With the rapid development of the automotive industry, automotive air-conditioning systems have significantly enhanced driving and passenger comfort, and there is growing emphasis on their functional requirements and technological innovation. The performance of the air-conditioning system relies on the connections within the piping system, such as high-pressure and low-pressure lines; consequently, the design requirements for air-conditioning piping are of particular importance. This paper explores the technical development process of air conditioning piping by providing a detailed overview of the composition, operating principles, piping design, manufacturing processes and testing requirements of automotive air conditioning systems. Furthermore, it analyses common failure modes in automotive air conditioning piping and proposes corresponding corrective measures and maintenance recommendations, thereby providing a reference for future project development and design.

Introduction

As a vital component of a vehicle’s interior, the air conditioning system enhances the comfort of both driver and passengers and plays a significant role in the vehicle’s overall performance. The air conditioning piping, as the core component of this system, acts much like the ‘blood vessels of the human body’, connecting key components such as the compressor, condenser, evaporator and expansion valve to form a closed-loop system. This ensures the orderly flow of refrigerant within the system, thereby enabling the air conditioning system to provide both cooling and heating functions.

With the rapid development of the Chinese automotive market, consumers are placing ever-higher demands on the performance, reliability and energy efficiency of vehicle air-conditioning systems. The design, manufacture and maintenance of vehicle air-conditioning piping systems present numerous challenges, necessitating continuous innovation and optimisation. A thorough examination of the relevant technologies and solutions for vehicle air-conditioning piping systems is of significant practical importance for enhancing the overall performance of these systems, reducing energy consumption, minimising failure rates and improving the user experience.

An Overview of Automotive Air Conditioning Systems

1. Components and Operating Principles of the Car Air Conditioning System

A vehicle’s air conditioning system primarily consists of a compressor, cooling fan, condenser, blower, desiccant drier, air conditioning piping, evaporator, expansion valve and refrigerant. In new energy vehicles equipped with liquid-cooled battery packs, a radiator is also required.

The primary function of a vehicle air conditioning system is to provide cooling and heating, ensuring a comfortable environment for passengers inside the vehicle. The cooling process of the air conditioning system primarily comprises compression, condensation, throttling, evaporation and circulation. Firstly, the compressor compresses the low-temperature, low-pressure gaseous refrigerant into a high-temperature, high-pressure gas, which is then fed into the condenser. Secondly, within the condenser, the refrigerant is cooled and liquefied, transforming into a medium-temperature, high-pressure liquid, before flowing into the receiver-drier for storage and drying. Next, after passing through the expansion valve where pressure is reduced, the refrigerant becomes a low-temperature, low-pressure liquid and enters the evaporator. Finally, within the evaporator, the refrigerant boils and absorbs heat, cooling the air flowing through it and thereby achieving the cooling effect; the gaseous refrigerant is then drawn back into the compressor, completing a cycle. During the cooling process, the air conditioning piping provides a flow path for the refrigerant.

The heating mechanisms in automotive air conditioning systems primarily involve utilising engine waste heat and employing independent heating units. Traditional petrol and diesel vehicles mainly rely on the heat generated by the engine, whereas new energy vehicles utilise PTC thermistors for heating.

2. Functions and classifications of automotive air conditioning piping

Air conditioning piping plays a crucial role in automotive air conditioning systems by connecting various components and conveying refrigerant, ensuring the smooth circulation of refrigerant within the system. Automotive air conditioning piping assemblies can be categorised into compressor piping assemblies, condenser piping, heater core piping and ventilation system piping, amongst others. Automotive air conditioning piping can be classified by material into copper tubing, aluminium tubing and rubber hoses; by pressure into high-pressure and low-pressure lines; and, based on the state of the refrigerant during the cycle, into gas-phase and liquid-phase lines.

As aluminium tubing is lightweight, it plays a positive role in automotive weight reduction design; consequently, aluminium tubing is now widely used in automotive air conditioning systems. Automotive air conditioning piping systems primarily consist of aluminium tubing, fittings (clamps, connectors, nuts, etc.), flexible hoses, corrugated hoses, aluminium sleeves, charging ports, O-rings, pressure switches and plastic caps. To ensure that the air conditioning refrigerant does not leak, the quality of the piping fitting design is of paramount importance. Fittings in automotive air conditioning piping are key to ensuring airtightness; the main types of fittings currently in use are threaded connections and clamp connections.

Threaded connections involve joining aluminium tubes to one another, or aluminium tubes to other components, using nuts and external threads. Clamp connections use clamps and bolts to secure pipe joints tightly together, ensuring both sealing and stability. When tightening threads, the hose may become twisted; hoses subjected to torsional shear stress are prone to premature fatigue failure, and this torsional force also tends to cause the joint to loosen. Consequently, clamping structures are now preferred for air conditioning piping.

Design Requirements for Automotive Air Conditioning Piping

1. Requirements for the installation and routing of automotive air conditioning pipes

Automotive air conditioning pipework is subject to vibration, impact and temperature fluctuations whilst the vehicle is in motion; therefore, the secure installation of the pipework is of paramount importance. Proper securing prevents loosening, wear and leakage, ensuring the normal operation and long-term reliability of the air conditioning system. Where two pipes run parallel to one another, welded nut holes are typically designed at suitable positions on the front bulkhead outer panel, and multi-pipe clamps are used to secure the pipework, with fixing points generally spaced at intervals of 300 mm. At the same time, cable ties are often used to assist with securing the lines. For rigid pipes, the distance between two fixing points should be between 100 and 400 mm to prevent excessive vibration caused by overly long sections. The addition of fixing points on flexible hoses should be minimised to reduce stress and wear on the hoses. Additional fixing points should be added at bends to ensure stability at these points.

When designing air conditioning ductwork, a series of layout requirements must be met. The angle of bends in rigid ducting should be greater than 90°; the bend radius should be 1.5 to 2 times the diameter of the duct; the minimum straight section following a bend should be no less than 15 mm; and the connection between flexible and rigid ducting should be greater than 35 mm. The clearance between the ductwork and surrounding components should be no less than 6 mm to prevent wear caused by contact between the ductwork and surrounding components.

3. Testing requirements for air conditioning ductwork

To prevent refrigerant leaks during the circulation process, automotive air-conditioning systems must meet stringent airtightness requirements; during the design and development phase, numerous tests must be conducted to verify the soundness of the design, if the test is passed, this indicates that the airtightness of the piping meets the requirements.

Failure Mode Analysis of Automotive Air Conditioning Piping

According to relevant statistics, faults in air conditioning systems caused by incorrect refrigerant charging rank as the most common issue, with leaks at the joint between the evaporator outlet pipe and the compressor suction pipe accounting for as much as 90% of these cases. Consequently, the primary failure mode in automotive air conditioning piping is refrigerant leakage at the joints, which is attributed to the following specific causes.

1. Ageing of pipework

After prolonged use, the rubber components of a car’s air conditioning system gradually age, harden and crack, leading to refrigerant leaks through these fissures. As the air conditioning pipes are mainly located in the engine compartment, where they are constantly exposed to high temperatures and vibrations, the ageing process is accelerated.

2. Loose connection

The joints in the air conditioning pipework may become loose whilst the vehicle is in motion, due to vibrations and other factors. Should a joint become loose, the seal will be compromised, making it likely for refrigerant to leak from the joint.

3. Component failure

Components in an air-conditioning system, such as the compressor, condenser and evaporator, can also cause refrigerant leaks if their internal seals are damaged or if the components themselves develop defects such as cracks or pinholes. For example, a damaged shaft seal on the compressor can cause refrigerant to leak from the seal into the external environment.

4. Traumatic injury

Whilst the vehicle is in motion, the air conditioning pipes may be subjected to external forces such as impacts from stones or scrapes from branches, which can cause damage to the pipes and result in refrigerant leaks. Furthermore, improper handling during vehicle maintenance and servicing may also damage the air conditioning pipes.

5. Abnormal pressure

If the pressure in an air-conditioning system is too high or too low, it can damage the pipework and components, increasing the risk of refrigerant leaks. For example, if non-condensable gases such as air enter the refrigeration system, this can cause the system pressure to rise excessively, leading to the failure of seals in the pipework or components and resulting in refrigerant leaks.

To prevent refrigerant leaks caused by the above factors, the following points should be observed. Firstly, during vehicle use, the exterior of the air conditioning piping should be inspected regularly for signs of ageing, cracking or damage, particularly at bends in the piping and in areas close to heat sources such as the engine. Secondly, the pipe joints should be checked frequently for looseness or leaks; this can be done by applying soapy water to check for the formation of bubbles, which indicates a leak. Furthermore, the operational status of all components within the air conditioning system should be checked regularly, such as whether the compressor is running normally and whether there is abnormal frost build-up on the condenser or evaporator. Finally, the air conditioning system should be used correctly in accordance with the vehicle’s owner’s manual. Avoid running the air conditioning for extended periods whilst the engine is not running, as this places an unnecessary strain on the compressor. Finally, during vehicle servicing, ensure the air conditioning system is properly maintained. This includes replacing the air filter to keep the system clean, preventing dust and other contaminants from entering the system, which could impair cooling performance and damage components. Furthermore, during vehicle repairs, take care to avoid damaging the air conditioning pipes and components. If it is necessary to remove the air conditioning pipes, follow standard operating procedures; after removal, protect the pipe joints and other areas to prevent foreign objects from entering.

Conclusion

This paper explores the technical development process of air conditioning piping by providing a detailed overview of the composition, operating principles, piping design, manufacturing processes and testing requirements of automotive air conditioning systems. Furthermore, by analysing and addressing leakage issues in air conditioning pipe joints, it proposes corresponding corrective measures and maintenance recommendations, thereby providing a reference for future project development and design. The technical development of air conditioning piping and the resolution of leakage issues not only affect the performance of the air conditioning system but also directly impact passenger comfort and the overall quality of the vehicle. Therefore, the design, fabrication and maintenance of air conditioning piping should be given due attention.

Car air conditioning, an indispensable ‘must-have’ for driving in the sweltering summer heat, provides us with a comfortable environment whilst on the road.

If the compressor is the heart of the air-conditioning system, then the vehicle’s air-conditioning piping is its circulatory system, connecting the various air-conditioning components scattered throughout the vehicle to form a complete and efficiently functioning air-conditioning system.

Car air conditioning pipework typically consists of aluminium pipes, flexible hoses and other fittings.

Unlike other car components, air conditioning pipes do not need to be replaced very often, which means they are easily overlooked; as a result, some car owners fail to notice leaks in the pipework in good time.

Generally speaking, there are typically two causes of leaks in air conditioning pipes: 

•  A blockage in the air conditioning system’s circuit, leading to prolonged high-temperature and high-pressure conditions between the compressor and the condenser, causing the PA layer on the inner wall of the rubber pipe to age and crack.  
•  During the crimping of the aluminium sleeve, if the pipe is not positioned correctly, gas can escape from the top of the crimped area into the braided layer, penetrating the rubber layer and causing a general leak. This phenomenon is also known as a gas leak.

Although air conditioning hoses do not need to be replaced very often, over time they can accumulate dirt and grime that is difficult to clean out; it is therefore advisable to fit new ones. When replacing air conditioning hoses, be sure to choose products of guaranteed quality to avoid system faults caused by substandard hoses.

Turbocharging is a technology that uses the exhaust gases produced by an internal combustion engine to drive an air compressor. The primary function of turbocharging in cars is to increase the volume of air entering the engine, thereby boosting engine power and torque and making the vehicle more responsive. However, following turbocharging, both the pressure and temperature within the engine rise significantly; consequently, advancements in materials are also crucial when implementing turbocharging technology in engines.

High-temperature resistance

The gas in a turbocharger generates high temperatures due to compression and intense friction; even after cooling, the gas temperature generally exceeds 100 °C. Consequently, the materials used for hoses in turbocharger systems must be capable of withstanding high temperatures. Ordinary natural rubber, styrene-butadiene rubber (SBR) and polybutadiene rubber (BR) are unable to meet the requirements for use under high-temperature conditions; therefore, specialised high-temperature-resistant rubber materials must be employed. As turbocharger pressures continue to rise, the temperature of the gas passing through the hoses also increases. If the pressure reaches 3.5×10⁵ Pa, the temperature of the gas passing through the hoses can exceed 250 °C, and there are very few types of rubber capable of withstanding such high temperatures.

Oil Resistance

The gas passing through the hoses in a turbocharging system is generally contaminated with oil vapour; therefore, the hoses must possess a certain degree of oil resistance, particularly resistance to high-temperature oil vapour. Some rubbers with good high-temperature resistance (such as silicone rubber) have poor oil resistance, so an inner lining must be added to the inner wall of the silicone rubber hose to prevent corrosion from the oil vapour.

Strength

Turbocharging systems are not only subject to high temperatures but also to a certain degree of pressure; in particular, the pressure on the high-temperature sections of the piping is relatively high. Although reinforced hoses are generally used in turbocharging systems (with the reinforced layer constituting the primary pressure-bearing component), the rubber itself must also possess a certain degree of strength to enhance the overall strength of the hose. Furthermore, to meet the requirements of the manufacturing process and assembly, the rubber must also exhibit high tensile strength and tear strength.

Compression set

Generally, turbocharger hoses are connected to metal pipes using clamps to form a piping system. At high temperatures, the rubber must possess good resistance to deformation; otherwise, excessive compression set may cause the clamps to loosen and the hose to detach, leading to a safety incident.

Cold resistance

Although the hoses operate in a high-temperature environment once the engine has started, they are exposed to cold air once the engine is switched off. When the engine is started in cold conditions during winter in cold regions, the rubber hoses vibrate at low temperatures. If the rubber has poor low-temperature resistance, the hoses may become hard and brittle, leading to problems such as tearing, detachment and loss of vibration-damping capability.

Adhesion Strength

The rubber layer of a hose must maintain good adhesion to the reinforcement layer and the inner lining under harsh conditions such as cold, heat, and exposure to oil and gas, and must possess sufficient adhesion strength to ensure that delamination does not occur. Adhesion strength is dependent on the properties of the rubber itself and the rubber formulation, and is also closely related to the impregnation and pre-treatment of the reinforcement layer, the choice of adhesive, and the bonding process; therefore, all these factors must be thoroughly considered.

Hardness

The rubber should have a suitable hardness. If the hardness is too high, the hose will be too rigid to provide effective vibration damping, and will be difficult to fit and prone to coming loose; if the hardness is too low, sufficient strength cannot be guaranteed.

If you hear a ‘creaking’ noise coming from the car’s chassis, accompanied by the steering wheel pulling to one side whilst driving, it could be a problem with the suspension bushings. Today, we’ll take a look at what you need to know about car bushings.

What is an automotive bushing?

In mechanical design, the connection of moving parts is a common requirement; however, relative motion between components can easily lead to friction and wear. To address this issue, flexible coupling solutions are widely adopted—not only do they effectively reduce wear, but they also make replacement more convenient and cost-effective should wear and damage occur later on. It is for this reason that industrial bushings have come into being. 

In the automotive sector, bushings are elastic, flexible connecting components installed at the joints of moving parts such as the chassis suspension system and control arms. They are typically made from elastic materials such as rubber or polyurethane (or a composite structure combining a metal skeleton with an elastic material), and their core function is to replace rigid connections, thereby resolving the issue of friction and wear caused by the relative movement of components. 

Put simply, they act as a ‘shock absorber and wear-resistant joint’ between chassis components.

What is the purpose of a bushing?

Chassis bushings play a crucial role in the construction of a vehicle’s chassis. Their primary function is to connect the chassis to the suspension system, preventing rigid connections, protecting metal components and absorbing shocks, thereby ensuring the vehicle’s stability and comfort whilst in motion. Chassis bushings must not only bear the vehicle’s weight and inertia but also cope with a variety of complex road conditions and driving scenarios. A high-performance chassis bushing can significantly enhance the vehicle’s ride quality, reduce tyre wear and suspension fatigue, and provide the driver with a more enjoyable driving experience. 

Chassis bushings can be categorised into various types based on different classification criteria: these include front and rear axle bushings, tie rod bushings, control arm bushings, subframe bushings, hydraulic and non-hydraulic bushings, as well as metal and nylon bushings, and open and closed bushings. Although the classifications vary, the functions they perform are similar.

Points to note when replacing bushings

1. Selection of press-fit sleeves

When removing or pressing in components, select a sleeve of the appropriate size to ensure that the force is applied to the outer ring of the bushing, whilst other parts remain unloaded. Wear safety goggles and gloves when carrying out this work. The dimensional information provided in the Codic product manual can assist in selecting the correct sleeve. 

 2. Press-fit Force

To facilitate installation, remove any burrs from the inner bore prior to pressing and apply a small amount of lubricant (such as 4240 grease) to the inner bore and the initial section of the outer diameter. Ensure that the press-fit force is not too low; for bushings with an outer tube diameter of 40 mm, the press-fit force for metal outer tubes should generally exceed 6 kN, and for nylon outer tubes, it should exceed 20 kN, with values varying according to the diameter of the outer tube. If the pressing force is found to be too high or too low, check the condition of the inner bore and verify that the correct bushing has been selected.

3. Confirmation of Installation Position
During installation, ensure that the solid section is aligned with the horizontal direction of travel. If the product features an arrow, ensure that the arrow points in the horizontal direction of travel. Ensure that the pressing position is centred, with equal lengths protruding from both ends.

4. Stress Relief
Once installation is complete whilst the vehicle is raised off the ground, stress concentrations often develop in the chassis system. To resolve this, the vehicle must be lowered to the ground and the steering wheel centred. The fasteners should then be loosened to the specified torque, before being retightened to the standard torque to release the stress and allow the chassis to return to its original state. At this point, the vehicle is as though fitted with a brand-new pair of running shoes, ready to roam freely.

• Contact Us for Vehicle-Specific Bushings: natasha@nafurancar.com