Week-4 Challenge WOT Condition Part-2

Problem Statement:

1. What is the difference between mapped and dynamic model of engine, motor and generator? How can you change model type?
2. How does the model calculate miles per gallon? Which factors are considered to model fuel flow?
3. Run the HEV ReferenceApplication with WOT drive cycle. Change the grade and wind velocity in the environment block. Comment on the results.
4. Keeping all other parameters same, compare the simulated results of hybrid and pure electric powertrains.

Solution:

1. Difference between mapped and dynamic model of Engine, Motor and Generator:

Mapped Engine	Dynamic Engine
Uses Steady-state look-up tables to characterize the engine performance	The engine behavior is broken down into engine characteristics seperated by lower level components. By combining these subsystems, the dynamic effect is captured.
Engine data from Dynamometer or GT-power is required.	Component level data is required.
For quasi-steady state simulation	To analyze the impact of engine components on overall performance

Mapped Motor	Dynamic Motor
It is based on the tabulated Torque-Speed envelope	It is based on the detailed design parameters of the motor
The block assumes that the speed fluctuations due to mechanical load do not affect the motor torque tracking	Transient effects are taken into account
The output torque tracks the torque reference demand and includes a motor-response and drive-response time constant	The block uses the three-phase input voltages to regulate the individual phase currents, allowing control of the motor torque or speed

To change the mode type:

A subsystem will be having numerous variant as shown in the image below:

The HEV multimode reference application engine variants are shown in the above image. The passenger car block has the engine, electric powerplant and drivetrain block. The engine subsystem has numerous options or variants to select from. However, only one variant can be active at a time and other variants are disabled. The active variant is darker as shown in the above image and the application runs with the active variant under consideration.

Go to the passenger car block and select engine block. The engine block gets highlighted. In the toolbar, under the Variant Subsystem section, Active Choice shows the variant that is active. Click over the drop-down menu. Then the numerous variants available within the subsystem are listed. Select the variant to activate it. This automatically disables the previosly active variant.

2. Calculation of Miles per Gallon and factors in fuel flow model:

• BattPwr (Battery Power) is taken as input. It is the value of battery power consumed by the vehicle at the end of the drive cycle in terms of Watts. Using Product block, power is converted to kilo Watts. Product block is then used to divide battery power with 33.7 to obtain US gallons per hour as output. Then again Product block is used to convert the value to US gallons by dividing input by 3600. Using Gain block, the gallons are converted to m^3 by multiplying gallons with 0.00378541, since 1 Gallon=0.00378541m^3.
• FuelVolFlw (Fuel Volume Flow) is taken as input, representing the value of the fuel used by the vehicle in m^3 per second. This fuel volume is added to the fuel volume equivalent of battery power. The total rate of flow of fuel is integrated over the complete drive cycle to obtain the total volume of fuel consumed in m^3 by the vehicle using an Integrator block.
• VehSpd (Vehicle speed) is taken as input. The square root of square of vehicle speed is obtained and then using the Integrator block, the distance travelled by the vehicle in meters in obtained.
• The distance in metres is fed into Gain block to convert it to miles (1 m=0.000621371 mile). The total fuel volume is put through Gain block to obtain the US gallon equivalent (Using 1 m^3=264.172 US gallons). The Product block is used to divide total miles with total fuel in gallons to give miles per gallon as output.

Factors considered in fuel flow model of:

1. mapped engine:

2. Dynamic SI engine:

3. Dynamic CI engine:

3. HEV ReferenceApplication with WOT drive cycle:

• Theory:
  • 
  • The above image shows the Hybrid Electric Vehicle configuration. It has three prime movers: Engine, motor and generator. It has two power source: A fuel tank and battery. The engine power can be seen transmitted to a generator which in turn charged battery and engine power is also transmitted to the wheels directly through an intermediary transmission line: gears, clutch and differential. The motor is taking battery stored electricity as input and transmits power to the wheels through intermediatory gears.
• Procedure:
  1. Go to Powertrain blockset documentation>Get started>Hybrid and Electric Vehicle Reference Application Projects>Explore the Hybrid Electric Vehicle Multimode Reference Package
  2. To open the working copy of the application, click over 'autoblkHevStart'
  3. This opens the Simulink model of the HEV multimode model as follows:
  4. RefSpd block refers to the reference speed block, that is the drive cycle data. Click over the block and select the required drive cycle.
  5. As shown in the above figure, Wide Open Throttle is selected from the drive cycle source.
  6. The start time is set to 5 seconds. This is the time after which vehicle starts moving.
  7. The initial reference speed is set to 0 m/s. It signifies that wide open throttle is applied when the vehicle is initially at rest.
  8. Nominal reference speed is set to 60 m/s or 135 mph. This represents the top speed of the vehicle, which the vehicle is intended to achieve.
  9. Time to start deceleration is set to 70 seconds. This is the time up to which the Wide Open Throttle condition is in place. After the lapse of this time, the vehicle starts decelerating rapidly.
  10. Final reference speed is set to 0 m/s. This represents the speed that vehicle should achieve after rapid deceleration.
  11. Simulation time is set to 100 seconds.
  12. Now select the Environment blockset. Click over the grade constant block and enter value 10. This is the degree of slope of the gradient. Then click over the wind_x constant block and set the speed to 10. It is the speed of the wind blowing against the vehicle. The set values can be seen below.
  13. Now run the simulation. After the completion, click over the Visualization block and then over performance and FE scope to get the output plots.
• Output:

From the above plot, it can be observed that the HEV powertrain is not able to meet the demanded speed on the grade and against the wind due to the battery power output constraint. Point 1 represents the time when the vehicle is about to start moving and 2 when the vehicle is about to start decelerating. The actual top speed achieved by the vehicle is 61.7 mph in the time span of 65 seconds. Trace selection is done using the measurement option in the toolbar.

During the WOT, the battery SOC reduces from 80% to 57%.

Point 1 is the start of deceleration from top speed and 2 is the end of deceleration at which the vehicle is stationary. From the right panel, it can be seen that, the vehicle takes 3 seconds to stop with the deceleration rate of 20.315 mph/s or 9 m/s^2. In the speed plot, the motor speed (blue line) drops from 7500 rpm to 0 rpm, indicating that the wheel torque was provided by the motor. The electricity generated by the motor is feb back to the battery. In the torque block, it can be observed that the motor torque is negative.

In the battery current plot, the constant current drops from 200 A to -200 A during the deceleration phase. This indicates that the direction of current flow is reveresed. The wheel torque is utilized to run the motor as generator in the regeneration mode.

In the first phase of acceleration, shown by points 1 and 2, it can be seen in the Battery SOC plot, the SOC drops from 80% to 68.8% rapidly with steeper slope. During this phase, in the motor speed plot, it can be seen that only motor is active, obtaining energy from Peak Power Source: Battery. In the torque plot, the motor can be seen working in constant torque region at 200 N-m. This motor torque is limited by the peak current output of the battery. The peak current is set at 500 A.

The acceleration in this phase can be measured using trace selection. Delta Y represents the change in velocity in this rapid acceleration phase. The vehicle reaches a speed of 44 mph or 70 kmph in the span of delta T=17 seconds. Accelertion is given by the slope of the line: delta Y/ delta T = 2.618 mph/s or 1.16 m/s^2.

The points 1 and 2 represents the slow acceleration phase during which, the motor works in constant power region as the torque falls and speed of the motor increases. In the torque plot, engine torque can be seen reflecting the generator torque. It signifies that the whole engine power is transmitted to run the generator, which then recharges the battery. During this phase, the motor is absorbing the battery energy while the engine is charging the battery through generator. Since the power required by the vehicle is more than power generated by the engine, battery SOC keeps dropping but with lower slope.

The magnitude of the slower acceleration phase is measured by this trace selection. The slope measured is 0.226 mph/s or 0.1 m/s.

Here the trace selection points 1 and 2 represents the increment of battery SOC from 57% to 66% in the duration of 29 seconds. During this time, the vehicle decelerates and comes to rest. So the motor speed and torque reduce to 0. However, the engine keeps running and transmits the power to generator, which feeds the electricity back to battery and recharges it. As the vehicle consumes no energy, the battery SOC stops discharging. During this period, the current is negative, indicating that the battery is being charged.

4. Electric Vehicle Reference Application under WOT condition and comparison with HEV results:

• Theory:
  
  • 
  • The Electric Vehicle configuration is displayed above. It has battery as power source and an electric motor as the prime mover. The motor takes charge from battery as input and generates rotational kinetic energy, which is transmitted to wheels through intermediatory gears.
• Procedure:
  1. Go to powertrain blockset documentation>Get Started>Hybrid and Electric Vehicle Reference Application Projects>Explore the Electric Vehicle Reference Application and then click over 'autoblkEvStart'
  2. Select Refspd block in the Simulink and select WOT drive cycle with the same speed and time conditions as in HEV appliaction.
  3. Go to environment block and set the grade and wind velocity to same values as earlier.
  4. Set the simulation time to 100 s and after the completion, go to 'Visualization' block and click over 'Performance and FE scope' to get the result plots.
• Output:

The trace selections represent the Wide Open Throttle condition. The EV power train is able to achieve a top speed of 51.7 mph against wind velocity of 10 m/s and grade of 10 degrees. This is due to the peak current output constraint of about 200 A from the battery. From the battery SOC plot, the battery is seen discharged to 74% with initial charge of 80%.

It is shown that the motor provides instantaneous torque of up to 280 N-m which is the maximum torque of in constant torque region of motor operation. As the motor speed increases to attain the top speed, the motor operation transits to constant power region where the torque output drops to 131 N-m.

The trace selections show the deceleration phase. The time taken to bring the vehicle from top speed of 51.7 mph to rest is 4 seconds. The vehicle decelerates at the rate of 13 mph/s or 5.77 m/s. The current flows in opposite direction with a peak magnitude of -170 A. This current is bed back to the battery by the motor which works as a generator by absorbing the kinetic energy of the wheels.

Here, it is evident that the battery is charged during deceleration. The battery SOC increases from 74.02% to 74.15%. It can be inferred that the charge is insignificant. This is due to the fact that vehicle is brought to rest naturally by the environmental condition. The gradient force and aerodynamic drag stops the vehicle and hence the motor does not receive good amount of wheel torque.

• Comparing the WOT of HEV and pure EV:
  • As HEV consists of engine and motor, utilizing the combined power of both the prime movers with fuel tank and battery as power source, HEV accelerated to top speed of 61.7 mph. Whereas, EV with only one power source: battery and one prime mover: motor, could generate sufficient power to accelerate to 51.7 mph.
  • At the end of deceleration, when the vehicle stopped, the HEV had consumed miles per gallon equivalent of 6. Whereas pure EV consumed battery energy of 19.2 MPGe.
  • In the HEV case, it can be seen that before the engine is activated, the peak current drained from the battery is around 500 A. Immediately after engine activation, the current drained from the battery drops to approximately 200 A. This shows that the torque demanded by the vehicle is partially fulfilled by the engine. Hence peak current output varies in HEV, while it remains the same in pure EV case.
  • In the HEV case, the battery SOC falls 23% during WOT and in the Pure EV case, the battery SOC falls 6%. This implies that HEV has a smaller battery size and Pure EV has bigger battery. This is due to the fact that HEV has an alternative power source and battery dimensions are limited.

Reference:

• Internal Combustion Mapped and Dynamic Engine Models - MATLAB & Simulink - MathWorks India