Project

Study 1: BAJA ATV with CVT

From this model, the components present are:

Brake and throttle input signal:

• No brake is provided
• A constant throttle of 0.3 for the first 20 seconds, and full throttle of 1 for the remaining
• These two parameters are fed into the engine

Generic engine:

• The engine uses a programmed relationship between speed and torque, modulated by the throttle signal. The throttle input is seen above (fed into port T) and the speed-torque data points are extracted from the graph shown below and fed into the engine through creation of column vectors in MATLAB.
• Crankshaft (Port F) is connected to an engine sensor, which simply senses the angular velocity and angular displacement of the crankshaft.
• The simple gear between engine and the CVT gear box is to slow down the engine RPM (in case it is too high) before it is fed to the CVT. An appropriate gear ratio needs to be selected so that over or under-revving isn't an issue.

Continuously Variable Transmission (CVT):

• As evident in the picture above, two cone-shaped pulleys connected by a steel or composite belt, whose diameters are continuously variable, work together to provide infinite gear ratios.
• Optimizing engine speed in a variety of road conditions, according to the speed and torque requirements of the terrain.
• Differ from automatic transmissions in the sense that there aren't gears the vehicle needs to step up/down to for different speed/torque requirements. Similar to automatic transmissions in the sense that the gears are shifted automatically.

CVT subsystem block:

• The input and output shaft inertia represent primary pulley (driver) and secondary pulley (driven), respectively.
• The variable ratio gear block represents the multiple gears of the CVT, with the gear ratio input given to Port R (shown below)
• Two rotational motion sensors are for tracking the angular velocities of the two pulleys.

Vehicle body subsystem block:

• The Vehicle Body accounts for body mass, aerodynamic drag, road incline, and weight distribution between axles due to acceleration and road profile.
• The Double-Shoe Brake is to apply brakes to the rear wheels. With this component, two brake blocks are applied at opposite ends of the drum. This prevents an unbalanced force from acting on the shaft and bearing connecting the drum to the wheel, which would be the the case with a single-shoe brake.
• All of the above is applied to the four wheels

Vehicle speed output based on throttle and brake signals:

• There is constant acceleration at throttle of 0.3 from 0 to 20 seconds
• After 20 seconds the car accelerates until peak velocity as throttle hits a max value of 1

CVT shaft speed output based on CVT gear ratio input:

• The CVT gear ratio input is above 1 throughout, which is why we see a lower secondary RPM than primary, because the output shaft will rotate slower to provide more torque to the wheels.

Varying CVT gear ratio:

• As soon as the CVT gear ratio falls below 1 (at about 28 sec), the secondary RPM becomes higher than the primary RPM as the velocity of the vehicle also increases with the reduction in torque.

Study 1b: BAJA ATV with CVT and Dashboard

• The only difference between Study #1a and #1b is that the throttle and brake inputs are given via graphical representation (dashboard control knobs) blocks while the simulation is running for an indefinite period. This allows us to input various scenarios.

Study #2: BAJA ATV with Lookup table

• The only difference with this study is that the CVT gear ratio isn't fed in via a signal builder, but instead via a lookup table (refer below).
• This lookup table is data regarding the relation between CVT gear ratio and vehicle speed, which is incorporated through a simple MATLAB script.
• In Study #1, a CVT gear ratio had to be inputted manually to see the effect on the vehicle's velocity. However, this isn't feasible in reality and doesn't mimic what happens in real time. Through this lookup table, the CVT can be modeled based on the vehicle speed as feedback is taken from vehicle velocity to calculate gear ratio (refer to the feedback loop above - vehicle velocity input into CVT subsystem block).

Conclusion:

The BAJA ATV with CVT was modelled using two main studies. With the focus purely on CVT, this project is able to show the effect of CVT on vehicle velocity, although multiple situations for different models of ATV design.