Date of Award

December 2019

Document Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

Committee Member

John Wagner

Committee Member

John Wagner

Committee Member

Todd Schweisinger

Committee Member

Yue Wang

Committee Member

Richard Miller


The development of Hybrid Electric and Unmanned Ground Vehicles (HEV and UGV) offer various benefits including improved vehicle performance, compatibility with high level control systems, reduced fuel consumption, and less environmental pollution. According to the International Energy Agency (IEA), the number of HEVs and EVs is expected to reach 20 million by the year 2020 (Green Car Congress, 2017). Compared with traditional Internal Combustion (IC) engines, hybrid powertrains are more complicated due to additional electronics including the electric motor, battery pack, and control units. However, these additional components introduce new challenges for the powertrain thermal management system design since they have different operating temperature requirements and modes of heat generation. In a hybrid vehicle, the modes of heat generation, apart from the IC engine, include the electric motor, battery pack, and some electrical subsystems, which lead to a more demanding thermal control system.

A traditional vehicle cooling system is composed of a mechanical water pump, radiator fan(s), hoses, and other mechanical actuators such as a thermostat valve. In recent times, however, computer-controlled actuators such as an electric water pump, variable speed fan(s), and smart valve(s) are being used for higher efficiency and performance. This approach, although effective and efficient for the common IC engine, may pose problems when it comes to the hybrid powertrains owing to limited space, different operating conditions, heat generation rates, etc. In this dissertation, several innovative designs, optimizations, and control strategies using heat pipes in the thermal management system targeted to hybrid powertrain applications will be analyzed.

First, an integrated electric motor air cooling system based on radial heat pipes was designed and the performance was explored through computer simulations. A reduced order electric motor thermal model was introduced to simulate the motor’s internal temperatures. Heat pipes were modeled based on the vapor flow and heat transfer processes, and also selected as the cooling system thermal bus to efficiently remove heat. Mathematical models for the thermal cradle and heat exchanger were developed to complete the cooling system. A series of simulation tests based on the Urban Assault and Convoy Escort driving cycles were used to test the cooling system performance. Numerical results show that the proposed cooling system saves up to 52.1kJ of energy within a 1,800s simulation in comparison to a traditional liquid cooling design (e.g., 67.8% energy saving).

Second, an electric motor liquid hybrid cooling system, for HEV applications, using integrated heat pipes and traditional liquid was designed and simulated. The innovative design features two parallel heat transfer pathways allowing optimal heat removal. Detailed mathematical models were developed for the electric motor, heat pipes, liquid cooling system, and heat exchanger. A classical controller was designed for the heat pipe heat transfer pathway while the liquid cooling pathway was adjusted using a nonlinear controller. Cooling performance was again evaluated based on the Urban Assault driving cycle for various road grades and ambient conditions. Results show that the electric motor temperature can be maintained around the target value of 70°C with 399kJ cooling system energy consumption compared to approximate 770kJ energy consumption with the conventional liquid cooling system (e.g., 48% energy saving).

Third, a smart HEV battery pack thermal management system using heat pipes as a thermal bus to remove heat efficiently was developed. The battery cooling system couples a standard air conditioning (AC) system with traditional ambient air ventilation. A lumped parameter battery thermal model was created to predict the battery core and surface temperatures. A nonlinear model predictive controller (NMPC) was developed to maintain the battery core temperature about the reference value. The system performance and power requirements were investigated for various driving cycles and ambient conditions. Results showed that the proposed thermal management system can maintain the battery core temperature within a small range (maximum tracking error of 2.1°C) using a suitable cooling strategy based on the ambient temperature conditions and battery heat generation rate. Furthermore, the system showed the ability to remove up to 1134.8kJ of heat within the 1200s simulation.

Fourth, a holistic thermal management system for an Unmanned Autonomous Ground Vehicle (UAGV) with a series hybrid powertrain was developed. The use of heat pipes combined with advanced controllers for the vehicle’s electric motors, battery pack, and engine generator set cooling was examined. A series of mathematical models were developed to describe the dynamics and thermal behavior for these elements. Controllers were designed to maintain the components temperatures about their reference values and minimize energy consumption by regulating multiple actuators (e.g., pump, radiator fan, smart valve, blower, and compressor). A vehicle level simulation was conducted which combines the cooling system power consumption with the vehicle power bus. An Urban Assault driving cycle with various road grades and ambient conditions were used for the simulation to show the robustness of the proposed cooling system. Results show that the component temperatures were maintained around their reference values with small errors (2.1°C) and up to 2,955kJ cooling system energy was saved over the 1,800s simulation using heat pipes and the proposed controllers (e.g., 19.8% energy saving).

Overall, this research has developed the basis for the holistic control of HEV powertrain thermal management systems. A suite of model-based advanced controllers was used to simultaneously regulate the cooling actuators for the battery, e-motors, and IC engine. For electronics, heat pipes were introduced to reduce the cooling system energy consumption due to their high effective conductivities. Numerical studies have been conducted using vehicle model under various driving cycle, road grade, and ambient conditions to show the advantages of heat pipes and the proposed controllers. The next generation of thermal management system will feature multiple heat transfer pathways to help reduce energy consumption for a better use of fossil fuel and electric power resources.



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