Mild hybridization and electric boosting: Improving diesel emissions and fuel efficiency with premium performance

30. September 2015 | Engineering Service

Diesel engines are facing various challenges related to further improvements in fuel efficiency and reduction of NOX emissions in the same time. FEV and Valeo have been running a joint program for several years now, investigating the potential of mild hybridization and electrical boosting for a downsized passenger car diesel engine.

Vehicle electrification is a key to compliance with future emission and CO2 limits. Aside from conventional 12V systems and full hybrid powertrain architectures, 48V architectures offer fuel economy potential and advanced emissions control without the need for a complete powertrain redesign. A particularly interesting technology within a 48V electrical system is the application of an electric charger (E-Charger).

Strategies for an E-Charger

The 48V E-Charger can be used across a wide range of different control strategies. For example, it can be activated only during transients as a means of compensating turbo lag, enabling engine exhaust soot and NOX emissions to be kept at nearly steady-state values. Another option is the application of transient overboosting as a means of supporting Exhaust Gas Recirculation (EGR). With this strategy, the NOX emissions can be significantly reduced even under more severe accelerations during on-road cycles.

Further advantages

In addition to advantages in engine exhaust emissions, there are some benefits related to CO2 reduction. “Although it’s the smallest part among these benefits, the direct effect on the pumping work of the engine under transient conditions is nonetheless far from negligible in real-world driving”, explained Jürgen Ogrzewalla, Director Hybrid Development at FEV. A second important feature is an additional degree of freedom in the base turbocharger layout, which can be optimized for higher efficiency. The resulting compromises in inertia and, therefore, transient response are compensated by the E-Charger. A base turbocharger layout that targets increased full-load performance (this is especially relevant for downsized engines) also becomes feasible, because the low-end torque can be addressed by the E-Charger. Since the response time of the 48V E-Charger to rated speed is only about 250 ms, very good torque response is possible, even at very low engine speeds. This allows “Downspeeding” to be adapted much more aggressively than for standard one-stage or even two-stage turbocharger

System Simulation Results

To quantify the benefits in terms of CO2/NOX/soot emissions, a selected series of load steps were evaluated on the engine test bench. The results were then used to calibrate an engine model including the E-Charger and a 48V belt starter generator (BSG). Based on this engine model, a downsizing scenario was investigated in which a 2.0-liter diesel engine was replaced with a smaller 1.6-liter version which was derived from the well-known FEV-HECS (High Efficiency Combustion System) engine concept. The E-Charger was activated in areas of the engine map where the base turbocharger was not able to achieve the desired boost pressure. The electrical energy required for the operation of the E-Charger was generated during overrun and braking phases. When the recuperated energy exceeded the energy demand, the excess energy was used to provide boost via a belt-driven starter generator. The simulation results demonstrated that the use of a 48V E-Charger in combination with the 48V BSG not only improves the driving dynamics, performance and comfort, but also significantly reduces fuel consumption and hence CO2 emissions (11 percent along the WLTP).


Air path layout with E-Charger

Simulation results for a D-segment vehicle: CO2 emissions and acceleration performance

Simulation results for a D-segment vehicle: CO2 emissions and acceleration performance


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