Environmental Engineering Reference
In-Depth Information
Chapter 9
Hybrid vehicle characterization
This chapter will describe how passenger vehicles are characterized first as to drag
and rolling resistance coefficients and second according to fuel economy over
standard drive cycles. Vehicle data necessary to compute rolling resistance and
aerodynamic drag coefficients are taken from coast down tests. This procedure is
further described in Chapter 11 along with some actual test data about how this
applies to characterizing the hybrid propulsion system. In this chapter the various
standard drive cycles employed in various geographical and demographic areas are
compared and rationale given for their selection.
Coast down testing is a procedure long in existence to extract the vehicle tyre
and body aerodynamic drag characteristics. A coast down test procedure consists of
accelerating the vehicle to a prescribed speed on a straight and level road course,
holding a set speed and then, with the vehicle in neutral, allowing it to coast down
naturally. The test is then repeated in the same manner from the opposite direction
to average out any inconsistencies due to relative wind velocity and road grade.
It is important to understand the need to accurately characterize a vehicle in
terms of fuel economy and emissions by also understanding how efficiently the
fuel, derived from primary energy, is delivered to the vehicle's fuel tank in the first
place and the type of fuel being used.
There remains clear need for cleaner vehicles [1], particularly larger passenger
vehicles, sport utility vehicles and heavy trucks including line haul and over the
road (OTR) trucks. There are now some 200M passenger vehicles licensed in North
America that are fuelled by gasoline. Add to this another 270,000 propane, LPG,
fuelled vehicles, and many others operating on E85 ethanol, compressed natural gas
(CNG) and other alternatives. Most estimates put the total electrified vehicle
penetration globally at 2.5% of new vehicle sales. In 2015 there are expected to be
some 88M light duty vehicles sold of which only 2M will be electrified.
Regulated test cycles are used to characterize vehicles for performance and
economy and are typically set by national governing authorities. Today's hybrids,
including fuel cell vehicles (FCVs), in fact may not be characterized using the
proper cycles for hybrid vehicles according to Dougherty [2] because these are
basically a carry-over from the past. Changing demographics, shifting populations
and modern traffic patterns demand a fresh look at what an appropriate drive cycle
may be. This chapter summarizes the present drive cycles used by automotive
testing labs. Drive cycles used by independent testing labs include (1) for the US,
