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The Alfa Romeo 156 JTS (“Jet Thrust Stoichiometric”) engine uses Bosch direct injection. This gives the performance of a 2.3-litre unit, yet consumes 10% less fuel. The 2-litre engine has the same capacity as the Twin Spark engine it replaces, yet power is up from 114kW to 121kW and torque rises from 187Nm to 206Nm. [Alfa Romeo] By Julian Edgar Direct Petrol Injection Complex electronic control and direct fuel injection can provide fuel savings of up to 40%. The injectors work at much higher fuel pressures and must inject the fuel in extremely short times, within each cylinder’s intake or compression stroke. C ONVENTIONAL ELECTRONIC fuel-injected engines use injectors to squirt fuel into the intake ports. In a sequentially injected engine, each injector opens a short time before its associated intake valves, while in simultaneous-injected engines the injectors all open at the same time. In either case, the cloud of small fuel droplets is drawn into the engine only 22  Silicon Chip when the intake valves open. But a new breed of fuel-injected engines is now appearing. As the name suggests, Direct Petrol Injection uses injectors that add fuel directly to the combustion chamber. Like diesel engines, the air/fuel mixing occurs inside the combustion chamber, rather than in the inlet ports. Taking this approach gives far greater control over the combustion process, allowing for a variety of combustion operating modes, including those having ultra-lean air/fuel ratios. However, the degree of electronic control required to make smooth transitions from one combustion operating mode to another is complex and engine operating processes need to be monitored far more closely than is the case with conventional port fuel injection. Manufacturers are deciding that the added complexity is worth it – Mitsubishi has produced more than 700,000 direct-injected petrol engines (although none has been sold outside Japan) and BMW and Alfa Romeo are now selling cars with direct injected engines in Australia. Direct injected engines have even won major motor sports events, with siliconchip.com.au HIGH PRESSURE PUMP CARBON CANISTER FUEL RAIL CANISTER PURGE VALVE AIR-MASS METER WITH TEMPERATURE SENSOR ACCELERATOR PEDAL MODULE INJECTOR THROTTLE DEVICE (EGAS) IGNITION COIL PRESSURE CONTROL VALVE INTAKE MANIFOLD SENSOR PHASE SENSOR OXYGEN SENSOR (LSU) FUEL PRESSURE SENSOR KNOCK SENSOR PRE-CATALYST EGR VALVE EXHAUST GAS TEMP. SENSOR ELECTRONIC CONTROL UNIT NOX CATALYST DIAGNOSIS INTERFACE MALFUNCTION INDICATOR LAMP DELIVERY MODULE, INCLUDING LOW PRESSURE PUMP IMMOBILISER BOSCH COMPONENTS SPECIFICALLY FOR DE CAN BOSCH COMPONENTS OXYGEN SENSOR (LSF) Fig.1: the Bosch MED Motronic engine management system, which uses direct fuel injection, looks similar to conventional engine management. However, it uses a new Electronic Control Unit with many different functions, a second high pressure fuel pump, new design injectors, a fuel rail pressure sensor, and a second NOx catalyst. Some of the sensors are also used in unconventional ways. [Bosch] Audi’s direct-injected 455kW twin turbo 3.6-litre V8 triumphing at Le Mans. FUEL INJECTOR CROSS-SECTION 1 System mechanicals Fig.1 shows the layout of the Bosch direct injection system. Its mechanical elements differ from conventional port injection in several ways. First, the fuel supply system uses two fuel pumps – a conventional electrical fuel pressure pump (in the past dubbed a high-pressure pump but now referred to in this system as a low-pressure pump) and a mechanically driven high-pressure pump. The low-pressure pump works at pressures of 0.3-0.5MPa while the high-pressure pump boosts this very substantially to 5-12MPa. The high-pressure fuel is stored in the fuel rail that feeds the injectors. The fuel rail is made sufficiently large that pressure fluctuations within it are minimised as each injector opens. The pressure of the fuel in the injector supply rail is controlled by an siliconchip.com.au 2 3 The fuel injectors used for direct injection are subjected to enormous pressures. Minimum injector opening time is just 5ms and droplets are on average smaller than 20µm – just one-fifth the size of traditional injectors and one-third the diameter of a human hair. [Bosch] Fig.2 (right): cross-section of a direct petrol injection injector. (1) fuel inlet with strainer, (2) electrical connections, (3) spring, (4) solenoid, (5) injector housing, (6) nozzle needle with solenoid armature, (7) valve seat, (8) injector outlet passage. [Bosch] 4 5 6 7 8 August 2005  23 TORQUE (M) THROTTLE-VALVE POSITION/ EXCESS AIR FACTOR (l) Stratified Mode STRATIFIED MODE HOMOGENEOUS MODE Fig.3: although at least six different modes of combustion can occur in a direct injection engine, this diagram shows the two main modes. In Stratified Mode, the injector adds the fuel during the compression stroke, just before the spark plug fires. In the period between the injection finishing and the spark plug firing, the airflow movement within the combustion chamber transports the air/fuel mixture towards the spark plug. This results in a portion of relatively rich air/fuel mixture surrounding the spark plug electrode while the rest of the combustion chamber is relatively lean. In Homogenous Mode, injection starts on the intake stroke, so there is sufficient time for the air/fuel mixture to be distributed throughout the combustion chamber. Stratified Mode can result in air/fuel ratios as lean as an incredible 55:1. [Bosch] electronically-controlled bypass valve that can divert fuel from the highpressure pump outlet back to its inlet. The fuel bypass valve is varied in flow by being pulse-width modulated by the Electronic Control Unit (ECU). A fuel pressure sensor is used to monitor fuel rail pressure. a 1 0 Ivm b Ih tvm c 0 ton INJECTED FUEL QUANTITY NEEDLE LIFT CURRENT Imax 0 toff d INJECTION DURATION Fig.4: direct injection injectors use a peak/hold system of operating; (a) is the triggering signal from the ECU, (b) is the actual injector current, (c) injector needle lift, (d) injected fuel quantity. A booster capacitor is used to provide the high opening current. [Bosch] 24  Silicon Chip Second, compared with a conventional port fuel injection system, the fuel injectors must be capable of working with huge fuel pressures and also injecting large amounts of fuel in very short periods. Fig.2 shows a cross-sectional view of an injector. The reason for the much reduced time in which the injection can be completed is due to the fact that all the injection must at times occur within a portion of the induction stroke. Port fuel injectors have two complete rotations of the crankshaft in which to inject the fuel charge – at an engine speed of 6000 RPM, this corresponds to 20ms. However, in some modes, direct fuel injectors have only 5ms in which to inject the full-load fuel. The fuel requirements at idle can drop the opening time to just 0.4ms. Direct injection fuel droplets are on average smaller than 20µm – one-fifth the droplet size of traditional injectors and one-third the diameter of a human hair. Finally, the very lean air/fuel ratios at which direct injection systems can operate result in the production of large quantities of oxides of nitrogen (NOx). As a result, direct-injected cars require a primary catalytic converter fitted close to the engine and also a main catalytic converter – incorpo- Homogeneous Mode a b l=1 0 ACCELERATOR PEDAL POSITION Fig.5: during Stratified Charge Mode, the throttle is held wideopen, irrespective of the driver’s accelerator pedal input. In this mode, the air/fuel ratio (Bosch refer to an increased ‘excess air ratio’) is very lean when the torque request is low, with the air/fuel ratio gradually becoming richer as more torque is required. At a certain point, the engine changes to Homogenous Mode. With the change in modes, the throttle valve opening becomes related to the driver’s torque request and the air/fuel ratio holds a constant stoichiometric ratio (ie, 14.7:1 or Lambda = 1) across the rest of the engine load range. [Bosch] rating an NOx accumulator – fitted further downstream. Combustion modes The really radical nature of direct siliconchip.com.au OPERATING MODE COORDINATION Priority OPERATING MODE SWITCHING Function 1  Monitoring concept 2  Component protection 3  Limp-home functions 4   Adjustability of torque      and oxygen content limits 5   Catalytic converter heating 6  NOx accumulator      regeneration 7  SOx accumulator      regeneration 8  Starting/warming up 9   Diagnostic system manager 10   Operating mode map Required operating mode Changeover Function Control Functions • • • • Exhaust recirculation Fuel tank venting Swirl flap EGAS Fig.6: the direct injection system incorporates an operating-mode co-ordinator which uses a 10-stage priority system. Before the selected combustion mode starts to occur, control functions for exhaust-gas recirculation, fuel tank ventilation, charge-movement flap and electronic throttle settings are initiated as required. The system waits for acknowledgement that these actions have been carried out before altering fuel injection and ignition timing to provide the appropriate combustion mode. [Bosch] fuel injection can be seen when the different combustion modes are examined. There are at least six different ways in which combustion can take place. • Stratified Charge Mode: at low torque output up to about 3000 RPM, the engine is operated in Stratified Charge Mode. Here, the injector adds the fuel during the compression stroke, just before the spark plug fires. In the period between the injection finishing and the spark plug firing, the air-flow movement within the combustion chamber transports the air/fuel mixture into the vicinity of the spark plug. This results in a portion of relatively rich air/fuel mixture surrounding the spark plug electrode, while the rest of the combustion chamber is relatively lean. The gas filling the rest of the chamber Current operating mode For example: • Homogeneous mixture • Homogeneous lean mixture • Stratified mixture • Double injection often comprises recirculated exhaust gases, resulting in a reduced combustion temperature and decreased NOx emissions. In Bosch direct injection systems, the air/fuel ratio within the whole combustion chamber can be as lean as 22:1 - 44:1. Mitsubishi states that total combustion chamber air/fuel ratios of 35:1 - 55:1 can be used. Compare this with a conventional port fuel injected engine that seldom uses an air/fuel ratio leaner than 14.7:1. • Homogenous Mode: Homogenous Mode is used at high torque outputs and at high engine speeds. Injection Left & above: Audi has won Le Mans using direct-injected engines. The twinturbo 3.6-litre V8s developed 455kW, despite having to breathe through two 32.4mm restrictors and having the boost of the twin turbos limited to just under 10 psi. The adoption of direct petrol injection allowed the compression ratio to be lifted a point and gave a fuel reduction of 8-10%, a torque increase of 9% over a wide RPM range and quicker starting and better driveability. [Audi] siliconchip.com.au August 2005  25 CONTROL UNIT HIGH PRESSURE PUMP PRESSURE CONTROL VALVE PRESSURE SENSOR FUEL DISTRIBUTOR INJECTORS Fig.7: the high-pressure pump is driven directly from the engine and develops fuel pressures as high as 12MPa. This high pressure is regulated by a fuel pump bypass valve which is pulse width modulated by the Electronic Control Unit. The injectors are opened with a burst of high current from a capacitor that delivers up to 90V. A dedicated sensor monitors fuel rail pressure. [Bosch] The BMW 760 uses a direct injection 6-litre V12 developing 327kW and 600Nm. It complies with the EU4 emission standard in Europe and the LEV (Low Emission Vehicle) standard in the USA. Fuel consumption in the EU test cycle is 13.4 litres/100 km. The car accelerates from 0-100km/h in 5.5 seconds and continues on to 200km/h in 17 seconds. [BMW] 26  Silicon Chip starts on the intake stroke so there is sufficient time for the air/fuel mixture to be distributed throughout the combustion chamber. In this mode, Bosch systems use an air/fuel ratio of 14.7:1 (the same as with port fuel injection at light loads), while Mitsubishi use air/ fuel ratios from 13:1 – 24:1. • Homogenous Lean-Burn Mode: during the transition between Stratified and Homogenous Modes, the engine can be run with a homogeneously lean air/fuel ratio. • Homogenous Stratified Charge Mode: initially, this mode appears nonsensical – how can the combustion process be both homogenous and stratified? However, what occurs is not one but two injection cycles. The initial injection occurs during the intake stroke, giving plenty of time for the fuel to mix with the air throughout the combustion chamber. Then, during the compression stroke, a second amount of fuel is injected. This leads to the creation of a rich zone around the spark plug. The rich zone easily ignites, which in turn ignites the leaner air/fuel ratio within the remainder of the combustion chamber. Of the total fuel addition, approxisiliconchip.com.au mately 75% occurs during the first injection and 25% during the second. The Homogenous Stratified Charge Mode is used during the transition from Stratified Charge to Homogenous Modes. In addition there are at least two more modes: Homogenous Anti-Knock and Stratified Charge Cat-Heating. The first is used at full throttle and the second to rapidly heat the catalytic converter to operating temperature. A final mode – mentioned in only some of the literature – is Rich Homogenous Mode, which is used to regenerate the NOx accumulator in the main catalytic converter. (The NOx accumulator deposits oxides of nitrogen in the form of NHO3 nitrates. When the accumulator is regenerated, the nitrates – together with carbon monoxide – are reduced in the exhaust to nitrogen and oxygen.) Fig.3 shows the two primary combustion modes. Electronic control systems As indicated earlier, the injectors must be opened against very high fuel pressures. This is done using a peak/ hold technique, whereby the opening current is very high and the “hold” current much reduced. A dedicated triggering module is used to control the injectors, with a booster capacitor providing 50-90V to initially open the injector. Fig.4 shows this process. The sensing of the mass of cylinder charge is more complex on a directinjected engine than a conventional port-injected engine. This is because at times recirculated exhaust gas forms a major component of the total cylinder charge. As a result, two cylinder charge sensors are used. These comprise a conventional hot-film mass air-flow sensor (similar to a hot-wire air-flow meter) and a manifold pressure sensor (MAP sensor). The flow through the air-flow meter is used as an input into the calculation of the pressure within the intake manifold and this is then compared with the actual intake manifold pressure measured by the MAP sensor. The difference between the two indicates the mass flow of the recirculated exhaust gas. As with many conventional engine management systems, direct injection requires the use of an electronically controlled throttle. In conventional systems, the actual throttle opening siliconchip.com.au Fig.8: fuel is introduced directly into the combustion chamber by the highpressure injectors. Depending on the operating mode, the fuel can be added during the intake stroke, the compression stroke or during both the intake and compression strokes. [Bosch] Fig.9: there are two main approaches to aiming the fuel spray. In Wall Guided (left), the air movement within the combustion chamber guides the area of rich mixture in the direction of the spark plug. In Spray Guided (right), the fuel is injected directly into the vicinity of the spark plug. The latter approach gives improved emissions and fuel economy but thermally stresses the spark plug. [Bosch] more or less follows the driver’s accelerator pedal torque request. By contrast, in direct injected engines, the throttle is fully open for much of the time and varying the fuel delivery regulates engine torque output. Fig.5 shows how this occurs. During Stratified Charge Mode, the throttle is held wide-open, irrespective of the driver’s accelerator pedal input. When the torque request is low, the air/fuel ratio is very lean (Bosch refer to this as an increased ‘excess air ratio’), with the air/fuel ratio gradually becoming richer as more torque is required. At a certain point, which corresponds on an engine-specific basis to engine speed and the amount of torque required, the engine changes to Homogenous Mode. (For simplicity, the transitional Homogenous LeanBurn Mode is ignored in this diagram.) With the change in modes, the throttle valve opening becomes related to the driver’s torque request and the air/ fuel ratio holds a constant stoichioAugust 2005  27 losses – the engine is no longer trying to breathe through the restriction of the nearly-closed throttle. However, the downside of this is that the partial vacuum that’s normally available for the brake booster will be lacking. To overcome this problem, a vacuum switch or pressure sensor monitors brake booster vacuum and if it is necessary, the combustion mode is altered so that vacuum again becomes available. Increased efficiencies This photo shows the BMW V12 injectors and fuel rail. Fuel rail pressure varies from 30-100 Bar. The injection pumps are fitted above the outlet camshafts and are driven by an additional cam. [BMW] metric air/fuel ratio (that is, 14.7:1 or Lambda = 1) across the rest of the engine load range. The system incorporates an operating-mode co-ordinator which maps operating mode against engine speed and torque request. Fig.6 shows a schematic diagram of the functioning of this controller. As can be seen, a 10-stage priority allocation is used when determining the required operating mode. Before the selected combustion mode starts INJECTION to occur, control functions for exhaustgas recirculation, fuel tank ventilation, charge-movement flap (ie, port tumble valves or variable length intake manifold) and electronic throttle settings are initiated as required. The system waits for acknowledgement that these actions have been carried out before altering the fuel injection and ignition timing. The advantage of having the electronic throttle valve fully open at low loads is a huge reduction in pumping IGNITION COMBUSTION & ENGINE START As well as the huge reduction in pumping losses at low loads (due to the throttle being wide open), thermodynamic efficiencies are also increased during Stratified Charge Mode. This is because the rich cloud of combustible air/fuel mixture around the spark plug is thermally insulated by the layer of air and recirculated exhaust gas that surrounds it. Together with the much leaner air/fuel ratios than can be used in a conventional port injected engine, the result is a fuel efficiency improvement that can be up to 40% at idle. Mitsubishi state that at 35km/h, their direct injected engines use 35% less fuel than a comparably-sized conventional engine and that in the Japanese 10-15 Urban Driving Cycle (albeit a slow speed cycle), the direct-injected engine uses less fuel than even a comparable diesel engine. During homogenous mode operation, both the use of an air/fuel ratio that is never richer than 14.7:1 and the higher compression ratios normally associated with direct injection engines result in a fuel saving of about 5%. Conclusion Fig.10: another advantage of direct injection is that it is possible to start a hot engine without using the starter motor. This approach reduces startup hydrocarbon emissions, making it more attractive to use engines that automatically switch off if the car has been stationary for a period. [Bosch] 28  Silicon Chip Mitsubishi has been building direct injection petrol engines since 1996 but they have not been able to achieve worldwide success with their designs due, it is said, to the engines’ reliance on high-quality fuel. Now Bosch has developed technology which is allowing direct injected engines to be sold around the world. Combine direct injection technology with the hybrid cars being widely developed – plus perhaps downsized and turbocharged engines – and we’re certainly in for some interesting times ahead. But without the sophisticated electronics working behind the scenes, none of these engines would be posSC sible. siliconchip.com.au