Hybrid Transmissions Two to tango
Volume production of hybrid cars is still relatively new. Developers hope that by combining combustion engines with electric motors, they can meet ever more stringent regulatory emissions targets. Once again, ZF offers suitable solutions for the effective reduction of emissions.
By 2021 at the latest, a new EU regulation will come into force. It will impose an average emission limit of just 95 grams per kilometer on manufacturers’ car fleets. Similar regulations are also being enacted outside the EU. But experts agree that there is no way all-electric solutions can be implemented fast or comprehensively enough to meet these new targets. Electric cars are still having a hard time matching the costs, range and convenience offered by fossil fuel-powered vehicles and expected by customers. Consequently, they are not going to conquer our streets, vehicle fleets and home garages overnight – not until they become the single most obvious answer to all personal mobility issues. Experts believe all-electric automobiles will only start to establish themselves worldwide starting in 2020 at the very earliest.
So what should we do while powerful engines are still required in certain segments – engines incapable of meeting the new emission limits? “Hybridization is and remains a key technology,” confirms Dr. Ralf Kubalczyk, Head of Hybrid Transmission Series at ZF. “That’s why, as one of the European pioneers of this technology, we’re already carrying everything in our portfolio that can help combustion engines save fuel, from mild-hybrid systems to plug-in hybrid systems, plus all the necessary power electronics. And we’re continuing to work on hybrid innovations alongside
our all-electric drives and drivelines.”
Cutting consumption with electric power
So hybridization is clearly the right tool for making the transition to pure e-mobility. Cars in which a combustion-powered driveline is supplemented by an electric motor are known as Hybrid Electric Vehicles, or HEVs for short. ZF offers the entire range of products required to build and optimize the many different types of hybrid solutions.
HEVs can be categorized according to how much power the electric motor contributes to the combustion engine. At the bottom of the performance spectrum is the “mild-hybrid” system. This is based on a low-voltage power supply of, for example, 48 volts. More and more cars are being wired to support this standard in any case, because of the growing number of power-hungry electrical devices they now carry; 48 volts simply makes common sense. This is why entry-level hybrid variants based on this standard are considered very inexpensive and yet effective. An electric motor that outputs 10-15 kilowatts supplements the combustion engine by adding features such as coasting, boosting, energy recuperation, electric starting and “creeping”, as well as stop-start functionality. So even the “mild” type of hybrid can reduce fuel consumption by 10-15 percent.
Range of up to 30 miles
Driving longer distances powered purely by electricity is only really possible with a high-voltage, full-hybrid system – and in particular, the latest development of this concept, the Plug-in Hybrid Electric Vehicle (PHEV). A PHEV can be recharged either as it is driving along, by the combustion engine, or once it is parked, by plugging it into a power outlet. It is perfectly realistic for commuters driving PHEVs to make their daily journeys to and from work on electric power alone. PHEVs fitted with ZF plug-in hybrid transmissions – the electric motor is installed in the transmission’s bell housing to save space – are capable of driving at speeds of 80 mph or more without producing any local emissions. If the potential efficiency of the combustion engine-electric motor combination is exploited to the full, fuel consumption over short distances drops by 100 percent in ideal cases.
Future evolution of Plug-In hybrids
Ultimately, it’s not just about augmenting combustion engines with electric drives. It’s about integrating both power units as intelligently and efficiently as possible so they form a seamless, all-in-one system. “In any case, integrated electronics plus a smart drive management system embedded in driver-assist functions are important prerequisites for making further progress,” explains Harald Deiss, Head of ZF’s Electronic Systems business unit. This includes drivelines that are capable of recognizing the route in advance and adjusting the vehicle’s speed as it approaches bends – without activating the standard service brakes. Another example is the interaction between the electric motor – capable of switching over to generator mode to slow the car while simultaneously enerating electricity – and the car’s service brakes. To ensure that the transition from one to the other is smooth and efficient, you need a smart brake management system. This is known as “brake blending”.
But there is a further stage of development, in which both the combustion engine and electric motor in the hybrid driveline become steadily more specialized. Meaning that in the future, the dimensions and operating ranges of both drive units will be optimized to deliver precisely the system performance required, while minimizing their consumption of energy and fuel. ZF is also working hard on reducing their installation footprint. “Among other things, we’re working to package all electrical components in the transmission so they’re as space-efficient as possible,” is how Ralf Kubalczyk describes one of the company’s current development projects.