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AQMD Approves $3M to Replace Old Diesel School Buses with CNG

The California South Coast Air Quality Management District (AQMD) recently approved nearly $3 million to replace 15 of the region’s oldest diesel school buses with new, natural gas-powered buses.

AQMD is the air pollution control agency for Orange County and major portions of Los Angeles, San Bernardino and Riverside counties.

Health studies continue to show the importance of reducing children’s exposure to air pollution and particularly harmful diesel exhaust.

Today’s action puts us one step closer to our goal of replacing all dirty diesel school buses in the Southland with cleaner technology.

—William Burke, Governing Board Chairman of the South Coast Air Quality Management District.

AQMD approved the funding to purchase new compressed natural gas (CNG) powered buses as well as CNG fueling stations for five school districts. The program is funded by $2.1 million from the state and $857,750 from AQMD. The California Legislature last year approved $12.5 million to replace 89 of the oldest buses statewide.

AQMD has a number of policies and incentives to replace dirty diesel school buses with alternative fuel models, including:

  • Rule 1195, adopted in 2001, requiring that school districts purchase alternative fuel buses, when they replace older ones, if outside funding is available;

  • Approval of $55.8 million to date to replace 357 older diesel buses with 271 new CNG and 86 lower-emitting diesel buses, and retrofitted more than 2,100 buses with particulate traps;

  • Soliciting applications for $14 million in funding to replace older buses and $5.45 million to retrofit additional buses with particulate traps.

In a recent evaluation of the condition of school buses by the Union of Concerned Scientists, California received a “C” grade on soot pollution but a “Good” rating for its clean-up program. (Earlier post.)


tony chilling

I would think they would error on the side of safety and chose a diesel hybrid bus.
What happens in a servere accident with a CNG tank?


California shows once again that they simply have no clue. The new clean diesels beat CNG by 40% to 60% across the board, in nearly every category you can think of. Diesel hybrids would have even greater efficiency.


I think California does know what they are doing - keeping kids from getting sick by taking old, foul diesels off the road and replacing them with something way cleaner - whether that's CNG or clean diesel isn't the important part here.

But to address the CNG vs Diesl issue: a March 15 2006 GCC post states: "The improving baseline emissions of comparable diesel buses with advanced emission control technologies has led to some questioning over whether or not CNG still retains its clean advantage. According to a recent study by NREL, it generally does."

Where did this diesel is cleaner than CNG data come from?

Rafael Seidl

Clean diesel in a MDV/HDV currently implies the use of a continuously regenerating particulate trap plus an SCR scrubber for the NOx. These systems are not cheap and, the SCR requires a urea additive. You also need sufficiently high temperatures in the exhaust (> ~200 deg C) to achieve the desired results. Nevertheless, the system is cheaper than CNG alternative. Average NOx emissions are low enough in both cases, especially if EGR is used on the diesel option:

Opinion has shifted in favor of clean diesel since 2003.

Both clean diesel and CNG busses can be hybridized. This permits aggressive downsizing of the ICE (from ~11L to ~3.5L) and a substantial improvement in fuel economy (from ~3MPGe to 5-8MPGe, depending on layout). Serial hybrids architectures are often used for low emissions and, to enable a low floor in the passenger compartment of city buses that must stop frequently. However, parallel and compound hybrid drivetrains for buses do exist as well. These are better choices for suburban and rural routes.


damn, a disel hybrid would be cheaper and reduce more emissions. the biggest problems with school buses are that they idle. only a hybrid system is going to resolve that.

Anand Kumar

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Richard Cutts

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