Higher voltage to optimize start-stop savings

It is expected that the number of vehicles powered by internal combustion engines will continue to rise worldwide despite the continued electrification of vehicle drives – unless a new totally disruptive technology is introduced. Full hybrids and all-electric vehicles call for a considerable technical outlay, and for this reason, car manufacturers’ highest priority is fuel economy in conventional vehicles.

Automatic start-stop systems can achieve an efficient and cost-effective reduction in fuel consumption using relatively few technical changes, especially in the city. An engine does not need to be firing when coasting downhill or slowing down. But, at the same time a reliable electrical power supply to the vehicle has to be maintained because modern safety-critical systems such as power steering and ABS are powered electrically.

A single conventional 12V lead-acid battery simply cannot manage that. A 48V/12V dual voltage powernet is one solution for the next generation of micro hybrid vehicles with coasting function. HELLA has developed a DC/DC converter for 48V/12V, which ensures this power supply throughout the switch-off periods.

Automotive Industries (AI) asked Dr. Carsten Hoff, head of the product segment Energy Management at HELLA, what savings are offered by the next generation of micro hybrids?

Hoff: If a coasting functionality is also included in the system then we can reach savings of up to 1 litre/100km. Switching off the combustion engine whenever no propulsion is needed can save up to 10% of the fuel consumed by a conventionally powered vehicle, depending on the specific driving cycle. Further potential savings are obtained by optimization of regenerative braking by using a lithium-ion battery. We have tested prototype vehicles in typical driving cycles.

AI: What has changed in the vehicle functionality?

Hoff: The start-stop mode is greatly extended, as the engine switches off as soon as the vehicle slows down when approaching a red light, greatly increasing the length of time that the engine is at rest. The core feature of the next generation of micro hybrids will be the coasting function. As soon as the driver’s foot is taken off the gas pedal the vehicle switches the engine off. Equipment such as steering, air conditioning and lights then have to be supplied with reliable electrical power without running the generator. The second step towards optimized fuel efficiency is regenerative braking, which will offer significantly more savings, because we can now use energy storage equipment with a high charge acceptance, such as lithium-ion batteries and super capacitors. A third option is the addition of the creeping functionality.

In the fully configured next generation micro hybrid, cars start driving electrically supplied from the 48V battery. This technology’s potential CO2 saving is considerable, and integrating it would be significantly cheaper than hybrids and fully electric vehicles.

AI: What new components are installed in the vehicle?

Hoff: Newly introduced possibilities include an integrated starter-generator, a 48V lithium-ion storage battery and a bi-directional DC/DC converter with a continuous output of up to 3kW. This means that our converters are scalable for all vehicles in the performance range 1kW to 3kW. This key feature allows for power electronics to be able to convert between 12V and 48V. 48V architecture for example, enables the EPS system to be moved from a 12V to a 48V power system, the functionality will be the same but the efficiency will be improved with high voltage level and this also means less power loss at the alternator.

AI: What are the advantages of the 48V electrical system?

Hoff: Manufacturers can equip their vehicles with new efficiency technologies that offer excellent cost-benefit ratios and go way beyond simple start-stop functions, such as coasting or creeping. Consumers not only achieve fuel economies of up to 15%, but the coming conversion to high-power loads based on 48V also offers entirely new convenience functions. Manufacturers in the premium segment in particular are struggling with new features in the vehicle. The largest alternator you can buy is in the range of 3.0 kW. Using existing technology it is not possible to increase the alternator power to more than 3kW. One of the core problems of the large vehicles is that once you have a nice idea for a new feature you will struggle to power it. By increasing the voltage you can easily achieve four times the energy. When you have four times higher voltage for the same energy then you have four times lower current which means the diameter the wires is reduced and you have less weight on the wiring harness.

AI: Which components are operated at 48V?

Hoff: Primarily it is higher-powered equipment that is supplied with 48V: windshield heating, active suspension systems, and aircon compressors. It doesn’t make sense to supply components with 48V that are using only limited power. A lot of CPUs run on 5V or 3V, so it doesn’t make sense at all to supply 48V. I would assume that the OEMs will start with a very limited number of devices on the 48V side.

AI: Why is the introduction of 48V so much more successful than 42V was 10 years ago?

Hoff: There are a lot of things that indicate that 48V will be a success. The first is the CO2 targets. Ten years ago everybody was aware that there was going to be fuel efficiency targets, but there were no penalties. Carmakers now need new features to bring fuel consumption down or face penalties – and 48V is one of these features. Number two is the storage technology. A 42V system required three lead- acid batteries. Just imagine the size and the weight. Now we have the lithium-ion technology. On the power electronics side there are a lot of new developments for the semiconductors available.

AI: What is HELLAs contribution to next-generation micro hybrids?

Hoff: Alongside our battery sensors for conventional batteries, energy storage modules and the associated control and power electronics we are focusing on DC/DC convertors to power 12V electrical systems from the 48V network. It is one of the core components. This is a business opportunity for HELLA. I believe that all the companies that are entering the lithium-ion battery world will also supply 48V battery. The efficiency of the DC/DC convertor is really crucial because of the energy loses. To control costs you need scalable solution for all vehicles. We aim to be one of the first suppliers offering high efficiency DC/DC convertors for 48V high power systems.