Digital Combustion Rate Shaping Control as a Tool to Identify Modern Fuel Injection Strategies

Autoren

Dipl.-Ing. Christian Jörg, Marius Zubel, MSc., Daniel Neumann, MSc.,
Institut für Verbrennungskraftmaschinen, RWTH Aachen University, Aachen;
Prof. Dr.-Ing. Alexander Heufer, Dr.-Ing. Joschka Schaub,
FEV Europe GmbH, Aachen;
Dr.-Ing. Jost Weber, Dr.-Ing. Olaf Herrmann,
DENSO AUTOMOTIVE Deutschland GmbH, Wegberg

Zusammenfassung

Combustion rate shaping (CRS) represents a novel fuel injection management strategy to optimize the thermodynamic engine behavior. CRS is characterized by complex fuel injection strategies with an increased number of injection events at reduced injection quantities, resulting in a significant reduction of the direct combustion noise level. This enables the application of greater indicated engine efficiencies. Higher railpressures are applied, which realizes an additional soot reduction potential at same combustion noise level.

The thermodynamic potential of CRS is evaluated for three certificationrepresentative steady state engine operation points using a single cylinder Diesel engine equipped with the DENSO G4P servo piezo injector, which enables up to 2500 bar injection pressure and allows close to zero pilot dwell times. The eexperimental results are extrapolated to the WLTP certification cycle for a typical Diesel engine passenger car of 1590 kg equivalent inertia class with an 1.6 L, 4 cylinder Diesel engine. In comparison to a conventional fuel injection strategy, CRS achieves simultaneous improvements in all relevant calibration targets: NVH, fuel economy and pollutant emissions. The averaged combustion sound level can be reduced by 2.6 dB(A) due to an optimized peak cylinder pressure gradient and start of combustion. Since CRS decouples the cylinder pressure gradient from the applied railpressure, a soot reduction of 26.6% is achieved as an outcome of a higher railpressure. The improved mixture formation further affects the CO emission level, which is reduced by 3.9%. A reduction of hydrocarbon (HC) emissions is further achieved by 7.7% in the WLTP cycle, which is due to the fact that the individual CRS fuel injection strategy contains a higher number of injection events in comparison to the conventional fuel injection strategy. This shortens the individual injection duration and hinders the fuel jet propagation towards colder cylinder wall regions. Hence, the
occurrence of wall quenching effects is diminished, which gives HC emission benefits especially in the medium part load region. A CO₂ emission reduction is also achieved by 1.7% as a consequence of an optimized combustion phasing position, which is enabled by the lower combustion sound level.

However, the identification of a CRS fuel injection strategy is not trivial. The conventional DoE-based fuel injection calibration approach, using injection timings and quantities as DoE parameters, is not suitable to calibrate CRS fuel injection
strategies due to their complexity. Instead, a CRS feedback controller is applied for an automated identification of the fuel injection strategy, consisting of the number of injection events, timings and quantities. Hence, the resulting CRS fuel injection
strategy is not limited to a pre-defined injection strategy anymore, which amplifies the calibration related degree of freedom to exploit the thermodynamic optimization potential of the engine. Instead of calibrating the parameters of the fuel injection strategy, an optimized crank angle resolved combustion rate profile is set and the controller calculates the corresponding fuel injection strategy. Besides other options, the target combustion rate profile can be synthesized in terms of a cylinder pressure profile, which only includes a very limited number of physical calibration parameters.
These calibration parameters are related to physical characteristics of the combustion process, such as cylinder pressure gradient, peak pressure limitation, gross indicated mean effective pressure or start of combustion, which significantly reduces the number of design of experiment (DoE) parameters. As a result of the reduced number of DoE parameters and their physical origin, the CRS-based
calibration procedure achieves an enormous reduction potential in calibration effort by up to 89% in comparison to a conventional fuel injection management calibration approach.

Once the CRS-based calibration procedure has generated an optimal fuel injection strategy for the entire engine operation range, its timings and duration can be applied to conventional, map-based, open-loop ECU software architectures. Hence, seriesuse capability is given by the considered CRS approach without the need for an
additional cylinder pressure sensor.

Finally, special attention needs to be drawn on the fuel injection system. The need for a low friction high pressure fuel pump is undisputed to enable the application of higher railpressures without the deterioration of brake engine efficiency. The fuel injector shall be capable in realizing hydraulic dwell times below 150 μs to achieve a sufficient CRS accuracy. CRS increases the number of applied fuel injection events by 30 - 40% per working cycle in comparison to conventional fuel injection pattern of around 4 injection events, which has to be assured over life-time. The robust
realization of small injection quantities at small hydraulic dwell times over the entire engine lifetime within the whole engine map area (up to 2500 bar rail pressure) is a vital requirement for the achievement of the desired CRS performance. Conventional
mechanisms like the zero fuel injection quantity correction are not expected to achieve the required level of injection accuracy for CRS. In this context, an injection pressure-based feedback correction of the fuel injection process shows a possibility to maintain a sufficient injector accuracy under all circumstances and driving
manoeuvres.

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