Systematically Developing the “Powertrain 2025”

An example vehicle fleet shows how a CO2 emission target of 75 grams per kilometer can be reached

Tighter CO2 limits will be in force from 2020. A further reduction of 15 percent is being discussed for 2025, and even of 30 percent by 2030. The spectrum of powertrain types then needed will remain broad, and the manufacturers are planning to launch further vehicle derivatives. And, ultimately, there are widely differing legislative requirements to be met on the global markets. This is why a method must be found that provides a key to meeting all of the boundary conditions in a technically and economically meaningful way. Using an example vehicle fleet, IAV shows how OEMs can systematically develop the “Powertrain 2025” tailored to their requirements.

The ambitious CO2 emission target of 75 grams per kilometer cannot be achieved with combustion engines alone. On the other hand, it is not possible to say when e-vehicles will offer a comprehensive solution to low CO2 emissions. For this, the foreseeable future will not be able to provide the necessary charging infrastructure and grid capacity in residential areas. The high costs and currently unfavorable life cycle assessment (LCA) of electric vehicles are also a problem. It is therefore expected that from 2025 onwards, OEMs will have to offer fleets which, in addition to a small percentage of battery electric vehicles, include above all hybrid vehicles for reducing CO2 emissions.

But what exactly can a fleet of this type look like when it is not only required to meet the CO2 limits but also be cost-effective to produce? This is where IAV Powertrain Synthesis comes in. Under given boundary conditions, the method determines which parameter combinations for combustion engine, transmission, e-motor, power electronics, battery, operating strategy, vehicle and topology will produce the desired results, taking into account aspects such as “maximum number of carry-over parts”. This allows a recommendation tailored to specific fleet scenarios to be computed for any OEM. Taking the example of a realistic European vehicle fleet, IAV has carried out this process to find a powertrain platform optimized for a fleet value of 75 g/km.

The point of departure is a fleet made up of vehicles ranging from subcompact car to sports car. The vehicles weighing between 926 and 2,100 kilograms cover the smaller models with front-wheel drive and the two top segments with all-wheel drive. Possible powertrain topologies are a conventional drivetrain as well as a P0 and a P2 hybrid. Further parameters concern the vehicles’ specific performance characteristics relating to acceleration times and top speeds, among others.

Comparison of 40 million configurations

Proceeding from this input, IAV Powertrain Synthesis was used to compute and compare around 40 million different configurations, including combustion engines with differing numbers of cylinders, displacement and technology, transmissions with between three and ten speeds as well as e-motors with an output of between 15 and 200 kilowatts. For each variant, IAV Powertrain Synthesis determined consumption in different driving cycles, the driving performance and costs. The technical details for the three components of combustion engine, e-motor and transmission are the result of subsequent layout and optimization computations using other IAV tools for advance engine, transmission and e-motor engineering.

The result: to obtain a CO2-optimized fleet, the fictitious OEM was to use a base engine with three cylinders and 1.1-liter displacement for all segments – in the form of a naturally aspirated engine with an output of 72 kilowatts for the smaller models, and a supercharged version delivering 120 kilowatts of power for the larger vehicles. It is of distinctively longstroke design with a stroke-bore ratio of 1.28 and has electrically operated auxiliaries. “The base engine is optimized in terms of function and costs and offers a very good modular structure”, explains Dr. Mirko Leesch, Head of the Combustion Engine Simulation and Validation department at IAV. “For example, it permits the addition of a turbocharger, balancer shaft and variable valve lift.” For the latter, IAV developed the patented SlideCam solution that manages with just a few additional components and produces lower gas exchange losses at low loads. A further option is a module for all secondary air systems that pools individual systems and therefore manages with fewer components – leading to lower costs and demanding less package.

Compact transmissions with few speeds

The transmission can also be of simple and compact design. Four speeds (A to D segment) or six speeds (high-end and sports car segment) are perfectly adequate for the entire fleet. “This is enough to optimize the operating points of combustion engine and e-motor for good levels of efficiency in the respective map”, says Dr. Ralf Tröger, Head of the Powertrain Configuration department at IAV. The dedicated planetary hybrid transmission (DHT) is configured for a maximum torque of 750 newton meters at an overall length of less than 380 mm, including 90-kilowatt electric motor. “It contains two planetary gear sets and four shifting elements”, Leesch says. “Among over 475.000 synthesized transmissions, this variant was shown to be the best, providing optimum efficiency and requiring little package.”

The e-motor selected emerged as the optimum solution from over 250,000 potential configurations. In combination with the four-speed DHT and depending on version, the scalable, permanent-magnet synchronous motor can reach an output of up to 90 kilowatts and torque levels of up to 300 Nm. Being highly integrated into the transmission, it takes up little package so that the hybridized powertrain also fits into small vehicles. Although it is possible to reach 75 grams per kilometer for the example fleet, the optimization shows 81 grams per kilometer as a good compromise between low extra costs, performance and emissions. “If the OEM sells ten percent of its models in each class as battery electric vehicles or as plug-in hybrids, it can reach the target of 75 grams per kilometer”, Tröger says. “This is a conservative estimate and therefore absolutely realistic.”

From recommendation to realization

This fictitious example demonstrates how IAV customers can be provided with individually tailored recommendations for the makeup of their fleets in 2025 and beyond – i.e. for the optimum percentage of hybrid, battery electric vehicles and plug-in hybrids as well as their technical configurations. The starting point is always IAV Powertrain Synthesis for simulating and optimizing drive versions in the entire fleet. Other special IAV tools are then used for defining the layout and concept of the various components. But that is not the only unique selling proposition to set IAV apart: customers not only get a recommendation – on request, IAV will turn the results into engines and transmissions as well as entire powertrains that are ready to go into volume production.