Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
turbocharger. In order to flow an adequate amount of EGR in a high pressure loop EGR
system, the pressure in the exhaust manifold must be higher than the pressure in the intake
manifold. This situation is referred to as running a negative ∆p. If an efficient conventional
turbocharger is used, the intake manifold pressure is often higher than the exhaust (positive
∆p). The VGT is adjusted to maintain a negative ∆p to the extent required to provide the
correct EGR flow.
Two-stage turbocharging, which utilizes a high pressure and a low-pressure turbo, also can
increase engine efficiency over a wide range of operating conditions. These two units can be
connected in series, or flow can be directed to only one unit depending on how the engine is
operating. A two-stage turbocharger system requires the use of an EGR pump, a
backpressure device, or a turbocompound system to flow EGR in an engine that uses high
pressure loop EGR. Both VGT and two stage turbo charging reduce lag effects during
transient operation, allow increased power output, and reduce NOx emissions when used in
concert with control technologies such as exhaust gas recirculation.
Improved materials and designs may eventually permit higher cylinder pressures and
therefore improved fuel efficiency, without a durability penalty. Higher pressures also
provide higher power density. Such improvements are constrained by NOx formation,
however. While high-compression ratios and combustion temperatures lead to high
thermal efficiency, durability concerns, and emissions regulations force a tradeoff by
limiting compression-related forces and allowable NOx emissions. Technologies to
increase engine efficiency must pursue the benefits of high combustion temperatures
without the associated increases in engine-out NOx when possible. Exhaust
aftertreatments are employed to bring final emissions down to allowable levels, although
these devices are generally not as cost-effective as in-cylinder design changes. This is a
field of continued study, and it is expected that as diesel NOx aftertreatment becomes
more effective, cylinder pressures will be increased in search of efficiency gains and
increased power density.
Improved fuel injectors also are the focus of research to improve engine efficiency.
Recently, manufacturers have begun moving to common rail systems whereby all injectors
are fed fuel from a single reservoir at extremely high pressure. Higher pressures generally
allow more effective mixing of air and fuel, leading to reduced PM emissions and greater
efficiency. These designs also allow flexibility with multiple injection strategies whereby
fuel is injected at more than one point during cylinder compression and expansion. This
approach can allow fuel to burn longer into the compression stroke without reaching as
high a flame temperature, thereby reducing NOx formation. There are a variety of
multiple injection strategies being pursued.
In addition to combustion improvement strategies, reducing engine accessory loads can
improve the operating efficiency of HDVs. Accessory loads require around 4 percent of
engine power output from a truck traveling at 65 mph (ORNL, 2000). The air conditioner
compressor, alternator, air compressor, cooling fan, and water pump are generally
connected directly to the engine by means of gears or a belt, so these devices are driven at
a speed directly proportional to the speed of the engine. Because accessories need to
function acceptably at all engine speeds, they are typically driven faster than necessary at
high engine speeds. Decoupling accessories from engine speed can reduce power
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