Comparison of Sealed and Sealless Pumps for Automotive Applications
Pumps are required for several key functions on a vehicle, oil pumps for lubrication, coolant pumps for cooling and refrigerant pumps for air conditioning systems to name three. 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