AutoSpeed - The Bosch ME-Motronic System, Part 1

The Bosch ME-Motronic system takes a radically different approach to previous engine management systems. While at first it appears little different to any electronic management system - fuel injectors, input sensors, an Electronic Control Unit and so on - the use of accelerator position sensing and an electronic throttle actuator makes this system very different to the average. For example, the relationship between accelerator pedal position and throttle opening is adjustable - not only can this system control injection and ignition, but also the cylinder charge.

Making ME-Motronic even more of a sea change is the underlying operating logic. Unlike older engine management systems, ME-Motronic determines how much engine torque is required in any given situation, and electronically opens the throttle blade sufficiently to allow the engine to develop that much torque. The accelerator pedal travel becomes just the driver's "torque request" input, to be weighed up against other torque requests that may be generated by the traction control system, speed limiter, engine braking torque control, and others. Additionally, 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 to the halfway position - but under the bonnet, the throttle blade can have snapped wide open! But in what type of situation could this possibly be advantageous?

In turbocharged cars, the maximum available torque output of the engine can substantially vary across a quite narrow band of engine speed. For example, the current model Audi S4 twin turbo V6 (pictured as the opener to this story) develops a torque maximum of 300Nm at 1400 rpm and 400Nm at 1850 rpm. So, as can be seen in this Audi graph, across just 450 rpm of engine speed, the peak torque output varies by 33 per cent. This characteristic is caused by the two turbos rapidly coming on boost. To a greater or lesser degree, a similar shaped torque curve is associated with all turbocharged engines.

A driver of a turbo car that is equipped with traditional engine management tends to automatically compensate for this steeply varying torque curve. When wishing to accelerate moderately hard, he or she will initially open the throttle a long way, manually backing it off as turbo boost and torque rises. But the driver of an Audi S4 V6 - equipped with Bosch ME 7.1 - doesn't need to do this. When engine response is relatively poor - ie the turbos are yet to generate appreciable boost - the ME-Motronic system can open the throttle far further than the driver directly requests, and then as revs rise, automatically adjust the throttle angle to retain a linear response. In this way, driveability, emissions and fuel consumption can all be improved.

Inputs and Outputs

As indicated, at first glance the ME-Motronic system looks very similar to other management systems. This Bosch diagram 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 either 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 the following one is:

Accelerator pedal position.

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 unique addition is the:

Electronic throttle control actuator

Given that the additional hardware comprises the accelerator pedal position sensor and electronic throttle control actuator, let's have a look at these two components in more detail.

Accelerator Pedal Position Sensor

Two approaches are currently used in the design of this sensor, but they are electrically identical. The throttle pedal assembly from an Audi S4 is shown here. Movement of the accelerator pedal manipulates two rotary potentiometers; unlike some electronic throttle engines, no back-up Bowden cable exists to connect accelerator pedal movement to the throttle blade. Two potentiometers are fitted to the sensor to allow redundancy - if one should fail, the other still lets the system operate.

As can be seen here, 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.

In the event that the accelerator position sensor fails, the lack of any mechanical connection between the accelerator and the throttle blade requires that sophisticated 'limp home' techniques are in place. The Audi S4 uses two techniques:

Emergency running program #1

This occurs when a single accelerator position potentiometer fails.

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;
The brake light signal is used to indicate when idling speed should be enacted;
The fault lamp is illuminated.

Emergency running program #2

This occurs when both accelerator position potentiometers fail.

The engine runs only at idle speed;
The fault lamp is illuminated.

Interestingly, if in the Audi 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 accelerator 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!

Electronic Throttle Control Actuator

The Audi S4 electronic throttle valve consists of a DC motor, reduction gear drive and dual feedback angle sensors. It is again for reasons of redundancy that two potentiometers are used for angle feedback. However, unlike the accelerator position sensor, these sensors have opposite resistance characteristics to one another, as shown below.

While continuous sensing of throttle blade position does occur, the ECU recognises four key functional positions of the throttle blade:

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 blade is fully open.

The control system has a self-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.

As with the Accelerator Pedal Position Sensor, sophisticated 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. Required is an intact throttle angle sensor and plausible mass airflow measurement.

Torque increasing requests from other systems are ignored (eg from the Engine Braking Control);
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 recognize the Emergency Running Position 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 driver torque demands.
The fault lamp is illuminated.

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!) defaults to the small emergency running opening;
The engine speed is limited to approximately 1200 rpm by fuel injection control;
The fault lamp is illuminated.

An Audi schematic diagram showing the operation of the electronic throttle system is shown here.

As you can see, Bosch engineers have been very careful to ensure that a failure of the electronic throttle system will not cause the car to suddenly have full power - or a stalled engine.

Next week we'll cover the operating logic of the ME-Motronic system.

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