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requirements. Since the propulsion needs are met by an electric motor, this results in the com-
plete decoupling between the diesel engine and the wheels, meaning that engine control is not
dependent on vehicle speed so offering additional flexibility [18]. This is a major advantage of
series hybrid drivetrains, where the engine can operate at any point on its speed-torque map,
which is impossible for conventional vehicles. Therefore, the engine is capable of constantly
operating at near optimum load, which minimises fuel consumption and emission [19].
The parallel hybrid configuration maintains the direct mechanical link between the diesel
engine and the wheels, using the battery for regenerative braking and supplementing the
peak power demands. The main advantages over the series hybrid are that the additional gen-
erator is no longer needed so has higher efficiency as well as reducing the size of the required
drive motor. The parallel configuration, however, does not decouple the diesel engine from
the wheels and hence operation is directly linked to the vehicle speed hence for low speed
city operation the ICE will often operate at a low efficiency [20]. As a result, the parallel con-
figuration is more appropriate for longer distance and higher speed routes. The series-parallel
hybrid can operate in either the series or parallel configurations and so can utilise the advan-
tages of both systems; however, the additional complexity and capital cost of the system
mean that they are currently not a viable option for transportation applications [19]. The most
popular option for city buses is the series configuration due to the simplicity of a single drive
system as well as higher efficiency during city driving where buses have a start-stop traffic
pattern with generally low speed operation [19].
The benefits offered by the hybridisation of the drive system relate to the increase in fuel
economy and reduction in emissions compared to a diesel bus and can be attributed to the
following points.
β’ On average buses idles for around 30β44% of urban driving time [21]. By using a hybrid
system, the vehicle can turn off the engine to prevent idling and low loads because it can
use the electrical energy storage and motor for initial acceleration. This can save 5β8% of
fuel consumption [17].
β’ A significant amount of energy is lost and dissipated by heat due to friction during conven-
tional braking. When a hybrid vehicle is braking, the drive motor can work as generator
to charge the electrical energy storage system and thus recycle some of the energy used to
propel the bus. Typically, 10β20% of the kinetic energy is recovered.
β’ In a conventional bus, the diesel engine needs to be large enough to provide for all of
the peak transient power demands. A hybrid vehicle is able to use the electrical system
to provide for a portion of these peak demands, and therefore, the engine can be down-
sized [17, 19].
β’ A diesel engine operates at its lowest efficiency during low load and low speed operation.
The electrical system can drive the electric motor to power the bus during low load and
start-up to avoid this. It is expected that diesel hybrid technology can achieve reductions of
between 24 and 37% CO2 emission [22], 21% to NOx emission and 10% to fuel consumption
compared with conventional diesel buses [7, 15].
Development of Bus Drive Technology towards Zero Emissions: A Review
http://dx.doi.org/10.5772/68139 37
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book Hybrid Electric Vehicles"
Hybrid Electric Vehicles
- Title
- Hybrid Electric Vehicles
- Author
- Teresa Donateo
- Editor
- InTech
- Location
- Rijeka
- Date
- 2017
- Language
- English
- License
- CC BY 4.0
- ISBN
- 978-953-51-3298-1
- Size
- 15.5 x 22.5 cm
- Pages
- 162
- Keywords
- Physical Sciences, Engineering and Technology, Engineering, Vehicle Engineering, Automobile Engineering
- Category
- Technik