2. May 2017 | Engineering Service

FEV electrifies diesel

FEV HECS ECObrid with 48V hybridization

Over the next few years, a significant increase in 48V “mild hybrid” concepts is expected, since they offer considerable potential to reduce fuel consumption as well as pollutant emissions. At the same time, they are relatively easy to implement, since they do not require a complete redesign of the powertrain.

As part of a joint project with Valeo, FEV has equipped a D-segment test vehicle with a 48V electrical system, a belt-driven starter generator (BSG), and an electrically driven compressor (e-Compressor). The result: the optimizations contribute to a CO2 reduction potential of approximately 11% in the Worldwide Harmonized Light-Duty Vehicles Test Procedure (WLTP).

Thanks to highly efficient exhaust gas aftertreatment systems, current diesel engines can comply with the lowest NOx emissions. However, in very transient operating situations – for instance, during strong acceleration – a significant, short-term increase of NOx raw (engine-out) emissions can occur which to some extent reach the tailpipe. The reason for this is that the Exhaust Gas Recirculation (EGR) system and the turbocharger can only deliver the necessary EGR rate or the required charge pressure after a certain delay time (“turbo lag”).

Potential of Electric Components

Electrically driven components offer meaningful support; a belt-driven starter generator (BSG) can provide additional torque, allowing the engine to be operated in a lower load range. In addition, an e-Compressor can generate the desired boost pressure with virtually no delay and independently of EGR, thus bridging the “turbo lag” of the turbocharger. As an additional measure, the exhaust gas turbocharger can alternatively be optimized with regard to higher efficiency or higher rated power.

HECS ECObrid Concept

The HECS ECObrid test vehicle uses the third generation of the FEV-HECS – a 1.6 liter 4-cylinder engine with single-stage supercharging and combined high-pressure / low-pressure EGR. A 48V e-Compressor and a 48V BSG were also integrated. The complete electronic control system for the engine and the hybrid system are handled by a proprietary FEV model-based software, which was implemented on a dSpace MABX II Rapid Control Prototyping System (RCP).

The 48V e-Compressor is installed downstream of the water-cooled charge air cooler (WCAC) and the turbocharger, which has variable turbine geometry. This installation position leads to a reduced volumetric flow downstream of the e-Compressor, which improves transient behavior. The reduced flow also allows the selection of a smaller compressor with additional response time improvements. Intercooling also reduces the power requirements of the e-Compressor, since the compression takes place at a lower temperature level.

The test vehicle has a 12V/48V electrical system with 2 batteries. The 48V system consists of the BSG in “P0” position, a lithium-ion battery, and the e-Compressor. A bidirectional DC/DC converter is used to establish the connection to the 12V system, which was taken over from the production vehicle, which also powers the Diesel engine’s electric water pumps.

48V Components

The Valeo 48V BSG is based on a claw pole generator and is equipped with power electronics unit for boosting and motor control. The machine is air-cooled, which restricts the continuously available power to 4 kW, while a short-term peak power of up to 12 kW can be delivered. The BSG can deliver up to 160 Nm to the crankshaft via the belt gear ratio. The belt is tensioned for bidirectional power transmission with a double arm tensioner from Mubea.
The air-cooled bidirectional DC/DC converter has a nominal power of 2.7 kW. It handles the supply of the 12V electrical system and is therefore only operated in this case as a low-voltage regulator. The supply of the 48V electrical system is realized through a circuit in the 48V battery unit.

The 48V prototype battery from Voltabox is based on a NMC/LTO cell chemistry and has a nominal capacity of 20 Ah. This comparatively high capacity enables a high degree of freedom for demonstrator applications and, in conjunction with air cooling, enables peak currents of up to
15 C. The battery module consists of 20 cells connected in series, resulting in an operational voltage of 44 to 48V in the relevant SoC area (SoC = State of Charge).

The Valeo 48V e-Compressor is driven by a switched reluctance motor and can be supplied with power of up to 6.5 kW. The low mass inertia leads to a very short response time of less than 150 ms.


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Air path of the HECS ECObrid












Graphics - diesel hybrid engine

Powertrain of the FEV HECS ECObrid










Special Control Concept

The additional hybrid components of the HECS ECObrid require new control functions, which have been integrated in the software environment of the existing engine control unit.

The Powertrain (PT) area includes the various hybrid operating states and functions. These states can be used for any type of hybrid powertrain (Px, series, or power-split) and specify the high-priority driving modes (e.g. Hybrid or e-Drive) and the high-priority transient modes (for example, engine start or engine stop).

The software architecture includes functions for all common components of the 48V system, such as batteries, starter generators, e-Compressors, DC/DC converters, and their interfaces. Individual functions for component management, system coordination, and diagnostics are also included. The interfaces of the individual hardware components are transferred to universal and hardware-independent signals via input / output functions that are analogous to the basic functions for a diesel engine. This allows easy exchange of components. In the case of a missing software feature, it can be added within the software module of the respective hardware component.

Hybrid mode is the most important mode of operation for mild hybrid applications. In this mode, the energy management module (EgyMgt) defines the battery power split for the 48V BSG and e-Compressor. The EgyMgt collects all power requests and prioritizes them according to the operating mode. This is how, for example, the BSG is prioritized for the engine start and launch assist.

The torque and speed setpoints are calculated in the modules of the BSG and the E-compressor, considering charge/discharge current, the limitations of the 48V strategy module, and torque request from the driver of the vehicle.

Simulation Results

In order to demonstrate the potential of additional electrification to reduce NOx engine-out emissions and fuel consumption, simulations were carried out for example load steps, as well as for the future WLTP certification cycle.

In this manner, a distinction is made between boost pressure build-up with an unchanged setpoint value and a transient boost pressure increase (overboost). The boost pressure setpoint is achieved with virtually no delay thanks to the e-Compressor, whereby the EGR rate, which is calibrated for stationary operation, can be adjusted without an increase in soot emissions during the load step. For a short-term overboost, the EGR rate can be further increased, which leads to an additional soot-neutral reduction of NOx emissions.

Significantly Reduced CO2 Emissions

Overall, there is a CO2 reduction potential of around 11% in the WLTP. The majority, around 5%, is achieved via recuperation and electrical support of the BSG. However, through various effects, the e-Compressor also delivers an additional CO2 reduction of 2% when the required electrical power is fully provided by recuperation. To begin with, the E-compressor reduces the charge-exchange work due to the rapid pressure build-up, and secondly, the larger exhaust-gas turbocharger (ATL) can be designed for a higher efficiency and lower back-pressure. Additionally, the larger turbocharger also enables an increase in specific engine output and thus a downsizing strategy, which leads to an additional CO2 reduction of over 4%.

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Calculated CO2 reduction through 48V hybridization in WLTP









Graphics - diesel hybrid engine

Control concept for the e-Compressor











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