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CARB Doubts Medium-Term Diesel Bus NOx Improvements; Considers Mandating Alternative Fuels

Projected reductions in NOx emissions from buses based on the 3 different proposed options.

The California Air Resources Board (CARB) is proposing a new rule that could mandate the purchase of only alternative-fueled urban transit buses beginning in 2007. Natural gas and other gaseous fuels, electric and hybrid vehicles fall under the aegis of “Alternative”—but not biodiesel.

The proposal, first circulated last month and which will be presented at the upcoming ARB board meeting on 15 September for discussion, could potentially significantly amend the existing exhaust emissions standards for 2007–2009 model years or purchasing requirements from 2007 on.

Some background to this is necessary first.

CARB has two programs in place specifically designed to reduce emissions from urban buses. One program affects emissions from new urban buses and the other program affects emissions from in-use urban buses operated by transit agencies (Fleet Rule for Transit Agencies).

In February 2000, the Board adopted a fleet rule for transit agencies operating urban buses, and more stringent emission standards for new urban bus engines and vehicles (ARB 1999; ARB 2000b).

The rules were designed to reduce emissions of NOx and PM by setting fleet emission reduction requirements that would require the purchase of cleaner engines and the retrofit of existing engines.

By 1 January 2001, transit agencies had to choose to follow either a diesel path, or an alternative fuel path. The decision was to be non-revocable, and would establish purchasing requirements for the transit agencies through 2015.

The rule also promoted advanced technologies by adopting requirements for zero-emission bus (ZEB) demonstrations and acquisitions applicable to larger transit agencies.

New, more stringent mid- and long-term emission standards were also adopted that apply to new urban bus engines. The rule also encourages the purchase of diesel hybrid electric buses from 2004 through 2006 by diesel path agencies.

Both the EPA and California have established increasingly stringent emissions standards for heavy-duty vehicles and urban buses. The California standards for buses are, however, more stringent than the EPAs in the area of upcoming NOx limitations for 2007.

Federal and California Diesel Emission Standards (g/bhp-hr)
Model YearFederalCalifornia
20042. / 2.20.01

CARB is beginning to doubt, based on their discussions with diesel engine manufacturers, that achieving this 0.2 g/bhp-hr level of NOx emissions from diesel engines in 2007 is possible.

The California and national heavy-duty truck new engine standard for 2007, which includes urban buses for all but California, is also 0.2 g/bhp-hr. Flexibilities in the heavy-duty truck rule, however, result in the option of certifying all engines to an average NOx standard of 1.2 g/bhp-hr between 2007 and 2009. This is what engine manufacturers have stated they plan to do, so it is unlikely, in CARB’s current opinion, that diesel engines meeting California’s urban bus NOx standard will be available in 2007.

If that were to prove to be the case, then without some modification of the ruling, transit fleets that had chosen the diesel path would be unable to purchase new diesel buses for those three model years (2007, 2008, 2009). The assumption is that by 2010, when both the Federal and California standards are the same, diesels meeting those requirements will be available.

To avoid the of mandating the purchase of engines that don’t exist, ARB is proposing three options.

  1. Keep the current new urban bus emissions standards as they are. Diesel agencies would buy no new diesel for MY 2007–2009, and make up for those deferred purchases in 2010–2012 when diesel engines are again available.

  2. Realign the NOx emission standard for 2007 through 2009 model year new urban buses with the federal standard by raising it from 0.2 g/bhp-hr to 1.2 g/bhp-hr.

  3. Require all transit agencies to purchase/lease only alternative fuel buses. This option would mandate that at least 85% of a transit agency’s annual purchases be alternative-fueled through 2015.

    Although theoretically a transit agency could purchase only alternative-fuel buses from 2007–2009 and then switch back, the practicalities of supporting an alt-fuel infrastructure lead ARB staff to conclude:

    If a requirement were adopted that required all transit agencies to follow the alternative fuel path, staff believes transit agencies would be forced to purchase alternative fuel engines in 2007 and later.

CARB evaluates the options, in terms of impact on emissions, as follows:

  • Option 2 (revise the NOx standard to 1.2 g/bhp-hr) provides reductions in emissions in each year from 2007 through 2010 compared to option 1 (no change), culminating in cuts of some 1.6 tons per day in 2009. This reduction occurs because 1.2 g/bhp-hr new engines replace higher emitting older engines, whereas in option 1 no new engines are purchased by diesel path agencies until 2010 or later, and higher emitting buses remain in operation.

  • However, once the deferred cleaner diesel purchases are made in 2010 and beyond, Option 1 provides slightly greater reductions than option 2 from 2012 on, reaching up to about 1.2 tpd. By deferring purchases until 2010 or after, all purchases are 0. 2 g/bhp-hr engines, whereas the buses purchased in 2007–2009 (option 2) have higher emissions (1.2 g/bhp-hr). CARB estimates that by 2025, there will be no difference in emissions because all engines remaining in the fleet are 0.2 g/bhp-hr (i.e. any 1.2 g/bhp-hr engines are over 15 years old and have been replaced).

  • Option 3 (require all diesel path agencies statewide to switch to the alternative fuel path) provides the lowest emissions. Compared to option 1, option 3 provided emission reductions in each year 2007 through 2011, reaching about 2.6 tpd in 2009. Compared to option 2, option 3 will provide lower emissions until 2025, reaching a maximum of about 1.0 tpd in 2009.

    CARB does note that agencies previously on the diesel path will have to invest in new alternative fuel infrastructure, and this could result in deferred or forgone purchases, which would reduce the emission benefit of this option.

Forty-eight (63%) of the state’s 76 transit agencies that report to ARB under the fleet rule are using diesel and would be affected by a change. The remaining 28 agencies on the alternative fuel path (37%) would continue as originally conceived, as CARB believes that conforming engines will be available in the 2007–2009 period.

The proposal of the alternative fuel option evoked a strong response (not surprisingly) from transit agencies that had gone down the diesel path, and now feel that CARB is proposing to change the rules of the game at potential great cost to the agencies.

The San Jose Mercury News, reported that the Bay Area Metropolitan Transit Commission is estimating that the new rule would cost the region roughly $300 million in the short term to buy, to operate and to maintain the required CNG buses.

The San Francisco Chronicle notes that AC Transit, the agency serving Alameda County (Oakland), has already spent some $120 million on improving its diesel fleet, and is a fierce opponent of the alternative fuel option.

The Staff presentation to the Board will be available on the ARB site the day of the meeting.




The Answer is Biodiesel

CARB's refusal to acknowledge the essential role of biodiesel in California's transportation emission problems is simply confounding. Biodiesel, a commercially produced renewable fuel, works in any existing diesel engine with little to no modification. Biodiesel use reduces particulate matter (PM) by 55%, and greenhouse gas emission by 93%. With the addition of "bolt on" filters, the results improve even more dramatically. CARB does not recognize biodiesel as a alternative fuel, even though every major regulatory agency worldwide does so. In fact, biodiesel is the first alternative fuel in the U.S. to successfully complete the EPA’s strict Tier I and Tier II Health Effects testing under the Clean Air Act.

Much of CARB's anti-biodiesel stance is due to old information regarding NOx emissions. Let's set aside that the role of NOx in health under serious debate. The fact is, the most recent comprehensive studies by the Dept of Energy's Renewable Energy Lab show a *reduction* in NOx when using biodiesel in a 20% blend.

Forcing California's transportation agencies to acquire all new equipment will surly lead to a drastic reduction in services and increased automobile traffic. Progressive urban areas world-wide are choosing biodiesel as the most practical near-term path to greenhouse gas and particulate reductions.

Natural gas is a fossil fuel, even more prone to price shocks than petroleum. The US is already importing more natural gas than it produces. From vehicle reliability to foreign source dependence; natural gas is not the answer to our collective transportation fuel problem.

Forcing transit agencies to spend millions purchasing natural gas powered equipment is a boondoggle for the gas industry. Dictating this "choice" to California's citizens without exploring the most effective solution is simply tragic.


It used to be that public transit were paid for through federal and state grants at a 80%/20% ratio. The transit systems paid nothing. How they arrived at these cost figures is a mystery to me.


It seems like NOx is Biodiesel's current holdup. Figuring out how to reduce NOx emmissions, demonstrate that reduction, and then get that demonstration to propegate through the regulation has to happen if Biodiesel is to be widely embraced by US gov't agencies.


One has to wonder how CARB has backed into this corner of obsession over NOx; yet no positive matrix for greenhouse gases, renewables vs fossil fuels, or fuel & equipment costs.

I'm still waiting for an explaination of SoCal's "weekend effect" when diesel emissions drop 50% (and theoretically NOx emissions), yet smog increases 50%.


Rob - I agree. CARB's obsession with NOx continues to amaze me in light of mounting evidence that NOx reductions make ozone (smog) worse.


What's interesting is that CARB apparently disregarded its own study of diesel vs. CNG:

"...Aside from NOx, average gram per average gram of emissions, the trap-equipped diesel still appears to have a slight advantage over the OC-equipped CNG...."


So they (CARB) are proposing to mandate a fuel/engine that is "dirtier" just to get slightly lower NOx emissions? Sounds fishy to me!

Robert Pyle

International Trading Pertners LLC is marketing a Bio Diesel Modifier Fuel Additive that reduces all emissions across the board and increase fuel econony. Testing has been going on for fithteen years and the results are conclusive. Learn more about this technology at

Parviz Nazem

Who is selling garbage trucks that meet the EPA 2010 air quality standards? please and thanks.

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