P 619 - Qualification of steel housings for compressors of a new generation of automotive air conditioning systems
A compressor whose housing is made of steel has been developed and tested for a new generation of environmentally neutral automotive air conditioning systems operated with CO2 as a refrigerant. All current compressors …
Content
- Authors
- E. Roos, K. Maile, S. Offermanns, X. Schuler, W. Stadtmüller
- Publication
- 2007
A compressor whose housing is made of steel has been developed and tested for a new generation of environmentally neutral automotive air conditioning systems operated with CO2 as a refrigerant. All current compressors in motor vehicle air conditioning systems using the conventional refrigerant R134a are designed with a die-cast aluminium housing, as are the compressors built for initial tests as part of feasibility studies on CO2 systems. The housing made of steel was thus a new approach. The reason for this was that the new compressors are expected to have significantly higher internal housing pressures of up to 132 bar and temperatures of up to 150°C for air conditioning systems, which are likely to cause creep stress on the aluminium, resulting in a possible fatigue strength problem and intolerable creep expansion.
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With steel, on the other hand, creep in the temperature range up to 300°C is negligible. A cylindrical shape was chosen for the steel housing in order to minimise production costs. In order to minimise mechanical processing, this was preformed using cold extrusion. Several development stages of the housing were trialled as part of the project. The final stage was a welded construction. The cylindrical area of the housing with base (housing tube) is cold extruded from material S420NL (1.8912), the so-called insert, which essentially takes over the bearing of the drive shaft, is machined from material S460N (1.8901) and welded into the housing base via a laser circular seam. The strength and service life potential of the housing was investigated both experimentally using burst and vibration tests and numerically using finite element analyses. The characterisation of the housing material, the seam quality and the condition of the housing after the experimental tests was carried out using mechanical-technological, non-destructive, metallographic and fractographic testing methods. At the same time, the compressor manufacturer adapted the proven compressor technology used in the R134a systems to the cylindrical housing shape, redesigned it for the significantly higher operating pressures and temperatures and tested it and and optimised in test bench tests. Two test series of twenty housings each were produced for the final housing tests of the project. Nine of these were analysed non-destructively and metallographically, particularly in the area of the laser circumferential weld, which was the focus of particular attention. Apart from pores, which primarily occurred in the seam outlet, there were no negative findings. The burst and vibration tests carried out did not reveal any failure initiated by the pores. The service life of the housings was assessed on the basis of the results obtained from finite element analyses and component vibration tests. The two operating load cases "extreme operation" (switching on the air conditioning system under very high ambient temperatures) and "AC normal operation" were used for this purpose. For the former, based on the specified operating cycles, a fatigue strength of S 300 was obtained. For "AC normal operation", a housing failure probability of 0.1 to 1.0 % is to be assumed based on the definition of the housing fatigue strength with N 107 fatigue cycles for an operating time of 5000 h. Burst tests were carried out at 150°C and -40°C with two housings that were tested for the load case "extreme operation" and withstood N 107 cycles, which is virtually equivalent to the end of life (EOL). The maximum pressures achieved in each case exceeded the minimum failure pressures required by SAE for the housings in the EOL state. In no case did failure occur due to bursting, but in each case due to leakage at the sealing of the screw connection between the housing and cylinder head. The test results thus describe a status of the steel compressor housing that can be regarded as a good basis for optimising the housing for series production. Compared to aluminium housings, the fact that a possible creep behaviour influencing the service life is not to be expected with steel at the given operating temperatures should prove to be an advantage.
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P 619 - Qualification of steel housings for compressors of a new generation of automotive air conditioning systems—
€30.00
