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Optimisation of oil flow in a dry-sumped gearbox

oil-pumpsIn the rarefied atmosphere of Formula One, engineers are often given the resources to investigate engineering issues that would be beyond the scope of other racing organisations. With the regulations limiting engineering freedom to an extensive degree, teams will go to great lengths to gain any advantage.

One example of this is the development work undertaken by the now defunct Minardi team in the production of a new gearbox for the 2005 Formula One season. The team had been working on its own gearbox design in 2003, which was then raced in 2004. During the ongoing development programme, the team's engineers identified the gearbox oiling system as an area for potential performance gain.

The original iteration of the gearbox incorporated a built-in lubricant pump, with the oil stored in a sump section. The volume of oil in the gearbox was singled out as an area where gains could be made, both in terms of reducing power loss due to friction and reducing weight in terms of capacity. This potential was narrowed down to the following areas:

  • Reduce the quantity of oil used
  • Improve gearbox lubrication
  • Reduce parasitic power loss
  • Increase the rigidity of the casting

It is in the approach taken to achieve these goals that things begin to get really interesting. Unhappy with the capability of its simulation software, the team decided that being able to visualise fluid flow within the transmission case would be beneficial. To this end, it contracted an outside company to produce a rapid-prototyped centre gearbox section, in clear material. Initially the team had tried to produce the component using its own in-house SLA (Stereo Lithography) facilities, but the properties of the SLA material made it unsuitable for such a structural application. It was for this reason that an outside contractor was called in to produce the section using a much stronger SLS (Selective Laser Sintered) material.

The aim of this approach was to visually asses the movement of lubricant inside the casing, with the gearbox mounted on a low-power dynamometer. The first key change to the system was to relocate the oil pump from inside the casing to an external position, and eliminate the sump, replacing it with an oil tank. This made the system 'dry' and immediately allowed a reduction in the casing's internal size. The effectiveness of this approach was verified by testing a rapid prototype of the original sumped system, which showed that the new dry sump provided ample lubrication.

oil-pumps-gearbox

During these initial tests it also became apparent that the volume of oil being supplied to the main gearbox bearings was excessive, which entailed a reduction in the size of the oiling holes. This proved to be effective, and the lubricant pressure increased with no subsequent increase in pump flow rate. The original oiling holes were 2 mm in diameter, which was later reduced to 1 mm. Under test conditions this equated to an increase in oil pressure of 0.18 bar at 6000 rpm, while still maintaining a pump flow rate of 3.08 litres per minute.

During this testing it was also noticed that, as a consequence of the casing's internal architecture, oil was being distributed to areas of the gearbox where it wasn't needed. Several iterations of the casing design using alternative geometries were tried, until an optimal solution was found. The end result was that the volume of oil in the casing, and hence overall weight, was reduced, with the added benefit of a stiffer casing structure.

With the case design and dry-sump oil system optimised, the design was subsequently produced using a rapid casting process, and using rapid-prototyped cores to produce a titanium casing. Although this is only one strand of a complex full-car development story, the project provides a valuable insight into what can be achieved with a little engineering ingenuity.

Fig. 1 - The rapid-prototyped gearbox containing red hydraulic fluid to aid visibility of flow patterns

Written by Lawrence Butcher

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