The science of engine development is one of incremental steps; small but measurable increases in power over a period of time. Improvements in port flow, increasing the engine rev limit or changes in intake or exhaust system sizes will, no doubt, help. But while that is undoubtedly the high profile, glamorous part of development, as far as engine performance goes, it is only half the story. The other, perhaps less glamorous part is that of minimising the parasitic losses; the friction in all the sliding components and the power to drive all the auxiliary drive systems - which includes of course, the oil pump.
In earlier articles we have established that oil pumps are invariably of the positive displacement type and it therefore follows that the power needed to drive them will be proportional to the volume flow rate produced and hence the engine speed. This flow rate will also vary with the temperature (and hence viscosity) of the oil and the clearances in the bearings but essentially for a given temperature the power consumed by the pump increases more or less linearly with speed. Traditionally the size of the oil pump is based around the flow requirement of the bearings at or near the engine idling speed. At this condition, classical bearing analysis assumes that the void between the journal and bearing will run full of oil but that any leakage from the sides will be made up by the flow from the oil pump.
As the speed increases most oil pumps therefore will be sized and have sufficient capacity to more than make up for the increasing leakage losses until the pressure reaches a certain pre-set value. At this point any further oil pumped will be directed back to the sump via the pressure relief valve. And while the output of the pump rises in proportion to the engine speed the demand from the engine bearing system does not. This additional pressure over and above that needed at the bearings therefore causes unnecessary extra pumping work. If we could tailor this output to that closer to the bearing requirement then pumping efficiency could increase and energy savings could be made.
The lube system in a modern engine can consume somewhere around 3-5% of the overall engine power output. Since these powers can be relatively small (a matter of a few bhp) and the complexity of producing a reliable variable flow pump onerous, manufacturers have often kept to the traditional approach of a simple pump and pressure relief valve. However as continuing improvements in fuel economy or performance becomes more difficult, the need to match the flow output of the pump with the demands of the engine assumes a higher level of importance.
One way of achieving this could be by using an electrically operated independent oil pump. While these have been suggested, I am not aware of anyone save on some kind of research engine who has as yet taken up this approach. Another way more recently introduced into the world of heavy-duty diesels is the split gerotor principle. Flow variation is achieved by dividing the rotor set in two along it’s axis and altering the radial position of the offset (the difference between the centres of the inner and outer rotors) of each of them but in the opposite direction. Much more complicated than the traditional approach, in this way however, the actual displacement can be varied to suit the demands of the engine.
Written by John Coxon.
Tags: oil-pumps
Assuming oil temperatures and pressures are constant, oil flow rates through crank journal bearings don’t change appreciably with rpm, since the bearing clearance (ie. the orifice flow area) doesn’t change. Journal bearing oil flow rates are based on cooling requirements, so bearing oil flows should be highest when engine power is highest.
As noted, a variable displacement oil pump would be beneficial for automotive engines. But not really so much for a racing engine. The reason for this is that racing engines run mostly at WOT, and automotive engines run mostly at limited part throttle.
A typical automotive engine oil pump might use less than 2 HP. The pump is a fixed displacement device, runs at a fixed speed ratio relative to engine speed, and is sized for flows needed for WOT operation. But that same engine also only produces about 20 HP most of the time it operates. So the oil pump loss typically amounts to 10% of the BHP output of the engine.
With a 600 HP race engine that operates mostly at WOT, a 2 or 3 HP oil pump loss is not a big deal.
So what was the solution Chevrolet provided with the Ecotec 1.4 L ? The only description I’ve found in a brief search almost sounded like double-speak that somewhat explains “what” it supposedly does but doesn’t really touch on the “how”. It reads:
Variable-Flow Oil Pump
The Ecotec 1.4L turbo uses a unique variable-flow oiling system that helps maximize fuel efficiency. Rather than the linear operation of a conventional fixed-flow pump, it is accomplished with a crankshaft-driven oil pump that matches the oil supply to the engine load. The Ecotec 1.4L turbo’s variable-flow pump changes its capacity based on the engine’s demand for oil. This prevents using energy to pump oil that is not required for proper engine operation.
Just curious. ?_?