In recent years, the fastest developing area in automotives is electronics. From LEDs to engine monitors, driver controls to braking systems, functionality has increased and improved steadily. But one drawback to these advances is the rapid increase in the heat being dissipated from the new electronics. For today’s cars and trucks, there’s a driving need for advanced thermal management.
Thermal issues for trucks are even more daunting than for cars. While most automobiles are mass-produced, large trucks are typically custom-designed. In fact, successive trucks leaving an assembly line typically will have different engines and different cooling system requirements, making the design and optimization of truck thermal management systems even more difficult. Engine manufacturers, truck manufacturers and equipment suppliers each have a role to play [1].
Now that there are so many electronics components in cars and trucks, a number of studies are underway to boost their performance. Such research often involves fan systems, particularly in heavy trucks.
Yet, fan power requirements in large trucks can be 35 to 50 kW. This high energy consumption can have a dramatic effect on fuel consumption. In these circumstances, axial fans are directly driven by the engine and an optimized fan shroud is used with a viscous clutch and a thermostat to control the fan [1].
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Under the hood airflow management is of utmost importance, because all the heat generated by an automobile’s components must be removed by the motion of air in the compartment. A challenge to this is the design of more aerodynamic vehicle body shapes, which can result in less available open grill area and under hood space.
This lack of space for airflow requires the fan system to create more pressure and that the overall system include more efficient heat exchangers. With less external aerodynamic drag, the needed contribution of under hood air flow has been shown to be significant [2].
The automotive engine control module (ECM) is a component that is exposed to extreme temperatures, typically from 105º to 125ºC. The power dissipation of these modules is typically 10-30 watts. The components on an ECM are generally designed to withstand 105oC so the thermal needs are clear.
Figure 1 shows a ceramic-based ECM where low temperature co-fired ceramic (LTCC) and the thick film alumina power substrate are bonded directly to the case with thermal adhesive tape. Because alumina has significantly better thermal conductivity than LTCC, it is used for the power substrate [3]
Figure 1: A ceramic-based engine control module, ECM. [3]
A ceramic-based engine control module, ECM. [3]
A ceramic-based engine control module, ECM. [3]
The case-to-ambient thermal resistance of some ECM components is approximately 1-3oC/W [3]. In very high power applications, such as power steering controllers, where the case-to-ambient thermal resistances should be less than 1oC/W, immersion cooling has been used. Figure 2 shows a controller for an electro-hydraulic steering motor.
Figure 2: An immersion-cooled ECM for an electro-hydraulic steering motor. [3]
An immersion-cooled ECM for an electro-hydraulic steering motor. [3]
An immersion-cooled ECM for an electro-hydraulic steering motor. [3]
Figure 3 shows the anticipated power dissipation for the most common automotive electronic systems. The starter-generator control and hybrid/EV motor control both dissipate 10 to 50 times more heat than other electronic systems. These devices need special thermal management techniques to dissipate such high heat fluxes.
Figure 3: The power dissipation of different electronic systems in a conventional automobile. [3]
The power dissipation of different electronic systems in a conventional automobile. [3]
The power dissipation of different electronic systems in a conventional automobile. [3]
Amodeo et al. used the lattice Boltzmann method (LBM) to simulate the flow aerodynamics and flow distribution under the hood of a car. [4] The LBM is an alternative approach to the Navier-Stokes solvers, equations for describing the motion of fluid substances. LBM methods do not need any special iterative procedures, and are very efficient when designing for mass, momentum and energy conservation.
Amodeo and associates applied the LBM method to a Ford Mondeo, whose geometry is shown in Figure 4. The computational domain included the exterior body, under the car and under hood.
Figure 4: Models of a Ford Mondeo that were used in an LBM simulation. [4]
Models of a Ford Mondeo that were used in an LBM simulation. [4]
Models of a Ford Mondeo that were used in an LBM simulation. [4]
The lattice size was around 18 million voxels (volumetric picture elements in three dimensional space) and contained the same number of surfels (surface elements). The heat transfer between the airflow and coolant in the heat exchanger was modeled with the NEMO 1D-tool.The 1D-tool does not model all the details of the heat exchanger. The heat transfer coefficient, which is a function of both airflow rate and coolant mass flow rate, was interpolated using a sandwich formula as shown in Figure 5.
Figure 5: Heat transfer coefficients for a radiator fit to experimental data [4]
Heat transfer coefficients for a radiator fit to experimental data [4]
Heat transfer coefficients for a radiator fit to experimental data [4]
The actual data was not shown by the authors because of confidentiality issues. The above heat transfer coefficients were used in their modeling.
The thermal management of vehicles is a complex subject consisting of interactions between the aerodynamics of flow at the exterior of a car, under the hood flow distribution, and thermal coupling between different components in the engine compartment. Some major government agencies, including the Argonne National Laboratory and Oak Ridge National Laboratory are conducting research on various features of automobiles. They have been working with companies such as CD-adapco, and organizations such as USCAR (US Council for Automotive Research) to develop computational techniques for modeling and analysis.
Computational fluid dynamics (CFD) packages can be used to model the flow around and under the hood of automotives. Sound engineering judgment is needed to properly model the components of major influence, while omitting non-factor components and thus reducing the complexity of CFD analysis.
References:
1. Wambsganss, M., “Thermal Management of Heavy Vehicles: A Review Identifying Issues and Research Requirements,” Argonne National Laboratory Report, 1999.
2. Carr, G., “The Influence of Engine-Cooling Airflow on Car Performance and Stability”, Vehicle Thermal Management Systems, Institution of Mechanical Engineers, London, 1995.
3. Myers, B. “Cooling Issues for Automotive Electronics,” ElectronicsCooling, August 2003.
4. Amodeo, J., Alajbegovic, A. and Jansen, W., “Thermal Management Simulation for Passenger Cars - Towards Total Vehicle Analysis,” Exa Corporation and Jaguar Cars.
5. PowerCOOL User’s Guide, Release 4.0, Exa Corporation, Boston, MA, 2006.