below is Dave Dobbs’ argument that natural gas buses may be no better than diesel buses.

As both a bus user and a transit researcher who has examined the pros and cons of natural gas buses, I can tell you they are far from the “clean machines” that Max would have you believe. Like everything else, there are trade-offs.

Max’s idea that replacing Capital Metro’s nearly brand-new fleet with 400 natural gas buses will somehow magically improve Austin’s air quality not only fails the smell test, it has a number of implicit assumptions that aren’t so. Among them are:

  1. “Bus exhaust is a major factor in Austin air quality.”
    1. In fact, Cap Met bus daily VMT is something less than 150 k a day including the UT shuttle while automobile VMT for the region is somewhere around 2.7 million. Mostly these are SOVs (single occupancy vehicles). Pollutants per passenger mile on buses is miniscule compared to the amount produced by cars using the same measure.
    2. According to Texas Commission on Environmental Quality (TCEQ formerly TNRCC) ozone is far and away the most critical factor in Austin’s air quality and is largely the product of internal combustion engines and the businesses that service these engines (read cars, off road equipment, and related servicing facilities). Particulates are rarely a factor except when there is burning in Mexico. (Brian Lambert at TCEQ told me today that this is going to be bad over the next few days. He expects it to go to level “orange” i.e., harmful to sensitive people. In 1998 Mexican smoke over Austin reached the “red” level – harmful to everyone – for several days, so stay tuned.)
  2. “Natural gas buses (CNG) is cleaner than diesel”
    1. In fact, CNG and diesel bus engines both easily meet federal air quality standards.
    2. Diesel is dirtier in NOx (a factor in ozone) and particle emission, but cleaner in carbon monoxide and hydrocarbons (both a factor in ozone).
  3. “Other than being more expensive to buy, a CNG bus is (a) either cheaper to operate or (b) about the same as a diesel bus and is really just a bus, but cleaner.”
    1. In fact, nominally CNG buses cost about $300-$325 k (each) while conventional diesels cost about $250 k-$275k.
    2. Maintenance costs on CNG are more than twice that of diesel according to Cap Met maintenance figures. *
    3. Capital Metro experience shows that diesel buses averaged 7270 miles between road calls while natural gas buses averaged only 3038 miles between road calls. In 1997 this meant that a natural gas bus averaged 80¢ per mile while diesels averaged 39¢ per mile to operate. *
    4. Put another way, CNG buses broke down more than twice as often: On an annual basis CNG averaged about 32,000 miles while diesel buses annual average was 51,000 miles. *

*(SOURCE: Memorandum “CNG and Diesel Comparative Analysis” BY Elaine Timbs, CMTA Chief of Maintenance, dated September or October of 1997)

  1. As any bus driver will tell you, diesel buses greatly out-accelerate CNG buses in any situation, and if you’ve ridden on the two different types of buses here, you know this is so. In frequent stop and go service, especially under heavy loads in hot weather with the AC going full blast with doors opening and closing, this is a very important consideration.
  2. Because of the extra heavy-duty fuel tanks, CNG buses are 50% heavier than diesels and thus raise the cost of street repairs (i.e., more VHCs from hot paving mix and off-road equipment).
  3. CNG buses have far less range than diesels and require more frequent refueling. There are more BTUs in less space with diesel in much lighter tanks.

Far from being better, CNG is more problematic and less reliable. Note that Detroit Diesel (now part of Daimler Chrysler), the primary builder of both CNG and diesel bus engines, gives extended warranties on diesel engines but does not do so with its CNG engines.

And for the record, Capital Metro’s maintenance department is the winner of numerous commendations for its efficiency and quality of service in all kinds of vehicle engines including CNG power plants.

Reliability and least-cost-per-passenger-mile are two important keys to making public transit an alternative to driving from whence most of Austin’s air quality problems stem. Converting our bus fleet to CNG at this point in the technology is neither cost-effective nor commensurate with Austin air quality goals. Bicycles and reliable buses are cost-effective and commensurate with these goals, but until we are more than a bus system, everyone, including the driving public, is going to find the situation increasingly untenable.

Like so many politicians in the last 17 years, Max is just the latest in a long list trying to hijack a Cap Met bus into office and after mugging the driver and diverting the bus, screaming that it didn’t run on time.

I love politics!

-Dave Dobbs, May 2, 2003