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 (CO2

). 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 refrigerant 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.

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14 ATZ 07-08I2008 Volume 110

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1 Introduction

On January 31, 2006, the European Union (EU) made the decision to ban refrigerants 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 automotive industry was the first in the world to 

decide to utilize the particularly ecofriendly R744 refrigerant in future A/C 

systems. In order to achieve this goal, 

they are accompanied by the development partner Behr based in Stuttgart, 

Germany, which has extensive experience 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 manufacturers, 20 vehicles were equipped 

with R744 circuits. To do lifecycle testing 

some fleet vehicles were also converted 

and endurance-tested. Altogether, components for 100 vehicles and vehicle bodies were delivered. Objectives of the fleet 

tests were:

– testing the performance and efficiency of the R744 A/C system

– testing the pressure and leak resistance of the refrigerant circuit

– gaining manufacturing experience 

through small-scale component production

– studying the integration of the refrigerant circuit into the vehicle

– derivation of specifications from actual driving operations for component 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 benches were developed. Subsequently, to meet 

the growing demand for air conditioning systems, test stands were also constructed 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 accumulated up to March 2008 was approximately 20,000 h. The component test stands 

for compressors, evaporators, gas coolers, 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 investigations and fleet trials were used to develop the R744 technology as well as upgrade the test benches themselves. So not 

only A/C circuit performance was measured on the refrigerant circuit test benches, 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 technology enables an accurate determination to 

be made of fuel consumption. In this way, 

additional potential increases in air conditioning efficiency can be investigated, 

which will ultimately reduce its fuel consumption 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 cabins, 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 development of R744 refrigerant circuits in the advanced development 

of A/C systems at Behr 

GmbH & Co. KG in 

Stuttgart (Germany).

Dipl.-Ing. (FH) 

Julian Helfen 

is development engineer R744 refrigerant 

circuits in the advanced development 

of A/C systems at Behr 

GmbH & Co. KG in 

Stuttgart (Germany).

Wolfgang Schmidt 

heads the development of service systems for R744 systems 

at Behr Service GmbH 

& Co. KG in Stuttgart 

(Germany).

Dipl.-Bw. 

Anja Müller-Koelbl 

leads the department 

new markets and marketing 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 Volume 110 15 
Additionally, real and virtual thermal dummies are used to evaluate vehicle climate comfort. These dummies 

mimic the behaviour of human skin, 

Title Figure. Depending on the air temperature and air motion, and the solar 

radiation temperature, the sensors on 

the measurement surface yield a temperature that correlates with the comfort perceived by test subjects. This comfort correlation is also used in three-dimensional flow simulations (CFD). 

Through the use of this virtual thermal 

dummy, initial statements regarding 

passenger comfort in the vehicle can already 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 tunnel is distinguished by several special features, 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 simulate cloud formation and other shade 

conditions.

Dynamic cycles are run in the climatic wind tunnel. The dynamics and flexibility of this tunnel are highly advanced, 

because environmental and driving conditions can be quickly altered. In addition, different climatic extremes (tropics, 

desert, Arctic) can be input in quick succession. 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 entire measurement chamber and test duration.

The relative humidity can also be input 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 attainable 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 primarily of vehicles from German automakers. 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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16 ATZ 07-08I2008 Volume 110

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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 consumption. However, the increase in efficiency diminishes as the ambient temperature increases. But the performance 

and dynamic advantages of the new refrigerant circuit can also be used to 

achieve a higher degree of climate comfort; a vehicle cabin heated by the sun, 

for example, can be cooled down much 

more rapidly. Figure 3 shows the A/C temperature pull down curves for a compactclass vehicle equipped with an R134a 

and an R744 refrigerant circuit. The comfort 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 vehicle classes are included in the comparison, the comfort range with R744 refrigerant 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 CO2 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 CO2 emissions 

caused by leakage. In this way, the overall emissions from an R744 air conditioning 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 component. The safety status of the R744 refrigerant 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 evaporators. At Behr, this technology was replaced 

in 2002 with evaporators manufactured 

from extruded flat tubes. These evaporators 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, although in a modified and more robust 

form. In contrast to present-day evaporators, which are designed for a burst pressure of 30 bar (at room temperature), for 

R744 evaporators this must be 260 bar (at 

90 °C). Owing to the higher specific cooling capacity of R744 compared with 

R134a, 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 header 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 evaporator against the high pressures of the 

refrigerant circuit; there must also be 

safeguards against leakage. Since the 

evaporator is installed in the interior beneath the cross member, it is probably 

located in the safest position in the vehicle. Special protective measures against a 

crash are therefore unnecessary, especially 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 CO2 emissions; basis of comparison: Frankfort/Main, Germany climate profile, NEFZ 

(New European Driving Cycle), mid-range vehicle, 40 g/year leakage 

ATZ 07-08I2008 Volume 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 increases up to about 60 cm3/min. During 

the same one-minute interval, an adult 

human being exhales approximately 333 

cm3 of CO2, more than five times the 

maximum assumed corrosion leakage. 

Approximately 350 g of CO2 are contained in an R744 refrigerant circuit at 

65 bar. A human being takes approximately 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 consists 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 prescribed 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 conditions for a given time period. “Confidence level” indicates how trustworthy a 

statistical value is. There is, therefore, a 

probability of 97.5 % that the 99.4 % reliability stated above will be attained during each test.

6 Production Experience

Small-volume production of R744 components 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 positively indicate that the tube connections 

are sufficiently protected from damage 

during the course of production. To this 

end, they must demonstrate a certain rigidity against bending, e.g. during shipment, which is checked in this test.

The additional leak test is necessary 

because of the high pressures in the refrigerant 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. Manufacturing defects can be corrected in this 

way. Defective components are not repaired; the evaporator in question is removed 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 refrigerant circuit. Furthermore, in the 

R744 A/C circuit with the IHX, an additional component is used. The R744 components 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. Consequently, 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 available space envelope in each vehicle model must preferably also be utilized as flexibly as possible. To this end, the same 

components were developed in different 

sizes, or integrated into modules; examples include:

– gas cooler: available as a separate component and as a gas cooler module 

with accumulator and IHX, Figure 6

– evaporator: similar to the R134a evaporator; integrated expansion valve, 

e.g. as an orifice tube with bypass

– IHX: separate, Figure 7, and in accumulator-integrated configuration or as 

part of the gas cooler module

– accumulator: stand-alone version and 

as combi unit consisting of accumulator and IHX, Figure 8.

These packaging space alternatives influence 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 larger units also has the goal of reducing the 

number of assembly costs and connection points.

8 Service

Although the conventional refrigerant 

R134a may no longer be used in new vehicle models after 2011, no automobile 

manufacturer is obliged to wait that 

long before introducing R744 air conditioning systems. Behr Service will therefore offer auto workshops a complete 

R744 service system package correspondFigure 5: Comparison 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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18 ATZ 07-08I2008 Volume 110

Thermo Management
ing 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 certification of shop personnel, technical support, 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. Today, a fully automatic unit is available, 

which is presently being evaluated on 

test vehicles. Its operability is comparable 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 operating errors, saving time and money. 

Individual functions such as the controlled 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 detected.

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 temperature-controlled cylinder heater, and also 

overheating safeguards. With an evacuation time of 30 min, the complete servicing 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 obtained to date, a smooth transition from 

R134a refrigerant to R744 can be expected. 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 likewise ensured. A fully automatic servicing 

unit was developed, and a training program 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 Volume 110 19 
A/C circuit is currently being prepared. 

An efficient national and international 

organization for marketing air conditioning components is also available.

Regular active participation at meetings and conventions (e.g. Society of Automotive Engineers (SAE) and German 

Association of the Automotive Industry 

(VDA) conventions) facilitates the broader dissemination of know-how and professional 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 development, advanced engineering, industrialization, and validation on a number of 

test stands, in the acoustics lab and the 

climatic wind tunnel, and with road trials. Together with joint-venture company 

BHTC (Behr-Hella Thermocontrol), a complete control system is also being developed and validated.

References

[1] Heinle, D.; Riegel, H.; Weinbrenner, M.: Fahrzeugintegration des R744-Kältekreislaufs. In: ATZ 108 

(2006), Nr. 6, S. 446–455

[2] Lochmahr, K.; Baruschke, W.; Britsch-Laudwein, A.: 

Regelsystem für R744-Kältekreisläufe. In: ATZ 107 

(2005), Nr. 9, S. 796–799

[3] Heinle, D.; Riegel, H.; Weinbrenner, M.: Klimatisierung mit dem Kältemittel R744. In: ATZ 107 

(2005), Nr. 9, S. 790–795

[4] Winterhagen, J.: Diskussion um Klimaanlagen 

flammt auf. ATZonline vom 18.05.2007

[5] N. N.: COP. http://de.wikipedia.org/wiki/Coefficient_Of_Performance vom 16. Mai 2008

Figure 8: R744 refrigerant circuit with the internal heat exchanger 

integrated into the accumulator

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20 ATZ 07-08I2008 Volume 110

Thermo Management
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