Lubrication in Formula One
The role of the lubricant in any tribological system is to eliminate component wear and minimise the frictional drag. This is as true for any road-going touring car engine as it is for one used in Formula One - indeed, in many ways the former is a far harder task since the engine has to be protected over a much wider range of operating conditions, summer and winter and often up to 20,000 miles between oil drains. In the case of the latter, the oil simply has to last only the race.
However, irrespective of the duty cycle of either engine, in any reciprocating engine there are three main critical regions - the bearings, piston rings and valvetrain - with different relative velocity, temperature and pressure within them. The engine lubricant (base stock, viscosity modifiers and additives) will therefore be the result of the compromises in blending the oil for these three zones.
The most critical property in any lubricant is its viscosity, and this is highly dependent on the temperature, the rate of shear and the pressure within it. Variations in viscosity with temperature are described by the Vogel equation, a principle that we all use when draining engine oil, say, as the oil drains more freely at higher temperature. The shear rate - the rate at which the successive layers of oil slide across each other in the lubricant film - has been described by MM Cross in his paper "Rheology of Non-Newtonian Fluids", while the variation in viscosity with pressure can be approximated using the Barus equation.
Combined together, and with knowledge of the various coefficients of each equation, a reasonable assessment of the viscosity within a given lubricant can be calculated in each of the systems above. Using this, a minimum oil film thickness in each of the zones can be modelled and the frictional forces therein derived. By blending a number of oil components together in the virtual world, a numerical model of an oil can be developed at a basic level before the actual blending and physical bench testing in the real one.
While this might sound straightforward - and, dare I say it, even pedestrian - nothing could be further from the truth. To generate more accurate data, the precise geometry of the components under study needs to be known. For instance, in the case of the piston ring pack, we need to know the gas pressures on either side of the piston rings, the temperature of both ring and liner at the point of contact and any possible effects as a result of any restrictions to the flow of the lubricant by the oil control ring.
In the bearings this will mean the temperature around the bearing and the elasticity of the components under the loads generated. As both temperature and pressure change as we travel around the bearing, so too will the oil viscosity. Indeed, while exact figures may be difficult to produce, the real benefit of such analysis is to investigate trends that will point towards improved design.
However, from an engine performance perspective, the real interest may lie in the total friction within these units. In a typical touring car at high speed using techniques described above, the total engine friction was estimated at 7.5 kW using an SAE 15W/40 oil. Of this, 55% came from the bearings, only 39% from the pistons and 6% from the valvetrain. Reducing the lubricant viscosity lowered the friction in the bearings and piston ring pack but brought about an increase in the friction in the valve train.
Using similar techniques at the maximum speed of a Formula One engine, the optimised figure was a total of 65 kW, of which 46% came from the piston assembly and 38% from the bearings. The remaining 16% (or 10 kW) was attributed to the valvetrain.
The approach to minimising engine friction in Formula One, or indeed any engine, would therefore appear to be elementary. For high levels of boundary lubrication (when relative sliding velocities are low), high viscosities should be used together with friction modifiers. Where an engine has less boundary lubrication (and therefore higher relative velocities), lower viscosities may be more acceptable. All fine in theory but in any complex lubrication system to establish the minimum viscosity acceptable requires regular sampling of the lube and subsequent screening for wear metals.
Fig. 1 - Variation of oil viscosity with shear rate and temperature
Written by John Coxon