# R744 A/C Systems Test Stand and Field Test Experience

For 14 years now, Behr has been investigating and developing refrigerant circuits operating on the alternative refrigerant R744 (CO 2 

## ). During this time, extensive staff and technical resources have been built up for the development, testing, and validation of these circuits in A/C (air conditioning) systems. These include a number of circuit and component test benches, test labs, climatic and acoustic chambers as well as a modern climatic wind tunnel. Also counted are simulation tools and the capability for industrialization and initial application of the refriger ant circuits in motor vehicles. Extensive field trials were carried out in conjunction with automobile manufacturers. These revealed that the performance, efficiency, vehicle integration, and reliability of R744 refrigerant circuits have attained a level that corresponds to, and in some cases even exceeds, that of present-day R134 systems. In addition, initial experience in the manufacture of R744 components was gained through small-scale production runs. 

## D e v e lopm e n t

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## Thermo Management

### 1 Introduction On January 31, 2006, the European Un ion (EU) made the decision to ban refrig erants with a global warming potential (GWP) of over 150, which also includes the currently used R134a refrigerant, which has a GWP of 1,430. Accordingly, as of 2011, all new vehicle models in the EU must be equipped with eco-friendly climate control systems, and, after 2017, all new vehicles. Following intensive testing of all alternative refrigerants, the German automo tive industry was the first in the world todecide to utilize the particularly ecofriendly R744 refrigerant in future A/C systems. In order to achieve this goal, they are accompanied by the develop ment partner Behr based in Stuttgart, Germany, which has extensive experi ence with the R744 refrigerant, and has carried out series-production projects through to the design validation phase additionally with fleet tests. 

### 2 Objectives of the Fleet Tests For the reason of in-house testing, and for trials conducted by automobile man ufacturers, 20 vehicles were equipped with R744 circuits. To do lifecycle testing some fleet vehicles were also converted and endurance-tested. Altogether, com ponents for 100 vehicles and vehicle bod ies were delivered. Objectives of the fleet tests were:

– testing the performance and efficien cy of the R744 A/C system

– testing the pressure and leak resist ance of the refrigerant circuit

– gaining manufacturing experience through small-scale component pro duction

– studying the integration of the refrig erant circuit into the vehicle

– derivation of specifications from ac tual driving operations for compo nent validation

– testing and application of the R744 servicing unit. 

### 3 Testing and Validation Resources Behr has great experience in testing and a big number of test benches for R744 [1–4]. In the following test benches and run times, simulation tools and thermal dummies, climatic wind tunnel and the vehicle fleet are discussed. 

### 3.1 Test Benches and Run Times First of all, refrigerant circuit test bench es were developed. Subsequently, to meet the growing demand for air condition ing systems, test stands were also con structed for individual components. Even after the deactivation of one older test stand, there are still three refrigerant circuit test benches in operation for the R744 fleet trials. The run time accumu lated up to March 2008 was approximate ly 20,000 h. The component test stands for compressors, evaporators, gas cool ers, and accumulator IHX (Internal Heat Exchanger) units were in use for 3000 h. An acoustic chamber is available for noise measurements. The fact that the operating time of the refrigerant circuit test benches is much higher than that of the of the component test rigs is due to the fact that many findings about the components had already been obtained on the refrigerant circuit test benches, **Figure 1** . The results from the test bench investi gations and fleet trials were used to de velop the R744 technology as well as up grade the test benches themselves. So not only A/C circuit performance was meas ured on the refrigerant circuit test bench es, but also the efficiency of the A/C circuit that means the power take-up of the compressor at different cooling outputs (and various boundary conditions) in the vehicle. Modern measurement technolo gy enables an accurate determination to be made of fuel consumption. In this way, additional potential increases in air con ditioning efficiency can be investigated, which will ultimately reduce its fuel con sumption demand. 

### 3.2 Simulation Tools and Thermal Dummies Simulation tools, such as Behr’s own “Behr Integrated System Simulation” (BISS), were also continuously upgraded. Today, complex circuits, such as the heating up and cooling down of vehicle cab ins, can be reliably modelled with BISS. Even air conditioning systems with two evaporators, as found in truck cabs or luxury vehicles, can be simulated. 

### The Authors

Dr.-Ing. Harald Riegel is head of develop

ment of R744 refriger

ant circuits in the ad

vanced development of A/C systems at Behr GmbH & Co. KG in Stuttgart (Germany). Dipl.-Ing. (FH) Julian Helfen is development engi

neer R744 refrigerant circuits in the ad

vanced development of A/C systems at Behr GmbH & Co. KG in Stuttgart (Germany). Wolfgang Schmidt heads the develop

ment of service sys

tems for R744 systems at Behr Service GmbH & Co. KG in Stuttgart (Germany). Dipl.-Bw. Anja Müller-Koelbl leads the department new markets and mar

keting for R744 service technology at Behr Service GmbH & Co. KG in Stuttgart (Germany). Dipl.-Ing. Peter Kroner is head of advanced development of A/C systems at Behr GmbH & Co. KG in Stuttgart (Germany).

ATZ 07-08I2008 V olume 110 **15**Additionally, real and virtual thermal dummies are used to evaluate vehi cle climate comfort. These dummies mimic the behaviour of human skin, **Title Figure**  . Depending on the air tem perature and air motion, and the solar radiation temperature, the sensors on the measurement surface yield a tem perature that correlates with the com fort perceived by test subjects. This com fort correlation is also used in three-di mensional flow simulations (CFD). Through the use of this virtual thermal dummy, initial statements regarding passenger comfort in the vehicle can al ready be made in the early development phase. 

### 3.3 Climatic Wind Tunnel With the climatic wind tunnel, installed only a few years ago, Behr’s development engineers, as well as the OEMs, have one of the world’s most advanced facilities of its kind at their disposal. The wind tun nel is distinguished by several special fea tures, primarily its superior air flow quality and low noise level. These properties enable a uniform air flow onto the vehicle as well as acoustic optimization of the interior noise. Additional special features include a movable solarium, **Figure 2** , and the fitting of the climatic wind tunnel with two powered and brakeable pairs of rollers. The solarium is equipped with a shutter system to simu late cloud formation and other shade conditions. Dynamic cycles are run in the climatic wind tunnel. The dynamics and flexi bility of this tunnel are highly advanced, because environmental and driving con ditions can be quickly altered. In addi tion, different climatic extremes (tropics, desert, Arctic) can be input in quick suc cession. The temperature can be reduced from +50 to –20 °C in less than 2 h. The lowest attainable temperature is –30 °C. The input temperature is controlled with a precision of ±0.5 °C throughout the en tire measurement chamber and test du ration. The relative humidity can also be in put very precisely and varied quickly, on the one hand through the continuous flow of extremely dry air, and on the other through the controllable injection of water vapour. The maximum attaina ble relative humidity is 95 %. 

### 3.4 Vehicle Fleet The vehicle fleet consists of two groups: Behr’s test vehicles and vehicles provided by the OEMs. The first group consists pri marily of vehicles from German auto makers. While the test bench trials are coordinated and conducted in Stuttgart, the OEMs are responsible for the vehicle tests. Both groups together account for over 1.2 million kilometres driven, and probably even more, because not all of the test kilometres logged by the OEMs could be recorded. **Figure 2:** Cooldown measurements in the climatic wind tunnel: with the help of the adjustable solarium, every possible type of solar radiation into the vehicle cabin can be simulated **Figure 1:** Climatic chamber for simulating the vehicle environment during the measurement of R744 A/C circuits; the climatic chamber is part of the R744 system test facility 

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## Thermo Management

### 4 Cooling Performance and Efficiency The significant value COP (Coefficient of Performance [5]) of an R744 refrigerant circuit is comparable to that of an R134a circuit, and in some cases is even higher. The cool down dynamics and cooling performance are higher in each case, which can be utilized to reduce fuel con sumption. However, the increase in effi ciency diminishes as the ambient tem perature increases. But the performance and dynamic advantages of the new re frigerant circuit can also be used to achieve a higher degree of climate com fort; a vehicle cabin heated by the sun, for example, can be cooled down much more rapidly. **Figure 3** shows the A/C tem perature pull down curves for a compactclass vehicle equipped with an R134a and an R744 refrigerant circuit. The com fort temperature of 25 °C was reached 14 min faster with the R744 refrigerant circuit than with the R134a circuit, starting from an initial temperature of 70 °C in the heated interior. On average, that means when all vehi cle classes are included in the compari son, the comfort range with R744 refrig erant circuits is reached 8 min earlier than with R134a circuits, **Figure 4** . Here is also shown that the use of R744 refrigerant circuits makes a key contribution to reducing CO 2 emissions attributable to air conditioning. The 1000-times-lower greenhouse potential of R744 compared with R134a leads to the elimination of the proportion of total CO 2 emissions caused by leakage. In this way, the overall emissions from an R744 air condition ing system are reduced by about 30 %. 

### 5 Safety Regarding the safety of the refrigerant circuit, that means pressure safety and leak safety, the evaporator located in the vehicle interior is the most critical com ponent. The safety status of the R744 re frigerant circuit is therefore demonstrated for this component. 

### 5.1 Pressure Safety Today, auto A/C systems around the world still mainly use brazed plate-type evapora tors. At Behr, this technology was replaced in 2002 with evaporators manufactured from extruded flat tubes. These evapora tors have a higher power density than all other types of evaporator known today. The flat tube, corrugated-fin technology can also be used for R744 evaporators, al though in a modified and more robust form. In contrast to present-day evaporators, which are designed for a burst pres sure of 30 bar (at room temperature), for R744 evaporators this must be 260 bar (at 90 °C). Owing to the higher specific cool ing capacity of R744 compared withR134a, it was possible to reduce the crosssections of the evaporator tubes. Relative to an R134a evaporator, the force exerted on the aluminum walls of R744 flat tubes is also decreased. **Figure 5** shows that the relative force, acting on the tubular head er of an R744 evaporator, is approximately 15 % lower than that on an R134a header. 

### 5.2 Leak Safety It does not suffice to safeguard the evap orator against the high pressures of the refrigerant circuit; there must also be safeguards against leakage. Since the evaporator is installed in the interior be neath the cross member, it is probably located in the safest position in the vehi cle. Special protective measures against a crash are therefore unnecessary, espe cially since, in the event of a crash, the refrigerant escapes into the atmosphere before it can penetrate into the cabin. Any refrigerant escaping from the evaporator can therefore only be caused

**Figure 3:** Comparison of cooldown measurements in a compact vehicle with an R134a and an R744 refrigerant circuit (43 °C ambient temperature, 15 % releative humidity)

**Figure 4:** Comparison of R134a and R744 refrigerant circuits: left: rate of cooldown; right: lower CO 2 emissions; basis of comparison: Frankfort/Main, Germany climate profile, NEFZ (New European Driving Cycle), mid-range vehicle, 40 g/year leakage ATZ 07-08I2008 V olume 110 **17**by a corrosion hole. Assuming the correct material is used, such a leak begins insidiously, usually with a small quantity of about 1.5 cm3 of R744 per minute. However, it seldom remains a small hole; instead, several small holes will appear on a larger corroding surface after a short time. In this case, the leakage in creases up to about 60 cm3/min. During the same one-minute interval, an adult human being exhales approximately 333 cm3 of CO 2 , more than five times the maximum assumed corrosion leakage. Approximately 350 g of CO 2 are con tained in an R744 refrigerant circuit at 65 bar. A human being takes approxi mately 10 h to generate this amount. It would take 84 days for this quantity to escape from a small corrosion hole; for several small holes as described above, it would take two days. In other words, the refrigerant circuit would be emptied in between 2 and 84 days. In order to guarantee the evaporator’s leak safety, it is subjected to an extensive, combined, long-duration test that con sists of the following individual tests: connecting tube bending test, leak test, pressure cycling test, SWAAT (Sea Water Acetic Acid Test) corrosion test, vibration test (with shakers), and temperature cycling test. A second pressure cycling test is conducted upon completion of the pre scribed test cycles, up to the destruction of the evaporator. On the basis of these tests, a reliability of 99.4 % at a 97.5-% confidence level was demonstrated. “Reliability” is defined as the ability of a component to perform its required function under given condi tions for a given time period. “Confi dence level” indicates how trustworthy a statistical value is. There is, therefore, a probability of 97.5 % that the 99.4 % reli ability stated above will be attained during each test. 

### 6 Production Experience Small-volume production of R744 com ponents has shown which additional tests are necessary compared with those for the production of R134a components: a tube-bending test and a high-pressure leak test. The bending test should posi tively indicate that the tube connections are sufficiently protected from damage during the course of production. To this end, they must demonstrate a certain ri gidity against bending, e.g. during ship ment, which is checked in this test. The additional leak test is necessary because of the high pressures in the re frigerant circuit; it is conducted at a test pressure of 150 bar. If a leak is detected during this preliminary leak test, the root cause can be determined. Manufac turing defects can be corrected in this way. Defective components are not re paired; the evaporator in question is re moved from production. 

### 7 Packaging and Vehicle Integration The R744 refrigerant differs substantially from today’s R134a refrigerant in terms of its thermodynamic and chemical properties. This has consequences for the functionality, size, and performance of the components installed in the R744 re frigerant circuit. Furthermore, in the R744 A/C circuit with the IHX, an addi tional component is used. The R744 com ponents require less packaging space when substituted for R134a components, because, as a result of the higher density of R744 compared with R134a, the same cooling performance can be achieved at a lower volumetric flow rate than with present-day air conditioners. Conse quently, R134a-based HVAC and cooling modules can easily be converted for R744 applications. However, this cannot be accomplished by means of smaller size alone. The avail able space envelope in each vehicle mod el must preferably also be utilized as flexibly as possible. To this end, the same components were developed in different sizes, or integrated into modules; exam ples include:

– gas cooler: available as a separate com ponent and as a gas cooler module with accumulator and IHX, **Figure 6**

– evaporator: similar to the R134a evap orator; integrated expansion valve, e.g. as an orifice tube with bypass

– IHX: separate, **Figure 7** , and in accumu lator-integrated configuration or as part of the gas cooler module

– accumulator: stand-alone version and as combi unit consisting of accumula tor and IHX, **Figure 8** . These packaging space alternatives influ ence the thermodynamics (performance, refrigerant charge), the acoustics, and, of course, also the costs of the refrigerant circuit. These influences are determined during the development phase by means of simulation calculations, and the circuit is optimized accordingly. Aside from increasing packaging space flexibility, the integration of components into larg er units also has the goal of reducing the number of assembly costs and connec tion points. 

### 8 Service Although the conventional refrigerant R134a may no longer be used in new ve hicle models after 2011, no automobile manufacturer is obliged to wait that long before introducing R744 air condi tioning systems. Behr Service will there fore offer auto workshops a complete R744 service system package correspond **Figure 5:** Com parison of the relative force (R134a set to 1) exerted on the walls of the header of an R134a and an R744 evaporator 

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## Thermo Managementing to the start of large-scale production of R744 air conditioning systems, which provides not only for the delivery of spare parts and workshop equipment, but also for the training and certifica tion of shop personnel, technical sup port, and technical information, e.g. service and repair manuals, as well as new tools and a fully automatic R744 servicing unit. A preliminary manual R744 servicing unit was already developed in 1998 based on proven R134 service equipment. To day, a fully automatic unit is available, which is presently being evaluated on test vehicles. Its operability is compara ble to that of the R134a service unit. Menu-driven functions facilitate both the implementation of individual work steps and integral, fully automatic A/C servicing. This virtually eliminates all op erating errors, saving time and money. Individual functions such as the control led draining of refrigerant, evacuation and dehumidification of the A/C circuit, recharging, replenishing of the refrigerant oil (as needed), and leak testing can be carried out by service personnel in the usual manner. For leak testing, a test gas is used which can be electronically de tected. The controlled removal of refrigerant by means of a discharge hose is necessary, so that it doesn’t concentrate in the workshop. The refrigerant thus recovered is vented to the outside air. Workshops equipped with a vacuum system can use this facility to discharge the refrigerant. The R744 service station is equipped with digital pressure gauges and electronic pressure sensors, a pressure and tempera ture-controlled cylinder heater, and also overheating safeguards. With an evacua tion time of 30 min, the complete servic ing operation takes about an hour, as with the R134a equipment. 

### 9 Summary Given the fleet testing described by Behr, as well as the production experience ob tained to date, a smooth transition from R134a refrigerant to R744 can be expect ed. The new refrigerant circuits surpass the present-day R134a systems in terms of eco-friendliness, cooling performance, and energy efficiency, and meet at least the same standards as current systems when it comes to operational safety, acoustics, packaging, vehicle integration, and service life. Easy servicing of the R744 A/C circuit in the workshop is like wise ensured. A fully automatic servicing unit was developed, and a training pro gram to facilitate handling of the new **Figure 6:** R744 refrigerant circuit with internal heat exchanger (upstream of the gas cooler) and accumulator integrated into the gas cooler module

**Figure 7:** R744 refrigerant circuit featuring the internal heat exchanger as a separate component ATZ 07-08I2008 V olume 110 **19**A/C circuit is currently being prepared. An efficient national and international organization for marketing air condi tioning components is also available. Regular active participation at meetings and conventions (e.g. Society of Au tomotive Engineers (SAE) and German Association of the Automotive Industry (VDA) conventions) facilitates the broad er dissemination of know-how and pro fessional exchanges with customers and suppliers. One such meeting was, for example, this year’s (February 2008) VDA Winter Meeting in Saalfelden, Germany, on advances in the introduction of alternative refrigerants for automotive air conditioning systems. Behr stands ready as a competent partner for the automobile industry – a partner in simulation, concept develop ment, advanced engineering, industriali zation, and validation on a number of test stands, in the acoustics lab and the climatic wind tunnel, and with road tri als. Together with joint-venture company BHTC (Behr-Hella Thermocontrol), a com plete control system is also being devel oped and validated. 

### References [1] Heinle, D.; Riegel, H.; Weinbrenner, M .: Fahrzeug

integration des R744-Kältekreislaufs. In: ATZ 108 (2006), N r. 6, S. 446–455 [2] L ochmahr, K.; Baruschke, W.; Britsch- L audwein, A.: Regelsystem für R744-Kältekreisläufe. In: ATZ 107 (2005), N r. 9, S. 796–799 [3] Heinle, D.; Riegel, H.; Weinbrenner, M .: Klimatis

ierung mit dem Kältemittel R744. In: ATZ 107 (2005), N r. 9, S. 790–795 [4] Winterhagen, J.: Diskussion um Klimaanlagen flammt auf. ATZonline vom 18.05.2007 [5] N

. N .: C OP . http://de.wikipedia.org/wiki/Coeffi

cient_ O f_ P erformance vom 16. M ai 2008 **Figure 8:** R744 refrigerant circuit with the internal heat exchanger integrated into the accumulator 

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