Under the microscope: 48 volt system: More power, performance, flexibility, comfort and efficiency

Jul 19, 2017

The 48 volt on-board power network also celebrates its world premiere with the introduction of the Integrated Starter Generator in the S 450 and S 500 models of the new S‑Class. It offers four times the power of a 12 volt system at identical currents, and makes fuel savings possible.

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. Thanks to the use of a 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.

The innovative functions and components of the 48 volt network and the Integrated Starter Generator include:

  • Hybrid functions such as the
    • Boost function: for a short period, the generator is able to support the internal combustion engine with up to 250 Nm 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 (see "ECO Assist" chapter).
  • Engine start/stop: the engine start is scarcely perceptible. 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, manoeuvring 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.