REQUIREMENTS FOR OPERATING MATERIALS APPLIED IN HYBRID POWERTRAINS
Auto Tech Review|July 2020
REQUIREMENTS FOR OPERATING MATERIALS APPLIED IN HYBRID POWERTRAINS
The fundamental change in the automotive industry from conventionally-powered vehicles to increasing electrification and fully electric powertrains is a challenge also for the development of operating materials. In this challenging environment, APL contributes with core expertise for internal combustion engines, as well as for hybrid electric vehicles in the areas of electric motors, power electronics and batteries, to support the evolution of alternative fuels and lubricants.
DIPL-ING. CHRISTIAN, DR-ING. MARCUS GOHL, DIPL-ING. MORTEN KRONSTEDT, DR GUNTHER MULLER

DEVELOPMENT OF OPERATING MATERIALS FOR FUTURE POWERTRAINS

In order to sustainably meet global demand for mobility within the framework of legal and societal requirements on the basis of renewable energy, the focus in the development of new mobility concepts is on reducing emissions and greenhouse gases. By exploiting technological developments and the properties of future fuels, further optimisation of non-hybridised internal combustion engine (ICE) powertrains is possible. However, the rules of physics limit the efficiency of the ICE, so that future fleet target values cannot be achieved. For this reason, the conventional engine is combined with an electric machine in a hybrid powertrain. The concepts range from micro-hybridisation to plug-in hybrids with high-voltage technology. Due to the many possible combinations of ICE, gearbox, electric motor and energy storage in hybrid powertrains, there is a great deal of complexity in the configuration of these systems, 1.

The proportion of electrified drivetrains and the degree of electrification will vary greatly across the different vehicle segments and regions. Although forecasts change frequently, most recent studies show that about 85 % of all vehicles produced worldwide by 2030 will still be powered by an ICE [1]. In addition, the great added value of proven combustion engine technology can be leveraged by alternative fuels from renewable sources – such as biofuels or e-fuels – for some applications and markets. Especially for commercial vehicles, the electrification will remain limited in the long-haul heavy-duty use in the foreseeable future. In the context of the energy density of the battery, payload, infrastructure and total operating costs, the focus here is on range optimisation.

The current range of fuels and lubricants, which was developed especially for ICEs and their operating profile in the conventional powertrain, often cannot fully meet the requirements of an electrified powertrain. In the future, worldwide demands of various markets and legislations with a multitude of powertrains from the conventional ICE to full hybrids must be considered. Due to a lack of standard laboratory test procedures in the automotive industry, APL has developed its own characterisation methods. Thus, different operating material qualities for use in hybrid applications can be differentiated.

CHANGED OPERATING BEHAVIOUR OF THE ICE

The extended functionalities of the electrified powertrains compared to the conventional vehicle lead to a new requirement profile for the ICE and its operating materials. The possibilities of recuperation of braking energy, fully electric driving, torque assist by the electric motor, the load point shift to increase efficiency and the increased number of engine starts produce changed operating boundary conditions.

2 shows the comparison of the operating behaviour between a conventional and a hybrid powertrain under real driving conditions. Due to the electrical assistance, the ICE can be switched off more frequently, which leads to a higher number of start-stop events. However, the intended fuel economy benefit can bring new challenges.

The starting process required during start-stop operation should ideally take place when the engine is warm. Under colder operating boundary conditions, due to the low wall temperatures in the combustion chamber, only low-boiling fuel components evaporate, which is why the mixture is enriched to comply with the ignition limits. In addition to unburned fuels, water vapour can also condense at low temperatures, get into the engine oil and accumulate there. Nevertheless, the lubricant changed in this way must allow sufficient support of the tribological systems for minimal frictional torque and wear.

3 shows the influence of the battery state of charge on the starting phase in a WLTP cycle. It becomes clear that the vehicle, with a 68 % charged battery, requires about 900 s of initial electric travel to reach temperatures comparable to those of low-battery combustion engine operation. In this context, it is important to note the increased emissions under the colder boundary conditions. Basically, it is important to exceed the light-off temperature of the catalyst as soon as possible, so that it reaches its ability to convert. These interrelations may become more important in plug-in hybrids.

NEW REQUIREMENT PROFILE FOR THE OPERATING MATERIALS OF COMBUSTION ENGINES

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July 2020