Semester

Summer

Date of Graduation

2023

Document Type

Thesis

Degree Type

MS

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Andrew Nix

Committee Member

Scott Wayne

Committee Member

Brian Woerner

Abstract

Coastdown testing and road load determination are pivotal parts of the automotive design process. Vehicle manufacturers and independent companies perform and analyze road loads determined through a coastdown or similar method to determine a vehicle’s road load for modeling and EPA certification. For a traditional coastdown, the vehicle’s drivetrain must be disconnected through a clutch between the engine and the transmission while traveling at a high rate of speed to place the vehicle in neutral. This changes for hybrid and electric vehicles. Some hybrid, and most electric, vehicles delivered to customers do not have this clutch action to grant the luxury of a traditional neutral. In fact, when coasting, most hybrid or electric vehicles, the electric drivetrain is charging the battery through regenerative braking with negative torque commands. Vehicle manufacturers can successfully disconnect the electric motors and drive units to perform a traditional coastdown with no negative torque. This disconnection is important to isolate the rolling resistance and air resistance without need to account for losses in the electric drive unit. This research aims to test and analyze hybrid and electric vehicle coastdowns where this disconnect is not possible/provided by the manufacturer. The conditions tested enable a hybrid or electric vehicle to coast as intended from the factory with the negative torque through the electric drive unit to recapture energy. The goal of this research is to provide methodology and results to justify testing a hybrid or electric vehicle with its electric drive unit clutch engaged. The testing for this thesis was performed on a 2019 Chevrolet Blazer that West Virginia University’s EcoCAR team converted into a P4 parallel hybrid electric vehicle with an internal combustion engine on the front axle and an electric drive unit on the rear. This vehicle’s electric drive unit has a clutch that can be disconnected through the team-implemented controls system. To test two post-processing methods to account for the forces of the drive unit outlined by an independent testing organization, the vehicle was subjected to 4 different coastdown conditions. The first condition was a traditional coastdown with the transmission from the engine to the front axle in neutral (all conditions had the transmission in neutral) and the electric drive unit disconnected. The second condition had the electric drive unit engaged with no torque commanded. The last two conditions were the regenerative braking conditions with a “low” torque condition of -200 Nm and the other was a “high” torque condition of -400 Nm. The two regenerative braking condition results were adjusted through two post-processing methods to account for the forces of the drive unit. The second coastdown condition is not statistically the same as the traditional coastdown condition at low vehicle speeds and served as the control for the final two. There all small differences between the two results but none that exceed one standard deviation of the control. Only one post-processing method was viable for the lower regenerative braking torque condition and the calculated road load matches the control. For the higher regenerative braking torque condition, the methods match the road load at vehicle velocities above 15 m/s. Below this velocity, they failed to match the control’s road load.

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