Oils for hybrid electric vehicles
R. David Whitby | TLT Worldwide September 2018
Conventional oils can be used, but specialty products offer additional benefits.
Under-development lubricants will better ensure smooth functioning of electrified drivetrain components.
© Can Stock Photo / chesky
Three alternative configurations can be used for gasoline-electric hybrid vehicles: series, parallel and series-parallel.
With series types, the gasoline engine powers a generator that feeds electricity to either a battery or an electric motor, which then powers the wheels through a conventional hypoid gearbox. The battery also can feed electricity to the electric motor. The speed of the engine is disconnected from the speed of the wheels.
In the parallel configuration, either the gasoline engine or the electric motor can drive the wheels using a double-gearbox arrangement. The electric motor is fed from a battery, which is charged separately while the vehicle is stationary.
With the series-parallel types, the gasoline engine can either drive the wheels directly or can power the generator indirectly, with the generator feeding electricity to either the electric motor or the battery. Again, the electric motor can also drive the wheels using a double gearbox arrangement.
In all three configurations, the gasoline engine can be lubricated using conventional gasoline engine oils. However, the use of newer oils can deliver additional benefits. Hybrid drivetrains take advantage of normal driving to increase efficiency. At lower driving speeds, hybrids can operate exclusively on the battery pack and electric motor. Regenerative braking functions to recharge the battery by capturing the kinetic energy while slowing down and stopping. Start-stop systems shut down the engine while stationary, cutting down the energy required for idling.
Electric-only and gasoline-only power allows the series-parallel design to provide the most fuel-efficient operation. The system can act as a series drive at lower speeds but then turn to gasoline only at higher speeds.
However, in the parallel and series-parallel configurations, the gasoline engine is required to kick in instantly when the computer control detects the higher speed required. Because response times need to be rapid, lower viscosity engine oils produce less resistance to the engine.
Lower viscosity engine oils also are beneficial for the series configuration. Because the engine is not connected directly to the drivetrain, it can be run at a constant speed to charge the battery pack. Constant engine speed does not put the same stresses on the engine oil as with conventional gasoline or diesel engines operating at variable speeds.
A number of lubricant manufacturers in Japan and Europe are now offering 0W-16 viscosity engine oils for gasoline-electric hybrid cars. Toyota and Honda are now recommending the use of 0W-16 oils for selected hybrid vehicles, although they will allow 0W-20 viscosity grades if the lower viscosity oils are not available. It is likely that 0W-16, 0W-12 and even 0W-8 viscosity grades will become more widely available for hybrid vehicles.
Toyota’s Prius hybrid has a series drivetrain, which uses a power split unit that consists of a planetary gear set, the generator, the electric motor and an oil pump. These components have a higher system voltage, a greater presence of copper and electronics, higher temperatures and speeds and varying friction requirements. Lubricants are being developed that are better able to protect and ensure the smooth functioning of the electrified drivetrain components. The main considerations are the electrical conductivity of fluids and related safety concerns, corrosion protection, high-speed bearing protection, thermal transfer properties, material compatibility and oxidation and sludge control.
The electrical conductivity of fluids is an issue because, if it is too high, there is a risk of current leaking. If it is too low, static charge can build up, resulting in electrical arcing in oil. This leads to the degradation of the fluid, compromising its protective properties. Additionally, conductivity increases as an oil oxidizes.
Some gasoline-electric hybrids use a dual clutch transmission, a multiple-speed automatic gearbox that uses an e-motor in addition to the twin clutch for seamless increases or decreases in speed. Because of the electrical components and wiring involved, a gearbox fluid would have to offer copper corrosion protection while at the same time being compatible with the insulation and other materials of construction.
Specialist oils are likely to become more widely available as the number of hybrid vehicles increases.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.