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The Electronic Control Unit for a four cylinder common rail BMW diesel. [BMW] Diesel Engine Management L ast month we looked at the mechanical make-up of the common rail diesel fuel injection systems that have revolutionised diesel-powered cars. The systems used extremely high fuel pressure, electronically controlled injectors and complex exhaust aftertreatment to provide very high specific torque outputs with low fuel consumption and low emissions. But how does the electronic control system work? In this article we look at the electronics of the system. Requirements The engine management system in a diesel common rail engine needs to provide: • Very high fuel injection pressures (up to 2000 Bar) • Variation in injected fuel quantity, 24  Silicon Chip intake manifold pressure and start of injection to suit engine operating conditions • Pre-injection and post-injection • Temperature-dependent rich air/fuel ratio for starting • Idle speed control independent of engine load • Exhaust gas recirculation • Long term precision As with current petrol engine management systems, the driver no longer has direct control over the injected fuel quantity. Instead, the movement of the accelerator pedal is treated as a torque request and the actual amount of fuel Part 2 by Julian Edgar injected in response is dependent on the engine operating status, engine temperature, the likely affect on exhaust emissions, and the intervention by other car systems (eg traction control). Figure 1 shows an overview of the inputs, outputs and internal processes in the Bosch common rail management system. Management Functions • Starting The injected fuel quantity and start of injection timing required for starting are primarily determined by engine coolant temperature and cranking speed. Special strategies are employed for very cold weather starting, especially at high altitudes. In these conditions, the turbocharger operation may be suspended as its torque demand – alsiliconchip.com.au Fig.1: an overview of a common rail diesel engine management system. The input signals to the ECU are on the left and include accelerator pedal position, intake mass airflow, fuel rail pressure and engine speed. Not shown here but also often included is a wideband exhaust gas oxygen sensor. The outputs (right) include the control of the fuel injectors, exhaust gas recirculation (EGR) and fuel rail pressure. Inside the ECU (middle) control strategies are implemented for idle speed, smooth running control, quantity of fuel injected, starting point of injection, and many others. [Bosch] siliconchip.com.au May 2006  25 • Idle Speed Control The set idle speed depends on engine coolant temperature, battery voltage and operation of the air conditioner. Idle speed is a closed loop function where the ECU monitors actual engine speed and continues to adjust fuel quantity until the desired speed is achieved. Fig.2: along with many other variables, three dimensional ECU maps are used for both injection start timing and smoke limitation. [Bosch] though small – may be sufficiently great as to prevent the car from moving off. • Driving In normal driving, the injected fuel quantity is determined primarily by the accelerator pedal sensor position, engine speed, fuel and intake air temperatures. However, many other maps of data also have an effect on the fuel injection quantity actually used. These include strategies that limit emissions, smoke production, mechanical overloading and thermal overloading (including measured or modelled temperatures of the exhaust gas, coolant, oil, turbocharger and injectors). Start of injection control is mapped as a function of engine speed, injected fuel quantity, coolant temperature and ambient pressure. Figure 2 shows example data maps for start of injection and smoke control. • Rev Limiter Unlike a petrol engine management system which usually cuts fuel abruptly when the rev limit is reached, a diesel engine management system progressively reduces the quantity of fuel injected as the engine speed exceeds the rpm at which peak power is developed. By the time maximum permitted engine speed has been reached, the quantity of fuel injected has dropped to zero. • Surge Damping Sudden changes in engine torque output can result in oscillations in the vehicle’s driveline. This is perceived by the vehicle occupants as unpleasant surges in acceleration. Active Surge Damping reduces the The parts that make up a BMW four cylinder diesel engine. The fuel injection system components are at bottom right – visible are the injectors and common rail, the high pressure pump and the ECU. [BMW] 26  Silicon Chip siliconchip.com.au tity for that cylinder is increased. If the engine speed is above the mean, the fuel injection quantity for that cylinder is decreased. Figure 4 shows this process. Fig.3: Surge Damping is used to prevent unwanted oscillations in acceleration. The top diagram shows the change in accelerations without surge damping (a) and with it (b). This alteration in car behaviour can be achieved in two ways. The lower diagram shows (1) the effect of electronic filtering of the accelerator pedal travel sensor output signal, and (2) the active correction of surge by increasing the injected fuel quantity when the engine speed drops and decreasing it when the speed increases. [Bosch]    Fig.4: Smooth Running Control addresses the fact that the torque output of each cylinder is not identical. To counteract this, the system compares the engine speed immediately after a cylinder’s injection with the average engine speed (in this case 800 rpm). If the speed has dropped, the fuel injection quantity for that cylinder is increased. If the engine speed is above the mean, the fuel injection quantity for that cylinder is decreased. [Bosch] • Closed Loop Oxygen Sensor Control As with petrol management systems, diesel management system use oxygen sensor closed loop control. However, in diesel systems a broadband oxygen sensor is used that is capable of measuring air/fuel ratios as lean as 60:1. This Universal Lambda Sensor (abbreviation in German: LSU) comprises a combination of a Nernst concentration cell and an oxygen pump cell. Because the LSU signal output is a function of exhaust gas oxygen concentration and exhaust gas pressure, the sensor output is compensated for variations in exhaust gas pressure. The LSU sensor output also changes over time and to compensate for this, when the engine is in over-run conditions, comparison is made between the measured oxygen concentration of the exhaust gas and the expected output of the sensor if it were sensing fresh air. Any difference is applied as a learned correction value. Closed loop oxygen control is used for short- and long-term adaptation learning of the injected fuel quantity. likelihood of these oscillations occurring. Two approaches can be taken. In the first, any sudden movements of the accelerator pedal are filtered out, while in the second, the ECU detects that surging is occurring and actively counteracts it by increasing the injected fuel quantity when the engine speed drops and decreasing it when the speed increases. Figure 3 shows this process. • Smooth Running Control Because of mechanical differences from cylinder to cylinder, the development of torque by each cylinder is not identical. This difference can result in rough running and increased emissions. To counteract this, Smooth Running Control uses the fluctuation in engine speed to detect output torque variations. Specifically, the system compares the engine speed immediately after a cylinder’s injection with the average engine speed. If the speed has dropped, the fuel injection quansiliconchip.com.au Fig.5: this diagram shows the relationship between solenoid valve (ie injector) current, solenoid valve needle lift and injected fuel quantity. At (a) the injector is opened with a rapidly rising (but controlled) rush of current, at (c) the current is decreased but is still sufficient to hold the injector open, at (e) the current is switched off and the injector closes. The sawtooth pattern of low current flow that can be seen at (f) is explained in Figure 6. [Bosch] May 2006  27 This is especially important in limiting smoke output, where the measured exhaust gas oxygen is compared with a target value on a smoke limitation map. Oxygen sensor feedback is also used to determine whether the target exhaust gas recirculation is being achieved. • Fuel Pressure and Flow Control The pressure in the common rail is regulated by closed loop control. A pressure sensor on the rail monitors real time fuel pressure and the ECU maintains it as the desired level by pulse width modulating the fuel pressure control valve. At high engine speeds but low fuel demand, the ECU deactivates one of the pistons in the high pressure pump. This reduces fuel heating in addition to decreasing the mechanical power drawn by the pump. Other Management System Outputs In addition to the control of the fuel injectors, the diesel engine management system can control • Glow plugs for sub-zero starting conditions • Glow plugs that heat the coolant, providing adequate cabin heating in cold climates Fig.6: this diagram shows how the high voltage capacitor used to rapidly pull open the injector is also in turn charged by the injector’s solenoid coil. (1) battery, (2) current control, (3) injector solenoid windings, (4) current boost switch, (5) capacitor, (6) diodes, (7) cylinder select switch. In phase (a), the injector is opened rapidly by the supply of high current from the 100V booster capacitor. In phase (b), the current supply for the injector switches from the capacitor to the battery. A pulse width modulated holding current is then used to maintain the injector in its open state (phase d) and during the transition to this phase (c), the inductive spike generated by the reduction in current through the injector is routed to the booster capacitor, so starting its recharging process. When the injector is switched off (phase e), the inductive spike is again routed to the booster capacitor. Between injector opening events, a sawtooth waveform is applied to the closed injector (phase f1 and f2). This current is insufficient to open the injector but the generated inductive spikes are used to further recharge the booster capacitor until it again reaches 100V. [Bosch] 28  Silicon Chip A cutaway view of a BMW common rail diesel 6-cylinder in-line engine. The electronically controlled fuel injectors can be seen at the top of each cylinder while one of the glow plugs (rarely used except in very cold ambient conditions) can be seen angled into the combustion chamber. [BMW] siliconchip.com.au So what are these co mm on ra il die sels actually like on the road? The Audi All roa d us es a 2.5 litr e tur bo ch arg ed , inj ec ted an d int ercooled diesel engine. Its maximum torque is 370Nm from 15002500 rpm and peak power is 132kW at 4000 rpm. The car has a mass of 1825kg and uses a 5-speed automatic transmission. Off the line there’s a no tic ea ble he sitation as the turbo bu ild s bo os t, the n – wh oo oo sh – the torque arrives and the engine rockets around to the 4500 rpm redline, pulling ha rd all the way. The auto slides to the next ratio – which puts engine revs back in the middle of that torque plateau – and sh e’s off again. Audi claim 0-100 km/h in 10.2 seconds, but the times are far faster tha rolling n this standing start tim e would sug- gest. You really only notice the absence of top-end power when climbing long hills at high speed. In a wide mix of driving biased more towards freeway than climbing mountains, we avera ge d 10 .1 litres/100km. An d wh at ab ou t that horrible diesel rat tle ? Th ere ’s no ge ttin g aw ay fro m it – the TDi Allroad is noisier than the equivalent petrol engine version. Despite extensive so un dproofing – including a rubber bonnet seal right around the engine bay – a distinctly different en gine note can be heard inside the cabin. It’s more of a wh ine that a rattle – thoug h passers-by hear a normal diesel. Bu t at cruise the car is co mmendably quiet – the unusual en gine note can really on ly be heard when accelerating. The Audi Allroad • Switchable intake manifolds, where at low loads air is forced through turbulence ducts to provide better in-cylinder swirl • Turbocharger boost pressure control • Switching of radiator fans Injector Operation The triggering of the injector can be divided into five phases: • In the first phase, the injector is opened rapidly by the supply of high current from a 100V booster capacitor. Peak current is limited to 20A and the rate of current increase is controlled to allow consistent injector opening times. • The second phase is termed ‘pickup current’. In this phase, the current supply for the injector switches from the capacitor to the battery. In this phase, peak current continues to be limited to 20A. • A 12A pulse width modulated holding current is then used to maintain the injector in its open state. The inductive spike generated by the reduction in current through the injector in the change from ‘pickup’ to ‘holding’ phases is routed to the booster capacitor, so starting its recharge process. • When the injector is switched off, the inductive spike is again       siliconchip.com.au routed to the booster capacitor. • Between actual injector events, a sawtooth waveform is applied to the closed injector. The current used is insufficient to open the injector and the generated inductive spikes are used to further recharge the booster capacitors until they reach 100V. Fig.5 shows the relationship between injector current, needle lift and fuel flow. Fig.6 shows the five phases of injector operation. Conclusion European car manufacturers and consumers have thrown their weight heavily behind passenger cars equipped with diesel engines. The major improvement in specific torque outputs and the reduction in fuel consumption and emissions have been achieved with sophisticated electronic control of very high pressure, individually controlled injectors. SC The direct injection system of a Jaguar 2.7 litre diesel V6. The mechanical high pressure fuel pump can be seen, as can the two banks of injectors fed by their individual fuel rails. [Jaguar] May 2006  29