Fuel Injection for Clean Diesel Engines

Hannu Jääskeläinen

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• Introduction
• Injection Timing
• Injection Pressure
• Rate Shaping
• Multiple Injections
  • Overview
  • Pilot Injection
  • Post Injections
  • After Injections

Introduction

As is apparent from the earlier sections on diesel fuel injection, diesel fuel injection systems have seen monumental changes starting in the later part of the 20^th century. The P-L-N injection systems that characterised the diesel engine from the 1920s has all but disappeared from diesel engines intended for the most advanced markets. This evolution has been almost entirely driven by the need to reduce exhaust emissions to levels that were not though possible even as late as the 1990s. These advances in fuel injection system hardware have enabled such features as:

• completely flexible injection timing,
• higher fuel injection pressures and the ability to adjust fuel pressure over the engine speed/load map to suit the particular engine operating conditions,
• tailoring the injection rate over the course of a single injection event and
• multiple injection events.

While these features have been fundamentally driven by the need to lower emissions, in many cases, they can also be utilized to reduce noise, increase specific power and manage exhaust temperatures for improving the performance of aftertreatment systems that can be used to achieve further reductions in exhaust emissions.

Injection Timing

NOx Emissions Control. Adjustments in injection timing are one of the fundamental means of achieving reductions in NOx emissions. Mechanical fuel injection systems were the first to incorporate variable injection timing. However, as electronics become more prevalent in diesel engine control, electronically controlled injectors became the preferred means of achieving variable injection timing and offered unprecedented flexibility in injection timings settings. The mechanisms for reduced NOx by injection timing retard are discussed elsewhere.

While NOx reduction via injection timing retard can be effective, there can be significant trade-offs in terms of fuel consumption and PM emissions. In many cases, these trade-offs must be dealt with through additional engine design enhancements. One early approach to reduce the fuel economy penalty associated with retarding injection timing was to reduce ignition delay by using a high compression ratio and higher injection pressures [685]. Additional measures such as reductions in oil consumption, increases in charge air pressure, increases in injection pressure, reductions in injector nozzle hole size, reductions in engine friction losses, reduction in intake manifold temperature, etc. can also be taken to control fuel consumption and PM emissions increases.

Prior to the introduction of electronically controlled injection systems, fuel injection timing was typically fixed at a constant value over the entire engine operating map. However, variable injection timing systems were sometimes used for additional flexibility and to compensate for shortcomings in engine performance. Some PLN systems incorporated a variable timing mechanism to compensate for changes in ignition delay with engine speed in order to maintain a more constant, and optimum, combustion phasing. In other cases, the fixed injection timing required to ensure that NOx emissions over the certification cycle were met could lead to excess hydrocarbons at light load, acceleration smoke, cold smoke and idle roughness that could be overcome by advancing injection timing at light loads with only a minor increase in duty-cycle NOx emissions.

Between 1987 and 1998 when electronically controlled injection timing retard was the primary means for reducing NOx emissions, one means that many North American engine manufacturers commonly used to offset fuel consumption penalties associated with retarded fuel injection timing was a dual-mapping strategy in electronically controlled engines. In this approach, a nominal injection timing setting that assured regulatory compliance with NOx emission standards was used in transient operation such as during emission certification test cycles. However, when it was determined that a vehicle was in cruise mode, injection timing was advanced to improve fuel economy. This provided a significant fuel economy improvement under the highway cruise condition commonly encountered by heavy-duty trucks but also increased NOx emissions significantly.

Injection timing on its own is limited in its ability to reduce NOx emissions. In addition to the trade-offs already discussed. NOx emissions can start to increase again if timing is retarded sufficiently or the engine can start to misfire [2138][2145][2135]. This places a practical lower limit of around 4 g/kWh NOx that can be achieved with injection timing retard [2139]. Further NOx reductions have required additional measures such as injection rate shaping, pilot injections, intake valve timing control, EGR and NOx aftertreatment. While injection timing retard is no longer the primary means of NOx control, it is still an important tool that can be used in conjunction with other control measures to ensure regulatory NOx limits are met.

Thermal Management. With the introduction of high efficiency NOx aftertreatment, retarded injection timing is less important for NOx emission control. However, it is an important tool that can be used to increase exhaust gas enthalpy and temperature for thermal management of exhaust aftertreatment systems. It is especially useful during cold starts before the aftertreatment temperature is sufficiently high to provide significant emission reductions. The lower NOx emissions associated with retarded injection timing are especially important under these conditions to limit overall drive cycle emissions.

Figure 1 illustrates the effect of retarded injection timing on turbocharger turbine outlet temperature for a light-duty diesel engine operating at a low load representative of the NEDC cycle. At this light load condition, exhaust temperature can be raised to 235°C for both cold and warm coolant temperatures. This represents an increase of about 45°C at 30°C coolant temperature and about 25°C for at 90°C coolant temperature. It should be noted that the EGR rate is lowered at retarded injection timing in this example in order to maintain constant NOx emissions [4852].

While the primary reason for increased exhaust temperature is an increase in exhaust losses due to a later combustion phasing, several other factors also contribute to the increased exhaust temperature. The retarded combustion phasing decreases engine efficiency and thus requires a larger amount of fuel to be burned to produce the same brake torque that will contribute to the higher exhaust temperature. It is also reported that for the data in Figure 1, air fuel ratio decreases as injection timing is retarded [4852]. A reduction in air fuel ratio would further contribute to increased exhaust temperature.

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