District using 20 compressed natural gas school buses with plans for more
(L-R) MISD Transportation Director Charles Stone, Atmos Energy representatives, Walter Miller, CEM, and Steve Mohr pose for demo on CNG school bus fueling. |
Mansfield, TX – Mansfield ISD is taking steps to help the community breathe a little easier with the use of compressed natural gas powered school buses and other cleaner engine technology.
The district resides in the Dallas/Fort Worth area, which has been classified as a non-attainment area by the Environmental Protection Agency (EPA) for failing to meet the National Ambient Air Quality Standard for ozone. Because of the area's classification, the district became eligible for special grant funds that target cleaner tailpipe emissions.
In the continuing effort to provide the best options for school buses to the students and the community, the district sought out and received grant funds to use cleaner and more fuel efficient school buses.
First, grant funds enabled 59 school buses to be retrofitted with particulate traps. These traps act like the catalytic converter on a car, super-heating the exhaust particles so that they turn into ash, which is then collected instead of exhausted into the air. Next, the district used grant funds to assist in the purchase of compressed natural gas (CNG) buses. Currently, the district has 20 CNG buses in use with six for more on the way for 2008-2009. These dedicated CNG engines are much more efficient than conversions and extremely reliable.
The use of CNG is not new, there are 100,000 vehicles a year added to the CNG user-base worldwide. Mansfield ISD is using engines that are dedicated to CNG, meaning that they were designed only to run on compressed natural gas. Other Texas school districts converted gasoline engines to run on CNG in the early 1980's. Those engines were notoriously inefficient and many districts chose to return fully to diesel.
Benefits
MISD benefits from monetary savings in using CNG buses in three areas: fuel cost differential; reduced maintenance costs; and reduced refueling labor.
The fuel cost differential refers to the lower cost of compressed natural gas as compared to the continued rising cost of diesel.
A reduction in maintenance costs comes from the inherent cleanliness of the engine process (particularly soot) that will require less maintenance effort. A longer oil life which reduces oil and filter time/materials plus a simpler engine design than the 2007 diesel engines should provide fewer engine compartment issues.
The Transportation Department also looks forward to reduced refueling labor. Refueling of CNG buses takes place at the assigned parking space using the time-fill stands instead of sitting in line waiting to get through the fuel station. “On a busy day, we have vehicles and drivers waiting for 10-20 minutes just to begin fueling,” said Transportation Director, Charles Stone. “The cost of refueling a CNG bus is absorbed in the post operational check minutes that the driver does anyway. The process of hooking up the fuel lines (and unhooking at the start of the next run) is less than a minute and done inside the assigned parking space.” Once hooked up, the compressor station reads the pressure in the bus tanks and decides whether or not to begin running/pressurizing. It only turns on if any of the hooked up fuel tanks are at less than 2700 psi. Once low pressure is detected, the time-fill stand turns on and brings all buses up to 3600 psi (once it kicks on, all tanks get to an equal pressure and then they all rise together). This process takes approximately three hours, so it allows another bus to arrive while the compressor cycle is running and also get refueled.
Natural gas is a Texas based product, rather than foreign based. Each CNG bus replaces about 2000 gallons of diesel use each year. “Twenty buses at 2000 gallons per year equals 40,000 gallons times the 20 year life equals about 800,000 gallons not imported,” said Stone. “Not much, but it is a start. In addition, the engine is significantly quieter, which won't show up on any spreadsheet, but will be noticed in the neighborhoods and campuses.”
Safety
The district takes the safety of all students, staff, and community members very seriously and many considerations were taken prior to implementing the CNG buses. The compressor station does not have a storage tank. It just compresses natural gas from the ATMOS feeder line. Also, there are pressure check valves so that none of the tanks can expel anywhere around the ground level. The bus design has a pressure relief channel from the engine compartment to the roof as an added safety feature should a failure occur at the engine. |
