The Bosch ME-Motronic System, Part 2

As discussed last week, the Bosch ME-Motronic engine management system is a radical departure from previous electronic engine management. The relationship between the accelerator pedal position and the opening angle of the throttle valve is no longer fixed; instead the Electronic Control Unit (ECU) determines how much torque the engine is required to produce and then opens the throttle valve to the appropriate angle. The chosen throttle opening is based on complex software that models the engine's instantaneous torque output and compares this with the required torque output, as requested not only by the driver but also by in-car systems.

The Audi twin turbo V6 (pictured above) is an example of an engine equipped with ME-Motronic management.

Torque Control Logic

The ME-Motronic system prioritises and coordinates torque demands in order that it can implement an overall torque control strategy. As this Audi diagram shows, torque requests are categorised as 'Internal' or 'External'. External torque requests include those made by the driver, cruise control system and driving dynamics systems like Automatic Stability Control. Internal torque requests are those made by the internal programming of the ECU - factors such as engine governing and idle speed control. The total requested torque is then modified by strategies such as those which take into account catalytic converter temperature or driving smoothness.

In older engine management systems, the driver - via the mechanical alteration of the throttle blade angle - exercised direct control over the mass of cylinder charge, while the management system was limited to torque reduction strategies (eg by fuel cuts) or minor torque increases through manipulation of the mass of air bypassing the throttle. However, this approach does not cope very well with competing and contrary torque demands that may well occur simultaneously. This Audi diagram shows some of the required torque variations found in current cars, excluding those requested by the driver.

The ME-Motronic system internally models the net torque development of the engine. This model takes into account losses through internal friction, pumping losses, and parasitic loads such as that of the power steering and water pumps. Internal mapping within the ECU allows optimal specifications for charge density, injection duration and ignition timing for any desired net torque value, taking into account the often conflicting requirements of best fuel economy and emissions. These requirements dictate that the system must perform well in transients (ie sudden changes in torque), as well as when being subjected to steady-state loads. To allow good performance in both constant and transient load conditions, two different control approaches are taken.

The first control strategy is termed by Bosch the Charge Path. 'Charge' in this context refers to the mass density of air trapped in the cylinder. At a given air/fuel ratio and ignition advance, the mass of this air is directly proportional to the force generated during the combustion process. The Charge Path, controlled by the opening angle of the throttle blade (and boost pressure in a forced induction car), is used to control engine torque output in static operations. The ability of this control system to change quickly is limited by the regulating speed of the throttle actuator and the time constant of the intake manifold, which can be as high as several hundred milliseconds at low engine speeds.

The other technique used to control torque output is termed, somewhat oddly, the Crankshaft Synchronous Path. This refers to torque variations able to be rapidly created by changes in ignition timing and injection operation, with the latter used to effect the air/fuel ratio. Examples of when this approach is employed include torque reduction during automatic transmission gear changes and when Vehicle Stability Systems are operating.

Getting confused? This Bosch diagram puts it all together. On the far left is the driver, who (at least on the diagram!) is still given pride of place. The driver request for torque is prioritised and processed in terms of driveability functions. These include filtering and slope-limiting, dashpot (to ensure that torque changes do not occur too quickly) and anti-jerking. These functions can be calibrated to suit a wide range of applications - for example a high level of anti-jerk to suit a luxury car, or very quick throttle response to suit a sports car.

In addition to the driver's torque request, other torque variations (for example, an increase in torque to operate the air conditioner compressor, or a reduction in torque required by the load change damping system) are processed, with the final request then fed into the 'Torque to charge density conversion' box. When a torque request is made, the ECU must calculate how much fresh air mass is required to be inhaled by the engine to meet this demand. The actual mass of air that is needed will be dependent on ignition timing (eg if the engine is running relatively retarded ignition to decrease oxides of nitrogen emissions, more air will be needed because efficiency will be lower), internal engine friction, the instantaneous air/fuel ratio and other factors.

Once a mass airflow that will meet the requirements is quantified, a throttle valve opening angle is calculated. However, in all engines, the required angle will be dependent on the manifold pressure, and in forced aspirated engines, manifold pressure will be quite critical to the mass of air actually inhaled. Thus, in these engines the turbocharger boost pressure and throttle valve opening are both specified such that the appropriate charge density required for the prescribed torque output is reached.

Calculating Cylinder Charge

As can be seen from the above, the accurate calculation of cylinder charge is vital if the torque modelling strategy is to be effective, and if appropriate amounts of fuel are to be accurately added to this air. Traditionally, a mass airflow meter positioned between the airfilter box and the throttle body has been used to measure intake airflow. However, the mechanical design of engines is now taking advantage of techniques that maximise cylinder charge in a way in which an averaged mass airflow measurement may not be able to accurately sense.

In the ME-Motronic system the available sensors are used as inputs to a charge air model, rather than being evaluated directly. The requirements for such a charge air model are:

• Accurate mass charge air determination in engines using resonant tuned and/or variable length intake manifolds, and engines using variable valve timing;
• Accurate response to Exhaust Gas Recirculation conditions;
• Calculation of required throttle valve aperture (and required turbo boost in forced induction engines).

While the engine is subjected to a constant load, mass airflow measurement is relatively accurate: ie if Xkg of air per second is passing through the airflow meter, it can be assumed that all of it is ending up in the cylinders! However, during transients, the situation is much more complex. For example, if the throttle blade is abruptly opened, the intake plenum chamber will rapidly fill with air. For an instant this will give an inaccurately high reading from the airflow meter - the meter will indicate a higher cylinder charge than has actually had time to occur. It is only when intake manifold pressure has risen that the flow will commence into the cylinders.

As a result of this characteristic, the ME-Motronic system generally uses both manifold absolute pressure (MAP) and hot wire airflow meter (HFM) inputs. (In some cases the MAP sensor is not fitted; further software modelling duplicates its function.) The HFM is a further development of the design used by Bosch and other management systems for about 15 years. Its improvements result in better accuracy; for example, it is capable of differentiating reverse flow pulses from airflow passing into the engine.

Conclusion

The Bosch ME-Motronic system represents a major change in management systems - very likely, as great a change as the combining of fuel injection and ignition timing controls into one system in 1979. Instead of the management system simply responding to the engine load changes indicated by varying intake airflows or rpm and manifold pressures, the control architecture now revolves around assessing the instantaneous torque requirements. How the engine goes about fulfilling that requirement is now very largely determined by the ECU.

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