Formula One scavenge pump development
In last month’s article we looked at the work by the Honda Formula One team in developing the pressure feed pump for its V8 engine. However, this was only half the challenge when it came to developing a reliable oil supply system for its car, so it is worth examining the development of the scavenge side of the system as well.
As its name suggests, the primary role of the scavenge system is to scavenge oil from the engine and pump it to the oil tank, ready to be pumped back into the engine by the pressure pump. In most dry-sump systems, particularly those that are additions to previously wet-sump engines, one pump body will take care of both pressure and scavenge operations. From a packaging perspective, this can be less than ideal, particularly within the tight confines of a Formula One engine. These engines, whether they be the previous generation of V8s or the new V6s, therefore tend to have separate pumps for scavenge and feed duties.
In the first iterations of Honda’s V10s the system relied on a single-stage scavenge pump. This drew oil from a sump pan that was split into chambers to reduce pumping losses associated with the movement of air and blow-by gases from cylinder to cylinder, caused by the rise and fall of the pistons. In later iterations of the V10, and then the V8, Honda moved to a multiple scavenge pump set-up as this was shown to provide a more efficient solution, scavenging more oil for less power consumption.
In effect this meant that each sump chamber had its own dedicated scavenge pump, which then fed into a common centrifugal oil-air separator before feeding into the oil tank. The layout Honda settled on until the end of its involvement with Formula One featured five scavenge pumps located on the right-hand side of the crankcase and a single oil feed pump on the left-hand side of the front of the motor.
On particular problem the team encountered during the development of the scavenge system was ensuring the consistent presence of oil at the pump inlets. Using the analogy of a straw, if the end of the straw is not completely immersed in liquid then it is almost impossible to suck the liquid up. The same applies to the inlets of the scavenge pumps, which have to be completely immersed in oil to operate.
Initially, Honda’s scavenge pump relied on the volume of oil displaced by the crank and rod rotating to fill the pump inlet. Unfortunately this approach could not guarantee a consistent supply, particularly under cornering where the g-forces were such that they pushed the oil away from the inlet. To combat this, Honda incorporated an oil trap into the inlet of the pump. This took the form of a small cavity just next to the pump inlet that acted as a reservoir that was filled by the oil thrown from the rotating components, ensuring a constant supply regardless of the g-forces.
Interestingly, owing to high operating pressures of 20-40 kPa, Honda also opted for a compression-type pump design rather than a positive-displacement pump for the scavenge system, the reason being the increase in pumping efficiency such a design offered. However, it did mean having to include a pressure relief system in each scavenge. This was because the pumps were specified to transport the light oil-gas mix that the crankcase contained; if they pumped no aerated oil, such as when the engine was initially started, the pressure increase caused by the higher-viscosity fluid could have caused damage.
As with the feed pump system examined last month, the level of development that went into optimising this relatively mundane engine subsystem is impressive, and highlights the level of refinement necessary in engines such as those in Formula One to ensure optimum performance and reliable operation.
Written by Lawrence Butcher