Controls for an Electrically-Driven AC Compressor for the Challenge X Entry

Photo of Brandon Bounds, Gary McGregor, and Shaun Noorian Students: Brandon Bounds, Gary McGregor, and Shaun Noorian

Sponsor: The University of Texas Society of Automotive Engineers

Date: Fall 2007

Requirements:

In the Challenge X competition, a rules committee will award points to the different teams based upon the performance of their AC system. In order to receive full credit, the air conditioner must cool the cabin to at least 72°F. There is also a geometric requirement that needs to be completed in order to have a functioning air conditioner. The Air Conditioning Controller (ACC), with dimensions of 7 3/8” X 4 11/16”, must be installed in the engine bay. Additionally, the battery in the trunk of the Chevy Equinox must be no larger than 38 5/8” X 10”. The design team focused on quality control and signal requirements because the Equinox is not equipped with a typical combustion engine operated air conditioning system. When the vehicle comes to a stop at a light or in traffic, the engine will shut down to conserve fuel. In doing so, the Engine Control Unit (ECU) must send a signal to the ACC to switch the compressor's power supply from the Belt-Driven Alternator/Starter (BAS) to the 36V battery and the opposite signal needed to be sent when the vehicle was in motion. Similarly, the ECU also must send proper signals to the blower and compressor when the user changes settings in the cabin such as the fan speed and temperature.

Problem:

The senior design team's goal was to research, develop, and test the controls for The University of Texas Challenge X air conditioning system. The team was selected to work with the Society of Automotive Engineers on the Challenge X project. The problem presented was to develop the controls for an all electric vehicle air conditioning system, replacing the previous mechanical system.

Solution:

The design team used analytical methods to first determine the ideal method of running the compressor. Thermodynamic and efficiency analyses of the compressor and DC motor were performed. The compressor and DC motor were determined to run most efficiently in a particular rpm range, while providing sufficient cooling capabilities and consuming the least amount of power. Once the best way of running these components was determined, the physical system was constructed and tested at the conditions determined through the analytical analysis. The design team found that running the DC motor and compressor in real time at their predetermined states matched the expectations of the analytical analysis. The next step in the project was developing the control model. By determining the best method of running the air conditioning system at different environmental states via sensors, the air conditioning controller can determine when to run the air conditioner. The control strategy also determines when to run the air conditioning system when the engine is shut off as well as managing the level of charge in the vehicle's battery. In addition, several safety features are embedded into the controller's logic flow by disabling the air conditioning system during conditions that would cause catastrophic failure of the system. The controller is also programmed to add a specified amount of torque to the engine when the air conditioning system demands engine power, thus smoothing out engine operation and keeping a sufficient amount of power available to the operator.

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