1(^2) %
There are two possible operating strategies for a parallel hybrid:
Use the electric motor for base (light) loads, while using the engine to provide power at higher
loads. Depending on vehicle load requirements, the engine is turned on and off.
Use the engine for the light load and the electric motor for short-term peak loads. In this case,
the engine operates steadily.
VW has chosen the first approach, and has used a small electric motor with 9 kW peak output to
aid a diesel or gasoline engine. The motor is used exclusively at all loads below 7 kW,
corresponding to a cruise speed of 40 mph on a level road; the engine is started instantaneously
when more power is needed. This vehicle, based on the VW Golf, consumes 2.8 litres of diesel
per 100 km, and 15.8 kWh of electric power, on the FTP urban cycle.^65 If the electricity were
generated (for example) at 34 percent energy efficiency at the wall plug from primary fuel the
hybrid would have a fuel consumption of 4.05 litres/100 km diesel equivalent, which is 35.8
percent better fuel economy than the conventional Golf diesel.
.
Project staff had an opportunity to drive the hybrid Golf, and the impression was that the
vehicle behaved quite differently (uncomfortably so) from a conventional auto. In particular, the
transitions between electric motor operation and engine operation during city driving were
disconcerting, although this impression may disappear with driving experience or with a more
advanced design. For this type of vehicle, the diesel is the more suitable engine because its hot
restart occurs in half a revolution of the engine, whereas hot restart on a gasoline engine is slower
and could have significant emission penalties. With a diesel engine, however, emissions over the
driving cycle are reduced significantly. It seems possible that a diesel-based parallel hybrid
using this operating strategy might be capable of meeting the ultralow emission vehicle
(ULEV) standard.
In the second type of strategy, where the ICE is on continuously (except possibly at idle, where
it could be turned off) and the electric motor is used for peak loads, most of the fuel economy
gains are associated with engine downsizing, at least on the FTP cycle, where hard accelerations
are not required. For a “type 2“ parallel hybrid, the electric motor power and battery storage
capacity are relatively small; coupled with the smaller engine, the overall vehicle weight should
decrease.
Two alternative specifications for mid-size parallel hybrid vehicles that provide near equal
performance (at speeds below 70 mph) to the baseline vehicle are shown in Table 4-14. The first
hybrid uses a 2.0-litre engine and a flywheel for energy storage, while the second uses a 1.0 litre
engine with a battery for energy storage. Either type of strategy can be incorporated with both
hybrid vehicles. The type 2 strategy of using the engine for peak loads could provide fuel
economy gains of approximately 25 to 30 percent in the first vehicle, and 30 to 35 percent
in the second, compared with equivalent vehicles with conventional drivetrains. However,
drivability and hot restart problems (with a gasoline engine) with these configurations
could be daunting. The fuel economy gains are estimated to be half as much using a type 2