Controllers

Electronic controllers main function is to adjust parameters for the smooth operation of

the parts and select the optimum mode of operation at each point. Three types of

controllers are often employed.

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A bang-bang controller is essentially an on/off switch, much like a thermostat that

controls the temperature a room when it gets cooler or warmer than preset values. When

thermostatic controllers are used, the engine continues to run as long as the state of

charge of the battery falls below a set value. Once charge in the battery reaches a safe

limit, engine shuts off and the hybrid works essentially as a pure electric vehicle. Since

most engine emission is during cold start and transient operation, this kind of control

does not necessarily reduce emission to the maximum extent possible.

A thermostatic controller is introduced to minimize the shortcomings of bang-bang

controllers. In thermostatic control, the engine operates continuously to provide the

steady state (cruising) load demand. This type of hybrid system control typically uses the

battery State Of Charge _SOC_ or a filtered battery pack/cell voltage as the control

variable to determine the throttle command (Power generation command).

A load follower (power follower) follows the driver command. When the driver pushes

on the accelerator (throttle control), the engine cannot be operated on its optimized

operation point (sweet spot). Load follower strategy (such as used in the Prius) allows the

power to be modulated either by throttle control or engine speed, and ensuring most

efficient engine operation by providing the transient load demand, just enough to

maintain the battery_s state of charge.

Energy Management

Flexibility inherent in design of hybrid systems, allows hybrid vehicles to be operated to

achieve:

Maximum fuel efficiency

Minimal emissions

Combination of the two

These objectives can be achieved by a combination of proper hardware configuration and

a well-designed control algorithm. A proper power control strategy allows controlling the

flow of power while assuring adequate energy reserve in the storage devices. Obviously

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maintaining a reasonable cost and achieving minimum performance and handling is of

primary importance.

Hardware configuration and control strategies are designed together to achieve theses

objectives. We covered two hardware configurations, parallel and series hybrid. Each

configuration can be modified with a variety of control strategies to fit a particular need

(Figure 2). Examples are given below:

Figure 2. Energy management systems.

Power-assist (Electric-assist) Parallel

A power-assist HEV is driven by an engine, while the electric drive is mostly for starting

or high load demands. This allows the APU to operate in a more efficient region and

keep emissions low by moving away from the full throttle condition that is normally

required for acceleration and steep gradients. Regenerative power can also be used to

help boost the efficiency during urban driving. Power-assist configuration uses a large

engine with smaller battery pack.

APU-assist Parallel

In this configuration, the electric motor and batteries are used as the main power source,

while the APU is turned only on for acceleration, high speed, or steep roads. It operates

as zero emission vehicle most of the time when APU is turned off. The drawback is that

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APU comes on during high load conditions, where emission is the highest.

For this arrangement, engine is often undersized and operates closer to full load, where it

is most efficient. For meeting the vehicle requirement during transients, the electric

motor will be available to provide the additional power.

Range-Extender (Genset)

A range-extender HEV (Genset) is essentially an electric vehicle with an on-board

charging system. The objective is to allow the battery to deplete the battery to a very low

SOC before the APU is turned onto recharge the battery. Once recharged, the APU is

turned off again until such needs arise again. Range-extenders have larger battery

capacity and a smaller engine. Advantage of this control strategy is that the APU can be

set at an operating point (torque and speed) that is most efficient. The APU is off during

transients when the highest level of emissions is produced. The disadvantage of this

configuration is that batteries are in direct current and need to be converted to alternating

current before reaching the traction motor. Because of various elements in series, the

overall efficiency is lower than that of some other configurations.

Hybrids using genset (engine/generator) work on an on/off mode, i.e., they are either

switched off (zero emission) or operate at a predetermined output where they produce the

lowest emission, or achieve the best fuel efficiency (sweet spot). Typically, hybrid

gensets are not throttled for variable output, as is the case for conventional engines.

Gensets are designed to deliver average power. The battery functions to store the energy

from the regenerative braking and to supply peak power during acceleration. The battery

is normally downsized and reconfigured for maximum specific power, whereas a BEV is

reconfigured for maximum specific energy.

Range-extenders can qualify as zero-emission vehicles when operated only in electricmode

(city driving).

If the engine employs an exhaust catalyst for emissions control, the catalyst can be

electronically preheated before the engine is started to minimize startup emission.

Load-Levelers

Although, the propulsive energy is supplied by the fuel tank and the battery concurrently,

this configuration is usually considered a series configuration, because all the propulsive

power eventually passes to the driving wheels through an electric motor1.

As with the power-assist, the APU is smaller and sized to meet the average power

demand. As with the range-extenders, the engine does not need to follow the transients.

Batteries are used to provide additional power during power peaks. In this configuration,

the engine continuously runs at a steady state to produce power. If the power exceeds the

vehicle_s needs, the excess power is used to charge the battery. In cities, the engine

could be shut off, which allows the vehicle to operate as a ZEV for a limited range. The

advantage of this strategy is batteries are rather small and it always hovers around a midlevel

SOC. The engine is also relatively small. The disadvantage is that engine must

change its power output to adjust for changing load. The emissions increase as engine

deviates from its _sweet-spot_ operation.