All Charged Up – What difference does a 48V electrical system make?

Article Graeme Morpeth

Continuing our discussion of the evolution of automotive electrical systems that began in the November-December 2017 issue of The Star: What’s the next step? The answer, emerging from research and development into hybrid and fully electric vehicles, is the new 48-volt direct-current electrical system used for the first time in the United States on the 2019 CLS. So what, you say? Isn’t it just more of the same? Well, yes and no, but the implications are far-reaching, dramatically changing the design of the engine system.

To start, let’s dive into some simple electrical engineering, looking specifically at the component used to start the engine. A traditional starter motor may draw 200 amps to start a large engine from cold: given a 12-volt battery supply, that would rate the motor at 2,400 watts. Suppose, however, that the electrical supply were 48 volts (48V): The current required to drive the same motor would be reduced to only 50 amps (50A).

The important consequence here is the reduction in the size of the cables required, and by implication, their cost. For comparison, for a 36-inch run of cable, at 12V and 200A, a 50mm2 cross-section copper cable would be required. In contrast, at 48V and 50A, the cable size required would be only 6mm2 – a significant weight and cost reduction.

The use of 48V systems frees engineers from a number of other constraints imposed by the lowly 12V system. For instance, the stop-start system on current M-Bs uses a slightly larger-than-standard 12-volt battery, but still this system can cause “stoplight-anxiety” – that slight pause between flexing one’s right foot and the engine starting. Benz’s 48-volt stop-start system uses a different set of components to achieve the same result, without the delay.

The starter motor is replaced with a 48-volt device called a Integrated Starter Generator (ISG). This component, in essence, is a combined alternator and motor unit – often used on high-powered motorcycles – built into the flywheel of the new Mercedes-Benz engines. When not being used to start the engine, the ISG recharges a 48-volt lithium-ion battery pack, typically located in the trunk or under the floor, to lower the center of gravity and improve the front and rear weight balance of the vehicle.

A DC-to-DC converter is used to convert the 48V DC power supply to high-frequency AC; a small, light and low-cost (due to the high frequency) transformer is used to change the voltage back to 12V AC, which is then rectified to 12V DC for use in lighting and control circuits.

This mild hybrid power train works in parallel with the combustion engine, costs about 70 percent less than a full hybrid system and increases fuel economy by 15 to 20 percent.

There is more, though, much more to these new systems. Electrically driven accessories working on a demand-only basis have been developed to replace belt-driven components: Electric power-steering systems have been used for some time; air-conditioning compressors and water pumps can also be driven electrically.

There are (at least) three obvious advantages with this switch from belt-driven to electrically powered ancillary systems: Firstly, the system is more efficient because there is no parasitic energy loss due to the belt drives, which rely on tension and friction to transfer power. Secondly, since the components are driven from the battery, they don’t need to shut down during stop-start operation at stoplights, meaning engine-cooling and air-conditioning systems remain in operation. Thirdly, the engine won’t be subject to mechanical failure of key systems, such as cooling, due to belt failure.

The biggest gains, though, would come from using electrically driven superchargers instead of exhaust-driven turbochargers, eliminating the bugbear in these systems – “turbo lag” – by keeping the compressor spinning when it would otherwise be off-boost (thus allowing it to remain on-boost). Though this approach is not used in the first-generation 48V system found in the 2019 CLS and other models, these engines do use an electrically driven compressor/supercharger to maintain boost pressure until the exhaust-driven turbochargers spool up to operating speed.

The implications for increases in thermal efficiencies for such systems are startling. The current Mercedes WH50 F1 engine, a 90 cubic-inch turbocharged V-6 rated at more than 900 brake horsepower, uses electrically driven accessories and a partial hybrid system and can power the 1,548-pound Mercedes F1 car to 62 mph in approximately 2 seconds, achieve a top speed of up to 210 mph and, on faster circuits, achieve average speeds of 150 mph for up to 200 miles without refueling. This engine was tested recently at the Mercedes F1 Technical Center in the U.K. and achieved a thermal efficiency in excess of 50 percent. To put this into some sort of context, thermal efficiency on most modern road cars hovers between 25 percent and 39 percent.

Clearly, we’ve come a long way from the day when Gottlieb Daimler and Carl Benz decided that using electricity would be a major improvement on relying on a dangerous and temperamental  open flame to ignite the fuel-air mixture in the cylinders of their first automobile engines.

Shown above is the new generation Mercedes-Benz 48-volt inline-6 cylinder engine, looking at the engine from the front. Not shown at the rear of the engine is the Integrated Starter Generator (ISG) – combined starter/alternator, and the electrically driven supercharger. Electrically driven water pump, air conditioning compressor are integrated into the engine, rather then being belt-driven on traditional engines.