The hybrid development team at Siemens VDO has built a full hybrid vehicle as a demonstration of its Hybrid Tool Box—a portfolio of hybrid components including high- and low-voltage motor inverters, electric motors and vehicle control algorithms and integration capabilities.
Simulations on the prototype—based on a Mercedes C230 K Sport Coupé—in FTP driving modes show improvements in fuel economy of 30% to 38%, depending on driving strategy. The hybrid functionality includes stop/start, regenerative braking, pure electric drive, gear shifting optimized to electric drive and regeneration and operating mode shift.
The non-hybrid base vehicle—a C230 K Sport Coupé—features a 1.8-liter 4-cylinder gasoline engine with 141 kW (189 hp) of power and maximum torque of 260 Nm (192 lb-ft). Acceleration from 0 to 100 kph takes 8.1 seconds, fuel consumption is 9.2 liters/100 km (25.6 mpg US), and CO2 emissions are 219 g/km. The engine is mated with a 5-speed automatic transmission.
|The electric motors and the clutch. Click to enlarge.|
For the hybrid drivetrain, the Siemens team used a parallel architecture, with a primary motor, a clutch, and a second, smaller motor (Side-Mounted Starter Generator—SSG). The main motor connects directly to the input shaft of the automatic transmission and replaces the torque converter. The second electric machine is located in the belt drive, and is responsible for engine starts. Once the engine is synchronized to gearbox speed, the interconnecting clutch is closed.
The primary traction motor delivers peak power of 75 kW, with continuous torque of 97 Nm (71.5 lb-ft) over a wide speed range and peak torque of 276 Nm (203.6 lb-ft) for 30 seconds from 0 to 2,700 rpm.
The secondary motor also contributes to the operation of the hybrid vehicle when the battery is low and with high vehicle load at steep grades—conditions in which the electric clutch needs a lot of electric energy. The SSG can deliver 5 kW of power continuously.
The inverter electronics are one of the main focus points of the toolbox, according to the Siemens team. The prototype inverter in the Sport Coupé offers a voltage range of up to 450V, continuous phase current of 75°C. The inverter also houses the control board with the microcontroller. Monitoring, diagnostics and basic safety concept is included.
|Photograph of the battery pack. Click to enlarge.|
The Siemens team chose a nickel-cobalt-based lithium-ion battery with an average power density of 1.3kW/kg at the cell level. The cells have 6.8 Ah capacity, and a peak recharge power of 200A for 10 seconds.
The battery pack consists of 100 cells connected in series, and is 80 liters in size.
Siemens VDO terms its driving strategy for hybrids IPM (Integrated Powertrain Management). IPM comprises two main parts: Traction Energy Management (TREM) and Torque Management.
|Siemens VDO IPM.|
TREM receives the current battery status regarding state-of-charge (SOC) and state-of-health (SOH) from the battery management system. Depending on these characteristics the different driving modes are released or limited. TREM can deactivate the Boost function, for example, to prevent the battery from further discharge.
TREM sets charge and discharge limits depending on the current battery status. Those limits are specific for the type of energy storage.
Torque Management includes a driver and situation detection function, which includes a driver pedal interpretation for the accelerator and the braking pedal. The Operating State Control determines the different driving states: Launch, Pure Electric Driving, Boost, Regenerative Braking and several more.
A special challenge for the system is the transient behavior between different driving states. Dynamic transients must not cause shocks in the drivetrain and must not be recognized by the driver.
A typical driving cycle for the Siemens system is as follows:
After the power up the driver engages gearlever-position D and steps on the accelerator pedal. The Sport Coupé launches immediately in pure electric mode with the combustion engine declutched from the drivetrain.
During further acceleration the combustion engine is started by the Side-Mounted Starter Generator and synchronized to the transmission input speed. If the speed difference is minimized the interclutch is closed and the driving torque can be transferred from the combustion engine.
The electric machine is now either operated in generator mode to charge the battery or in motor-mode to support the acceleration. During braking phases the interclutch remains either closed or the clutch is opened and the combustion engine is shut off to optimize the potential for regenerative braking.
In cold-start situations, IPM can request gear neutral although gearlever-position D is selected by the driver. The interclutch is closed and the combustion engine is started by the motor. With the engine idling the interclutch is opened again, the gear is engaged and the vehicle is launched electrically.
If the high voltage battery is deeply discharged the drivetrain is operated as serial hybrid.
This powertrain configuration supports a high potential for fuel consumption reduction as well spontaneous and crisp driving behavior. One of the challenges is to design an electric drive system that performs in extreme situations like steep grades with multiple stop start sequences.
For the Sport Coupé Prototype we found a solution that promises to work well. The potential in fuel savings of 38% (simulated) is great and benefits from omitting the T/Q and the high efficiency of the electric drive including the battery.
The system can thus be transferred to other vehicle platforms like transversal engine, small, mid-size, or large sedans. Even SUVs are possible, the weight to torque ratio has to finely balanced and the serial hybrid mode needs to be adapted to cooling and battery capacity.—Mathias Deiml, et. al.
A Full Hybrid Vehicle with Parallel Hybrid Powertrain and Electric Clutch; Mathias Deiml, Martin Rampeltshammer, Matthias Toens, Ralf Kruse, Marko Turek; Siemens AG, Siemens VDO Automotive, Germany