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Lightweight materials in turbocharger development

When manufacturers of exhaust gas turbochargers and mechanical superchargers research and develop, the main focus is on reducing weight and friction as well as optimal flow. There are great hopes for new materials. But the road to market is long in view of the high thermal and mechanical loads in operation.

When Günter Krämer came to BorgWarner Turbo Systems 20 years ago, compressor wheels were still cast. "Today they are milled from solid", reported the marketing director. What only happened in one-offs at the time has become established in the volume market - machining centers are in use 24 hours a day to machine the flow-optimized paddle wheels.

In order to optimize the precision, reliability, weight and friction behavior of the turbocharger, to be able to optimally adapt their air throughput to the respective load range of the engines and to arm them against the extreme exhaust gas temperatures of gasoline engines, the technology has been continuously developed.

BorgWarner produces turbochargers from lightweight materials

Some of the tests are running in racing - BorgWarner has only just been awarded the contract to equip all engines of the IndyCar series with new turbochargers, which use new materials for the turbine wheels as well as for the bearings and housings: blades made of titanium aluminide (TiAl ), is stored with ceramic ball bearings and the whole thing is packed in a housing made of high-strength, extremely heat-resistant cast steel. The racing teams can use the chargers individually or have them integrated into bi-turbocharger concepts.

The new materials are not yet ready for large-scale production, in which, despite mass production, there is a need for trouble-free function over the entire life of the car and the budget is tight. "New materials are already in use in hand-picked series models such as the Porsche Panamera Turbo S or the V6 diesel from Daimler," said Krämer to VDI nachrichten.

Thanks to their lower weight and friction, the TiAl materials allow the turbos to respond quickly at low speeds. For large-scale production, however, there are still questions to be clarified about the connection between the new alloy and the steel shafts of the loader or about production technology. Detailed work, as done by the developers in flow simulations of the turbochargers, with coatings on the components or with those bypasses that prevent the turbochargers from overspeeding at full load.

In addition to minute improvements, the turbocharger manufacturers have also made leaps in technology in recent years. For example with variable turbine geometry (VTG), in which swiveling guide vanes regulate the flow of exhaust gas. At low speeds, the turbine inlet is narrowed in order to accelerate the flow and thus the turbine. It drives the paddle wheel on the cold side via a shaft, on which more air then gets into the cylinders.

While VTG technology has largely established itself in diesel engines and teething troubles such as its sensitivity to soot have been overcome, according to Krämer it is now also gaining ground in gasoline engines.

Turbochargers make a leap in development through bi or twin turbo concepts

Turbocharger technology has recently experienced a further leap through bi or twin turbo concepts. They serve higher elasticity by optionally combining a small, fast-responding turbocharger and a large one that permeates a lot of air at high speeds.

There are also manufacturers who use two medium-sized turbochargers, only one of which works at low speeds and both work at high speeds. Still others combine exhaust gas turbochargers with superchargers - that is, superchargers that are driven by the crankshaft.

Advantage of the supercharger: Since they only work on the cold side, the cooling effort of the intercooler is reduced and, thanks to the connection to the crankshaft, they respond immediately when the driver accelerates.

With the superchargers, further development is also the order of the day. According to sales director Klaus Dehnert, market leader Eaton is working on concepts for light commercial vehicles, among other things. In addition, the supplier's research laboratories are also concerned with new, lighter materials, especially for the air conveyor blades and shafts. “The aim is of course to increase efficiency,” he says.

The transition from three-wing to four-wing superchargers brought a leap in both aerodynamic and mechanical terms. Not only has the overall efficiency been significantly optimized, it is also now easier to adapt the technology to the respective application. "Now the task is to optimize the application in detail, as we are doing with the new 1.2-liter three-cylinder from Nissan," says Dehnert.

Turbocharger: With potential in the drive technology of the supercharger

Dehnert suspects great potential in the drive technology of the Supercharger. “We are working on needs-based control so that the charger really only runs exactly when the engine needs the air,” he explains. Among other things, one therefore deals with electric drives and combinations of easily controllable electric drives and conventional mechanics. To this end, the total energy balances are currently being evaluated.

Another important field of research is lightweight construction, according to Dehnert. In order to set the Roots blower - the four-winged fan that pushes cold, oxygen-rich outside air into the combustion chambers - in motion, the heavier they are, the more energy is required. “They are already made of aluminum and are hollow on the inside,” he explained. In future, they could be made of magnesium, the density of which is almost a third less than that of aluminum. Investigations with plastics are also already in progress, which Dehnert did not want to define more narrowly in conversation with VDI nachrichten.

A contribution by:

  • Peter Trechow

  • Wolfgang Pester