Environmental Engineering Reference
In-Depth Information
density of 8,835 Wh/L (32 MJ/L) and taking today's alternator efficiency at 45%
on average, one finds that the cost of on-board electricity generation today is $0.26/
kWh when the fuel price is $1.55/US gallon - significantly higher than residential
electricity cost and far too expensive for an HEV. Increasing the mechanical to
electrical conversion efficiency to 85% reduces the cost of on-board electricity
generation to $0.136/kWh. Another way of looking at the difference in efficiency is
to evaluate its impact on vehicle fuel efficiency for an 80 mpg (3 L/100 km) car.
When calculated for the average speed over the Federal Urban Drive Cycle
(FUDC) (23 mph and similar to EPA-city cycle) and for an average vehicle elec-
trical load of 800 W, today's alternator lowers the fuel economy by 5.86 mpg,
whereas the more efficient power assist hybrid M/G lowers fuel economy by only
3.1 mpg. Hence, the higher efficiency of the hybrid system is worth 2.76 mpg in an
80 mpg vehicle, or 0.35 mpg fuel economy reduction per 100 W of electrical load.
In the power assist HEV, the recuperation of vehicle braking energy via
regeneration through the electric drive subsystem to the battery partially offsets the
operating cost of electricity generation. As shown in Figure 1.21, the conventional
5-passenger vehicle requires an average of 11 kW input to the traction motor
inverter for a total of 4.3 kWh energy expenditure over the FUDC [9]. However, for
this drive cycle, an average of only 1 kW can be extracted from the vehicle kinetic
energy and made available to replenish battery storage at the traction inverter dc
link terminals for a total of 0.39 kWh or roughly 10% of the energy expended in
propulsion.
35 kW
35
Average driving
inverter power: 11 kW
25
15
5
-5
-15
Average regeneration
inverter power: -1 kW
20 kW
-25
0
200
400
600
800
1,000
1,200
1,400
Time (s)
Figure 1.21 Power assist hybrid propulsion and regeneration energy
The distribution of propulsion and regenerated energy at the traction inverter
dc link terminals is shown in Figure 1.22. In particular, note that the bulk of
regenerated power falls in the 5-10 kW regime. Conversely, tractive power over
the FUDC falls in the 5-15 kW regime with a distribution tail reaching out beyond
30 kW for vehicle acceleration and grade performance. Combined with data such as
