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Mercedes-Benz 48V system debuts in new S-Class

Mercedes-Benz’ new S-Class will enter the market this month with an all-new engine range (diesel and gasoline variants of 6-cylinder in-line engines and a new V8 biturbo gasoline engine) with a series of new technologies for electrification of the powertrain (earlier post), including a 48V system (S 450 and S 500 models) and Integrated Starter Generator (ISG) for mild hybrid functionality, an electric booster compressor, and an upgraded plug-in hybrid model.

The 48 volt system establishes a third on-board power network. It fills the gap between the 12 volt network already established for decades and the high-voltage network required by electric vehicles. The latter makes special safety precautions necessary, can therefore only be used to good effect for a small number of components and is limited to driving functions. As the power requirement increases, the limited voltage of the conventional 12 volt network leads to a constant increase in current levels, and therefore to supply lines with large cross-sections. It is however not being replaced, but rather supplemented by the 48 volt network.

The voltage is directly derived from the legally prescribed contact protection maximum of 60 volts, and can therefore be used without the precautions for a high-voltage on-board network. At the same time the voltage which is four times that of a 12 volt on-board network results in four times the output available. With its lithium-ion battery, the 48 volt network allows outputs of up to 16 kW and thereby considerably extends the range of applications for electrical consumers.

The power for the 48 volt network is generated by the ISG. Located between the engine and the transmission, this electrical machine combines the functions of a starter and alternator. The conventional 12 volt network is likewise supplied from the new network using a 48 volt/12 volt DC/DC converter. As there are high demands on the 12 volt on-board network in terms of system failure, a 12 volt battery continues to be used but in a smaller version than previously. The overall on-board battery capacity is increased by the 48 volt battery, allowing more electrical energy to be provided for innovative functions when stationary, e.g. in climate control.

Functions and components of the 48 volt network and the Integrated Starter Generator include:

  • Boost function: for a short period, the generator is able to support the internal combustion engine with up to 250 N·m of torque and 16 kW of output.

  • Recuperation: when decelerating, the kinetic energy is converted into electric power and used to recharge the battery.

  • Gliding mode: when the driving situation and charge level of the battery allow, the combustion engine is decoupled from the powertrain and switched off, so that the vehicle can coast freely.

  • Engine start/stop: A start at high engine speed is possible, for example (e.g. after a gliding phase: here the engine is immediately brought up to a speed appropriate to the driving situation by the starter generator) as is compensation of the alternating torques during the starting phase. During auto stop, the crankshaft is brought into the optimum position for the next comfortable restart of the combustion engine.

  • Stop/start system 2.0: when coming to a stop, the engine is already switched off while still on the move, i.e. during the stopping procedure, to save fuel. This is done according to the situation, e.g. on the basis of the driving profile to date, information from the radar sensors and other vehicle data: This avoids short stops of under two seconds duration. This is because when starting after a short stop, more fuel is needed than was saved during the stop. Short stop situations include e.g. joining a traffic light queue that is already starting to move, parking, maneuvering and driving in a tailback. Given an adequate downhill gradient, the engine start is suppressed when releasing the brakes to roll forward, so that it is possible to close up in a tailback without the engine running, i.e. with no fuel consumption. As usual the driver retains full control: the engine starts as soon as the driver presses the accelerator.

  • Shifting of the load point: the electric torque of the starter generator is regulated so that the combustion engine can operate at the most efficient load point.Electric water pump: doubling the maximum pump output (approx. 950 watts rather than 400 watts) increases the cooling performance. This allows a higher engine power density. It is often also possible to save energy by situation-related actuation of the pump irrespective of engine speed.

  • Electric auxiliary compressor: this ensures almost instant compression of the intake air when additional power is required, before the turbocharger responds. Turbo-lag is completely eliminated in this way.

  • Idling rpm control: for more efficiency and comfort, idling rpm control of the combustion engine is carried out by the electric motor. To this end the electric motor acts as a generator which regulates the charge current so that mechanical vibrations from the combustion engine are absorbed. This allows an energy-efficient idling speed of 520 rpm to be precisely and comfortably maintained.

  • Electric refrigerant compressor in the air conditioning system. This makes climate control independent of the engine speed. It also works in gliding mode and when stationary with the engine switched off (auto-stop or before setting off).

The electrification of these assemblies and integration of the generator into the powertrain also eliminates the conventional belt drives on the front face of the engine. This not only makes the engine shorter, but also lowers the noise level while making for improved reliability.

The new, systematically electrified in-line six-cylinder engines in the new S-Class initially come in two output levels. In the Mercedes-Benz S 450 (also as a 4MATIC model) it generates 270 kW (367 hp) and 500 N·m of torque. (combined fuel consumption: 6.6 l/100 km/ 35.8 mpg US; combined CO2 emissions: 150 g/km). The S 500 has an output of 320 kW (435 hp) and 520 Nm (combined fuel consumption: 6.6 l/100 km; combined CO2 emissions: 150 g/km). Over a short period, the 48V Integrated Starter Generator (ISG) makes a further 250 N·m of torque and 16 kW of output available. Compared to the similarly powerful S 500 predecessor with a V8 engine, Mercedes-Benz reduced the CO2 emissions of the engine by around 22%.

New, intelligent forced induction that includes an electric booster compressor as well as the ISG deliver power without turbo lag.



Looking at these particular points:

Boost function: for a short period, the generator is able to support the internal combustion engine with up to 250 N·m of torque and 16 kW of output.

Electric auxiliary compressor: this ensures almost instant compression of the intake air when additional power is required, before the turbocharger responds. Turbo-lag is completely eliminated in this way.

Idling rpm control: for more efficiency and comfort, idling rpm control of the combustion engine is carried out by the electric motor. To this end the electric motor acts as a generator which regulates the charge current so that mechanical vibrations from the combustion engine are absorbed.

In short, this gives the vehicle a quality of smoothness and responsiveness which rivals electric drivetrains.

EVs have permanently raised the bar.


Is it an EV? It is certainly a hybrid.
If you were to say that increased electrification has raised the bar, I would agree wholeheartedly.
The problem that I see is complexity and cost. This is fine in a Merc S class, but will it scale back to a Ford Focus - I hope so.
IMO, some kind of hybrid is the way forward as then you can size the battery to the median drive distance, rather than some unattainable maximum (say 12KwH instead of 60+), which ends up looking like a PHEV.
The only problem then is cost and gasoline going "stale".


Things like the idle-torque compensation are firmware, which ought to be transferable to any unit with a crankshaft-mounted ISG (not doable with BAS because of the play in the belt tensioner).  The boost function and electric aux compressor should transfer to anything, BAS or otherwise.

Thomas Pedersen

What is the maximum regen power? I suppose it is also 16 kW, limited by the 333 A running through the wires and power converter..?

It may take a few years but I definitely think this technology will percolate down to the Ford Focus class vehicles. Probably as an optional extra because these features appear to increase comfort almost as much as they improve fuel efficiency.

With time, this technology would ideally motivate development of much less sophisticated ICEs with higher peak efficiency and lower component count (no more cylinder de-activation, VTG, etc.) and thus reduce the premium of a fuel-saving 48V system.


The peak regen power may be higher, as the start/assist power is limited by the battery voltage and the regen power is not so limited at higher shaft speeds.  Whether this matters in any significant way I can't say, and certainly depends on the specific details of the system as designed and built.


It may be about time to develop a new specification for low voltage (48 V) system in cars. The system that uses both +48 Volt and -48 Volt. The idea is to have 2 identical blocks of 48 Volt battery, spatially separated, connected to chassis with mid point, the 2 battery modules are connected in series, providing 96 Volt that can be used for drive motor(s).
As the module providing 48V will be loaded more heavily (for 12V and 48V accessories), there will be need for one isolated DC-DC converter that takes power from negative battery module and charges positive battery module.
Drive motors that offer boost and regen are driven by inverters connected to 96V battery, and they charge both modules in series (96V).
Schaeffler 48V electric axle develops 20 kW with apparently one e-motor.

With 96V two e-motors could produce 2 x 40 = 80 kW which is sufficient for a small or mid size PHEV. With 3 such motors, it can be a BEV.
Expensive high voltage protection measures would only be applied to inverter-motor system, they are always one beside the other.

Re: Idling rpm control - Honda did something similar long ago in their Insight IMA hybrid. I read somewhere they used e-motor (P1, IMA configuration) to balance their 3-cyl engine.


IIUC the voltage limits are not conductor-to-ground, they're anything-to-anything so somebody can't grab the wrong two wires and get hurt.  You still have the 48V (nominal) limit so as not to exceed 60 V in operation.


Voltage limit rules "anything-to-anything" already don't apply to engine block heaters that have been used for decades in Canada. That option is available on many cars, people plug it into wall socket with AC 120V, 60 Hz.
The industrial rules for low voltage 60V could be relaxed for cars. They can be defined for +/-48V as anything-to-anything within 10 or 12 inches. Also a mechanical switch may be required at the battery connection for -48V, that is turned on only when 'key' is inserted, turned off when key is out, or airbag sensor detects crash.
It is already environment where hundreds and thousands lose their lives because of speed and stupidity.
Far more dangerous thing is to allow self driving cars when a malfunctioning sensor or software bug can kill people. No double or triple redundancy is required for self-driving cars, as it is a required in civil aviation, at least I haven't read about it. It would make such cars too expensive.


There may be exemptions for block heaters because they are de-energized when the vehicle isn't plugged in; the vehicle battery is on all the time unless there's a disconnector.


Triple redundancy of many/most ADV components and subsystems may become a necessity to raise ADVs safety to the level required. Cost is coming down fast and will become affordable for expensive cars (above $60K) by 2020/2025.

Electrified vehicles will not real need 48V batteries. DC/DC step down invertors will safely supply 12/48 Volts from the main battery bank as and where required.

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