332 Automobile electrical and electronic systems

Transcription

1 332 Automobile electrical and electronic systems Figure 9.65 njector(source:bosch Media) The quantity of fuel that is metered for injection at any time is computed by the engine ECU, which sends signals to the injection-pump ECU for control of the high-pressure solenoid valve. The electrical current for operating this valve is high and the two electronic control units are separated, in order to avoid high current interference, in the more electronically vulnerable engine ECU. The electronic diesel control units are provided with data signals from sensors and switches attached to the engine, the pump and other vehicle systems. The sensors are used for comparisons to programmed operating parameters and for calculations for metering the amount of fuel delivered and for controlling the injection advance. njection advance is obtained by rotation of the cam ring by pump body pressure in the injection advance mechanism. The injection advance mechanism consists of a transverse timing device piston and control components and an electrical solenoid valve. Maximum advance is 40 of crankshaft rotation. A needle motion sensor in the injector sends a signal to the engine ECU at the instant of opening of the injector (Figure 9.65). This point, relative to the crankshaft rotational angle before top dead centre, is used for load and speed injection timing calculations and for control of the exhaust gas recirculation valve. The Bosch, VR electronic diesel control system, uses a number of sensors and control actuators. This allows it to achieve optimum performance. However, even this sophisticated system has virtually been superseded by the common rail injection Common rail system The development of diesel fuel systems is continuing, with many new electronic changes to the control and injection processes. One of the most significant isthe CR 'common rail' system, which operates at very high injection pressures. t also has piloted and phased injection to reduce noise and vibration. The common rail system has made it easier for small high speed diesel engines to have all the advantages of direct injection. These developments have resulted in significant improvements in fuel consumption and performance.

2 Fuel control 315 njector Fuel filter Electronic control unit Bectric fuel pump Figure 9.40 LH-Jetronic LU-Jetronic This system is a further refinement of the LE systems but also utilizes closed loop lambda control. L3-Jetronic The ECU for the L3-Jetronic forms part of the air flow meter installation. The ECU now includes a 'limp home' facility. The system can be operated with or without lambda closed loop control. The air-fuel ratio can be adjusted by a screw-operated potentiometer on the side of the ECU. LH-Jetronic The LH system incorporates most of the improvements noted above. The main difference is that a hot-wire type of air flow meter is used. The component layout is shown in Figure Further developments are continuing but, in general, most systems have now developed into combined fuel and ignition control systems as discussed in the next chapter Bosch Mono Jetronic - single point injection The Mono Jetronic is an electronically controlled system utilizing just one injector positioned above the throttle butterfly valve. The throttle body assembly is similar

3 314 Automobile electrical and electronic systems Bosch 'L' Jetronic - Variations Owing to continued demands for improvements, the 'L' Jetronic system, along with all other injection systems, developed and changed over the years. This section will highlight the main changes that have taken place. L2-Jetronic This system is changed little except for the removal of the injector series resistors as the ECU now limits the output current to the injectors. The injector resistance is 16 Q. LE1-Jetronic No current resistors are used and the throttle switch is adjustable. The fuel pump does not have safety contacts in the air flow sensor. The safety circuit is incorporated in the electronic relay. This will only allow the fuel pump to operate when an ignition signal is present; that is, when the engine is running or being cranked. LE2-Jetronic This is very similar to the LE1 systems except the thermo-time switch and cold start injector are not used. The ECU determines cold starting enrichment and adjusts the injector open period accordingly. njector a \ Throttle position switch Fuel filter Electronic control unit Electric fuel pump Figure 9.39 L-Jetronic

4 Fuel control 313 Manifold pressure f---=:=-l ~Fuelinlet o Fuel outlet Figure 9.35 Pressure regulator Battery supply when engine is being cranked Cold start injector Figure 9.36 Typical cold start arrangement Earth contacts - Figure 9.37 Combination relay Combination relay (Figure 9.37) This takes many forms on different systems but is basically two relays, one to control the fuel pump and one to power the rest of the injection system. The relay is often controlled by the ECU or will only operate when ignition pulses are sensed as a safety feature. This will only allow the fuel pump to operate when the engine is being cranked or is running. Electronic control unit (Figure 9.38) Earlier ECUs were analogue in operation. All ECUs now in use employ digital processing. Figure 9.38 Electronic control unit

5 312 Automobile electrical and electronic systems n tt-'1l1.j.l Magnetic circuit Pintle Figure 9.33 Fuel njector Pressure relief valve Armature , Roller-cell pump! Check valve Figure 9.34 Fuel pump (high pressure) Fuel pump (Figure 9.34) Key fact The pump ensures a constant supply of fuel to the fuel rail. The pump ensures a constant supply of fuel to the fuel rail. The volume inthe rail acts as a swamp to prevent pressure fluctuations as the injectors operate The pump must be able to maintain a pressure of about 3 bar. Fuel pressure regulator (Figure 9.35) This device ensures a constant differential pressure across the injectors. t is mechanical device and has a connection to the inlet manifold. Cold start injector and thermo-time switch (Figure 9.36) An extra injector was used on earlier systems as a form of choke. This work in conjunction with the thermo-time switch to control the amount of cold enrichment. Both engine temperature and a heating winding heat it. This technique has been replaced on newer systems, which enrich the mixture by increasing the number of injector pulses or the pulse length.

6 Fuel control 311 Figure 9.28 Crank sensor in position Figure 9.29 Coolant temperature sensor Figure 9.30 Throttle position sensors Lambda sensor (Figure 9.31) This device provides information to the ECU on exhaust gas oxygen content. From this information, corrections can be applied to ensure the engine is kept at or very near to stoichiometry. Also shown in this figure is a combustion chamber pressure sensor. dle or fast idle control actuator (Figure 9.32) Bimetal or stepper motor actuators are used but the one shown is a pulsed actuator. The air that it allows through is set by its open/close ratio. Figure 9.31 Lambda sensor in the exhaust down pipe Fuel injector(s) (Figure 9.33) Two types are shown - the pintle and disc injectors. They are simple solenoidoperated valves designed to operate very quickly and produce a finely atomized spray pattern. njector resistors These resistors were used on some systems when the injector coil resistance was very low. A lower inductive reactance in the circuit allows faster operation of the injectors. Most systems now limit injector maximum current in the ECU in much the same way as for low resistance ignition on coils. Figure 9.32 Rotary idle actuator