Washington State University Extension Energy Program

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Diesel Engines
Beginning with the 2007 model year, new heavy-duty engines must meet a particulate matter (PM)
emission standard of 0.01 grams per brake horsepower hour (g/bhp-hr). The highway diesel program also
establishes standards for nitrous oxide (NOx) and non-methane hydrocarbons (NMHC) of 0.20 g/bhp-hr
and 0.14 g/bhp-hr, respectively. The NOx and NMHC standards are phased in over three years, while the
PM standard requires 100 percent compliance starting with model year (MY) 2007.
Future off-road diesel engine standards are considerably less stringent than the 2007 on-road standards.
However, they will require significant emissions improvements over existing off-road engines. The new
off-road PM standards are specific to model year and engine size, but in general, are about 20-60 percent
lower than current Tier 1 standards. For off-road engines above 175 horsepower, PM standards will drop
from 0.4 g/bhp-hr to 0.15 g/bhp-hr. For the most part, these standards will be phased in through 2005.



Natural gas
Natural gas is a gaseous fuel composed mostly of methane, with smaller amounts of propane, ethane,
helium, carbon dioxide and water. While natural gas is considered an effective alternative to diesel fuel, it
is not as convenient to use as a liquid fuel. Natural gas must either be compressed to 3,000-3,600 pounds
per square inch (psi), or liquefied through super cooling to -327.2 degrees F. In either case, refrigeration or
compression equipment is required for refueling purposes.
Compression is the most common method for delivering natural gas for vehicle use. Currently, there are
over 1,200 compressed natural gas (CNG) stations in the United States. Because of its lower fuel density,
CNG is not considered a practical fuel for long distance, heavy-duty truck applications such as Class 7 and
8 trucks. CNG is being used successfully in shorter range, heavy duty applications such as street sweepers
and refuse trucks, and has a long history of use in many medium-duty applications such as school bus and
transit fleets. For longer range applications, liquid natural gas (LNG) is the preferred fuel. While LNG
infrastructure is fairly limited at present, it is slowly being developed, primarily in Southern California, and
Arizona. Even still, it takes about 1.7 times the volume of LNG to provide an amount of energy equivalent
to a gallon of diesel.


Hybrid electric
An emerging alternative to conventional diesel engines is the electric hybrid system. Hybrid buses typically
utilize an electric drive coupled in series or operating in parallel with a combustion engine and traction
battery.
Hybrid technology allows the use of a smaller internal combustion engine which is designed to operate near
its optimum efficiency, thereby minimizing engine emissions and maximizing fuel economy. Typically, a
hybrid system also employs regenerative braking which transforms kinetic energy into electric energy,
again improving fuel economy. To a fleet operator, hybrid technology is attractive because it does not
require the development of new refueling infrastructure or modifications to existing maintenance areas.