Environmental Engineering Reference
In-Depth Information
E ob
J elec
J elec ¼ 38 : 322 kWh = gallon
E ob ¼ kWh supplied off -board
V eq ¼
ð gallon equiv Þ
ð 1 : 46 Þ
For example, suppose the HEV20 vehicle is driven over the specified highway
fuel economy test (HWFET) or the highway fuel economy drive schedule
(HFEDS), as specified in SAE J1711. If the vehicle consumes half of its fuel tank
capacity (8.4 US gallon) and half of its on-board energy storage (5.9 kWh) and
drives a total of 178.35 mi, half of the on-board stored electric energy in gallons-
equivalent. From (1.46) the fuel equivalent of off-board recharge energy amounts
to 0.077 gallon for a total fuel consumed of 4.277 gallon. The adjusted fuel econ-
omy is then 178.35/4.277 = 41.7 mpg.
A CS hybrid, on the other hand, will have a much smaller on-board energy
storage system that is replenished during driving when the opportunity arises
(downhill coasting, regenerative braking etc.). SAE J1711 makes provision for this
HEV0 vehicle by running two back-to-back Federal Urban Drive Schedules (FUDS)
cycles separated by a 10 min break or two HWRET cycles separated by a 15 min
break. The first run is a warm-up and the second is counted towards the procedure.
Today fuel economy simulations are performed with 'forward' models as
opposed to early 'backward' models in which all the road load and driveline losses
are accumulated and the engine and/or M/G then set to deliver the demanded
power. The reason for this was explained earlier in this chapter. A forward model in
contrast uses a feedback process to 'drive' the vehicle in simulation by mimicking
what a test driver would do to follow the driving schedule on a chart recorder as the
vehicle was running on a dynamometer. In effect, the forward model adds a driver
model and feedback to the backward model. The forward model therefore requires
a more refined and very accurate engine, M/G and vehicle system models, parti-
cularly of the energy storage system, to function properly. Battery models are now
tending to available energy dynamic models that are amenable to such forward
simulation work. A brief description of this work can be found in Reference 23.
A good description of CD versus CS is given in Reference 18 and this is
illustrated in Figure 1.48.
In this figure the CD mode transitions to CS somewhere during tests T4 and
T5. In one method the SAE J1772 task force members fit trend lines to the CD and
CS modes and call the intersection the termination of CD. In another method the
burden is placed on the PHEV RESS controller to compute instantaneous SOC
second by second, and the first occurrence of SOC equalling CS SOC is termed the
termination of CD mode. When blended mode is being used, the challenge is more
difficult because engine-ON operation starts somewhere in the T2 to T3 regions of
the test in Figure 1.48. So, a utility factor has been developed to account for dif-
ferent usage patterns and from this to compute an effective fuel economy. The
thinking now is that a true measure of PHEV fuel economy that properly accounts
for utility supplied electricity is to use ac line kWh as energy input. With this
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