School Districts Fleets

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CNG School Districts – Benefits

One of the saddest images of the modern era is children walking away from a school bus as it takes off leaving a cloud of diesel exhaust.

By switching to CNG School districts will greatly benefit themselves and local communities in more than one way. Aside from reducing the district’s fuel costs by up to 40 percent,  switch to natural gas will reduce emissions, including particulate matter, by 89 percent. It’s significantly cleaner for our environment, and it reduces our dependence on foreign oil The switch to natural gas will save the district millions of dollars over time,  — money that can be reinvested in the classrooms which makes it a triple win.

School and transit buses generally are powered with a medium duty diesel engine.  Currently diesel engines are required to achieve very clean tailpipe emissions which require two significant engine devices in order to meet EPA standards for NOx and particulate reductions.   Both operations add to engine workload and maintenance activities.

  • An exhaust gas recirculation process that essentially reuses some of the exhaust to reintroduce into the engine combustion chamber.  This allows the engine to more completely oxidize the fuel.  This in turn raises the operational exhaust temperature and increases the need for maintenance activities.
  • The other addition is a particulate trap system using a process which injects and burns Urea within the exhaust stream, raising temperatures so that the catalytic type of converter particulate trap can convert particulates from soot to ash.  The trap then captures the ash prior to releasing the exhaust out the tailpipe.  High exhaust temperatures are experienced during operations and occasionally while idling when the engine regenerates (automatically races to raise airflow and temperature) to ensure a clean burn from soot to ash.  The Urea must be stored in the shop and onboard the vehicle.

School and transit bus conversions to CNG are limited by the EPA to approved engines that have received a Certificate of Conformity, indicating compliance with existing emission standards.  Currently there are but two engines that have achieved this EPA approval status:  the Mercedes 906 and the Detroit DT466 engines.   Each of these engines has established a CNG fuel tested NOx and particulate rating that meets the 2010 standards.

A full EPA Certification is an expensive testing process that can only be achieved for a dedicated CNG engine, where only CNG is used.  This is a spark ignited fuel mode which meets 2010 emission standards without the need for exhaust gas recirculation or particulate traps (or Urea).  Currently the only dedicated CNG OEM engine is the Cummins Westport 8.9 liter ISL G model.  Unfortunately this engine is only designed to be placed into rear engine school and transit buses, due mostly to the size and weight of the engine system.  Two school bus manufacturers, Blue Bird and Thomas Bus, offer this engine in the transit, rear engine model in the 66 to 84 passenger sizes.

Bi-fuel (independent operation on either diesel or CNG) cannot be accomplished since each of these fuels is ignited differently, diesel by compression and CNG by spark.  It can be used with gasoline engines that have been designed for multi-fuel systems or independently hardened for CNG.

Dual fuel (simultaneous operation on both diesel and CNG) is accomplished by using diesel compression to ignite the CNG.  This mode allows operators to use CNG somewhat efficiently and to use diesel as a starter, low speed accelerant, and as a primary fuel when gas pressure is too low.  By somewhat efficiently, we realize that the stop & go operations, such as that of school buses, will significantly diminish the economic savings of CNG when used in a dual fuel mode.

A full EPA Certification is an expensive testing process that can only be achieved for a dedicated CNG engine, where only CNG is used.  This is a spark ignited fuel mode which meets 2010 emission standards without the need for exhaust gas recirculation or particulate traps (or Urea).

Currently only the Cummins Westport 8.9 liter ISL G model has the dedicated CNG approval.  Unfortunately this engine is only designed to be placed into rear engine school and transit buses, due mostly to the size and weight of the engine system.  Two school bus manufacturers offer this engine in the transit style, Type D school buses of 66 to 84 passenger capacity.

Production design has been initiated by Cummins to adopt the ISL G technology into the current ISB engine system in order to provide a front engine school bus option for 2015.  Once this is approved by the EPA and the school bus design engineers, an entirely fresh market for CNG will emerge.

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