Need for cooling and cooling types in diesel engines

Need of cooling system

During combustion the gas temperature in a diesel engine reaches about 2000 °C. About 25–35 % of this heat flows into the cylinder walls, head, piston and valves. Cooling is needed to:

• keep the cylinder wall temperature low (about 150–200 °C) so the lubricating oil film is not burnt;
• prevent seizure of the piston from uneven expansion;
• avoid cracking of the head and warping of valves from thermal stress;
• keep volumetric efficiency and strength of the materials.

Over-cooling is also bad (more heat loss, poor combustion, corrosion), so temperature is controlled with a thermostat.

Cooling systems

1. Air cooling (small sets)

Fins on the cylinder and head increase surface area; a fan blows air over them.

• Simple, light, no water needed.
• Uneven cooling, noisy; only for small engines.

2. Water cooling

(a) Open (once-through) system

 River/lake -> pump -> filter -> engine jacket
                                    |
            back to river <---------+ (hot)

Water from a natural source passes once through the jacket and is discharged. Cheap and simple, but needs a large water source; impurities cause scaling.

(b) Natural circulation (thermosyphon) system

   +--> radiator/cooling tank --+
   |   (hot water rises)        | (cool water sinks)
   +---- engine jacket <--------+

Circulation is by density difference only; no pump. Slow circulation; suits small engines.

(c) Forced circulation, closed system - used in diesel power plants.

 +-> engine jacket -> thermostat -> heat exchanger -+
 |                                     ^ raw water  |
 +-- pump <------- expansion tank <----+------------+
          cooling tower or spray pond
          cools the raw water

A pump circulates soft (treated) water through the jacket and a heat exchanger; raw water from a cooling tower or spray pond takes the heat away. A thermostat bypasses the cooler until the engine warms up. Water loss and scaling are small, and temperature is closely controlled.