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Drive By Wire The Bosch ME-Motronic System; Pt.1 The Bosch ME-Motronic system is the first “drive-by-wire” engine management system for cars. It breaks the direct link between the driver’s foot and the throttle position and it’s coming soon to your new car. By JULIAN EDGAR The new Bosch ME-Motronic system takes a radically differ­ ent approach to engine management systems. There is no accelera­tor cable. While it initially appears to have all the usual ingredients of a modern electronic management system – fuel injectors, input sensors, an electronic control unit (ECU) and so on – the use of accelerator position sensing and an electronic throt­tle actuator makes this system very different. In effect, the direct link between the driver’s foot and the throttle is broken. What the driver demands may not be what the driver gets. This situation has existed on some large trucks for some time now. However, it’s only now that the engine management 4  Silicon Chip system has be able to adjust the relationship between the car’s accelerator pedal and throttle opening. Not only can this system control fuel injection and ignition but also the cylinder charge. Making the advent of the ME-Motronic even more of a sea change is the underlying operating logic. Unlike other engine management systems, ME-Motronic determines how much engine torque is required in any given situation and then opens the throttle sufficiently to allow the engine to develop that much torque. The accelerator pedal travel becomes just the driver’s “torque request”, to be weighed up against other torque requests generated by the traction control system, speed limiter, engine braking torque control and others. Furthermore, at all times the engine management ECU models the engine’s instantaneous torque development, adjusting the throttle opening according to the relationship between the requested and developed torque. A quick example makes this easier to understand. In some situations, the driver may have only depressed the accelerator pedal halfway – but under the bonnet, the throttle butterfly valve can have snapped wide open! But why would this be an advan­tage? In turbo-charged cars, the maximum available torque can vary substantially over quite a narrow range of engine speed. For example, the current model Audi S4 twin turbo V6 develops a maximum torque of 300Nm at 1400 RPM and 400Nm at 1850 RPM. So, across just 450 RPM of engine speed, the peak torque varies by 33%. This characteristic is caused by the two turbos rapidly increasing in speed – ie, “coming on boost”. Fig.1 shows the power and torque curves for this particular engine; a similarly shaped torque curve is as- Fig.1: the torque curve of this twin turbo Audi 2.67 litre V6 shows the very rapid rise that occurs as the turbos start to boost at low engine speeds. Electronic throttle control by the Bosch ME-Motronic 7.1 engine management system allows good driveability, even with this massive torque change. [Audi] sociated to a greater or lesser degree with all turbo-charged engines. A driver of a turbo car equipped with traditional engine management tends to automatically compensate for this steeply rising torque curve. When wishing to accelerate moderately hard, he or she will initially floor the throttle and then back off as turbo boost and torque rises. But with the latest Audi S4 V6, equipped Where there are major torque changes over a small range in engine speeds, an engine management system that varies the relationship between accelerator position and throttle opening can yield major improvements in driveability. This Audi twin turbo V6 – fitted to the current S4 model – uses Bosch ME-Motronic management. The 2000 model Porsche on the facing page is also fitted with the Bosch ME-Motronic system. with Bosch ME 7.1, the driver need not do this. At low speeds when the engine response is relatively poor – the turbos yet to generate appreciable boost – the ME-Motronic system opens the throttle far further than the driver pushes the pedal and then as revs rise, it au- tomatically adjusts the throt­tle angle to retain a smooth linear response. In this way, drive­ability, emissions and fuel consumption are all improved. Inputs and outputs As indicated, at first glance the ME-Motronic system looks very Fig.2: The ME-Motronic system has inputs and outputs very similar to other engine management systems but it has two unique items – the accelerator pedal travel input sensor and the ETC (Electronic Throttle Control) actuator. [Bosch] August 2000  5 RESISTANCE IN OHMS RESISTANCE IN OHMS THROTTLE VALVE OPENING IN % ACCELERATOR TRAVEL Fig.3: two potentiometers are used in the accelerator position sensor (to give redundancy) and they are slightly offset to give characteristic shown here. [Audi] similar to other current management systems. Fig.2 shows the inputs and outputs of a typical ME-Motronic system. In addition to two-way diagnostics and Controller Area Network buses (the CAN buses communicate with other systems such as the automatic trans­mission ECU), the inputs comprise: • Vehicle speed; • Transmission gear; • Camshaft position; • Crankshaft speed and position; • Dual oxygen sensors (located eith­ er side of the catalytic converter – ‘V’ engines have four sensors); • Knock sensor; • Coolant temperature; • Intake air temperature sensor; • Battery voltage; • Intake air mass (plus frequently manifold pressure); • Throttle position. None of these inputs is unique to this system but it also includes a sensor for accelerator pedal position. Fig.4: the feedback mechanism of the throttle – which shows the actual throttle valve position – also uses two potentiometers. However, these have output character­ istics that work in opposite directions. [Audi] With one exception, the outputs are also very similar to other recent management systems: • Spark plugs; • Injectors; • Instrument panel tachometer; • Fuel pump relay; • Oxygen sensor heaters; • Intake manifold runner control (ie control of the position of valves within dual tuned length manifolds, or the length of infinitely variable intake runners); • Fuel system evaporative control, secondary air injection and exhaust gas recirculation (all emissions control approaches). The added component is the Electronic throttle control actuator. Let’s have a look at these two extra components in more detail. Accelerator position sensor Two approaches are currently used in the design of this sensor but they are electrically identical. Movement Fig.5: the Accelerator Pedal Position Sensor uses a dual poten­tiometer connected to the accelerator pedal by a rod and crank. [Audi] 6  Silicon Chip of the accelerator pedal rotates two potentiometers; as already noted, there is no Bowden cable to connect accelerator pedal movement to the throttle butterfly. Two potentiometers are fitted to the sensor to allow redundancy – if one fails, the other still lets the system operate. As shown in Fig.3, the outputs of the potentiometers are identical but for an offset. Cars equipped with automatic trans­missions do not have an additional kickdown switch in the assem­bly. Instead, a ‘mechanical pressure point’ is used to give the feel of a kickdown switch. Fig.5 shows the pedal assembly and sensor used in the Audi S4. If the accelerator position sensor fails, the lack of any mechanical connection between the accelerator and the throttle means that ‘limp home’ techniques are called for. The Audi S4 has two: Emergency running program 1: this occurs when a single accelerator position potentiome­ter fails. In this case, the throttle position is limited to a defined value. In the case of implausible signals from the two potentiometers, the lower value of the two is used. At the same time, the brake light signal is used to switch the engine back to idling and the fault lamp is illuminated. Emergency running program 2: this occurs when both accelerator position potentiometers fail. This more drastic condition causes the engine to run only at idle speed and the fault lamp is illuminated. Interestingly, in the Audi, if the accelerator and brake pedals are depressed together, the throttle valve is automatical­ly closed to a defined small opening. However, if the brake is pressed and depressing of the ac- celerator then follows this, the torque request is enabled. I assume that the latter provision is solely for those who like to left-foot brake, with applications of power used to balance the car! Throttle control actuator The electronic throttle valve consists of a DC motor, reduction gear drive and dual feedback angle sensors. Again to provide redundancy, two potentiometers are used for angle feed­ back. However, unlike the accelerator position sensor, these sensors have opposite resistance characteristics to one another, as shown in Fig.4. While continuous sensing of the throttle butterfly valve position does occur, the ECU only recognises four key functional positions of the throttle: • Lower mechanical limit stop – the valve is totally shut. • Lower electrical limit stop – the lower limit used in normal operation. This position does not totally close the valve, thus preventing contact wear of the housing and throttle blade. • Emergency running position – the position of the valve when it is not energised. This allows sufficient airflow for an idle speed a little higher than standard. • Upper electrical limit stop – the valve is fully open. The control system has a learning function, whereby the state of the mechanicals within the electronic throttle (eg, spring tensions) is determined by the evaluation of the throttle valve’s reaction speed. Fig.6 shows an internal view of the Electronic Throttle Control Actuator. As with the Accelerator Pedal Position Sensor, limp-home techniques are available should the Electronic Throttle Control Actuator develop problems. These include: Emergency running program 1: this occurs when an angle sensor within the throttle body fails or an implausible signal is received. The program still requires a throttle angle signal and a plausible mass airflow measurement. Torque increasing requests from other systems are ignored (eg, from the Engine Braking Control) and the fault lamp is illuminated. Emergency running program 2: this occurs if the throttle valve drive fails or malfunc­tions; it requires that both throttle valve potentiometers recog­ nise the Emergency Running Position Fig.6: The throttle valve is moved by means of a reversible DC motor acting through a reduction drive. In the event of failure, the valve defaults to a nearclosed position. [Audi] of the throttle blade. The throttle valve drive is switched off so that the valve defaults to the small emergency running opening. As far as possi­ble, ignition angle control and turbo boost control(!) are used to execute the driver torque demands. Finally, the fault lamp is illuminated as before. Emergency running program 3: this occurs if the throttle valve position is unknown and/or if the throttle valve is not definitely known to be in the Emergency Running Position. The throttle valve drive is switched off so that the valve (hopefully!) de- faults to the small emergen­cy running opening. The engine speed is limited to approximately 1200 RPM by fuel injection control and the fault lamp is illumi­nated. A schematic diagram showing the operation of the electronic throttle system is shown in Fig.7. As you can see, Bosch engineers have been very careful to ensure that a failure of the electronic throttle system will not cause the engine to suddenly have full power or to stall. Next month, we’ll take a look at the operating logic of the ME-Motronic SC system. ENGINE CONTROL UNIT INPUT SIGNALS THROTTLE VALVE DRIVE OUTPUT SIGNALS ACCELERATOR POSITION SENDER M CPU ACCELERATOR POSITION SENDERS SAFETY MODULE ANGLE SENDER FOR THROTTLE VALVE DRIVE Fig.7: the Bosch ME electronic throttle control system. Dual (redundant) potentiometers are used in both the accelerator position sensor and for the throttle angle feedback sensor. [Audi] August 2000  7