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Electronic Engine Management Pt.6: System Operation – by Julian Edgar The actual processes which occur within the ECM to allow the control of fuel injection, ignition timing, idle speed & so on are obviously complex. Various inputs trigger various outputs but what happens in between? The simplest to understand are engine management systems which use analog control processes. A good example of this type of engine management system is the Bosch L-Jetronic system which was developed back in the 1970s. This is a fuel-only system and so it can be more accurately referred to as an Electronic Fuel Injection (EFI) system. The Bosch L-Jetronic design was the first EFI system in common use and has now been largely dropped as more sophisticated engine management systems have been developed. Cars which used the L-Jetronic system (or variations of it) include many European cars from the 1970s (BMW, Mercedes Benz), many Japanese cars from the early-mid 1980s (Nissan, Toyota), and – in Australia – Ford and Holden with their first fuel injected cars (Falcon and Commodore) in the mid 1980s. The “L” in L-Jetronic is from the German word “luft”, mean­ ing air. Airflow measurement is critical in the operation of the EFI system and, as was subsequently proved, in all other engine management systems as well! The L-Jetronic ECM initially used discrete components, as was common in electronics at the time of its introduction. More recent versions of the L-Jetronic system use integrated circuits. Injector pulses An early Bosch L-Jetronic ECM. Note the use of discrete (& large) components in this mid-1970s Mercedes unit. Being an analog system, the ECM has no memory & doesn’t use an oscilla­tor. 32  Silicon Chip Fig.1 gives some idea of how the system generates its injector pulses. The ECM uses the ignition pulse as its starting point and this is derived from the low tension side of the igni­ tion coil. A pulse shaper is then used to generate rectangular pulses of the same frequency from this input. In this system, the injectors are fired simultaneously twice per engine cycle (two turns of the crankshaft). Because of this, it is necessary to divide the pulse train so that a single pulse is produced for each complete rotation of the crankshaft, regard­less of the number of cylinders. This is achieved by using a bistable multivibrator to divide the rectangular trigger pulses by two. The measured engine rpm and the Fig.1: how the fuel injector pulses are generated. A basic injection time (tp) is first of all derived according to engine rpm & airflow & this is then corrected for factors such as acceleration, engine temperature & battery voltage. signal from the vane air­flow meter are now used by the division control multivibrator to generate the base injection pulse width. This gives an injector opening time which is uncorrected for factors such as accelera­tion and engine temperature. A multiplier stage calculates a correction factor to take these aspects into account and this is added to the base injection time, giving an injector pulse width which is correct at the standard battery voltage. In practice, the response time of the fuel injectors is greatly in­fluenced by battery voltage, the latter varying during normal vehicle operation from about 11V to 14V. This gives rise to insufficient fuel delivery at low battery voltages, due to slow injector response times. To overcome this problem, a voltage compensation stage is used to appropriately extend the injector pulse width. This now gives the final injector opening time, with the injectors con­ trolled by power output transistors. Fig.2 shows a block diagram of the system. Analog systems are “programmed” using a hard-wired mathematical algorithm which is determined by the values of the components used. This means that the EFI computer is designed for a specific car and engine; changes have to be made by the manu­ facturer to the actual hardware before the ECM can be used in other cars. It also means that if fuel injection modifications are made with L-Jetron­ ic The Saab APC (“Advanced Performance Control”) is used to control turbo­ charger boost and ignition timing. This is also an analog ECM & was introduced in the early 1980s. March 1994  33 Sold in Australia only in the Ducati 851 motorcycle, this Weber-Marelli ECM has a 24Kb memory & a clock speed of 4MHz. This digital ECM is from a rotaryengined Mazda RX-7 Turbo & is typical of early 1980s Japanese designs. The clock speed is 4MHz & the memory capability is 12Kb. Motronic It was only a matter of time before the fully analog EFI systems like FULL ENGINE SPEED (IGNITION ENGINE SPEED (IGNITION PULSE)PULSE) PULSE SHAPING STAGE FREQUENCY DIVIDER systems, then analog circuit design procedures need to be undertaken. LOAD FULL LOAD SWITCH SWITCH BATTERY BATTERY VOLTAGE VOLTAGE FULL LOAD ENRICH VOLT CORR BATTERY BATTERY POSITIVE POSITIVE INJECTORS DIVISION CONTROL MULTIVIBRATOR (DSM) START ENRICH AIR AIR FLOW SENSOR FLOW SENSOR STARTER STARTER SIGNAL SIGNAL POWER STAGE MULTIPLIER FUEL CUTOFF IDLE IDLE SWITCH SWITCH ACCEL ENRICH AFTER START ENRICH WARM UP ENRICH COLD START CONTROL TEMPERATURE TEMPERATURE SENSOR SENSOR Fig.2: block diagram of the Bosch L-Jetronic EFI (electronic fuel injection) system, as used in mid-1980s Ford Falcons. 34  Silicon Chip L-Jetronic were replaced with digital systems, using microcomputers. These offer several important advantages, includ­ ing lower price, greater ease of programming, and more accurate control. The digital Bosch Motronic design is probably the most sophisticated engine management system currently in mass produc­ tion. Note that the “Motronic” name has been given to a number of different systems over the years – today’s Motronic is much more sophisticated than the system of five years ago. Fig.3 shows the basics of an early Motronic system, while Fig.4 is a block diagram of the ECM. Note that a large number of analog-to-digital converters are used on the input signals. This is because sensors such as the throttle position potentiometer, engine coolant thermistor and so on produce a varying voltage analog signal. This information must be converted to digital format before it can be processed. Other sensors – such as the crankshaft position and engine speed sensors – need to have their outputs fed through a pulse shaper before being fed to the microcomputer. The Motronic ECM calculates output data in two different ways. When in closed-loop mode, feedback signals are obtained from the exhaust oxygen sensor and the knock sensor. In this situation, the ECM uses digital Fig.3: diagram of a typical Motronic engine management system – 1 fuel tank; 2 electric fuel pump; 3 fuel filter; 4 pressure regulator; 5 electronic control unit; 6 ignition coil; 7 high-voltage distributor; 8 spark plug; 9 injection valve; 10 throttle valve; 11 throttle valve switch; 12 air-flow sensor; 13 air temperature sensor; 14 lambda (oxygen) sensor; 15 engine temperature sensor; 16 idle speed actuator; 17 engine speed sensor; 18 battery; 19 ignition switch; 20 air-conditioning switch. algorithms to calculate, in real time, the ignition timing and injector pulse width. Conversely, when in open-loop configuration (with the ECM not monitoring the results), the system uses a series of ROM-stored maps of informa­ tion. These are burned-in during manufacture but can be repro­grammed by after-market chip “cookers”. The sort of program information which is stored in the ROM is often shown in the form of 3-axis graphs. This ECM is from a 2.6-litre Holden Rodeo. Although it uses only a relatively small memory of 4Kb, this ECM shows current state-of-the-art construction with its VLSI chip. Its clock speed is 8MHz. March 1994  35 Extensive engine dynamometer testing is carried out by the manu­facturer to give precisely the best outputs at a variety of loads, engine speeds, engine coolant temperatures, and so on. Other systems This GM-Delco ECM is now used in all Holdens, whether they run 4, 6 or 8-cylinder engines. The program software is contained within a plug-in “MemCal” (memory calibration) unit, which is shown at the bottom of the picture. Fig.4: block diagram of the Motronic electronic control system. 36  Silicon Chip Almost all car manufacturers now use either Bosch compon­ents or technology in their engine management systems. However, the range of software and hardware available means that each manufacturer’s system is unique. Self-learning feedback is used in many systems, allowing changed engine parameters – like engine wear – to be compensated for. As an example of self-learning, when a fault has been fixed in some cars (and the fault code cleared), the car must then be driven for several kilometres before normal performance is restored. This is because the ECM needs to re-learn its new operating parameters! Another example of this self-learning process can be found in the Subaru Liberty. The Liberty uses an exhaust gas oxygen (EGO) sensor to monitor mixture richness, as is the case in most current cars. However, in many engine management systems, the EGO sensor is simply used to modify the base injector pulse width, which has been derived – according to engine load and rpm – from the memory. The greater the correction applied by the EGO feed­back, then the lower the control accuracy of the system. In the Subaru system, the air/fuel ratio correction factor is constantly memorised and is then applied directly to the base injector width, actually changing the stored base injec­ tion time. This process occurs after a few engine cycles. The subsequent correction to the mixture by the EGO feedback loop is therefore lowered, giving more accurate overall control. To put this another way, if the car is being driven hard on a hilly, open road, then rich mixtures will be required to give maximum power. The ECM will quickly “learn” that a wider than normal base fuel quantity is being required and so the need for feedback correction from the EGO sensor is lessened. When the car is once more being driven gently, the learned base fuel injector pulse will again shorten. Other cars also use self-learning procedures, all of which are aimed at realising optimal values quickly. SC