AMS takes a look at production of the new Honeywell turbochargers that feature ballbearing technology, examining the relationship between the company’s turbo and aerospace divisions and how this benefits production.
Place a jet engine and a turbocharger side by side and there is no apparent connection between the two. Look beyond this top-level comparison, says Craig Balis, Vice-President Engineering, Honeywell Turbo Technologies, and the similarities are plain to see. “The turbocharger business was born out of our aerospace division. We talk about turbochargers as our product, but our technology is turbo machinery. Turbine (jet) engines and turbochargers are both turbo machinery.”
In developing its range of turbochargers, Balis says that Honeywell has had to resolve similar problems to those encountered with turbine engine design, such as high temperature control, high rotation speeds, high levels of stress and precision balancing. “We collaborate with our aerospace colleagues about which materials can withstand the environment and at what cost,” explains Balis, adding that Honeywell is the only producer of turbochargers that has access to this type of resource, drawing information from related ‘technology domains’.
Far from being limited to material research, the information available from the aerospace sector covers a broad spectrum of specialist areas, including aerodynamics of the wheel design, the shape of the blades, and also the ball-bearing technology, which has now been incorporated in volume production of Honeywell turbochargers.
Where the shaft, holding both ‘wheels’ of the turbo, previously rotated in a metal sleeve, it now rotates on the individual balls. Like most turbochargers, it’s a lubricated system that takes engine oil taken directly from the sump. Balis explains that while there were technical problems to solve, largely surrounding selection of the ball-bearing material and rotation speeds that can reach 250,000rpm, there were also issues surrounding assembly of the turbocharger package. “Anything spinning at that speed is generating some type of vibration and that can translate into noise. We had to design the turbocharger to cancel the vibration, to make sure that it didn’t turn into noise.” According to Balis, the key to high-volume production is how the balance of each turbocharger is checked and adjusted, while also validating the unit’s characteristics for each engine application. Balancing each unit involves removing a small quantity of material, but he points out that it’s a normal part of the build process built into the manufacturing line.
“When assembling a turbocharger there are certain imperfections, measured in 10ths of a micron, that can affect unit behaviour. To perform the balance adjustment, we spin up the turbo and software senses where any balance imperfection lies. Adjustments are made by removing small pieces of mass from different areas of the wheels or shaft to achieve perfect balance. We do that on every unit, all within the production environment.” Depending on output volumes, the material removal process can be performed automatically, or the software can advise an operator where and how much material to remove.
The other adjustment made on the assembly line is waste gate calibration, the geometry mechanism. “The calibration is application-specific. Every engine, or model, has a different requirement. That is where we work with OEM to define the right behaviour of the turbocharger for that vehicle.” Engine block material or how the exhaust system is installed can also influence turbocharger characteristics. “We have a target level that we need to be below. We validate this with our customers; get below that level and there’s no noise. It’s largely vehicle-specific, so even with the same engine, if that is used in a different vehicle, the characteristics required could be very different.”
Turbocharger assembly starts with the casting or milling of the turbine and compressor wheels. Balis says that this is followed by the welding of the shaft, the ‘heart’ of the turbocharger, to the turbine wheel. The shaft is then subjected to a round of high-precision machining in order to both balance the part and add required features, such as the lubricating grooves.
The bearing assembly is then added to the centre housing. The shaft is then passed from the turbine side through to the compressor side and this completed assembly is tightened to pre-set tolerances.
“The units have to be tightened to very specific levels,” says Balis. “When the turbocharger is running at full speed, the wheel actually grows a little due to the centrifugal forces. As this happens, the wheel also gets a little thinner, so having the right tightening to accommodate that material change is a very delicate operation. If the assembly is overtightened, the shaft will break, either in production or over the lifetime of the unit.”
The full manufacturing process includes a mix of automated, semi-automated and manual steps. Balis explains the factors that can influence production choices: “There is an economic trade-off due to the high capital cost of buying automation. If it’s a low-volume operation, it can be more cost-effective to have steps such as final assembly completed by hand. The per-product cost will be higher, as we’re paying for labour, but the investment will be considerably lower. With a high-volume operation, the economics will tell you to invest in the machine.”
While each separate component that goes into a turbocharger is tested before assembly, and the assembly process is followed by a further test of the completed unit, there is no extended testing done on the production line. “We do not do any durability testing of units coming off the assembly line,” says Craig Balis. “This is all completed in the development phase of the process. We run multiple units for months at a time, simulating 200,000 miles for a passenger car and one million miles for a commercial vehicle. The design is proven before the start of manufacturing, so that quality can be assured as we go to high-volume production.” With regards to calibration of the turbochargers, Balis says that as production figures are calculated in the millions, changes have to be made in a highly-controlled, fast and repeatable process.
Balis says that efficiency was the underlying motive behind the switch to ball-bearing technology. “The turbocharger is effectively helping to improve engine breathing. The key to helping the engine to be more efficient is the turbo being more efficient, and the bearing system is one factor in this. There is a certain power loss, or drag, due to friction within the turbo and reducing this translates to increased engine efficiency.”
The switch to ball-bearings was supported by the aerospace division of Honeywell, which also helps with the sourcing of other materials. Balis says that it is the complexity of using ball-bearings in turbochargers and the difficulty of achieving high-volume production that have limited introduction of the technology One of the newest turbochargers in the range is also the smallest, designed for vehicle production in emerging markets. “This (unit) is being applied to a 0.8 litre, two-cylinder engine for India; it’s the first two cylinder turbodiesel engine,” says Balis. Although he declines to confirm which OEM will be using this model, supplying markets in these regions, in combination with the introduction of new technologies, can only serve to strengthen Honeywell’s global position.