Volvo Diesel Engine Cavitation

Volvo Diesel Engine Cavitation

Pump suction capacity depends on coolant pressure and temperature. An increase in coolant temperature reduces suction capacity. In order to avoid cavitation there must be a static pressure on the suction side of the pump. To achieve this there must be a hose from the expansion tank to the suction side of the pump. Cavitation occurs when static coolant pressure inside the pump falls to coolant vapor pressure. Some of the coolant then vaporizes and vapor bubbles are formed. These vapor bubbles follow the coolant a little way and when they reach an area in the pump with higher pressure they implode. A pressure pulse is formed at every implosion and if this is repeated often enough mechanical damage to pump materials may occur (above all to the impeller). At the same time, pump performance is impaired.

Thermostat

A thermostat is installed in the cooling system to maintain coolant temperature within a certain temperature range. The cooling system comprises an inner circuit and an outer circuit through the radiator. The thermostat is closed when the Volvo engine is cold and during the warm-up period. All coolant circulates in the internal circuit in order for the engine to reach the correct operating temperature as quickly as possible. When the coolant reaches thermostat opening temperature it begins to circulate through the external circuit. The thermostat controls coolant flow to the radiator and thus maintains coolant temperature at the correct level for different engine loads.

These cooling system differences have no influence on external cooling system design. The thermostat opens fully at extreme engine loads and high ambient temperatures. The internal coolant system is then fully isolated and coolant circulates through the radiator.

NOTICE! If the thermostat is removed, the following will happen:

• The warm-up period the engine requires to reach operating temperature will be much longer. Furthermore, the engine will not reach operating temperature at idle or low to medium loads at moderate ambient temperatures.

• Engine lubricating oil temperature will not reach the correct level, which increases fuel consumption.

Exhaust emissions will increase (more white smoke) and engine power will be reduced a little. It will also increase engine wear and shorten engine operating life.

• System cooling capacity will also be impaired as not all coolant will pass through the radiator (uncontrolled coolant flow).

Even if the temperature gauge shows the correct coolant temperature the coolant may boil in the engine water jacket.

• Engines that are operated without a thermostat are not covered by warranty.

Example

TAD1370 thermostat position at increased coolant capacity to the retarder cooler.

The thermostat location is moved if increased coolant capacity to the retarder cooler is selected. This unit may only be ordered as an option when ordering a new engine.

The thermostat is moved from location A (original position) to the new unit B. The original position A then has no thermostat.

Radiator

The radiator is a coolant-to-air type heat exchanger and forms part of both the coolant and cooling air systems. Hot coolant from the engine passes through the radiator pipes and is cooled by cooling air with the aid of a fan. The radiator must have external dimensions that are as small as possible, but still have good cooling capacity and small pressure drops on the air and coolant sides. The radiator core may not be too compact in extremely dusty conditions, as it may become clogged.

Heat from the coolant must be transferred to air through the radiator tube core. The heat transfer coefficient is much higher on the coolant side than on the air side and therefore the contact surface between the air and the tubes must be enlarged by means of fins. The radiator comprises an upper and lower tank, coolant tubes, fins and a frame.

Volvo Penta uses aluminum radiators:

The gills on the fins increase the heat transfer coefficient further.

The following factors are important when selecting a radiator:

- Engine heat transfer to coolant.

- Heat transfer from accessory components (e.g. torque converter oil cooler) to coolant.

- The maximum temperature of the air entering the radiator.

- Maximum coolant temperature at the radiator inlet

- Cooling air flow and direction (puller and pusher fans).

- The pressure drop on the coolant side.

- The pressure drop on the cooling air side.

- Radiator core surface area.

- Vane type and vane interval.

- The amount of dust and foreign particles in cooling air.

- The ease with which it can be damaged and the type of guard that may be used.