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This BMW 3-litre in-line 6-cylinder diesel uses twin turbochargers. It develops 200kW at 4400 RPM and a staggering 560Nm at 2000 RPM, with no less than 530Nm available from 1500 RPM. The twin overhead cam, iron block and alloy head design uses four valves per cylinder and has a mass of 228kg. Bosch DDE 6.0 engine management is used with common rail injection. [BMW] Diesel Engine While the principle of the diesel engine itself hasn’t changed much since it was invented by Rudolf Diesel more than a century ago (he patented the concept in 1892) the last couple of decades has seen enormous advances in the performance of diesel engines, particularly those used in cars. Julian Edgar explains: C ars equipped with diesel engines now comprise half of all new cars sold in Europe. The main reason for their popularity is fuel economy: a medium-sized diesel car can easily achieve a city fuel consumption of better than 6 litres/100km and on a highway, 4 litres/100km. 16  Silicon Chip Even the very high power diesel passenger car engines now available have exceptional fuel economy for their size and performance. The BMW 535d, equipped with a twin-turbo 3-litre diesel engine developing 200kW of power and 560Nm of torque, accelerates to 100km/h in just 6.5 seconds and has an EU combined cycle fuel economy of 8l/100km. At the other end of the power spectrum, the Smart Fortwo 0.8-litre diesel develops 30kW and 100Nm but has a combined EU cycle fuel consumption of only 3.4l/100km! In Australia, Audi, BMW, Citroen, siliconchip.com.au Extremely high fuel injection pressures are now being employed to provide excellent fuel atomisation. This nozzle is designed to work at 2000 Bar (29,000 psi!) fuel pressure. [Bosch] DaimlerChrysler, Peugeot and Volkswagen all sell diesel-powered passenger cars. But aren’t diesels noisy, smelly devices that puff black smoke at inopportune times and rev to only 3000 RPM? Not any more! A revolution has been achieved by the use of extremely high fuel pressures and electronically controlled common rail fuel injection that allows far more accurate control of the injection process. Diesel engines Although the basic designs of petrol and diesel engines are similar (both are 2 or 4-stroke designs with reciprocating pistons driving a crankshaft), a diesel engine does not compress its fuel/air charge and then ignite combustion with a spark plug. Instead, in a diesel engine, just air is highly compressed. When the piston is near Top Dead Centre, an injector sprays the fuel into the combustion chamber, whereupon it mixes with the hot compressed air and self-ignites. In order that the air within the diesel combustion chamber becomes hot enough for self-ignition to occur, the compression ratio needs to be much higher than in a spark ignition engine. Compression ratios in the range of 16:1 to 24:1 are commonly used, giving forced-aspirated diesel engines a compression pressure of up to 150 Bar. This generates temperatures of up to 900°C. Since the ignition temperature of the most combustible components of diesel fuel is only 250°C, it is easy to see why the fuel burns when it is injected after the piston has risen on the compression stroke. Diesel engines are designed to develop high torque at low engine speeds. In recent years, the use of turbochargers and common-rail direct injection has dramatically improved the specific torque output of diesel car engines. Fig.1 shows that specific torque has risen from about 70Nm/litre to more than 182Nm/litre over the last 20 years. Management Part 1 Fig.1: the very rapid development in diesel engine performance over the last 20 years can be seen in this DaimlerChrysler chart. Since the 1995 E300D model, specific torque has risen from about 70Nm/litre to more than 182Nm/litre while at the same time, specific fuel consumption has fallen by over 60%! [DaimlerChrysler] siliconchip.com.au April 2006  17 reducing efficiency and increasing fuel consumption. The sharp rise in cylinder pressure also increases noise. Too late an injection reduces torque and can result in incomplete combustion, increasing the emissions of unburned hydrocarbons. • Injection Duration Unlike a conventional port fuel injected petrol engine, where the amount of fuel injected can be considered to be directly proportional to the injector opening time, a diesel injector will vary in mass flow. This depends on the difference between the injection and combustion chamber pressures, the density of the fuel (which is temperature dependent) and the dynamic compressibility of the fuel. The specified injector duration must therefore take these factors into account. Fig.2: a simple common-rail diesel fuel injection system. A high-pressure mechanical pump (1) feeds the fuel to the common rail (3). A fuel rail control valve (4) allows the fuel pressure to be maintained at a level set by the Electronic Control Unit (8). The common rail feeds the injectors (5). Sensor inputs to the ECU comprise fuel pressure (2), engine speed (9), camshaft position (10), accelerator pedal travel (11), boost pressure (12), intake air temperature (13) and engine coolant temperature (14). (6) and (7) are the fuel filter and fuel tank, respectively. [Bosch] At the same time, specific fuel consumption has fallen by over 60%! Compared with petrol-powered engines that most often run with stoichiometric mixtures (ie, the theoretically correct air/fuel ratio for complete combustion, which is about 14.7:1), diesels use very lean air/fuel ratios. The air/fuel ratios for diesel engines under full load are between 17:1 and 29:1, while when idling or under no load, this ratio can exceed 145:1. However, within the combustion chamber, localised air/fuel ratios vary – it is not possible to achieve a homogenous mixing of the fuel within the combustion chamber. To reduce these in-chamber air/fuel ratio variations, large numbers of very small droplets of fuel are injected. Higher fuel pressure results in better fuel atomisation, so explaining the increase in injection pressures now being used. Injection Diesel engines are not throttled. Instead, the combustion behaviour is 18  Silicon Chip affected by these variables: • Timing of start of injection • Injection duration • Injector discharge curve Since the use of electronically controlled common rail injection allows these variables to be individually controlled, we’ll briefly look at each. • Timing of Start of Injection The timing of the injection of fuel has a major affect on emission levels, fuel consumption and combustion noise. The optimal timing of the start of injection varies with engine load. In car engines, optimal injection at no load is within the window of 2 crankshaft degrees Before Top Dead Centre (BTDC) to 4 degrees After Top Dead Centre (ATDC). At part load, this alters to 6 degrees BTDC to 4 degrees ATDC, while at full load the start of injection should occur from 6-15 degrees BTDC. The duration of combustion at full load is 40-60 degrees of crankshaft rotation. Too early an injection initiates combustion when the piston is still rising, • Discharge Curve Diesel fuel injectors do not add the fuel for a combustion cycle in one event; instead they operate in one of four different modes. The first is pre-injection, a short duration pulse which reduces combustion noise and oxides of nitrogen (NOx) emissions. The bulk of the fuel is then added in the main injection phase, after which the injector is turned off momentarily before then adding a post-injection amount of fuel. This post-injection reduces soot emissions. Finally, at up to 180 crankshaft degrees later, a retarded post-injection can occur. The latter acts as a reducing agent for an NOx accumulator-type catalytic converter and/or raises the exhaust gas temperature for the regeneration of a particulate filter. The injection amounts vary between 1 cubic millimetre for pre-injection to 50 cubic millimetres for full-load delivery. The injection duration is 1-2 milliseconds. Common rail system overview Unlike previous diesel fuel injection systems – even those electronically controlled – common rail systems use, as the name suggests, a common fuel pressure rail that feeds all injectors. (In this respect, common rail diesel systems are like traditional electronic fuel injected petrol engines.) By separating the functions of fuel siliconchip.com.au (1) hot film airflow meter (2) ECU (3) high pressure pump (4) common rail (5) injector (6) engine speed sensor (7) coolant temperature sensor (8) fuel filter (9) accelerator pedal travel sensor pressure generation and fuel injection, a common rail system is able to supply fuel over a broader range of injection timing and pressure than previous systems. Fig.2 shows a simple common rail fuel injection system. A high-pressure mechanical pump feeds the fuel to the common rail. A control valve allows the fuel pressure to be maintained at a level set by the Electronic Control Unit (ECU). The common rail feeds the injectors, which are electrically operated solenoid valves. Sensor inputs to the ECU comprise fuel pressure, engine speed, camshaft position, accelerator pedal travel, boost pressure (most engines are turbocharged), intake air temperature and engine coolant temperature. Fig.3 shows a slightly more complex common rail system mounted on an engine. More complex common rail systems use these additional sensors: • Vehicle speed • Exhaust temperature • Broadband exhaust oxygen sensor • Differential pressure sensor (to determine catalytic converter siliconchip.com.au Fig.3: this diagram shows the components of a more sophisticated common rail diesel injection system mounted on an engine. [Bosch] and/or exhaust particulate filter blockage) Not shown on these diagrams are the glow plugs. Common rail diesels still use glow plugs, however their use is not normally required except for starting in ambient temperatures below 0°C. Extra ECU outputs can include control of turbocharger boost pressure, exhaust gas recirculation and intake port tumble flaps. Common rail system components • High Pressure Pump A high-pressure pump, driven from the crankshaft, generates fuel pres- Fig.4: a mechanicallydriven three-piston pump provides the extremely high fuel pressure required for common rail diesel injection. [Bosch] (1) drive shaft (2) drive cam (3) pump piston (4) intake valve (5) outlet valve (6) fuel inlet April 2006  19 • Fuel Injectors The fuel injectors superficially look like those used in conventional petrol injection systems but they differ significantly. Fig.6 shows a common rail injector. Because of the very high fuel rail pressure, the injectors use a hydraulic servo system in which the solenoid controls not the pintle but the movement of a small ball which regulates the flow of fuel from a valve Fig.5: the fuel pressure regulator is electronically control chamber within the controlled. It comprises a fuel-cooled solenoid injector. valve driven by pulse width modulation at a When the injector is off, frequency of 1kHz. [Bosch] the ball seals the outlet from the valve control chamber. sures of up to 1600 Bar. The pump The hydraulic force acting on the end uses a radial piston design of the type of the plunger is then greater than that shown in Fig.4. It is lubricated by acting on a shoulder located lower on the fuel and can absorb up to 3.8kW. the plunger, so keeping the injector So that fuel flow can be varied with closed. engine load, individual pistons of The injector in this position is the pump can be shut down. This is shown in Fig.6(a). When the armature achieved by using a solenoid to hold is energised, the ball is lifted and the the intake valve of that piston open. pressure in the valve control chamber However, when a piston is deactivatdrops. ed, the fuel delivery pressure fluctuAs soon as the force on the shoulder ates to a greater extent than when all of the plunger exceeds the force on three pistons are in operation. the top of the plunger, the plunger rises, lifting the pintle and allowing • Pressure Control Valve fuel to flow out of the injector, as in The fuel pressure control valve comFig.6(b). prises a fuel-cooled solenoid valve, as The life of a common rail diesel fuel shown in Fig.5. injector is certainly a hard one. Bosch The valve opening is varied by pulse estimates a commercial vehicle injecwidth modulated drive at a frequency of 1kHz. When the pressure control valve is not activated, its internal spring maintains a fuel pressure of about 100 Bar. When the valve is activated, the force of the electromagnet aids the spring, reducing the opening of the valve and so increasing fuel pressure. The fuel pressure control valve also acts as a mechanical pressure damper, smoothing the high frequency pressure pulses emanating from the radial piston pump when less than three pistons are activated. • Fuel Rail The common fuel rail feeds each injector and is made sufficiently large so that the internal pressure is relatively unaffected by each fuel injector pulse. The rail is fitted with a fuel pressure sensor and a relief valve. 20  Silicon Chip (a) INJECTOR CLOSED tor will open and close more than a billion times in its service life. Emissions Five major approaches are taken to reducing diesel exhaust emissions. These have been effective in meeting current emissions standards, however car manufacturers state the proposed 2007 United States NOx limits for diesels will be hard to meet. This explains the attention currently being given to reducing NOx outputs. • Design Within the engine itself, the design of the combustion chamber, the placement of the injection nozzle and the use of small droplets all help reduce the production of emissions at their source. Accurate control of engine speed, injection mass, injection timing, pressures, temperatures and the air/fuel ratio are used to decrease oxides of nitrogen, particulates, hydrocarbons and carbon monoxide. • Exhaust Gas Recirculation Exhaust gas recirculation, where a proportion of the exhaust gas is mixed with the intake charge, is also used to reduce oxides of nitrogen emissions. It does this by reducing the oxygen concentration in the combustion chamber and the amount of exhaust gas passing into the atmosphere. Recirculation rates can as high as 50 per cent. • Catalytic Converter Diesel oxidation-type catalytic conv- (b) INJECTOR OPEN Fig.6: because of the very high fuel rail pressure, the injectors use a hydraulic servo system in which the solenoid controls the movement of a small ball (4) which regulates the flow of fuel from a valve control chamber (5) within the injector. (1) fuel return outlet, (2) solenoid coil, (6) pressure shoulder, (7) nozzle jet, (8) outlet restrictor, (9) high pressure fuel connection, (10) inlet restrictor, (11) valve plunger. [Bosch] siliconchip.com.au (1) diesel engine (2) optional exhaust heater (3) optional oxidation-type catalytic converter (4) temperature sensor (5) broadband oxygen sensor (6) NOx accumulator-type catalytic converter (7) NOx sensor or oxygen sensor (8) electronic control unit Fig.7: diesel exhaust “after-treatment” is becoming very complex. erters can be used to reduce hydrocarbon and carbon monoxide emissions, converting these to water and carbon dioxide. So they rapidly reach their operating temperature, this type of catalytic converter is fitted close to the engine. NOx accumulator-type catalytic converters are also used. This type of design breaks down the NOx by storing it, for periods from 30 seconds to several minutes. The nitrogen oxides combine with metal oxides on the surface of the NOx accumulator to form nitrates, with this process occurring when the air/fuel ratio is lean (ie, excess oxygen). However, this storage can only be short-term and when the ability to bind nitrogen oxides decreases, the catalytic converter needs to be regenerated by having the stored NOx released and converted into nitrogen. To achieve this, the engine is briefly run at a rich mixture (eg, an air/fuel ratio of 13.8:1) Detecting when regeneration needs to occur and then when it has been fully completed is complex. The need for regeneration can be assessed by the use of a model that calculates the quantity of stored nitrogen oxides on the basis of catalytic converter temperature. Alternatively, a specific NOx sensor can be located downstream of the accumulator catalytic converter to detect when the efficiency of the device is decreasing. Assessing when regeneration is complete is done by either a model-based approach or an oxygen sensor located downstream from the “cat”; a change in signal from high oxygen to low oxygen indicates the end of the regeneration phase. In order that the NOx storage converter works effectively from cold, an electric exhaust gas heater can be employed. Fig.7 shows this complex approach to exhaust treatment. • Selective Catalytic Reduction One of the most interesting approaches to diesel exhaust treatment is Selective Catalytic Reduction. In this approach, a reducing agent such as dilute urea solution is added to the exhaust in minutely measured quantities. A hydrolysing catalytic converter then converts the urea to ammonia, which reacts with NOx to form nitrogen and water. This system is so effective at reducing NOx emissions that leaner-than -normal air/fuel ratios can be used, resulting in improved fuel economy. The urea tank is filled at each service. • Particulate Filters Exhaust particulate filters are made from porous ceramic materials. When they become full, being heated to above 600°C can regenerate them. This is a higher exhaust gas temperature than is normally experienced in diesels and to achieve this, retarded injection and intake flow restriction can be used to increase the temperature of the exhaust gas. Conclusion Particulate filters fitted to Mercedes cars reduce visible soot and smoke emissions. Being heated above 600°C, achieved by retarded injection and intake flow restriction, can periodically regenerate them. [DaimlerChrysler] siliconchip.com.au As can be seen, dramatic changes in both the fuel injection system and exhaust aftertreatment have occurred in diesel technology. SC NEXT MONTH: we’ll look at how the electronic control system makes it all function. April 2006  21