An engine after-cooling system (10) comprises an after-cooler (11) for cooling compressed air which is output from a compressor for combustion by an internal-combustion engine, a heat exchanger (19) for cooling a coolant which is output from the after-cooler (11), a pump (21) for continuously pumping the coolant through the system (10), an outlet air temperature sensor (27) for sensing the temperature of the compressed air after being cooled by the after-cooler (11), a coolant temperature sensor (17) for sensing the temperature of the coolant output from the after-cooler (11), and a controller (26) for controlling the flow-rate of coolant through the system (10) in response to the sensed air and coolant temperatures.