Most modern liquid cooled engines employ a pump. The pump ensures a more positive circulation of the coolant and is necessary for engines of high power output and complicated cylinder block and head construction which would not receive adequate cooling from thermo-syphon system.
Many goods vehicles are fitted with tilt cabs which require a low-height coolant radiator to give adequate clear Angeles when swiveling the cap. A header tank mounted on the back of the cab is normally employed on those vehicles.
This type of pump employs a disc-shaped impeller with widely spaced vanes on one side. The vanes are usually, but not always, deeper at the impeller centre than at the edge, and they may be straight or curved. The impeller is secured to one end of its driving shaft and the drive pulley and fan assembly are secured at the other end, the shaft being mounted in bearings which are housed in the pump casing. A special grease is normally used to pre-pack the pump bearings during assembly, and they do not required any attention in service.
How impeller pump works
When an engine is running, the impeller rotates, coolant enter the pump inlet and passes to the centre of the impeller which forces the coolant by centrifugal force along the vanes to their outer edges. It is flung off the impeller and directed to the pump outlet. If the thermostat valve is closed, the coolant leaving the pump is returned to the pump inlet via a bypass passage which may be internal or external. This type of pump does not operate with high delivery pressure and it is not used with some of the larger heavy vehicle engines for this reason.