Advanced Automotive Technology: Visions of a Super-Efficient Family Car

The above analysis clearly indicates that restricting the engine in a series hybrid to
operating only at its most efficient point is not a practical strategy; the theoretical
advantage in efficiency is overwhelmed by both the requirement for a very large engine and
the energy and emissions penalties from turning the engine on and off during operation. A
more practical alternative is to use a smaller engine running at its most efficient point most of the
time, with short-term high-power needs met by the battery (or other storage device) and longer-
term power needs, such as hill climbing, met by allowing the engine to increase its output. In
other words, if high-peak loads persist for over 20 or 30 seconds, the control logic can allow the
engine to provide more power rapidly (albeit with lower efficiency) so that the batteries are not
taxed too heavily. To avoid too large an efficiency loss, the engine can be constrained to stay
within 10 percent of the maximum efficiency--a constraint that still allows a substantial increase in
available power. The only disadvantage of this strategy is that the battery must be somewhat
bigger, to provide maximum peak short-term power with the engine operating at lower power
than the previous, larger engine. Even this has some benefits, however, because the larger storage
capacity of the battery reduces the need to turn the engine on and off, thus reducing the adverse
emission consequences.

For the same Taurus example, we have the following HEV specification:^58

l Vehicle curb weight

l Engine peak output

l Continuous output

l Engine plus generator

l Battery

peak output

energy stored

weight

type

l Motor

output

weight

In other words, the hybrid
reasonable. Its engine is now
litres, and total vehicle weight

1385 kg

44.7 kW

19.0 kW

weight 167 kg

59.1 kW

8.3 kWh

197 kg

Semi-bipolar lead acid

79.3 kW

80 kg

with a relaxed engine-operating

quite small, with a 44.7 kW peak

very similar to the current Taurus.

strategy appears much more
rating and displacement of 1.0
On the urban cycle, the engine
would be on 28 percent of the time, and shut off during the rest of the cycle. On the highway
cycle, the engine is on for 62 percent of the time, and the engine would be operating continuously
at 70 mph cruise on level ground. This is favorable for fuel efficiency because the engine would be
operating at its near optimal point, and energy can flow directly from generator to motor without
going through the battery.

The effects on fuel consumption can be estimated with reasonable accuracy using the

methodology presented in appendix A. The major assumption here is that the engine can be

operated at close to optimal efficiency, or else be turned off. The computation, described in box

(^58) Assumptions: engine weighs 2.3 kg/kW,
generator weighs 1.0 kg/kW, peak specific power of the engine/generator combination is 284 W/kg.