This white paper will look at coolant pumps which are required for thermal management. Even in a highly efficient electric vehicle powertrain, there are still cooling requirements for the motors, power electronics, cooling, and heating for the battery systems and of course the vehicle occupants.
Whilst the thermal management heat rejection loads in an electric vehicle are massively reduced compared to an internal combustion engine due to the very high efficiency of the electric drivetrain components, the thermal management challenge is complex due to relatively low required operating temperatures reducing the temperature gradient to the outside air and also the need for highly efficient ancillary systems to maximise battery range.
Every EV requires an electric coolant pump and electrification of the coolant pump is also a key technique for improving internal combustion engine (ICE) efficiency in conventional and hybrid electric vehicles, whereby the traditional engine front end belt driven pumps are replaced by highly efficient and controllable electrically powered alternatives.
Coolant pumps are typically centrifugal pumps where an impeller spins in the coolant, fluid is drawn into the centre of the impeller and flung Radially outwards due to centrifugal action where it is directed by a shaped housing known as a volute into the vehicle pipework. This style of pump is favoured as it is able to operate across a wide operating condition range and is resistant to deadheading.
In an electric coolant pump, the impeller is driven by a motor, high-efficiency brushless DC (BLDC) motors sometimes known as EC motors which stands for electronically commutated are typically used due to their long life and high efficiency. There are 2 choices for sealing arrangement on the pump. 1) Simply to have a face seal which keeps the fluid out of the motor. 2) To have a sealless pump.
Firstly, sealed pumps are the classic design for automotive coolant pumps. They use a sprung face seal where two running surfaces are pressed together using a spring to create a seal.
The seal faces are continuously in contact and typically require a small amount of coolant to pass between the faces to act as a lubricant and coolant. Despite using self-lubricating materials and very resistant materials running these face seals dry will lead to seal failure as the contacting surfaces overheat and wear out. The seal can also be subject to damage from debris in the coolant. In general, the seal will always wear out and whilst improvements have been made to face seal technology to enable long lifespans this often does not meet the needs of the OEM especially in heavy duty vehicle applications such as off-highway machinery and on highway truck/bus.
In an electric or hybrid vehicle due to the lower temperature and pressure nature of the cooling system the small quantity of fluid that intentionally leaks past the seal can also be problematic. In a conventional ICE application, this leak by fluid is hot enough and under pressure so that it typically immediately evaporates when it leaks past the seal. However, in an EV or HEV cooling system the coolant tends to be at a much lower temperature, so the fluid does not evaporate this can lead to small droplets of brightly coloured coolant appearing on the pump or worse on the floor under the vehicle. This is often misdiagnosed as a fault with the coolant pump.
The advantages of a sealed electric coolant pump are that the motor can be designed to be very efficient with a very tight air gap and overall the pumps will be highly efficient and they are more tolerant of debris, particularly fine ferrous particles that can cause issues for sealless pumps.
There are two styles of sealless coolant pump.
- Magnetically coupled pumps where the motor is separate to the impeller/volute unit and coupled with a magnetic drive. The torque from the motor being transmitted to the impeller via a magnetic coupling
- Wet rotor pumps, where the pump motor is designed to run with the rotor exposed to the coolant
As the motor is separate from the pump impeller in a magnetically coupled pump a very efficient motor can be used, however the magnetic coupling requires costly additional magnets.
These magnets in the coupling can act as a magnetic filter on the cooling system and becomes clogged jamming the pump. The magnetic coupling also adds physical size and mass to the pump unit. For these reasons mag coupled pumps are not common in mainstream automotive applications, for although they eliminate some of the reliability issues of the seal they introduce new failure modes, additional costs, and packaging volume and mass.
The alternative and more common sealless pump is a wet rotor style coolant pump. In this style of pump, the motor is designed so that the stator is sealed off from the coolant by a static seal and sleeve arrangement and the rotor spins inside the sleeve completely submerged in coolant. A small amount of coolant is allowed to flow through the rotor cavity to act as a coolant and less lubricant for bearings.
This design eliminates the reliability issues of the seal and the complexity of the mag coupling. However, the key issue then becomes the reduction in efficiency of the motor due to the increased “air” gap and the additional design and manufacturing complexity required for the coolant flow through the rotor cavity and bearings and the sleeving design. In order to address the efficiency highly concentrated windings need to be used and high strength magnets which increase material costs compared to a dry running motor. Finally, care must also be taken to ensure that non-ferrous particles enter the pump which could build up and jam the rotor. In a well-designed system, the wet rotor pump should have a very long service life.
Overall the sealed pump is not preferred for low-temperature applications due to the issues of run dry and weepage past the seal. Despite its good base efficiency, the mag coupled pump adds cost, weight, and size which are always at a premium. The wet rotor pump is gaining considerable traction as the best type of coolant pump for these applications due to its lack of a seal and the associated issues. Well-designed it can have an efficiency close to that of the best in class sealed pumps and the additional material costs in the motor can be offset by savings elsewhere.
AVID Technology has experience in the design and manufacture of sealed and wet rotor coolant pumps for automotive applications. AVID has its own BLDC motor and motor controller power electronics technology both conventional high power density radial flux and ultra-high power density axial flux. We can provide a complete custom electric coolant pump design or standard off the shelf products.
For more information please contact us.