Air conditioning is essential for new cars. But the fluorocarbon refrigerant HFC-134a — the global standard — is considered an ozone-depleting substance and the European Union passed regulations phasing out the refrigerant and mandating the use of alternatives with less environmental impact. By 2018, HFC-134a is to be totally banned in new cars sold in the European Union. Japan is expected to follow suit.

One of the top alternatives is the refrigerant CO2, also known as R-744. This gas is a natural refrigerant and provides 25 percent faster cool-down. Moreover, CO2 systems can be reversed and thereby serve as a passenger cabin heater in cold weather — a definite plus in electric cars that otherwise drain power from batteries as well as internal-combustion engines (particularly diesels) that need high operating temperatures to run efficiently.

Small Eco-Footprint Presents Major Challenges

Moving to CO2 won’t be easy, however. Gas pressures can be 10 times greater than fluorocarbon-based systems, requiring compressors, seals, and other major components to be specially designed. For the most efficient performance, the CO2 refrigerant must be kept above its “supercritical” temperature of 31°C, so a gas cooler must replace a classic R-134a loop condenser. CO2 systems also require a special heat exchanger separating the high and low pressure sides. According to some industry observers, these design complexities — plus a longer development cycle and additional physical prototype testing to refine the designs — could mean that some of the first CO2-based systems could cost 30 percent more than conventional units.

Compounding these difficulties, CO2 has not been established as a global standard. On the contrary, the United States has no plans to discontinue HFC-134a, and China has made major investments in producing HFC-134a. Moreover, fluorocarbon suppliers are actively pursuing alternative blends such as HFO-1234yf that have lower ozone-depletion potential and are direct “drop-in” substitutes for HFC-134a in current air conditioning systems. Consequently, a/c suppliers must develop systems for different global regions based on refrigerant specifications that may or may not change at any time.

Right now, a/c suppliers are challenged to meet these varying requirements with systems that integrate smoothly into the total vehicle to provide optimal cool-down performance and passenger comfort with minimal engine drag and pollutant emissions. With faster and faster development cycles in the automotive sector, a/c suppliers are striving to develop these complex designs better, faster, and less expensively, and to be the first to demonstrate optimal system performance. Design speed is critical, as is the ability to account for all of the complex vehicle thermal, mechanical, and electronic control issues relating to an a/c system.

The Race for New Contracts

Both a/c suppliers and carmakers use LMS Imagine.Lab Vehicle Thermal Management to handle a/c system design and predict performance. The software lets engineers analyze component behavior in relation to engine temperature, exhaust levels, auxiliary equipment, cabin environment, and other factors to find the best combination. The solution runs on the LMS Imagine.Lab AMESim platform, allowing engineers to access the required tools and libraries to build simulation models, run simulations, and display results graphically.

One of the suppliers using this LMS solution is Japanese supplier Calsonic Kansei. The company is one of the few suppliers that designs and manufactures the complete a/c system.

Junichiro Hara, senior engineer and project manager for innovative a/c and engine cooling module systems development at Calsonic Kansei, noted that no expert programmers are needed to build the LMS models and run simulations. Engineers themselves drag, drop, and interconnect simple icons — pre-defined 1D elements selected from libraries in the different physical domains — to create a unified physics-based model. The block diagram is a fairly simple sketch, but underlying it is all actual validated dynamics information — a real working one-dimensional representation of all the different parts of the a/c system. Not only did the company’s engineers model all the parts of the system, they linked together the subsystems to simulate the complete interaction between the a/c, cabin, and engine.

Easy as 1-2-3

First, engineers selected the icons for each individual a/c component — the compressor, evaporator, and piping as well as refrigerant tubes in the gas cooler and evaporator. Different size components were swapped according to vehicle requirements and connected to represent the warm and cool refrigerant loops. Refrigerant thermal-physical properties for CO2 were then modeled, and the 1D simulation was run to study a/c thermal output performance as well as individual part behavior.

LMS Imagine.Lab AMESim lets engineers access a variety of plots and charts to evaluate different aspects of the a/c system. Mollier pressure-enthalpy diagrams show the state of the refrigerant in each phase of the cooling cycle. Pressure levels at the compressor inlet and outlet can be reviewed as well as heat exchange in the gas cooler. Changes in gas mass fraction levels can be plotted at various locations in the refrigerant circuit. With in-depth information like this, Calsonic Kansei engineers could easily perform sensitivity analysis on critical parameters to determine the best possible a/c performance.

Meanwhile, engineers worked in parallel on the cabin interior model, using various icons to represent blowers, air ducts, windows, thermal properties of walls, solar heat flux, internal and external convection, and ambient thermal radiation. By linking the model of the cabin and model of the a/c system, engineers could calculate cool-down rate, cabin temperature, and humidity — even incorporating different weather and driving conditions. Using the one-dimensional model this early in the process lets engineers accurately size critical system components, such as ducts and fans and test climate control strategies. The LMS Vehicle Thermal Management solution was also used to perform a durability analysis regarding the impact of corrosion and contamination on the heat exchangers and other parts.

After this, the team took the LMS solution one step further to examine the overall energy balance between the engine and lubrication and cooling systems, combustion chamber, air intake, and exhaust pipes. By linking the engine model with the a/c system and cabin models, engineers then could optimize engine performance, fuel consumption, and exhaust emissions while providing the best passenger comfort.

Business Value of 1D Simulation

“Using our previous internally-developed programs, creating engineering estimations and acquiring experimental data was very time-consuming — even to roughly approximate a/c performance under transient conditions such as cool-down and vehicle speed variations. Furthermore, including engine characteristics into the development process was impossible before,” Hara said. “The LMS Vehicle Thermal Management solution lets us quickly and accurately predict how the complete a/c system will operate, taking into account a wide range of conditions that otherwise would not be included.”

He continued, “The beauty of LMS Vehicle Thermal Management is that the model — once created and verified — can be used as a basis for a wide range of future designs without starting from scratch each time. Engineers merely enter new parameters to reflect different vehicle applications and run simulations to quickly predict a/c system performance. When fully operational and integrated into system development, indications are that the simulation-based process using LMS software will be at least 80 percent more accurate than current methods and reduce the number of physical prototypes by half — definite business benefits in lowering development costs and shortening the time to respond to customer requests of quotes.”

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Publication date: 05/07/2012