Analysis on Harness Design of Engine Management System

With the advent of the era of scientific and technological intelligence, the electronic and electrical system is becoming more and more complex, and the automobile manufacturers take the electronic and electrical architecture as the core technology. The core of the automobile electronic system is the control unit, which is responsible for the management and control of the whole control system. Automotive wiring harness is the hub carrier of the control unit, which is connected with the operation of the controller. Compared with other control units, the wire harness of the engine management system can ensure the higher reliability of the whole vehicle in various harsh environments and driving conditions.  

1 Introduction

Engine electronic control unit (ECU) plays the most prominent role in the vehicle control unit. The key components of the engine control management system are used to detect and control the normal operation of the engine system, calculate the torque required by the driver, ensure the optimal fuel economy and meet the national exhaust emission standards. The engine management system is equipped with sensors and actuators necessary for ECU to work, and related components are connected together through wiring loom.

The design, material selection, wire diameter and installation layout of the engine harness determine the correctness and stability of the engine control management system. The normal operation of the system depends on the transmission of correct signals, and the safe operation of the vehicle is also closely related to the performance of the harness, so the design of the harness is also an important link in vehicle development. According to the different standards of the industry and enterprises, the design of the wiring harness is different in each automobile factory. In order to ensure the stability of the engine control system and give full play to it, the engine control unit has made some suggestions on the selection of wiring harness, the size of wire diameter, wire resistance and installation, so as to meet the requirements of system stability.

2 Material Selection Suggestions

2.1 Conductor Selection

At present, major automobile manufacturing countries such as the United States, Germany, France and Japan have formulated their own standards or industry standards for automobile lines. United States: SAEJ1128, 1678; Germany: DIN72551；; Japan: JASOD611, JASOD608；; France: NFR13414, 13415; International standard: ISO6722. Commonly used conductor types include Japanese standard (AVSS, etc.), national standard (QVR), German standard (FLRY), American standard and so on. AVSS (AVS) conductor is characterized by thin skin insulation and good flexibility. QVR is characterized by thick insulation skin, softness and good ductility; German standard conductor has thinner insulation and good flexibility; American standard conductor insulation skin is generally thermoplastic or thermosetting elastomer, and it is also processed by irradiation process.

OEM can decide which type of wire to use according to the temperature, environmental conditions (humidity, wear resistance, etc.) and current carrying capacity of the wire harness, as well as the hole size, secondary repair and easy operation in the wire harness manufacturing process. Generally, it is recommended to use the wire with the largest wire diameter and high temperature resistance when the crimping conditions permit, and the insulation of the wire should be above 400V V.

The press joint of the two wire ends of the wire harness meets the national or industrial standards, and you can refer to QC/T29106—2014. The requirements of insulation, high temperature resistance and wear resistance should be considered in the treatment of the press joint to prevent the wire ends at the press joint from puncturing the adhesive tape and causing short circuit. The position to be heat-shrunk is located in the center of the heat-shrinkable tube. After heat-shrinking, there is no phenomenon of insulation layer melting, incomplete heat-shrinkable or puncture. When the double-layer heat-shrinkable tube is heat-shrunk, there must be glue on both sides, and there is no gap, and there is no scorching, copper wire sticking out and copper wire leakage.

2.2 Wire Specifications

EMS has recommended values for wire diameter on the provided wire harness diagram. Considering the material and quality of domestic wire harness, OEM should choose the recommended wire diameter or use wire specifications larger than the recommended wire diameter when designing the wire harness to ensure that the performance of the wire harness meets the system requirements.

2.3 Wire Resistance of Wire Harness

When there is a large wire resistance on the wire, it will produce a large voltage drop on the wire, which will affect the accuracy of the signal. The following requirements are put forward for the resistance of the wire (the resistance from the ECU female connector to the sensor and the actuator female connector).

1) Ignition driving signal, ignition grounding, electronic grounding and power grounding < 50mΩ.

2) other signals < 100mΩ.

2.4 Twisted Pair/Shielded Wire

Twisted pair and shielded wire are required for weak and sensitive signals in engine control management system, including knock signal, crankshaft sensor signal, camshaft sensor signal, CAN, LIN and vehicle speed signal.

1) The lay lengths of twisted pairs should be between 20 and 100 mm, and the lay lengths of different twisted pairs arranged in the same beam should be different.

2) There are two commonly used shielding wires: single-layer shielding and double-layer shielding. Single-ended grounding is required for single-layer shielding, both ends of the outer layer are grounded for double-layer shielding, and the inner layer is grounded at the same potential. The shielding layer in the shielding wire should be reserved at the connector end of the parts, and the shielding ground in the shielding wire should be on the same potential plane as the ground wire of ECU, but it is not recommended to be connected to the ground point of ECU.

2.5 Connector

EMS will provide the recommended female connector model when providing the parts list, and it is recommended to use the mating connector recommended by EMS.

The connector shall be tested with the sensors and actuators of the engine management system for waterproof, vibration and temperature resistance, so as to ensure the strength and reliability of the connection and meet the requirements of the system. Different parts have different waterproof requirements due to different installation positions, even for the same type of parts. Therefore, when using connectors of non-recommended models, please conduct corresponding tests to ensure that the performance of connectors meets the requirements.

2.6 Connector Pin Handling

The pins of ECU connectors are tinned, so it is required that the matching connectors selected are also tinned. The requirements of large pins and small pins in connectors are different, and OEM can put forward requirements to harness manufacturers according to its own specifications and industry standards to avoid system instability caused by crimping quality.

Inside the connector, generally, the female terminal plug-in has its own waterproof rubber pad in the mounting hole of the small terminal pin, which can be directly inserted into the interference fit after installation to achieve waterproof effect, but there is no waterproof rubber pad in the mounting hole of the large terminal pin, so the waterproof plug should be sleeved on the insulation of the wire for crimping first. The unused pinhole in the connector should be waterproof and sealed to prevent water from flowing into the ECU pin through the pinhole, which will corrode the pin and lead to poor contact and short circuit. The blind plug or pin provided by the manufacturer of mating connector can be used for plugging, and injection molding can also be used.

3 Harness Layout Suggestions

1) No wire in the wiring harness is allowed to be exposed in the air, and the protective measures for the wire should be less than 10mm for the size of the connector end.

2) The ECU shell must keep good electrical contact with the vehicle body, and the grounding point of ECU cannot be pressed together with the negative electrode of the battery. The grounding point of harness should be avoided in a place where it is easy to be corroded and dust accumulated. Electroplating should be avoided at the grounding point, but corrosion protection should be done.

3) The wiring harness layout point should be far away from the heat source, and the wiring harness should not be placed on the engine housing.

4) The installation position of the wiring harness should keep a certain safe distance from the moving parts and avoid being squeezed by other parts.

5) The installation of the wiring harness should avoid the situation of folding back and large bending angle.

6) The distance between the outlet end of ECU connector and the first wire harness fixing point should be about 15cm, and then it should be fixed every 20cm.

7) The direction design and fixed position selection of sensitive signals (engine speed sensor signal, phase sensor signal, accelerator pedal, throttle position, CAN, etc.) should be more than 15cm away from high-power interference sources such as ignition coil, generator, starter, air conditioner compressor and wiper motor.

8) Try to arrange the mating connector with the port upward, so that the accumulated water can flow away along the wiring harness.

9) The length of the wiring harness should be as short as possible, especially for heavy load signals such as ignition signal, fuel injection signal and electronic throttle body drive signal, which are generally less than 3m.

10) Try to avoid using connectors to transfer the wiring harness in the wiring harness.

11) The wiring harness between the two fixing points of the wiring harness should not be tight, and a certain degree of slack should be maintained to avoid the breakage of the wiring harness or poor contact of the connector due to pre-tension. If two adjacent fixing points in the wiring harness are fixed to the engine and the car body respectively, the length of the wiring harness left between the two fixing points should be larger.

12) The crimping point of the power cord with branches of the wire harness shall be selected as close as possible to the power supply end.

13) The crimping point of the grounding line branch of ECU should be as close as possible to the female connector of ECU.

14) The pressure contact of signal branch shall be as close as possible to the female connector of ECU.

4 Conclusion

To sum up, this paper introduces the selection of wire harness, wire diameter, wire resistance, installation and other contents in the engine management system, puts forward the evaluation path through material selection and wire harness layout, determines the selection scheme, and avoids unreasonable schemes through fault cases, hoping to provide reference for colleagues.