Why are car oil pumps configured that way?

Why do car oil pumps look so different on their inner-workings compared a car’s water pump? A water pump is pretty easy to understand why it looks that way, a fan-shaped impeller contained in a circular chamber, with an inlet port and an outlet port. It’s pretty easy to see how that configuration works to pump water.

But oil pumps for cars, I’ve never understood why they have that weird configuration. At least on the ones I’ve encountered, there’s a 4 equally spaced pronged gadget, fitted inside a chamber with not 4 holes, but 5 equally spaced holes. There’s an extra hole for some reason, which makes the 4 prong gadget seem like an awkward fit at best.

So how does this type car oil pumps work, and why do they use that 4-prongs-in-5-holes configuration vs the simple-to-understand configuration used in a water pump?

Here’s a link to a photo, you may have to cut and paste to get it to work.

Several differences.
First, the oil pump is positive displacement.
At a given rpm it moves a fixed volume of oil (gallons per minute) regardless of pressure.
The water pump will spin happily with moderate pressure even with the flow blocked off.
Do that with an oil pump and the pressure will rise until something breaks.
The oil pump develops much more pressure than a water pump, 10-100psi vs maybe <5 psi.
The extra lobe on the outer part of the oil pump means there’s always some open space between inner and outer.
As the whole thing rotates each pocket opens and closes, giving the same effect as a piston going up and down.
Holes on the side provide a path for oil to be pumped in and out of the pockets.

A water pump design would not pump oil as needed. That is to say…a lot of oil under consistent pressure. I’ve never tried it but I highly doubt that an oil pump design would pump water.

A water pump doesn’t provide pressure. It just needs to provide flow.

An oil pump needs to provide both pressure and flow.

So there has to be a means to displace the oil to provide that pressure. And this done by providing and extra slot in the driven gear than in the drive gear.


I understand a lot more now that I did 5 minutes ago, thanks. The animation in circuitsmith’s post is surprising to me, I didn’t realize the inner gear wasn’t locked in position w/respect to the outer gear. Instead, it slowly moves around. I guess as it moves from slot to slot, that is what squeezes the oil out. I can see w/that config you’d get a measured amount squeezed out per rotation.

Is it accurate to say the oil pump sort of “squeezes” the oil out, vs the water pump pushes the water?

Yes, “squeezes” is a good way to put it.

One more question for clarification. The animation above shows the center gear slipping exactly one notch per revolution. Is that an artifact of the animation, and the rate of slippage (in notches per revolution) actually varies depending on the rpm, oil flow rate, pressure, etc? Or are these two gears synchronized so that the slippage is always one notch per revolution?

The driving gear always moves one notch per revolution relative to the driven gear.

The oil pressure is controlled depending on the revolution either by an oil pressure relief valve built into the engine, or by a by-pass valve built into the oil filter.


George, I think the thing that causes the optical illusion effect is the fact that the inner and outer parts of the pump are turning on different axis and are different size “gears”, and each is turning at a different speed, as different size gears do. That offset creates a visual effect, however neither gear is slipping against the other. The surfaces are actually rolling smoothly against one another.

Different designs are simply to accommodate different goals. A water pump is designed to provide volume, an oil pump is designed to provide pressure. Each design is specific to its purpose.

There are a few different configurations of these pumps. Here’s a good video that describes several:

Good video, my friend.

I dunno, when I did my timing chain, I’m just glad the factory manual said to pack it with vaseline to prime it first or no oil would flow.

The wikipedia article as referenced by the OP is interesting re earlier discussions on timing belts and timing chains, as it uses a timing gear setup.


Good descriptions…so good in fact that there’s nothing to add.