Oil system CFD simulation

Thursday, March 27, 2014

Tags :  oil-system

Understanding the way oil moves around an engine is key to ensuring an efficient lubrication system. To this end, CFD simulation of oil flows can prove highly beneficial to the engine designer. While CFD in the racing environment is more commonly thought of in the context of vehicle aerodynamics, it also has extensive applications in the powertrain, particularly with regard to inlet charge and exhaust gas behaviour. However, this type of simulation, given the right tools and understanding, can be used wherever there are fluid or gas flows.

With this in mind, it should come as no surprise that some engine developers have harnessed the power of CFD to create fully dynamic simulations of engine flows, encompassing both the properties of the fluids running in and around the engine, and accounting for factors such as heat rejection. Such advanced levels of simulation are exceptionally complex though, requiring large amounts of computing power to obtain high-resolution solutions. Luckily, with the emergence of open source CFD packages such as OpenFoam, and cloud-based computing, the ability to analyse specific areas of interest have been brought within the reach of a far wider range of engineers.

One particular area of the lubrication system into which CFD can provide useful insight is the design of oil pumps. The pump is arguably the most important component in the lubrication system, so in a highly optimised race engine its operation needs to be as efficient as possible.

The most common pump used in race engines, which invariably feature dry-sump lubrication, is the gerotor type. This consists of an external and an internal rotor, with the outer rotor driven by the rotation of the inner one, which is connected to a driveshaft. The pump body will have one or more inlets and outlets, and as the rotor rotates, the teeth of the inner and outer rotors disengage on the inlet side, creating a partial vacuum that draws oil in. The oil fills the volume between the teeth, and as the teeth begin to re-engage, it is forced out of the outlets.

Theoretically, the flow rate of the oil is a function of the gears’ rotational speed, but in practice leakage through the gaps between the gears reduces efficiency. It is therefore beneficial for the pump designer to be able to model exactly what is going on inside a pump. While it is possible to physically test pumps on a rig, comparing different rotor geometries and body designs requires the production of multiple components. However, by using CFD to perform such analysis, changes need only be made in the CAD models, with the only limitation on the number of iterations that can be tested being the computing power available.

Provided the CFD model is properly validated, it can also be used to investigate other important phenomena, for example the formation of bubbles and cavitation in the pump body, or the impact that variations in features such as the shape and angle of the inlet and outlet ports has on oil pressure and flow. Being able to assess the benefits of changes in these areas can have a small but significant impact on overall engine efficiency, leading to potential gains in both lubrication system effectiveness and reductions in parasitic power losses.

It would be fair to say that, given the current state of development of CFD and its now widespread availability, such detailed investigations are no longer the preserve of the big engine manufacturers and are within reach of smaller engine developers.

Written by Lawrence Butcher

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