82 CHAPTER 5 FORMULATION OF CONTROL STRATEGY AND PROTOTYPE DEVELOPMENT 5.1 INTRODUCTION Plug-in hybrid implementation in a two-wheeler is a good tradeoff between an electric and IC engine power which ensure sufficient all-electric range and minimum emissions as well. For heterogeneous India’s traffic pattern, a single operating mode of the vehicle cannot satisfy the driving pattern. In order to formulate the control strategy, all types of driving modes need to be considered. The development of prototype vehicle involves the design of control system with control strategy suitable for Indian city driving conditions. This chapter discusses the formulation of control strategy and development of control system followed by the conversion of selected base two-wheeler into of plug-in hybrid electric two-wheeler. 5.2 FORMULATION OF CONTROL STRATEGY The integration of conventional vehicle components with electric propulsion components results in a vast number of potential hybrid electric configurations. The series hybrid electric configuration is an interesting solution for driving in urban areas with passenger cars, light duty vehicles as well as with heavy-duty vehicles like city buses. On the other hand, parallel hybrid electric powertrain configuration is more suitable for the family or higher class vehicle segment, while driving on highway and long
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CHAPTER 5
FORMULATION OF CONTROL STRATEGY AND
PROTOTYPE DEVELOPMENT
5.1 INTRODUCTION
Plug-in hybrid implementation in a two-wheeler is a good tradeoff
between an electric and IC engine power which ensure sufficient all-electric
range and minimum emissions as well. For heterogeneous India’s traffic
pattern, a single operating mode of the vehicle cannot satisfy the driving
pattern. In order to formulate the control strategy, all types of driving modes
need to be considered. The development of prototype vehicle involves the
design of control system with control strategy suitable for Indian city driving
conditions. This chapter discusses the formulation of control strategy and
development of control system followed by the conversion of selected base
two-wheeler into of plug-in hybrid electric two-wheeler.
5.2 FORMULATION OF CONTROL STRATEGY
The integration of conventional vehicle components with electric
propulsion components results in a vast number of potential hybrid electric
configurations. The series hybrid electric configuration is an interesting
solution for driving in urban areas with passenger cars, light duty vehicles as
well as with heavy-duty vehicles like city buses. On the other hand, parallel
hybrid electric powertrain configuration is more suitable for the family
or higher class vehicle segment, while driving on highway and long
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distances. In addition, a series-parallel hybrid powertrain system has a
complex transmission and increase in the number of components also makes
the integration more complicated (Montazeri-Gh et al 2006). As the
complexity of the vehicle configuration is increases, so do the demands for
control. As people may expect, there is no universal architecture that can be
considered superior in all practical aspects such as energy efficiency, vehicle
performance and range, driver comfort, manufacturing complexity, and
production cost. Therefore, in practice, automakers may choose different
architectures to achieve different goals and meet distinct transport segment
requirements.
Besides the powertrain configuration, a suitable power and energy
distribution system is also important. The control strategy plays a basic role.
A control strategy is an algorithm that manages the power split between the
IC engine and the electrical machine in order to reduce fuel consumption and
pollutant emissions. In a plug-in hybrid electric vehicle, the strategy will
attempt to use most of the energy from the battery pack. However, majority of
global two-wheelers population utilizes small displacement engines, generally
in the order of 50-150 cc. Hence, for two-wheelers of simple architecture with
low cost operation, there is a need to develop a simple powertrain design with
a simple control strategy which is less complex and easy for retro-fitment.
Electrification of kilometres through charge depleting operation in a
PHETW is expected to be a cost-effective way to continue to reducing fuel
consumption beyond HEV technology capabilities. The designed control
strategy does not necessarily provide maximum fuel savings over all driving
demand. This is because the national average daily travelled distance by
two-wheelers in India is close to 24 km/day. As per the survey, it is also
observed that about 61% of two-wheelers drive less than 25 kilometres per
day. Only 7% of two-wheelers travel more than 50 km per day and about 32%
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of two-wheelers travel in between 25 to 50 km per day. Therefore, choosing
the right electric range to handle the daily driving needs is essential. The
control system should utilize a strategy modified in real time depending on
the input from various sensors in the system.
Two control strategies can be applied to PHEV: the all-electric
strategy and the blended strategy (Markel 2007). In all-electric strategy, the
electric motor supplies all the power needed for the vehicle until the battery
reaches the predetermined minimum SOC level. In blended strategy, both
motor and engine work together to provide the power requirements. In this
work, both all-electric and blended strategies have been adopted to suit
two-wheelers in Indian cities to realize better driving performance and good
energy management. In all-electric strategy, it has been planned to cover
average daily travel distance with zero emissions. However, by selecting the
blended strategy at the beginning of the journey itself, the vehicle can travel
more than double the all-electric range with improved fuel economy and
minimum emissions. In both the strategies, the energy from the battery pack
has charge depleting mode. Therefore, three distinct modes were derived and
the switching logic was drafted for each mode of driving. The three modes of
operation are namely electric mode, hybrid mode and engine mode. The
electric mode uses all-electric strategy and hybrid mode uses blended
strategy, whereas engine mode is similar to conventional vehicle operation.
Figure 5.1 shows the simple flow chart of plug-in hybrid electric two-wheeler.
The rider can select the modes based on the driving range and battery pack
state-of-charge.
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Figure 5.1 Plug-in hybrid electric two-wheeler flow chart
5.2.1 Electric Mode
The electric mode of the prototype vehicle aims at providing an
eco-friendly transportation solution in urban driving. In this mode, the
converted plug-in hybrid electric two-wheeler utilizes power from the battery
alone with zero tail-pipe emissions. The charge-depleting all-electric strategy
emphasizes all electric vehicle operation over a desired distance in which
battery discharges to a minimum threshold. So, this mode has all the
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advantages of electric vehicle. The developed prototype vehicle has been
designed to provide an all electric range of about 25 km with further scope for
improvement, as range is a function of both battery size and amperage.
Increasing the energy capacity of the battery pack provides the ability to
extend the driving distance using electricity, but it would increase the
incremental cost of the battery.
5.2.2 Hybrid Mode
The effectiveness of fuel consumption in hybrid mode depends not
only on vehicle design, but also on the control strategy used. It defines how
and when power and energy will be provided or consumed by various
components of the vehicle (Markel and Wipke 2001). The charge-depleting
blended strategy of hybrid mode in plug-in hybrid electric two-wheeler aims at
providing the maximum utilization of the available energy - battery and IC
engine to run the vehicle. This mode is primarily meant for striking a balance
between emissions and the range of the vehicle.
In the hybrid mode, the control strategy is formulated in such a way
that the IC engine idling and low power modes could be eliminated to a great
extent. For the starting of the vehicle and at low speed-high torque region, the
battery pack supplies the power to the hub motor. The engine is OFF during
idling and low-load driving conditions where the engine efficiency would be
low. However, an electric motor provides high efficiency at low load when
compared to an engine. By adopting this control strategy, IC engine idling and
inefficient engine operation at low power modes are eliminated, which in turn
improves the overall efficiency and reduces the fuel consumption, thus reducing
emissions.
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SPEED >
SET SPEED
Figure 5.2 Hybrid mode flow chart
Figure 5.2 shows the logic flow chart of hybrid mode. The IC engine
takes over only when the speed of the vehicle exceeds the set-speed after a
delay of 5 seconds. The engine will be switched off when the speed of the
vehicle reaches below the set speed and remains in that state for about 5
seconds. However, the set-speed can be varied using the key pad built in the
control system. In the hybrid mode, the IC engine delivers power for high
speed driving and for hill climbing, while the electric wheel hub motor in the
front wheel is engaged for low speed driving. A unique feature of the control
strategy helps in eliminating the idling and low power operations of engine
for better fuel economy.
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5.2.3 Engine Mode
If the total distance to be travelled on a given particular day is more
than the daily average travel distance or if the battery packs SOC level drops
below the minimum threshold limit, the system will permit neither the electric
nor the hybrid mode. The conventional engine mode can be switched ON.
Hence, unlike the electric two-wheeler the range is not limited. This mode is
primarily meant for high speed driving and cruising wherein speed variations
are minimum. Better utilization of the IC engine is done at higher speeds and
the driver has the option of choosing the engine mode during off-peak traffic
hours etc. Since the electric and hybrid modes of a developed Plug-in hybrid
electric two-wheeler are designed for single rider with an average weight of
70 kg, whenever the pillion weight is added to the total vehicle weight the
engine mode will render support without losing the original performance of
the base vehicle.
5.3 DEVELOPMENT OF CONTROL SYSTEM
The control system is an important element in the development of
plug-in hybrid electric two-wheeler. It provides the path for flow of energy
between the various components when the vehicle is in motion. The main task
of the control system is to shift the power required by the vehicle between IC
engine and wheel hub motor. Figure 5.3 illustrates the block diagram of
control system with electronic accessories used respectively. The control
system utilizes a real time strategy depending on vehicle speed. Switching
from electric to hybrid mode and vice versa is facilitated by a microcontroller
which is provided with the above input. The control strategy is fed to the
controller in the form of a coded logic. So, based on the input signals, the
microcontroller decides the energizing of the corresponding relays so as to
actuate the respective relay. This microcontroller is programmed to work in
all the three modes of the control strategy.
Figure 5.3 Block diagram of control system
5.3.1 Microcontroller
The microcontroller is the heart of the control system that decides
the vehicle’s strategy and operation.
device, which integrates a number of the components of a microprocessor
system on a single chip. It has an inbuilt CPU (Central Processing Unit),
memory and peripherals to make it appear as a mini computer. The
microcontroller that has been used for this project is from the PIC (
Interface Controller) series. PIC microcontroller is the first RISC (Reduced
Input Set Computation)
(complementary metal oxide semiconductor).
separate bus for instruction and data, allowing simultaneous access of
program and data memory. EEPROM (Electrically Erasable Programmable