Design of the Daimler S400 Mild Hybrid System
11 June 2009
|Hybrid concept for the S 400 hybrid. Source: Daimler. Click to enlarge.|
Daimler’s S400 BlueHYBRID, which goes on sale this month, is the first series production passenger car featuring a traction lithium-ion battery pack. (Earlier post.) At the Advanced Automotive Battery Conference 2009 in Long Beach, Dr. Uwe Wiedemann from Mercedes-Benz Cars/Development, Hybrid Systems & Components, presented an overview of the design of the system and components, with a particular emphasis on managing the small 0.8 kWh Li-ion pack.
The S400 Hybrid uses a parallel mild hybrid system with a small 15 kW e-motor connected to the crankshaft between the motor and transmission, along with a Lithium-ion battery pack for energy storage, power electronics and a complement of electrified auximilary systems. The system offers start/stop functions, regnerative braking and electrical drive support.
Although the motor seems small from a power output perspective, “for the given architecture it turned out that the maximum fuel reduction occurred at electrification of 15-16 kW. That’s why we chose a lower system,” Wiedemann said.
|Combined torque curve for the S400 hybrid. Click to enlarge.|
The high torque of the e-motor at low speeds offsets the reduction in low-end torque resulting from applying the Atkinson cycle to the combustion engine.
Moving to a more powerful e-motor increased the weight of the hybrid system and decreased the fuel consumption. Furthermore, at a higher electrical to combustion power ratio, the e-motor operates increasingly in less favorable areas of the performance map as maximum requirements increase. Although relatively low in power, the e-motor delivers rated torque of 160 N·m (118 lb-ft), contributing to a combined system torque of 385 N·m (284 lb-ft).
Power electronics. The power electronics—from Continental—comprise a control unit which acts as the master of the E-drive system and a power unit that converts the direct current generated by the battery. The power electronics can cope with continuous currents of 150A, and short-term as high as 310A. Power is supplied to the e-motor by a bus bar.
The power electronics are situated in the engine compartment in the location of the conventional starter motor and are cooled by a separate circuit.
|The S400 hybrid battery pack. Click to enlarge.|
Lithium-ion battery pack. The compact Li-ion pack, developed by Continental and JCS Saft, comprises 35 cells and provides 19 kW of power, with a capacity of 6.5 Ah. The battery is connected to the vehicle air conditioning circuit so it can be cooled independent from the engine.
Cut-off valves are integrated into the system that allows the customer to switchoff the air conditioning without interrupting battery cooling. When the engine is not running, the electric A/C compressor not only provides air conditioning but also guarantees that the battery’s operating temperature limits are not exceeded. Battery pack temperatures do not increased above 50 °C in any operating state to prevent serious damage.
Operating strategy. The operating strategy of the S400 hybrid is based around start/stop, regenerative braking, boost and load point shifting—i.e., moving the load point of the combustion engine to less consumption-intensive ranges while offsetting the torque deficit with the e-motor.
When providing support for load-point shifting, the operating strategy only allows shallow discharge cycle of the Li-ion battery to maintain the cycle strength. Fuller deployment of the electrical support is only provided based on the driver’s request, as indicated by accelerator pedal position and a large pedal value gradient.
The focus of state of charge (SOC) swings is in the range of 5%. Values of up to 10% occur less frequently, while SOC cycle of more than 10% are rarely observed. This contributes to the 10-year expected service life.
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