Power follower control

In this strategy the ICE is the primary source of power and the EM is used to provide additional power when needed by the vehicle. Care is always taken to maintain the SOC of batteries within safe limits. The rule base that is generally used is:

i.  Below a certain minimum vehicle speed, only the EM is used.

ii.  If the demanded power is greater than the maximum power that the ICE can produce at its operating speed, the EM is used to produce excess power.

iii.  The EM charges the batteries by regenerative braking.

iv.  The ICE shuts off when the power demand falls below a limit at the operating speed. This is done to prevent inefficient operation of ICE.

This is a very simple and effective strategy but the major disadvantage is that the efficiency of the entire drivetrain is not optimized.

Modified power follower

In order to improve the power follower controller a cost function is introduced. The role of this cost function is to strike a balance between fuel consumption and emissions at all operating points of HEV. The rule base for the proposed strategy is as follows:

Define the range of operating points : The range of operating points (distribution of ICE and EM torques) is represented by the range of acceptable motor torques for the current torque request. The relation between the ICE, EM and requested torque is given by

Tice = Trequest - KTem where K = motor to ICE gear ratio

(1)

The greatest possible positive motor torque defines one extreme of the operating point range: This value is the minimum of three values:

a.  The driver's torque request

b.  The maximum rated positive torque of the motor at the current speed

c.  Maximum available positive torque from the EM, according to the limits imposed by the capability of the batteries

The greatest possible negative EM torque defines the other extreme of the operating point range. This value is the maximum of:

a.  The difference between the driver's torque request and the maximum positive torque available from the ICE

b.  The maximum rated negative torque of the EM at the current speed

c.  The maximum available negative torque from the EM, according to limits imposed by the capability of the battery.

For each candidate operating point, calculate the constituent factors for optimization: The following steps are involved in this step:

a.  Calculate the fuel energy that would be consumed by the ICE. The actual fuel energy consumed for a given ICE torque is affected by two things:

•  Hot, steady state ICE fuel maps

•  Temperature correction factors

For a given torque request and motor torque, equation 1 sets the ICE torque. At this torque and given speed, the ICE map provides the fuel consumed by the ICE when it is hot ( Figure 4 ).

A cold ICE uses more fuel than a hot ICE. A cold ICE correspondingly produces more emissions than a hot ICE. The outputs of the ICE for cold and hot operation are given by

(2)