Design must take into account not only manufacturing processes but also the ease of assembly for the OEM. During the pilot production phase for a new automotive model, frequent assembly difficulties arose with the air conditioning refrigeration piping, resulting in substantial costs for subsequent design modifications. By incorporating concurrent engineering into the final assembly process, virtual assembly analysis and design constraints were applied during the development of the air conditioning refrigeration piping. This effectively reduced production costs during the manufacturing process and improved production efficiency. This paper briefly outlines the assembly and design issues encountered in the synchronous engineering analysis of air conditioning refrigeration piping, along with their solutions, and provides valuable guidance for the development of air conditioning refrigeration piping in new vehicle models.

I. Introduction to Synchronised Engineering for Final Assembly

Synchronised Engineering (SE) for final assembly is a process in which final assembly processes are integrated into the design and development phase of vehicle development. It primarily involves conducting process analyses of assembly digital models, production lines, equipment and assembly processes, and provides feasible process design changes to support the design. Its primary objective is to review issues in product design during the drawing design and digital model generation stages, taking effective measures in advance to address potential problems that may arise during process implementation, thereby ensuring the new vehicle model is production-feasible and compatible with equipment and tools.

II.  Air Conditioning Piping Assembly and Design 

1. Composition of the Front Engine Compartment Air Conditioning Refrigerant Piping

The air conditioning refrigerant piping primarily comprises the air conditioning high- and low-pressure pipe assembly, air conditioning exhaust pipe assembly II, air conditioning exhaust pipe assembly I (which may be combined with air conditioning exhaust pipe assembly II, depending on assembly considerations), air conditioning low-pressure pipe assembly I, and air conditioning high-pressure pipe assembly I (which may be combined with the air conditioning high- and low-pressure pipe assembly, depending on assembly considerations).

2. Issues with the design and assembly of the air conditioning refrigerant piping

(1) At the connection between the high- and low-pressure pipe assembly and the HVAC expansion valve, the foam padding on the clamps attached to the pipes is too thick and too rigid, causing excessive interference with the front panel and making the piping difficult to fit.

(2) The air conditioning high- and low-pressure pipe assembly comes with its own mounting brackets (secured to the engine compartment side panels and longitudinal beams). The cut-outs are circular, but the allowance for offset in the X-direction is too small; due to the combination of fitting accuracy and cumulative tolerances, the bolt holes cannot be aligned.

(3) The air conditioning refrigerant lines are connected using bolts and nuts; during prototyping, there is insufficient working space for tightening tools (such as a cordless impact wrench). The interference persists even when a shorter socket is used.

(4) It is not possible to apply refrigeration oil to the clamps during assembly of the pipe joints, and refrigerant leaks occur once assembly is complete. There is no flexible hose section connecting the high- and low-pressure pipe assemblies to the high-pressure pipe assembly; the rigid pipes are difficult to connect and prone to deformation.

(5) The piping layout is not sufficiently well-designed, leading to frequent issues such as abnormal noises and poor assembly ergonomics; for example, the piping does not run close enough to the engine compartment, and the air conditioning filling port is positioned too low to allow for refilling.

3. Design Constraints for Air Conditioning Refrigerant Piping

Design constraints are guidelines derived from a compilation of common issues encountered during the introduction of new vehicle models and the prototyping process; they are intended to identify areas requiring improvement in subsequent product designs. In response to the assembly issues outlined above, the following design constraints have been established.

(1) The foam used in the clamping plate at the connection between the air conditioning high- and low-pressure pipe assembly and the HVAC expansion valve should be made of PUR material, with a thickness preferably less than 15 mm.

(2) With the exception of the primary locating holes, all holes in the brackets on the air conditioning high- and low-pressure pipe assemblies shall be elliptical in the X-direction (e.g. 8×10, depending on the bolt specification), to accommodate cumulative tolerances. A rotational restraint mechanism (such as a locking clip) must be provided at the point where the bracket connects to the vehicle body to prevent the bracket from rotating when the bolts are tightened, which could cause deformation of the piping. The brackets for the air conditioning pipes must be designed to be mounted on the rigid pipe sections to avoid scratching the flexible hoses.

(3) When designing the system, consideration must be given to the working space required for operating pipe connection fastening tools. When using an elbow gun, the distance between the rivet head and the end of the stud must be greater than 85 mm; when using a straight gun, the distance between the rivet head and the end of the stud must be 40 mm. 

 (4) The male end of pipe fittings must face upwards in the Z-direction (no requirement for the X-direction) to facilitate the application of refrigeration oil. Rigid pipes must not be connected directly to one another; a flexible hose must be used as an intermediate connection, and the joint must be properly sealed, for example by fitting a sealing gasket. 

 (5) Above the high- and low-pressure filling ports of the air conditioning high- and low-pressure pipe assemblies, there must be a clear space with a diameter of 50 mm and a height of 250 mm. Furthermore, the spacing between the high- and low-pressure filling ports must be reasonable (depending on the size of the filling nozzle).

III. Conclusion     

This paper summarises the common issues encountered during the final assembly of the refrigeration piping system for a particular automotive air conditioning unit. By incorporating SA constraints into the design phase through concurrent engineering during the early stages of new model introduction, this approach has helped to minimise design shortcomings, optimise the manufacturability of the final assembly process, and reduce production costs for the company. Furthermore, it provides valuable guidance for the development of refrigeration piping systems for new vehicle models.