As global pressure to reduce CO2 emissions increases, conventional powertrains are being heavily revised to improve efficiency. A key enabler for each leap in technology is the correct choice of lubricant, but in many cases, the choice is made too late in the design process.
Aaron K. Warner speaks to Nevil Hall, Joint Managing Director at UK-based Millers Oils.
“Why don’t powertrain engineers consider the lubricant specification at an earlier stage of the design?” asks Hall. “An optimised design cannot be achieved without matching the load, temperature and durability requirements to the available lubricant properties. The alternatives are inefficiency through overdesign, or early failure through underdesign.”
Hall is just one of many lubricant specialists who believes that the potential exists to improve performance throughout the powertrain and driveline, including engines, transmissions and axles. “All the new technologies tend to push up loads by increasing torque levels, but the available package space is shrinking at the same time,” he says.
Hall cites hybrid vehicles as a prime example of these trends:“The addition of an e-machine between the engine and transmission, to create a hybrid powertrain within an established vehicle, squeezes the package space available for the transmission yet increases the torque input by as much as 30% in some cases.
“The right oil can make a more compact transmission feasible, despite the higher torque.”
The ‘right’ oil in such circumstances would have higher film strength to provide greater protection of the loaded metal surfaces which would otherwise be forced into contact. “Under very high loads, it’s more about surface chemistry than conventional lubrication,” he explains. “When a fully laden EV, such as a delivery van, pulls away from a standstill, the maximum motor torque is available from zero rpm and is carried by a single pair of meshing teeth at the final drive. The contact stresses won’t break the teeth, but without suitably formulated oil they will lead to micro-pitting and, eventually, cracking. Developments like nanotechnology are allowing oils to be created that adhere more tenaciously to the metal surfaces, protecting them under more extreme loads.”
The superior load capacity of the latest oils also helps OEMs to minimise the overall size of the final drive assembly, helping to satisfy underfloor packaging requirements. The traditional approach to propshaft packaging offsets the shaft some distance below the differential centre line, creating gear geometry with high levels of sliding at the mesh. By adhering more strongly to metal surfaces, the best oils can provide greater wear resistance and lower friction under conditions of sliding contact.
The search for improved efficiency also leads OEMs towards reduced oil levels in transmissions and differentials, as churning losses are directly related to the depth of immersion of the rotating gears. Cooling becomes an issue if the oil level is set too low because the lubricant is the primary means of managing the temperature of the meshing gears.
Downsizing quality
Hall makes some interesting revelations about the latest engine technologies, too, beginning with the trend towards stop-start operation, now widely used. “Most people are aware of the effect of oil viscosity, in that efficiency is reduced at lower temperatures because the oil is thicker, but few realise that film strength is also affected by temperature,” he explains. “Many oil additives only react chemically from around
70 degrees Celsius upwards, so they don’t offer the same protection when cold. In developing oils with a consistent film strength across a wide temperature range, we have benchmarked some examples with quite obvious shortfalls in film strength at certain temperatures. This could be an issue when temperatures fluctuate, such as in stop-start operation.”
Downsizing strategies present a range of challenges for lubricating oil, as a result of the increased thermal and mechanical loads associated with a higher power output per litre of displacement. Engines with fewer cylinders typically also have smaller sumps with lower oil capacity, yet longer drain intervals are required by the market. At the same time, OEMs require thinner oils to reduce viscous friction within the engine.
“Even the shift from port injection to direct injection on a petrol engine can cause a 30% increase in bearing load,” Hall comments. “When you add the effect of turbo- or supercharging, cylinder pressures can double with higher torque across a wider rev range. When a four cylinder engine replaces a six, the number of bearings is reduced but package limitations constrain any increase in bearing width.”
For large multi-cylinder engines, deactivation of several cylinders at times of light load operation is a well-established method for improving efficiency, but according to Hall there are consequences for the engine oil. “Normally the oil film on the bore in a firing cylinder is 0.5 to 1.0 microns thick,” he explains. “In a motored cylinder it increases to as much as 30 microns and is expelled in a single firing event once the cylinder is re- activated. In order to avoid poisoning the catalyst, oil with a low sulphur, ash and phosphorous content – low SAP – must be specified.”
For Hall, the answer is clear: powertrain and driveline developers must consult lubricant specialists early in the design process to establish what can be achieved.
“It can make the difference between a robust design that handles the maximum loads within the minimum package envelope, or a compromised solution that is not fully competitive,” he says.
Aaron K. Warner
This article was first published in the Q4 issue of Megatrends magazine, to continue reading, simply download your free copy now.
