A correctly-working oil pump is vital to good engine health. If the oil pump starts to fail, the oil pressure will drop. And when the oil pressure drops too much, the end result can be catastrophic, such as seized mains or rod bearings. In this technical feature, we’re going to lift the lid on some oil pump design secrets!
Oil pumps are termed as positive-displacement pumps. The rotors within the pump move to create an open void that expands with movement and the expanding void is filled with oil. As the rotors move around, they take the oil with them, and the void then compresses, forcing the oil out of the rotors.
Internal and External Pumps
Broadly speaking, there are two types of positive-displacement rotary pumps – internal rotors and external rotors.
In an internal rotor pump, an inner rotor with external teeth runs inside an outer rotor with internal teeth. The inner rotor will have one less tooth than the outer rotor, and the two rotors run on fixed eccentric axis. As the two rotors rotate, the volume created between the meshing teeth expands and contracts the oil.
Most internal rotor pumps need to have a crescent shape in the space under the inner rotor. This crescent separates the two rotors, creating a seal that stops leakage from the high pressure volume back to the low pressure volume. However, this crescent is eliminated in a gerotor pump like our popular Cosworth YB 2WD oil pump (gerotor is short for generated rotor). The gerotor design does away with the crescent, creating a compact and simple solution.
External rotor pumps consist of two identical rotors running side by side. As with the internal rotor pump, one gear drives another, and oil is moved through the rotors by the expanding and collapsing void when the teeth mesh. Usually the two rotors have parallel teeth, but in other applications the teeth can be angled in a chevron pattern.
Rotor Teeth Profile
Did you know that the profiles of the inner and outer rotors in an internal rotor pump are anything but simple curves – in fact, the profiles belong to the “trochoid” family of mathematical curves.
If you’re old enough to remember playing with a Spirograph toy, then you’ll remember the curve created by pushing a pen through a hole in a small cog and then rotating it inside or outside a larger toothed disc. These curves are actually called epitrochoids and hypotrochoids, and these curves are the starting points for the design of the rotor profiles. When the oil pump rotors are first designed, the chosen curves for the rotors are carefully adjusted to reduce the build-up of stress concentrations in the roots of the profiles, and to also provide good resistance to debris without compromising volumetric efficiency.
Its not just the profile of the tooth that determines the how the pump will behave. The other important design consideration is the amount of eccentricity between the inner and outer rotors. This eccentricity influences the shape of the rotor profile.
The number of teeth chosen will have a major impact on pump performance. Fewer teeth mean that the overall diameter of the pump can be smaller, which in turn makes it easier to package in the engine. Fewer teeth also result in a reduction in power loss for a given flow rate, but the downside is that they have to run at a higher rotational speed, which can increase wear.
On the other hand, more teeth will give fewer pressure fluctuations and hence a smoother overall oil pressure. The fluctuation is sometimes referred to as pressure or flow ripple, and is an unwanted side-effect of any positive-displacement pump. The effect of pressure ripple is mechanical vibration in the oil circuit in the engine, which can damage components such as oil hoses, seals, and bearings, so any measures that can minimise pressure ripple are likely to be of great benefit.
The clearance between the rotors themselves and between the rotors and the end faces will also have a significant effect on pump efficiency.
Reducing these clearances will have a knock-on reduction in leakage of oil across the end faces from the pressure side to the suction side. However, not enough clearance can cause the rotors to bind and jam up, plus debris will become trapped and then damage the pump.
Fit Genuine Parts!
The rotor profiles and clearances in our oil pumps have been carefully designed by Cosworth engine designers to match the requirements of the YB engine, and that’s why we believe that its important to fit genuine Cosworth parts like these. If you’d like more information on the Cosworth YB oil pumps that we stock and sell then please get in touch via our Contact page.