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Drive By Wire The Bosch ME-Motronic System; Pt.2 Last month, we covered the unique hardware associated with this new engine management system which eliminates the accelerator cable and thereby any direct link between the driver and the throttle. In this story we explore some of the control system logic. By JULIAN EDGAR As discussed last month, the Bosch ME-Motronic engine man­ a gement system is a radical departure from current systems which control fuel injection and ignition timing. The relationship between the accelerator pedal position and the opening angle of the throttle valve is no longer fixed - there is no accelerator cable. Instead the Electronic Control Unit (ECU) determines how much engine torque is required 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 70  Silicon Chip output and compares this with the required torque output, as requested not only by the driver but also by other in-car systems. Torque control logic The ME-Motronic system coordinates the various torque de­mands in order that it can implement an overall torque control strategy. 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 factors such as catalytic converter temperature or driving smoothness. Fig.1 shows an overview of this approach. In previous engine management systems, the driver operated the throttle and thereby had 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 simul­ taneously. Fig.2 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 Fig.1: unlike a conventional management system, in the ME-Motronic system there are multiple torque request inputs, rather than a system that indirectly responds to the driver’s request by sensing increased intake airflow or changes in mani­fold pressure. The driver and external systems make the External Torque Requests, while the Internal Torque Requests are pre-programmed internal ECU factors. Actual developed torque is determined by control of the throttle valve angle, intake charge pressure (turbo cars), ignition advance angle, injection cut and injection pulse width. [Audi] loads such as that of the power steering and water pumps. Internal mapping within the ECU allows optimum charge density, injection duration and ignition timing for any desired net torque value, taking into account best fuel economy and exhaust emissions. These often conflicting requirements dictate that the system must perform well in transients, as well as when being subjected to steady-state loads. To allow good performance in both constant and transient load conditions, two different con­ t rol approaches are taken. Bosch call the first control strategy the “Charge Path”. “Charge” in this context refers to the 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 valve (and boost pressure in a super-charged or turbo-charged car), is used to control engine torque output in static operations. The dynamic nature of this control 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 called, 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 Traction Control systems are operating. Getting confused? Fig.3 puts all of this together. On the far left is the driver, who (at least on the diagram!) is still given pride of place. The driver’s torque request is processed in terms of driveability functions and given a priority level; ie, the driver may not get what he requested! Some of the driveability functions 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 a very quick throttle response to suit a sports car. The BMW M5 V8 has a switch that allows selection of ‘sport’ or ‘normal’ throttle modes. In addition to the driver’s torque October 2000  71 Fig.2: all the torque demands on the engine are assessed and given a priority before the ECU decides on an appropriate throt­tle opening angle to use. In addition to the request of the driver, there are a variety of requests that may need to be processed before the final decision is made. [Audi] 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 rela­ tively 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 worked out, a throttle valve opening angle is calculated. Howev­er, in all engines, the required angle will be dependent on The ME-Motronic ECU uses tiny surface mount components, with the board very similar in appearance to this Alfa Romeo ECU. 72  Silicon Chip the manifold pressure and in forced aspirated engines, manifold pressure will be quite critical to the mass of air actually inhaled. In these engines the turbocharger boost pressure and throt­ tle 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 air filter box and the throttle body has been used to measure intake airflow. However, engines are now taking advantage of techniques that maximise cylinder charge such that an averaged mass airflow measurement may not be sufficiently accurate. 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; Fig.3: two different decision making paths are used to specify the actual engine torque that is delivered. The upper path on the diagram shows how the entrance of the appropriate mass of air into the cylinders is determined, while the lower path controls torque via fuel and ignition changes. [Bosch] • 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 such as rapid use of the throt­tle, the situation is much more complex. If the throttle valve is abruptly opened, the intake plenum chamber will rapidly fill with air. For an instant, the airflow 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 management systems for about 15 years. Its improvements result in better accuracy. For example, it is capable of differentiating reverse flow pulses from air- flow (eg, in resonant inlet mani­folds) passing into the engine. Conclusion The Bosch ME-Motronic system represents a major change in management systems – very likely, it is 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 respond­ i ng to the engine load changes indicated by varying intake air­flows or RPM and manifold pressures, the control architecture now revolves around assessing the DON’T MISS THE ’BUS Do you feel left behind by the latest advances in com­puter technology? Don’t miss the bus: get the ’bus! Includes articles on troubleshooting your PC, setting up computer networks, hard disk drive upgrades, clean installing Windows 98, CPU upgrades, a basic introduction to Linux plus much more. instantaneous torque requirements. How the engine goes about fulfilling that requirement is now very largely determined by the ECU. Footnote: in the introduction to part 1 of this article, we stated that the Bosch ME-Motronic is the first drive-by-wire engine management system for cars. This is incorrect as some cars (eg, Lexus and BMW) have had drive by wire (ie, electronic throttle control) for some years. The ME-Motronic’s unique­ness comes from its variable relationship between accelerator pedal movement SC and throttle blade opening. www.siliconchip.com.au SILICON CHIP’S 132 Pages $ 95 * 9 ISBN 0 95852291 X 9780958522910 09 09 9 780958 522910 COMPUTER OMNIBUS INC LUD ES FEA TUR E LIN UX A collection of computer features from the pages of SILICON CHIP magazine o Hints o Tips o Upgrades o Fixes Covers DOS, Windows 3.1, 95, 98, NTAVANOW DIRE ILABLE C SILIC T FROM ON just $ CHIP 125O INC RT ORDER NOW: Use the handy order form in this issue or call (02) 9979 5644, 8.30-5.30 Mon-Fri with your credit card details. P&P October 2000  73