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Powertrains are growing increasingly diverse. Hybrids already belong to the mainstream and are increasingly sophisticated, while battery-driven electric mobility is picking up speed. This makes it all the more important to have a holistic overview of all topics and how they overlap. IAV uses a consistent tool chain for concepts and developments to optimize individual components as well as complete powertrains and fleets.

At the moment, e-vehicles are still an exotic species on our streets, but that will soon change. Following the trendsetters such as Nissan or Tesla, now the established OEMs are also putting models with electric drive on the roads. In addition, many new manufacturers are trying to take advantage of the currently favorable situation and get a foothold on the rapidly growing market. At the same time, the next generation of the first electric drive systems is emerging, with manufacturers aiming primarily to optimize costs, efficiency, NVH behavior and industrial feasibility. Corresponding modular systems will soon offer complete families of electric drives with e-motors in differing power categories. Here new functions such as torque vectoring and multi-speed transmission will play a major role.

At the moment, many of the current vehicles have a mechanically simple transmission with just one speed. “But the demands for torque and performance are meanwhile so high that it’s becoming indispensable to have several speeds, particularly in large vehicles”, says Jens Liebold, technical consultant at IAV. “As a general rule, you can say that the larger the vehicle mass, the greater the advantages of multi-speed transmission.” In these vehicles, such as full size electric SUVs with a high maximum vehicle speed, a high output flows through the powertrain so that a transmission with several speeds can clearly reduce losses by keeping the motor as close as possible to its ideal operating point.

Two, three or four speeds?

The demands made during start-up and when cruising at high engine speed can be worlds apart. The IAV experts therefore expect that in future, multi-speed transmissions will be the standard at least in some models. “One speed may just about suffice up to the C-segment, but two or three speeds are needed as soon as you go beyond that point”, says Dr. Klaus von Rüden, head of department at IAV. “I expect that large vehicles in particular will have two speeds. While a third speed has no more to offer in terms of range and performance, it can make a difference to driving comfort.” On the commercial vehicles sector, IAV expects to see transmissions with three to four speeds to provide the far greater torque and performance needed in these vehicles due to their high inertia. “A multi-speed transmission lets you downsize the electric motor”, explains Liebold, “with a trade-off in the costs between motor, transmission and battery.”

IAV is familiar with the topic from many real projects and provides customers with support from the initial concept (how many speeds are necessary?) through to the SOP. IAV Powertrain Synthesis has proven to be an important tool. It works on the basis of customer specifications, such as range, performance or costs, to produce an objective recommendation for the entire fleet. It takes account of aspects such as new developments in power electronics, including parts made of SiC and GaN compound semi-conductors, for example. The engineers always have an eye on comfort aspects as well: “In electric vehicles, anything that’s not running smoothly is noticed straight away”, reports von Rüden. “But the customer must never be aware of this, so that the comfort requirements are higher than in vehicles with a combustion engine.”

The future belongs to DHTs

The revolution in powertrains is also triggering an increase in hybrid vehicles. Automatic transmissions have always been widespread in Europe and the USA. In recent years, manufacturers have taken many aspects of exist- ing transmissions as the basis for deriving hybrid variants. China is ahead in this field. The country has more e-vehicles and manufacturers already started developing dedicated hybrid transmissions (DHT) at an earlier point in time. “We are now seeing more of this development in Europe and the USA. After all, the global trend is clearly towards DHT”, says Dr. Jörg Müller, head of department at IAV.

Dedicated hybrid transmissions offer a whole number of advantages. The output of the electric motor makes it possible to dispense with gears, managing with just four to six shift stages. New functions are also possible, such as fully variable coasting without combustion engine, purely electric start-up or purely electric reversing. However, detailed analysis is necessary to cope with the large choice of different topologies (parallel, serial, combined, power- split). Furthermore, the question arises as to how many electric motors are appropriate. And finally, a choice has to be made between different transmission technologies and actuation units (hydraulic, electric or pneumatic in commercial vehicles).

“Hybrid transmissions are far more complex than purely electric drives”, explains Müller. “You have to find a balance between the mechanical and electric effort, while also keeping an eye on costs, consumption levels and customer requirements.” Modular systems with many carry-over components are sure to catch on. IAV uses a specialized tool in this context: IAV Transmission Synthesis produces the best solution in objective terms under the prevailing general conditions, based on millions of variants.

Coherent driveability analysis, validation and documentation toolchain

Global leader for tools

IAV advises its customers and helps them to manage the growing complexity of powertrains. Here the experts use a tool chain that incorporates decades of experience and that has proven its worth in many customer projects. IAVPowertrain Synthesis begins by analyzing millions of different options, then uses the results to produce the drive configuration that is objectively speaking the best match for the customer specifications. Furthermore, a special modular tool can be used to examine combinations of drives for different vehicles and markets, resulting in an optimized modular set of drives for an entire fleet.

The special tools are then applied to the individual components: synthesis tools for combustion engine, electric motor and transmission. “We always trawl through the complete solution space to find the best powertrain”, says Dr. Christoph Danzer, team leader at IAV. “In doing so, we take due account of different layout and technology combinations of components and thus calculate the characteristics in various drive cycles such as the WLTP. At the same time, we keep our eyes on costs, performance and modularity of the drive solutions.”

The final product consists in objectively assessable solutions and a high degree of transparency when it comes to requirements, technology and costs. “In the past, the individual components were each optimized on their own. But today, the strict legal stipulations mean that we have to view the whole system. We lead the world market in this field with corresponding successful achievements to our name”, says Danzer.


The article was published in automotion 02/2019, the automotive engineering magazine of IAV. Here you can order the autmotion free of charge.