Each simulator focuses on a different aspect of the design and optimization of the hybrid-electric vehicle (HEV) architecture:
Start/stop modeling involves a more detailed and specific focus on the engine and belt dynamics found in cX-START.
Combining the results from each of these simulators allowed the Ohio State team to thoroughly evaluate a plethora of vehicle design and performance metrics prior to the integration on the physical system.
cX-SIM
Designing and tuning a hybrid-vehicle power train for maximum fuel-economy and minimum emissions encapsulates the primary motivation of the Challenge X competition. Ohio State used cX-SIM to effectively and efficiently accomplish this and evaluate basic performance specifications.
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The above figure shows the main user-interface of cX-SIM. Four main subsystems elements compose this simulator. Starting from the left of the figure, the driver subsystem is essentially nothing more than a PI-controller (proportional integral) that compares an input desired vehicle speed, to actual vehicle speed and adjusts the accelerator and brake pedal positions accordingly.
HEV power train
The controller and models of each of the power train components reside within the HEV power train block in the user interface figure above. The figure below shows the contents of this subsystem.
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This simulator employs torque-speed maps for the ICE (internal combustion engine (diesel)) and EM (electric machine) as this is a quasi-static simulator. Similarly, the torque converter model consists of polynomials and coefficients to determine output torque. The output from the torque converter is simply multiplied by the appropriate gear ratio to determine transmission output. In a similar fashion as the transmission, the output torque from the EM is manipulated by the gearbox ratio and sent to the rear brakes. Considering the brake pedal position and respective brake proportioning constant, the brakes subtract from the torque delivered by the transmission. Dividing by the wheel radius converts this torque into a force which acts on the vehicle as seen in the previous figure showing the user interface.
Vehicle
Aerodynamic forces, rolling resistance, and positive road grade reduce the magnitude of the force input to the vehicle from the power train. The "vehicle" subsystem contains each of these, as well as the mass factor. Dividing the resultant force by the vehicle mass results in vehicle acceleration. Integration of this value provides the vehicle speed feedback for both driver and power train. Thus, torque is the feed-forward term, and speed is the feedback term.
Exhaust aftertreatment
Similar to the power train actuators, the exhaust aftertreatment model uses emissions maps to predict the NOx, CO, and HC (hydrocarbon) emissions. Complex algebraic equations model the primary components of this custom system.
Assuming that all inertias within the driveline react infinitely fast (compared to the vehicle inertia) allows for cX-SIM to take a quasi-static approach to modeling. This constraint results in a computationally inexpensive tool for quickly and accurately assessing fuel economy, emissions, and acceleration.
