School Bus Diesel Versus School Bus Natural Gas Debate
Breaking down operational considerations to assist decision making for fleets By Robert T. Pudlewski
Today’s school buses carry a heavy cost
premium for emission controls and die- sel fuel is hovering at $4 per gallon. Once again, manufacturer and operator focus shifts to alternatives. Tough evidence is inconclusive, using
natural gas vehicles could lead to fewer par- ticulate and nitrogen oxide emissions, but it may increase “ultra-fine” particulate and greenhouse gas emissions. Because it mixes more uniformly in the ignition chamber, nat- ural gas combustion consequently has fewer pollution emissions than older vehicles us- ing diesel fuel. Conversely, using EPA 2010 compliant diesel engines reduces green- house gas emissions such as carbon dioxide and methane. In addition, technological ad- vances on EPA emission certified 2010 diesel engines have led to cleaner, “green” diesels. Terefore, just as with diesel fuel, the com- bustion process of natural gas vehicles may need help in getting cleaned up. Why all this focus on school bus and
truck fuels? In addition to the decades- long concern over dependence on foreign fossil fuels, older medium- and heavy-duty trucks and buses comprise about 10 per- cent of nitrogen oxide emissions in the United States. Nitrogen oxide helps form ground level ozone, smog and fine particu- lates. Advances in green diesel technology, such as the higher percentage of exhaust gas recirculation (EGR) and the chemically assisted selective catalyst reduction systems (SCR) reduces nitrogen oxide emissions, but controlling all harmful diesel emissions is dif- ficult, as evidenced in fuel combustion and exhaust treatment system changes to 2010 EPA emission compliant diesel engines. Te significant emissions reduction
system changes are; increased EGR, new high-pressure engine fuel system design, combustion chamber design; engine oil; closed crankcase ventilation systems; ex- haust after-treatment devices (DOC, DPF,
SCR) and Ultra-Low Sulfur Diesel. In spite of emission trade-offs, many
fleet operators believe that ULSD is the fuel of choice versus natural gas. Why? It is easy to get, and because cost conscience operators know that diesel engines are more efficient. Diesel engines operate at high compression ratios and convert a large percentage of the fuel’s available energy into usable work. Diesel engines’ higher fuel efficiency generally lowers the carbon dioxide emissions that contribute to the greenhouse effect. Another subject for this debate is meth-
ane (NGV) emissions. Te Harvard Center for Risk Analysis stated in its report, “Fueling Heavy Trucks: Diesel or Natural Gas,” that methane is approximately 20 times more potent as a greenhouse gas than carbon di- oxide, and that using natural gas from a life cycle perspective instead of diesel in heavy- duty vehicles may increase greenhouse gas emissions by 5 percent to 10 percent.
HEALTH AND SAFETY CONCERNS FMVSS 304 published more than a de-
cade ago specifies requirements for the integrity of CNG motor vehicle fuel sys- tems. Still, characteristics of natural gas could potentially make it a greater safety hazard than diesel. Natural gas is highly flammable, and its vapors at low tempera- tures are dense and can form clouds of flammable vapor concentrations. At higher ambient temperatures methane vapors dis- sipate rapidly. Te National Fire Protection Association gives natural gas the highest hazard ranking for flammability while des- ignating diesel as moderately flammable. Diesel fuel is less flammable because it usually does not form ignitable mixtures unless it is heated to plus 125 degrees ver- sus methane at minus 300 degrees. Visit the STN July digital edition at
www.stnonline.com/go/818 for a chart
62 School Transportation News Magazine July 2011
that shows the Fahrenheit temperature at which fuel will ignite. In addition to its flammable nature, natural gas poses
special storage haz-
ards. For example, liquefied natural gas (LNG) storage vessels must be equipped with pressure-release valves to prevent the pressure buildup that can occur when LNG warms and changes to a gas- eous state. Special care must be given to the transport, transfer and storage of compressed natural gas (CNG) and LNG to avoid leaks and tank ruptures. Main- tenance shops where NGV vehicles are repaired must be modified to detect CNG leaks, have emergency ventilation sys- tems, and be fitted with special heating and electrical fixtures to prevent sparks. Even using ULSD is not without health
risks. In some urban areas, district op- erators who
continue to use older
diesel-powered school buses without ex- haust treatment systems found on 2004 or newer vehicles, significantly contribute to ambient levels of particulates. When combined with trucks of the same vintage they comprise 10 percent to 30 percent of fine particulate emissions. Particulates reduce visibility and have been implicat- ed with adverse health effects. For those health risks, diesel emissions are the sub- ject of continued debate, controversy and scientific research.
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