Internal combustion engines in commercial vehicle applications draw fresh air from the surroundings through the air intake system. The design of the air intake systems is a critical factor influencing the engine performance. One important engineering criterion to evaluate the performance of the air intake system is the Rise over Ambient (RoA) value, which is the magnitude of the air temperature increase at the engine intake port over the ambient condition. Engine manufacturers usually specify an upper design limit for proper engine operation. In addition, a lower RoA value is better for engine performance due to better thermodynamic efficiency. It is a common practice to report a RoA value that is averaged over a predefined commercial vehicle driving cycle.
The air intake system consists of several key components including the intake snorkel, air box, air filter, and tubes. Because they are located in the underhood environment exposed to the hot air coming from the heat exchangers, you must place the intake snorkel properly to avoid drawing excessively hot air. In addition, you must control the heat transfer via conduction and radiation from the neighboring hot components into the intake air to keep the RoA value under the design limit. Furthermore, you should consider other commercial vehicle design requirements such as water/snow ingestment and noise characteristics when optimizing the air intake system.
Given the complexity of the flow and temperature fields in the commercial vehicle underhood, it is difficult to satisfy these engineering goals by designing the air intake system entirely based on limited measurement data available thru physical testing. Typically, several prototypes must be built and tested before an acceptable design can be found through trial and error. In addition, last minute design changes add significant cost to vehicle development. Therefore, it is very desirable to have a simulation methodology to predict the RoA value so you can optimize the air intake system in the early development stages. The simulation methodology must be sophisticated enough to capture the complex physics involved in the underhood flow prediction, but computationally efficient enough to handle the relatively long physical time scale associated with a vehicle driving cycle. (Below: validation data for inake ports.)
The Exa RoA simulation methodology uses PowerFLOW to calculate flow and the air temperature fields. All the geometry details of the underhood components can be fully resolved. For the common scenario where the heat rejected from heat exchangers is a primary source of the elevated air temperature in the underhood, you can couple PowerCOOL with PowerFLOW. The setup and simulation processes for a PowerFLOW/PowerCOOL-coupled run are integrated seamlessly. Depending on the vehicle driving cycle pattern employed in a particular RoA study, you might need to perform several PowerFLOW/PowerCOOL-coupled simulations to represent a few selected vehicle operating conditions on the cycle.
The underhood simulation flow results are interpolated to define the time-dependent flow and temperature boundary conditions. Given that a typical driving cycle for a RoA study spans more than a few minutes, a subset of the full geometry model including only the air intake system’s key components (snorkel, air box, air filter/cleaner, tubes) is created for the second step to achieve maximum computational efficiency. Using the available geometry and boundary conditions, along with material property information, the RoA simulation is handled by PowerTHERM, Exa’s conduction and radiation solver. The transient PowerTHERM simulation calculates the air temperature history at the intake port, and estimates the average RoA value for the given driving cycle accordingly.
Using Exa’s RoA simulation approach, you can:
- Estimate the RoA value for a predefined driving cycle in an early stage of the vehicle development process and compare it against the design limit imposed by the engine requirements.
- Identify the root causes of the elevated intake air temperature (such as flow leakage and recirculation, excessive thermal engine radiation, and conduction from other hot components), and use the information to design appropriate countermeasures (for example, seals, heat shields, and changes to the intake system configuration).
- Optimize the underhood component layout to determine the best location of the snorkel inlet.
EXA SOFTWARE USED FOR THIS APPLICATION