LO 11-6 Identify and describe the construction features, types, and applications of the diesel engine cooling components

Water Pumps
To ensure the positive circulation of coolant, an engine-driven water pump moves coolant through the cooling system. Pumps can be belt- or gear-driven Figure 46. A gear-driven pump does not rely on a belt, which minimizes service requirements, and is also more reliable than a belt for long distances. All water pumps are centrifugal-type turbine pumps, which means they become more efficient the faster they turn. The impeller, which transfers the pump’s energy to the coolant, is located in the coolant and may be made of plastic or metal. Impellers can wear if a significant amount of abrasives circulate through the cooling system Figure 47. Plastic and metal impellers that are pressed onto the water pump shaft can also become separated from the shaft and cause poor coolant flow. This problem is usually identified when the engine overheats but the lower radiator hose, which is normally hot to the touch, is instead cold or lukewarm because radiator coolant is not being pulled by the pump into the engine.

FIGURE 11-46: A gear-driven water pump.

FIGURE 11-47: Centrifugal metal-type water pump impellers. Abrasives in the cooling system have eroded the tips of the impeller on the left.

Coolant enters the center area of the pump impeller and is flung off the impeller vanes at high speed to create pressure and flow. The inlet to the water pump is the lower radiator hose or some point located near the lower radiator hose inlet. Water drawn into the pump is discharged into the engine. Under rapid acceleration, internal engine block pressures between the pump and the thermostat can reach as high as 40 psi (276 kPa). To reduce energy losses from driving water pumps, manufacturers are now introducing electrically controlled, variable-speed water pumps to help meet GHG targets Figure 48. A common configuration uses a clutch that controls the speed of the water pump and is signalled by the engine control unit (ECU) using a pulse width modulation (PWM) electrical signal. A speed sensor on the pump provides data for closed-loop feedback control of the water pump speed Figure 49.

FIGURE 11-48: An electronically controlled, variable-speed water pump reduces engine drag when coolant flow is minimal.

FIGURE 11-49: This 2015 Paccar MX engine uses an electrically controlled, variable-speed water pump. Note the speed sensor and another connector supplying a PWM signal, which regulates water pump speed to minimize parasitic loads on the engine.

The use of a centrifugal pump means that at low engine speeds with heavy loads, coolant flow can be dangerously slow. Construction equipment engines often encounter these conditions. Conversely, at high engine speeds with low loads, such as those encountered by an empty bus on a highway, coolant flow can be excessively high, consuming power and fuel unnecessarily. Hydraulic or electrically driven pumps turning at speeds independent of the engine are increasingly used to prevent these inefficiencies.

Water Pump Bearing Seals
Separating engine coolant from the water pump shaft support bearings is a seal. Generally made from long-wearing ceramic, failure of the seal can potentially allow a catastrophic internal coolant leak into the engine from gear-driven water pumps. Seal failure in belt-driven pumps causes an external loss of coolant when the cooling system is pressurized, which can result in overheating of the engine. The use of a weep hole in the bearing housing of both types of water pumps can limit the possibility of pressurized coolant leaking past the water pump seal and through the bearing, which can cause catastrophic bearing failure. In the case of gear-driven water pumps, the weep hole prevents coolant from leaking into the engine, outside of and away from the pump. If coolant reaches the bearings, a major failure of the water pump will occur Figure 50. To prevent this from happening, manufacturers add a weep hole between the seal and the bearings Figure 51. If the seal is worn and damaged, the weep hole allows coolant to leak from the cavity inside the water pump to prevent bearing damage. Some coolant leakage from a weep hole is normal, and some water pumps hold any small leakage of coolant in a small reservoir, where it evaporates.

FIGURE 11-50: The water pump seal prevents coolant from contacting the pump shaft bearings.

FIGURE 11-51: The weep hole of a water pump on a Mack E7.