Friction in the engine, transmission, tyres and brakes consumes a large share of the fuel energy of a conventional passenger car (Holmberg et al., 2012). An electric powertrain removes most of the engine-related losses, and the total frictional loss of a battery electric car is considerably smaller than that of a combustion-engine car (Holmberg and Erdemir, 2019). What remains matters more. With a fixed battery capacity, any loss in the driveline translates directly into range, and the reduction gearbox between motor and wheels becomes one of the few places where a lubricant can still make a measurable difference.

The simplest lever is viscosity. Thinner oil is easier to shear and to move, so the drag of gears and bearings falls. The same change thins the film that keeps loaded surfaces apart. Transmission engineers have managed this trade-off for decades, but the electric drive unit shifts its terms: motor speeds are much higher than engine speeds, torque is available from standstill, and the fluid is often asked to cool the motor as well as lubricate the gears (Farfan-Cabrera, 2019; Chen et al., 2020). This article reviews what the published literature establishes about low-viscosity lubricant behaviour under those conditions.

The review is narrative. It combines recent survey papers on electric vehicle tribology with the older and more fundamental literature on gear power loss, elastohydrodynamic lubrication (EHL) and gear surface failure, on which any assessment of thin oils must rest. No systematic search was performed and the selection reflects the sources most widely relied on in the field. Because little component-level test data from production drive units is openly published, the discussion is framed in terms of mechanisms and established trends, and specific loss or life figures are not quoted.

A typical electric drive unit couples a traction motor to the differential through a single-speed reduction, usually in two helical stages. The motor may run at speeds several times those of a combustion engine, so the input pinion and its bearings experience very high pitch-line and rolling velocities, while the output stage sees lower speed and high torque. Peak torque is delivered at or near zero speed, where no hydrodynamic film has formed, and regenerative braking loads the reverse flanks of the teeth. The duty therefore spans a wider range of lubrication regimes than a conventional gearbox, with the two extremes often present in the same housing at the same time (Farfan-Cabrera, 2019).

Integration adds further demands. In many designs one fluid lubricates the gears and bearings and is also sprayed or circulated over the stator end-windings and rotor to remove heat. The fluid then meets copper conductors, insulation varnish, polymers and electrical potential differences, and its thermal conductivity, heat capacity, electrical resistivity and compatibility with copper become design properties alongside the familiar tribological ones (Chen et al., 2020; Tung et al., 2020). Stray currents can also pass through bearings and gear contacts, producing electrical damage that depends partly on the lubricant (He et al., 2020). Lower viscosity helps heat transfer and pumping, which is a second reason, beyond efficiency, for the move to thinner fluids.

Gearbox power loss is conventionally divided into load-dependent and load-independent parts, each arising in gears, bearings and seals (Höhn et al., 2009; Michaelis et al., 2011). Load-dependent losses come from sliding and rolling friction in the loaded contacts. Load-independent, or spin, losses are present whenever the shafts turn: oil is churned, squeezed out of the meshing tooth spaces and dragged through bearings, and air and oil mist are pumped by the rotating gear bodies.

The separation of gear teeth and rolling elements is described by EHL theory, in which elastic flattening of the surfaces and the rise of viscosity with pressure combine to produce a thin, nearly parallel film (Dowson and Higginson, 1966). The regression formulae of Hamrock and Dowson (1977) and their line-contact counterparts show film thickness increasing with the product of entrainment speed and inlet viscosity raised to a power of roughly two-thirds, and with the pressure-viscosity coefficient, while depending only weakly on load. The ratio of film thickness to composite surface roughness, the specific film thickness or lambda ratio, has long been used to indicate the lubrication regime and the likelihood of surface distress (Tallian, 1967).

On this basis high speed appears to compensate for low viscosity, and at the motor-side pinion it partly does. Two effects limit the benefit. The viscosity that counts is that at the temperature of the surfaces entering the contact, and both bulk and flash temperatures rise with speed and sliding, so the effective viscosity falls just where the nominal calculation predicts the thickest film. Shear heating in the inlet and the difficulty of keeping the inlet supplied with oil at very high speed reduce the film further. At the output stage, speed is low and torque is high, and a thin fluid gives a film that may be smaller than the surface roughness. Figure 1 sets out the four combinations of speed and viscosity in simplified form.

If the hydrodynamic film is thinner, more of the protection must come from elsewhere. Table 1 summarises the requirements placed on a low-viscosity drive unit fluid and the tension each has with the others.

The standard instruments for rating gear lubricants, the FZG scuffing, micropitting and efficiency tests, operate at pitch-line speeds well below those at the input stage of an electric drive unit. Their rankings cannot be assumed to carry over, because the balance between flash temperature, film formation and additive reaction rate changes with speed. High-speed variants of these rigs and component tests on complete drive units exist in industry, but few results are openly published, and the review papers cited here repeatedly note the shortage (Farfan-Cabrera, 2019; Tung et al., 2020).

Several specific gaps follow. Film thickness and traction data for very low viscosity fluids at high entrainment speeds, where thermal and starvation effects dominate, are sparse. The micropitting behaviour of such fluids on gears with improved surface finish has not been mapped publicly in a way that would allow the benefit of superfinishing to be traded against viscosity. The effect of entrained air on both lubrication and cooling at high speed is poorly quantified. And the interaction between electrical stress and tribological film formation, relevant to bearings in particular (He et al., 2020), is at an early stage of study. Each of these would benefit from test methods agreed across the industry.

The literature gives a consistent account of what lowering lubricant viscosity does in a high-speed reduction gearbox. Load-independent losses fall, most clearly at low and moderate speed and less so where inertia and windage take over. Loaded contacts respond in a more complicated way, since their friction depends on base oil structure and on whether asperities touch. Film thickness declines, with thermal effects eroding much of the help that speed would otherwise give, and the risk of micropitting at the high-torque stage and of scuffing at the high-speed stage rises. The burden moves to surface finish, base oil selection and additive chemistry, and the last of these is restricted by the fluid's contact with the motor. Thin fluids are plainly workable, as their growing use shows, but the published evidence base at representative speeds remains narrow, and openly reported high-speed test methods are the most pressing need.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.