Week-11 Challenge: Braking

QUESTION -

1. For a defined driving cycle, calculate the energy required for braking.

Solution -

Here we are considering the Drive cycle and Calculating the Energy required for Braking.Following picture shows the drive cycle of Time Vs Velocity and its corresponding plot.

The equation for kinetic energy from braking is given as,

Where,

m = Mass of the vehicle in Kg.

Vi = Initial speed of the vehicle in m/s.

Vf = Final speed of the vehicle in m/s.

Here we are considering the Kerb weight of vehicle is 1000Kg and Payload on Vehicle is 500Kg.

From the above drive cycle, there are 3 different situations of decceleration of which the energy has to be calculated.

First Braking : 30 - 24km/hr in 5 seconds

Second Braking : 62 - 45km/hr in 3 seconds

Third Braking : 67 - 43 km/hr in 4 seconds

Energy, Power and Bracking power is calculated in the sheet as shown in above.From the above calculations we get the total Energy required for Bracking is 0.005969Kwh.

2. Why electric motor can’t develop braking torque at high speed similar to starting? How electric and mechanical brakes are coordinated?

Solution -

Regenerative braking allows electric vehicles to use the motor as a generator when the brakes are applied, to pump vehicle energy from the brakes into an energy storage device. Regenerative braking is an effective approach to extend the driving range of an EV.

• The regenerative braking torque cannot be made large enough to provide all the required braking torque of the vehicle as it might lead to the deterioration in stability of the vehicle. Also as compared to the torque values of acceleration torque of motor is much higher than decceleration torque.
• At high speed, the motor might be in its constant power region which causes less values of torques produced and this limits the amount of braking torque that is used for regeneration. Hence the peak vlaue of accleration torque and braking torque are not equal.

Electric and Mechanical brakes Co-ordination:

The regenerative braking system may not be used under many conditions, such as with a high State of Charge (SOC) or a high temperature of the battery. In these cases, the mechanical braking system works to cover the required total braking torque.

Thus, cooperation between the mechanical braking system and the regenerative braking system is a main part of the design of the EV braking control strategy and is known as torque blending.

There are 2 different control strategies for Braking Combinations:

1. Series Control Strategy
2. Parallel Control Strategy

Series Control Strategy:

• a combination of friction-based mechanical braking system with a regenerative braking system that transfers energy to the battery with an integrated control, which distributes the required braking force between the regenerative braking system and the mechanical braking system.
• Series Startegy could give an increase of 15-30% in fuel efficiency and has more consistent braking due to good torque blending capability.

Parallel Control Strategy:

• combination of mechanical and the regenerative braking system, operated simultaneously without an integrated control.
• The regenerative braking force is added to the mechanical braking force which cannot be controlled.
• The regenerative torque is determined by considering the motor capacity, battery SOC, and vehicle velocity. The regenerative braking force is calculated from the brake control unit by comparing the demanded brake torque and the motor torque available.

Application of Braking Combination : EDIB (Electric Driven Intelligent Brake) in NISSAN LEAF

• EDIB allows and controls optimization of the regenerative brake and regular friction brake (hydraulic brake).
• The system produces an adequate braking force keeping energy regeneration increased as much as possible.

• At higher speeds, a combination of regenerative and friction braking is applied.
• At lower speeds, regenerative braking is given a higher proportion for maximised energy conversion.

3. Make a MATLAB program which plots contour of given motor speed, torque and efficiency values. Attach the code as a .m file attach a screenshot of all the plots.

Solution -

Matlab Script -

Above MATLAB Script is used to plot the Efficiency contour. Firstly we are defining the speed and Torque using linspace command the we are defining the motor constants.Then we are calculate the Output power, Total losses and Input power from the formula as shown in the above script. Now the different values of Efficiency are selected for the contouring.Then graph is plotted by using "contourf" command. Labels and Title is provided to the plot as shown in the above script.

Output -

The contour plot for a motor with defined constants are plotted in the defined Speed Vs Torque Region based on efficiency. It can be seen that for increasing speed and torque, the efficiency of the motor gradually decreases. The colourbar indicates the different values of efficiency.As we go inside the efficiency increases and efficiency gets decreased at outside.