Design of Automobile Wire Harness and Selection of Raw Materials for Wire Harness

It is the main network of automotive Wiring Harness automobile circuit, and there is no automobile circuit without wire harness. Reasonable design of automotive wiring loom can greatly improve the overall performance of automobile. Firstly, this paper, by Kable-X, introduces the circuit design of the whole vehicle, then introduces in detail the selection of connectors, the selection of wires, and the design of the whole vehicle harness seal. Finally, it discusses the wrapping and fixing design of the whole vehicle harness.

Introduction

It is the main network body of automobile circuit in automotive wiring harness. With the improvement of people's requirements for the safety, comfort, economy and emission of automobiles, automotive wiring harness has become more and more complex, but the space given by car body for wiring harness has become smaller and smaller. How to improve the comprehensive performance design of automotive wiring harness has become the focus of attention, so the design of wiring harness is very important.

1. Circuit Design of the Whole Vehicle

1.1 Power Distribution Design

The power supply system of an automobile is generally composed of three parts.

(1) Battery direct power supply system (generally called constant power or 30 power). The load connected to this part of the power supply is the safety or important part of the car. Such as the working power supply of engine ECU and engine sensor, the working power supply of fuel pump, the power supply of ABS controller, the power supply of diagnostic interface, etc.

(2) Power supply system controlled by ignition switch (generally referred to as ON gear, IG gear or smart electricity). This part of electrical equipment is used only when the engine is running, and it is taken from the power supply of the generator, thus avoiding the possibility of competing for power supply when charging the battery. Such as instrument power supply, brake light power supply, airbag power supply, etc.

(3) When the engine starts, unload the power supply of the load (generally referred to as ACC power supply). Generally, this part of electrical equipment carries a heavy load, and it doesn't have to work when the car starts. Generally, there are cigarette lighter power supply, air conditioning power supply, radio player power supply, wiper power supply, etc.

1.2 Line Protection Design

Line protection is the protection of wires and the protection of circuit electrical devices. Protection devices mainly include fuses, circuit breakers and fusible links.

1.2.1 Selection Principle of Fuse

Fuses are divided into fast melting type and slow melting type. The main component of fast-melting fuse is thin tin wire, among which chip fuse is widely used because of its simple structure, good reliability, vibration resistance and easy detection. Slow-melting fuses are actually tin alloy pieces. Fuses with this structure are generally connected in series to circuits with inductive loads, such as motor circuits. Fuses must be set separately for electrical devices that are easily interfered by other electrical equipment, such as engine ECU, ABS, etc.

It also has a great impact on the performance and safety of the whole vehicle, but this kind of electric load is not sensitive to mutual interference. Such as engine sensors, various alarm signal lamps, external lighting lamps, horns, etc., can be combined with each other according to the situation, and a fuse can be used together. The electrical loads of common electrical appliances set for increasing comfort can be combined with each other according to the situation, and a fuse can be used together. Resistor-type load and inductor-type load should avoid using the same fuse as much as possible. The fuse capacity can be calculated and determined according to the maximum continuous working current of general electrical devices, according to the empirical formula: rated capacity of fuse = maximum working current of circuit ÷80% (or 70%).

1.2.2 Circuit Breaker

The biggest feature of circuit breaker is its recoverability, but its cost is higher and its use is less. The new circuit breaker uses PTC solid material as overcurrent protection element, which is a kind of resistor with positive temperature coefficient, which can be turned off or turned on according to the current or temperature. The biggest advantage of this kind of protection element is that it can be switched on automatically after troubleshooting, without manual adjustment and replacement.

1.2.3 Fusible Link

The characteristic of fusible link is that when the line passes through a great overload current, the fusible link can be fused within a certain period of time (generally ≤5s), thus cutting off the power supply and preventing a vicious accident. Fusible wire is also composed of conductor and insulation layer, which is generally made of chlorosulfonated polyethylene. Because the insulation layer is thick, it looks thicker than the same specification wire.

Fusible wires are generally connected in the circuit directly led out from the storage battery. Commonly used nominal cross sections of fusible links are 0.3mm², 0.5mm², 0.75mm², 1.0mm², 1.5mm², and even fusible links with larger cross sections such as 8mm². The length of fusible link is divided into (50±5) mm, (100±10)mm and (150±15) mm.

The fusible link should be clearly marked, and after it is blown, its mark should still exist for easy replacement.

1.3 Selection and Design of Relay

Relays are divided into current type and voltage type. Generally, according to the power of the electric appliance and the carrying capacity of the switch, whether to choose relay or not is decided. Commonly used relay devices generally include wipers, horns, defrosting, headlamps, fog lights, fans, blowers, turn signals (flashers), etc. There are three kinds of relays: 6V, 12V and 24V. The rated voltage of commonly used relays is 12V and 24V.

Technical requirements for selecting relay: ① Good reliability; ② Stable performance; ③ Light weight, small size, long service life and little influence on surrounding components; ④ Simple structure, good manufacturability and low cost.

1.4 Design Principle of Ground Distribution

Engine ECU, ABS, etc. have great influence on the performance and safety of the whole vehicle, and are easily interfered by other electrical equipment, so the grounding points of these parts must be set separately.

For the airbag system, its grounding point should not only be single, but also in order to ensure its safety and reliability, it is best to adopt multiple grounding. The purpose is that one of the grounding points fails, and the system can be grounded through the other grounding point to ensure the safe operation of the system. To avoid interference, the radio system should also be grounded separately.

The weak signal sensor should be grounded independently, and the grounding point should be near the sensor to ensure the true transmission of the signal. Other electrical devices can be combined with each other to share the grounding point according to the specific arrangement.

The principle is to ground nearby to avoid unnecessary voltage drop caused by too long grounding wire. Due to the large cross-section of the lead wire, such as the negative battery wire and the engine grounding wire, it is necessary to control the wire length and direction to reduce the voltage drop; In order to increase safety, the engine and the body are generally connected to the negative electrode of the battery and grounded separately.

Grounding method: First, grounding through hole joint. In this method, heat shrinkable tube insulation must be baked at the tail of the joint; Second, it is grounded directly through the sheath of internal short circuit.

2. Selection and Design of Connector

2.1 Selection and Design Principles of Connectors

Selection of connectors should ensure good contact with electrical devices, minimize contact resistance and improve reliability, and connectors with double-spring compression structure are preferred. Choose the Connector reasonably according to the cross-sectional area of the wire and the current. Because the temperature and humidity in the cabin are too high and there are a lot of corrosive gases and liquids, the waterproof jacket must be selected. If the same sheath is used in the same harness, its color must be different. Based on the overall coordination of automobile appearance, the black or dark jacket should be preferred in the engine compartment.

In order to reduce the types and quantity of sheaths for wiring harness butt joint, the hybrid parts are preferred to facilitate assembly and fixation.

For terminal connectors for airbags, ABS, ECU, etc., which require high performance, gold-plated parts should be preferred to ensure safety and reliability.

2.2 Performance Analysis of Raw Materials of Connectors

2.2.1 Sheath Material (Plastic Parts)

Commonly used materials are PA6, PA66, ABS, PBT, PP, etc. When designing the plug-in, you can choose different materials according to different requirements, and you can also add flame retardant or reinforced materials to the plastic according to the actual situation, so as to achieve the purpose of strengthening or flame retardant, such as adding glass fiber reinforcement.

2.2.2 Terminal Material (Copper)

Copper used in connectors is mainly brass and bronze (the hardness of brass is slightly lower than that of bronze), of which brass accounts for a large proportion. In addition, different coatings can be selected according to different requirements.

3. Selection and Design of Conductor

3.1 Selection of Wire Type

The wire type selected in the design of the harness focuses on the environment and function of the harness. For example, the ambient temperature around the engine is high, and there are many corrosive gases and liquids. Therefore, be sure to use high temperature, oil, vibration and friction resistant wires; The wires on the luggage compartment cover should keep their elasticity at low temperature, so cold elastic wires should be selected to ensure their normal work; The wire on the automatic transmission must be resistant to high temperature and hydraulic oil, and its temperature stability is good; Weak signal sensors should use shielded wires, such as knock sensor, crankshaft position sensor, ABS wheel speed sensor, etc. The bending resistance of the door inner line is required to be high.

The wires commonly used in automotive wiring harness usually use stranded copper wires with PVC insulation. Wire Harness should be heat-resistant, oil-resistant, wear-resistant, waterproof, anti-corrosive, anti-oxidation and flame retardant.

The types of conductors commonly used in automotive wiring harness are Japanese Standard (AVSS, etc.), National Standard (QVR), German Standard (FLRY), American Standard, etc. AVSS(AVS) conductor is characterized by thin skin insulation and good flexibility; QVR is characterized by thick insulation, softness and good ductility; German standard conductor has thinner insulation and good flexibility; American standard wire insulation sheath is generally thermoplastic or thermosetting elastomer, and some are processed by irradiation process. According to the user's needs and different working environments, the appropriate type of conductor can be selected.

3.2 Calculate the Selected Conductor Cross-Sectional Area.

Calculate the current of the circulating wire according to the power of the electrical device; Electrical equipment working for a long time can choose wires with actual current carrying capacity of 60%; Electrical equipment working for a short time can use wires with actual current carrying capacity between 60% and 100%.

According to different working environment and temperature, the cross-sectional area of the wire should be changed appropriately.

According to the direction of the wire and the number of connectors (i.e. the voltage drop), change the cross-sectional area of the wire appropriately.

On the calculation of conductor cross-sectional area: I=P/UsA=IρL/Ud

Where: I-current; P-power; Us-system voltage; A- cross-sectional area of conductor; Ud- maximum allowable voltage drop loss; ρ-resistivity of copper; L-conductor length

Generally speaking, several key allowable maximum voltage drop losses (Ud) are charging cable: 0.8V (at rated voltage and rated power) from B+ terminal of three-phase generator to battery; Main starter cable: 1.0V(+20℃) from the storage battery to the starter terminal 30; Use starter control wire: 2.0V from the start switch to the starter 50 terminal.

Other control circuits: such as switches, relays, horns, etc., 1.0V (at rated voltage).

4. Design of the Whole Vehicle Harness Seal (Rubber Parts)

Generally, rubber parts are used for transition when automotive wiring harness passes through holes, so as to play the role of wear resistance, waterproof and sealing. Mainly distributed in the following places: the interface between engine and cab, the interface between front cabin and cab, the interface between four doors (or back doors) and carriage, and the inlet of fuel tank.

Commonly used materials are natural rubber, neoprene, silicone rubber, ethylene propylene diene monomer, etc.

Characteristics of natural rubber: It has good elasticity and mechanical strength, excellent bending flexibility, high tearing strength and good cold resistance. Disadvantages: Poor aging resistance, oil and ozone resistance, and flammability.

The characteristics of neoprene: ozone resistance, heat aging resistance, oil resistance, flame retardancy and self-extinguishing; But the low temperature resistance is not good.

Characteristics of silicone rubber: good heat resistance, cold resistance and weather resistance; The disadvantage is that it is not oil-resistant.

The characteristics of EPDM: good weather resistance, ozone resistance, heat resistance, corrosion resistance, acid and alkali resistance, and high strength and expansion rate; Disadvantages: Poor adhesion, elasticity not as good as natural rubber, and poor oil resistance.

Comparatively speaking, the comprehensive performance of EPDM is better, so the rubber parts used in automotive wiring harness are generally made of EPDM.

5. Bundling and Fixing Design of the Whole Vehicle Harness

5.1 Harness Bandaging Design

The external binding of wire harness plays the role of wear resistance, flame retardance, corrosion resistance, interference prevention, noise reduction and appearance beautification. Generally, the following binding design scheme is formulated according to the working environment and space size.

(1) The working environment of the engine harness is harsh, so it is all wrapped with corrugated pipe with high flame retardancy, water resistance and high mechanical strength.

(2) The working environment of the front cabin line is also relatively poor. Most branches are also wrapped with corrugated pipes with good flame retardancy, and some branches are wrapped with PVC pipes.

(3) The working space of instrument line is small and the environment is relatively good, so it can be wrapped with tape or flowers.

(4) The working space of the door line and the ceiling line is small, so it can be wrapped with adhesive tape, and some branches can be wrapped with industrial plastic sheets; The thinner roof line can be directly attached to the car body with sponge tape.

(5) Because there are many contact parts between the chassis line and the car body, it is wrapped with bellows to prevent the harness from being worn.

5.2 Performance Analysis of Raw Materials for Bandaging

5.2.1 Bellows

Bellows generally account for about 60%, or even more, in the binding of wire bundles. Its main characteristics are good wear resistance, high temperature resistance, flame retardance and heat resistance in high temperature zone. The temperature resistance of bellows is between-40℃ and 150℃. Its materials are generally divided into PP and PA. PA is superior to PP in flame retardancy and wear resistance; However, PP is better than PA in bending fatigue resistance.

5.2.2 PVC Pipe

The function of PVC pipe is similar to that of corrugated pipe. PVC pipe has good flexibility and bending deformation resistance, and PVC pipe is generally closed, so PVC pipe is mainly used at the branch of the wire harness to make the conductor smoothly transition. The heat-resistant temperature of PVC pipe is not high, generally below 80℃.

5.2.3 Adhesive Tape

Tape plays the role of binding, wear resistance, insulation, flame retardant, noise reduction, marking, etc., and generally accounts for about 30% of the wrapping materials. Harness tape is generally divided into PVC tape, air flannel tape and duct tape. PVC tape has good wear resistance and flame retardancy; The temperature resistance is around 80℃, the noise reduction is not good, and the price is cheaper. The flannel and duct tape are made of PET. Velvet tape has the best bandaging and noise reduction, and its temperature resistance is around 105℃. Duct tape has the best wear resistance and the highest temperature resistance is about 150℃. The common disadvantage of flannel and duct tape is poor flame retardancy and high price.

6. Conclusion

In a word, whether the design of automotive wiring harness is reasonable or not greatly affects the overall performance of the car. Only by analyzing problems, summarizing in practice, and flexibly using the acquired knowledge in different environments can designers constantly improve their skills, thus achieving the goal of designing automotive wiring harness quickly and reasonably.