Starter-alternators and 48 V electrical system: Untiring helpers

Oct 28, 2016

Systematic electrification of the high-tech engines allows them to achieve a new level of efficiency. Key technologies include the integration of alternator and starter into starter-alternators as well as a 48 V electrical system. This enables hybrid functions such as boost, energy recovery and start/stop. The more powerful electrical system also allows the use of new technologies, such as electric auxiliary compressor (eZV) as well as the electrification of key components, such as the air-conditioning compressor.

The Integrated Starter-Alternator (ISG) combines starter and alternator in one powerful electric motor that is installed between engine and transmission and is also used for cold starting. The integrated electric motor assists the engine, for example during acceleration, and supplies the battery with power by means of high-efficiency energy recovery. Especially in combination with a 48 V electrical system, the necessary adaptations of the powertrain architecture pay long-term dividends in the form of higher fuel savings.

In addition, such a starter concept is so durable that the engine can be switched off much more often and whenever it is not needed: whether during decelerating or coasting, i.e. when rolling to save fuel at higher speeds, as soon as the driver lifts off the accelerator. And this is done with maximum comfort, because a 48 V system with starter-alternator is able to bring an engine to idle speed very smoothly. This benefits NVH (Noise, Vibration, Harshness), with the transition when the engine starts being made even less perceptible: the engine simply runs again as if it had never been off.

The benefits at a glance:

  • Boost: 10 kW (14 hp) to 15 kW (20 hp) extra output
  • Energy recovery: recovery of over 80% of the braking energy
  • CO2 emissions: reduced by 7 to 12 g/km
  • Significantly improved starting comfort
  • Coasting function
  • Allows the load point to be shifted to a more fuel-efficient area of the engine map

Many of the benefits of an ISG are also offered by the belt-driven starter-alternator (BSA). The system uses existing alternator mounts and thus does not affect the design of the powertrain.

48 V electrical system: a host of fuel-economy and comfort advantages

The introduction of the new engine generation blurs the lines between petrol and hybrid models. Looking to the future, all Mercedes-Benz passenger cars will be electrified, because the company is at the same time systematically advancing the development of the 48 V electrical system. The technology will be gradually rolled out in various model series, initially as a sub-system for the functions inside the engine compartment.

The 48 V electrical system supplies four times the power of its 12 V predecessor at the same current, but does not require the additional safety architecture of a high-voltage system. This allows fuel savings that were previously the exclusive domain of high-voltage hybrid technology, for the key hybrid functions of "energy recovery" and "boost" can thus be made available without high-voltage components. The engineers also enjoy new freedoms with regard to the design of the operating strategies for the high-tech engine, with the result that it can be operated even more frequently in an area of the map that is favourable in terms of fuel consumption.

There are also entirely new technologies, such as the electric auxiliary compressor (eZV), which is celebrating its premiere as part of the intelligent turbocharging of the new six-cylinder in-line petrol engine (M 256). This compressor delivers charge-air pressure in fractions of a second, independently of engine speed and engine load. Connected in tandem with a large exhaust turbocharger, the eZV ensures a balanced engine response in all rpm ranges and eliminates the turbo lag.

The integration of a 48 V electrical system also offers advantages for in-vehicle consumers, such as air conditioning, electric heating elements or extractor fan - these, too, could be operated at a higher voltage in future. The advantage is that the same power requires only a quarter of the current of a conventional system. This means that the wiring can be thinner and therefore lighter, which indirectly contributes to saving fuel. The 48 V electrical system also paves the way for the further expansion of infotainment and assistance systems.