Why water pumps will be electric (and oil pumps won't)

29 December 2018

Over the next ten years, water pumps will probably be replaced with electric motors.  They're more efficient, and easier to package up.  And for hybrid engines they are all but essential.  Oil pumps, on the other hand, will likely remain as pure mechanical pumps driven directly by the engine.  Stick with me while we start with the basics - you'll learn a thing or two about engines along the way!

Water pumps

The water pump propels coolant around the engine, and out to the radiator when the engine is up to operating temperature.  The coolant is needed to keep the engine from overheating - it carries heat away from the hottest parts of the engine. 

Although we call it a water pump, it doesn't actually pump water.  Or it shouldn't anyway - it should be pumping coolant, which is a mix of water and ethylene-glycol (aka antifreeze). 


Structure of a water pump

Water pumps are simple, inexpensive and they won't last the lifetime of the vehicle - they are replaced at the same time as the timing belt. As can be seen in this diagram, on 99% of today's engines, the water pump is driven by a pulley which is powered from the crankshaft.

Downsides of mechanical water pumps

There are two limitations with mechanical water pumps.  Because they are driven directly off the crankshaft, the water pump can only run when the engine is on, and if the engine is on then the water pump is definitely running.  The speed of a water pump is fixed in relation to the engine speed - the ratio of pulleys determines that, for example, each turn of the engine turns the pump twice.  

Enter the electric pump

Right now, there are a handful of engines using electric water pumps.  Volkwagen's W12 engine which powers the Audi A8 and R8 uses an electric pump in addition to a mechanical one, the VR6 engine (which is basically half a W12) also uses an electric water pump.  BMW's 3-series has had an electric water pump since 2006, and virtually all hybrids use motors - Toyota's Prius and Lexus' lineup.  

More control

As I mentioned above, the speed of a mechanical pump can't be varied based on need.  The pumps above are controlled from the ECU and that means they aren't using energy from the engine when it's not needed.  

With a mechanical pump, it's speed is determined by the engine speed.  But that's not what we want - we really want the pump to go faster when the coolant is hot.  That's what an electric water pump allows.  

Not only is there more control over the speed of a pump, but also over its placement.  A mechanical water pump needs to be driven by a belt and so it's got to be somewhere around the crankshaft with a big old pulley.  An electric pump, however, can be stashed anywhere that's convenient - it just needs a couple of wires.  This gives extra flexibility when packaging up the engine. 

And a bonus: Preventing heat soak

Drive your car hard for a half-hour and then turn off the engine.  It might surprise you to know that the engine temperature will actually rise after the ignition is switched off.  Why?  Because the coolant stops circulating immediately, but the cylinders and head are still full of heat from burning fuel just 1 second earlier. 

The bigger the engine, the bigger this issue because the hot parts are deep in a mass of metal.  And they don't come much bigger than VW's W12 monster, which is also designed to be driven hard.  So one of the main reasons the W12 includes an electric water pump is to allow the coolant to circulate after the engine is switched off.  

When the ignition is turned off, the water pump continues running for up to ten minutes to allow the heat to dissipate (unless the battery is low - smart!)

Stop-start technology and hybrids

With more and more vehicles adopting start-stop technology, the engine cycles on and off far more frequently.  And of course that means there are bigger issues with heat soak.  It's now expected that you'll drive at 60mph up to a set of lights and then kill your engine.  

Electric water pumps keep the coolant circulating during these stops - preventing the damage that comes with excess heat building up in parts.  Cylinder head warpage being one example. 

Long live the mechanical oil pump

The oil pump sends oil around the engine.  It's far more critical than the water pump.  If the water pump fails or stop the engine will probably overheat but no damage will be done.  A failed oil pump will lead to serious failure of the engine 100% of the time.  Just a short period running at full pelt with no oil, and the engine will be seriously damaged. 

Oil pumps are driven by the crankshaft - either directly or via gears.  If the engine is turning, then the oil pump is working.  An electric oil pump can stop working and leave the engine running - leading to an unacceptable risk of catastrophic damage.  

Additionally - an electric oil pump would need to be far more powerful than a water pump.  Cold oil is viscous (that means it's not very runny) and hard to pump.  And when an engine is cold, it's doubly important to get that oil pressure up.  Traditional oil pumps are super strong - they'll pump basically anything you can run through them.  

So there you have it - a little dip into the future of engines.  If you enjoy this level of detail then like How a Car Works on Facebook and checkout the video course for a total understanding of cars.