Bearings for thermal and kinetic energy recovery systems
11 February 2013
In automotive and industrial applications, super precision bearings are playing a key role in both thermal and kinetic energy recovery systems (KERS), helping OEMs develop greener, more energy efficient systems.
Although the internal combustion engine is likely to still dominate the automotive landscape for the next decade or so, the increasing mismatch between energy consumption and available resources, together with tighter legal restrictions on engine CO2 emissions, is pushing the boundaries of existing automotive technologies and accelerating the development of reduced friction, more energy efficient, ‘greener’ alternatives.
A key area where these requirements are being addressed is the development of energy efficient, low friction, super precision ball bearings for these emerging technologies.
In order to support the growing demand for more energy efficient, low carbon emission vehicles, manufacturers of turbochargers, particularly for passenger cars and commercial vehicles, are being asked to provide more compact, higher efficiency turbocharging systems that are both durable and affordable.
The author’s company first manufactured ball bearings for turbochargers in 2004, and it has since led the way in the development of new low friction, double row angular contact ball bearings for these systems. These super precision ball bearings are helping to set new performance benchmarks for the future, particularly in the high-end passenger car, light duty and heavy duty truck markets, where the individual cost of the bearing is less of an issue and where the fuel savings and reduced emissions are more immediate and noticeable.
Advantages of ball bearing turbochargers over more traditional hydrostatic oil-filled turbocharger bearing sub-systems stem from the fundamental change in the friction mechanism present in the system. Multiple rolling elements replace a thin oil film in high shear, significantly reducing system friction.
This results in a significant improvement in system friction at operating temperature (typically up to 50 percent) and even greater improvements during the first minute of an engine cold start. Turbocharger studies have shown that the ‘ball bearing effect’ is most pronounced at low engine speeds, just where a down-speeding or downsizing concept needs the most help from the turbocharger system.
With the more conventional hydrostatic, oil filled turbocharger systems, the oil is very viscous in cold conditions, particularly during engine start up. The time taken to heat up the oil means that engine emissions are going to suffer during this period. However, with ball bearing turbochargers, the air is available to the system immediately on cold start up, resulting in a more energy efficient system with reduced emissions.
Most ball bearings for turbochargers are of the angular contact type, typically comprising ceramic balls, cages, anti-rotation devices, an outer ring, a compressor inner ring, a turbine inner ring and a series of oil flow control jets or squeeze film dampers.
These types of bearings rotate up to six times faster than any other vehicle bearing. In hot shutdown conditions, they can also reach temperatures of up to 400oC. The bearing is designed to be cooled by the lubricating oil flow and its materials of construction must resist extreme conditions at all times over the complete life of the turbocharger.
Kinetic energy recovery systems
As well as thermal energy recovery systems such as turbochargers, super precision ball bearings are also playing a vital role in the development of innovative kinetic energy recovery systems (KERS).
In automotive applications, KERS normally involve some kind of motor generator unit and an electrical flywheel. These bearing systems accumulate energy as a result of the motion of the vehicle or system. This recovered or ‘free’ energy is then stored and re-used to assist vehicle acceleration.
The author’s company is working closely with a number of others using these same principles, including applications in heavy goods vehicles, elevators, lifting gear and cranes, through to public transport systems such as buses, trains and trams.
Most bearings for automotive KERS applications are angular contact ball bearings configured as double row cartridges, with built in axial play or preload. These bearings often have standard phenolic cages but typically use ceramic balls, which weigh 60 percent less than steel balls. Another key requirement for the bearings is that they provide long grease life with no re-lubrication required. Typical bearing speeds can reach 1.4 million Ndm.
In industrial KERS applications, single row angular contact ball bearings are normally provided. In a recent elevator application, for example, single row angular contact ball bearings were designed to support the weight of the flywheel and motor. The bearings were custom designed with special, larger grease reservoirs that maximise the bearing grease life.
Solid oxide fuel cells are simply another type of thermal energy recovery system, whereby the free energy from the fuel cells is utilised to power other parts of the vehicle.
Lower friction, higher speed ball bearings are also being developed for the latest hybrid-electric and all-electric vehicle drive systems, including bearings for electrical turbochargers that supply air to the fuel cell system. These types of bearings must provide stability, robustness, and should be lubricated-for-life to provide maximum durability and reliability over the life of the fuel cell system.
Chris Mitchell is project manager at The Barden Corporation
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