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Mild-Hybrid-Diesel-Powertrain with a Pre-Turbine Exhaust Aftertreatment
The potential to achieve a simultaneous reduction in both NOx and CO2 emissions via fitting of a pre-turbine exhaust aftertreatment system (PT-EATS) in combination with a mild-hybrid concept was investigated via simulation. The main engine and hybrid system hardware were specified and thereafter, the operating strategies for recuperation and turbocharger control were determined to enable the system to meet a defined tailpipe NOX emissions value of 40 mg/km with a conformity factor (CF) of 1 over all real-world driving cycles. The performance and drivability of the demonstrator are defined to be equivalent to the base vehicle.
The main concept was to locate the exhaust aftertreatment system (EATS) directly downstream of the exhaust manifold, but upstream of the turbine as shown in Figure 1, so that the best CO2 reduction potential and the best aftertreatment performances can be achieved simultaneously.
The engine hardware and PT-EATS were designed and optimized via simulation to identify the best layout of the catalysts and to quantify the potential benefits for CO2 and NOX emissions reduction. The 48V system, made up of a belt starter generator (BSG) with the associated control components, an electric assisted turbocharger (e-TC) and the 48V battery as well as PT-EATS, were integrated to the existing engine model. The simulations optimized EATS component sizes to achieve successfully the integration within the engine bay. The e-Turbo was dimensioned in GT Power and moreover the EGR strategy was optimized to meet the extremely low engine-out NOx emission targets. Furthermore, the recuperation potential was established by using the simulation model. The original exhaust manifold was rotated 180 °C to enable the integration of the turbocharger and a larger EGR cooler was inserted to allow EGR to be used during full-load operation. Additional design modifications were made to the intercooler bracket, the water lines and air lines to allow the complete packaging within the engine bay of the chosen J-Segment demonstrator vehicle.
Placing the aftertreatment system upstream of the turbine results in an altered enthalpy and thermal inertia profile over the turbocharger compared to a conventional arrangement (Figure 2). The introduction of a 48V electric system to the vehicle enables the incorporation of a pre-turbine aftertreatment system via the integration of an e-TC which compensates the loss of pressure and temperature caused by the increased thermal inertia of the PT-EATS.
In the early phases of operation, the temperature before turbine is significantly lower than without the PT-EATS, owing to the increased thermal mass, but as the exhaust system heats up, a thermal lag and overall temperature offset is seen as a result of the higher thermal mass upstream of the turbine. The heat loss profile over the PT-EATS leads to a calculated cumulative enthalpy loss of ~ 4 percent over a WLTC (Figure 2). In order to maintain the boost pressure levels in such low enthalpy phases, the electric turbocharger generates additional boost pressure, it is also used to recuperate excess energy whenever possible.
The recuperation potential of the system was investigated at two part load operating points, shown in Figure 3. The comparison of the brake specific fuel consumption for 2 recuperation strategies was investigated and shown in Figure 3. Recuperation at the turbocharger via VGT is compared with the extraction of the same power over the BSG via an operating point load shift. The latter strategy shows a more energy-efficient path by up to 3.3 percent, as closing the VGT increases the pump losses, as seen in Figure 3, right. It should be noted however that increasing recuperation increases fuel consumption as additional power is needed to generate the same effective power.
The sizing of the e-TC was considered, as a larger turbine could reduce fuel consumption via optimized pumping losses, but, as typical passenger car driving scenarios are not significantly impacted by pumping loss based fuel consumption penalties, there was minimal benefit to upsizing the e-TC as the electrical boost required during transient operation would be increased so a smaller turbine was chosen. The key criteria in determining the size of the e-machine used is the transient response behavior of the vehicle. An acceleration from a standstill to 100 km/h was simulated with different sizes of e-TC to see which could achieve a comparable acceleration behavior to the base vehicle (8.7 s in the sprint to 100 km/h). These simulations are seen in Figure 4. Illustrating that without electrical boost support, the acceleration time increases to 13.0 s, confirming that an e-Turbo is required. An increase in the E-machine power above 11 kW showed no significant reduction in response time (9.0 s to 9.4 s) as acceleration was limited by the electrical machine speed to 180000 min-1.
Regarding the potential to reduce the cost and complexity of the EGR circuit, the use of an HP-EGR-only strategy was investigated which would consist of recuperating excess exhaust gas energy, while controlling the VGT position to achieve the required back pressure to drive higher EGR rates at comparable boost pressure. The results showed, that including the LP-EGR path reduces the required electrical energy over the WLTC by about 30 percent, therefore both HP- and LP- EGR are used. As the PT-EATS volume was found to show only minor impacts on the fuel consumption, compared to the EGR strategy influence, Figure 5, the EATS volumes were chosen to fill the available package space. All the above variables were combined to create an optimised air path strategy. Combining LP and HP-EGR, with a comparatively small turbine and an 11 kW electric motor allows for the lowest possible boost requirement during transient driving conditions. The additional energy requirement for this configuration over a WLTC was approx. 52 Wh when omitting recuperation at the BSG or e-Turbo.
Air path control
The electrical VGT turbocharger requires a dedicated control strategy to optimize the different operating states. For this concept configuration, the electric machine is mainly used for transient support during boost pressure build-up and recuperation during deceleration or in overrun operation. The conventional boost pressure control for the VGT was extended with an advanced model-based control for the power, respectively torque of the electric machine. In this approach the torque of the electric machine is calculated based on the difference between desired and actual turbine torque. An additional e-boost control factor is introduced to balance and adjust the responsiveness of the model-calculated torque demand to the e-machine against the electric energy consumption. The fuel consumption penalty and NOx engine-out emissions as a function of the e-boost control factor are shown, in Figure 6 for the WLTC.
When applying small e-boost control factors, the electric machine only supports during very high differences between desired and actual turbine torque, while for higher control factor values, the e-machine supports for smaller deltas in turbine torque. As such, the NOx engine-out emissions are reduced at higher e-boost control factor values, whereas the fuel consumption increases significantly in consequence of the increased electric power demand. These trends were combined to determine the target operation area.
Overall hybrid strategy optimization
The additional benefits of the 48V mild-hybrid system architecture shown in Figure 7 were evaluated. A 48V belt starter generator replaces the conventional 12V generator, a 48V battery with a capacity of 0.5 kWh, and the electrically supported electrical VGT turbocharger were integrated with the 12V on-board power supply provided via a bidirectional DC/DC converter. When optimizing the control of the electrical VGT turbocharger, a priority manager governs available power for the different consumers, based on the current state of the electric system. The simulation model uses a higher-level energy management strategy to ensure reliable supply for the on-board 12V network under all operating conditions and simultaneously maximizes the potential of the various 48V components to balance transient support during boost pressure build-up and recuperation potential at engine overrun operation and high enthalpy flow upstream turbine.
The balance of the electrical energy within the 48V system over the WLTC is shown in Figure 8. Recuperation takes place almost exclusively by the BSG, whereas the energy consumption is split in roughly equal parts between the supply of the consumers in the 12V network and to support the electrical boosting. As the recuperated energy exceeds the electrical energy consumption, approx. 30 percent is used to charge the 48V battery.
The second part of this paper in an upcoming magazin article will detail the PT–EATS system optimization and the overall system performance over key RDE cycles.