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Variable-displacement oil pumps

Most automotive engines, whether they are use for road or racing use, have fixed-displacement oil pumps. Most are directly driven by the engine, so in order to ensure sufficient oil supply at low revs – when the pump is turning slowly – the pumps are oversized for what is required at high rpm, where the pump is turning faster.

Traditionally, the excess pressure is dealt with by an oil pressure release valve that bleeds oil back to the sump if the optimum pressure is exceeded. Ultimately, this means that when the pump is flowing more oil volume than required, it is contributing excessively to parasitic power losses.

One development that could provide improvements and reduce these losses is the advent of variable-displacement oil pumps. Rather than rely on a release valve to control oil pressure, the volume of the oil pump itself is actively controlled in relation to oil pressure. In the past two years, a number of production cars have started to feature this type of pump as manufacturers strive to improve engine efficiency in order to meet increasingly stringent economy standards.

Both VW/Porsche (VW Audi Group) and GM use variable-displacement designs in their roadcars, although those from VAG differ in operation from those from GM.

The VAG group pump is of the gerotor type, where inner and outer rotors are used to pump the oil. The inner rotor sits on a driveshaft and drives the outer rotor. Because the inner and outer rotors have different rotating axes, more space is created on the suction side due to the rotating motion. The oil is drawn in and transported to the pressure side. On the pressure side, the space between the teeth becomes smaller again, and oil is forced into the oil circuit.

In a variable-displacement pump, however, the outer rotor is enclosed by a control ring that can move the position of the outer ring relative to the inner ring. The control ring is kept in place by a regulator spring, and only begins to move once the oil pressure pushing against it exceeds the spring pressure. When this happens, the control ring turns, decreasing the space between the inner and outer rotors, resulting in less oil being transported from the suction side to the pressure side and forced into the oil circuit. With less oil, the oil pressure is lower, and the nature of the system means that the volume of oil pumped is constantly varied in order to maintain the correct oil pressure.

The unit from GM, currently found in its 1400 cc turbocharged I4 Ecotec engine, works on a similar basis but relies on a vane-type pump rather than a gerotor unit; its operation is shown in the video below. Vane-type pumps are typically used in situations where low-viscosity fluids need to be pumped, and because of this – plus their extra complexity and therefore higher cost compared with gear pumps – they have not tended to be used in automotive oil systems. However, they are beginning to be seen in an increasing number of automotive applications thanks to their more efficient operation compared to geared pumps. 

A vane-type pump generates flow using a set of vanes that are free to move radially within a slotted rotor that rotates in an elliptical chamber. A typical configuration uses an elliptical cam ring, with the rotor spinning in a cylindrical housing and a pair of side plates to form the pumping chambers. The changing volume of the cavity between adjacent vanes creates the pumping action as the rotor rotates. In the case of GM’s pump, the variable output is achieved in much the same way as with the VAG unit, with an oil pressure-controlled ring varying the pump’s displacement by changing the volume of the pump’s cavity.

It is early days for variable-displacement oil pumps, but with manufacturers claiming reductions of about 60% in their power consumption, it is only a matter of time before they see wider adoption. 

Video: GM variable-displacement vane pump

Written by Lawrence Butcher

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