There are two types of coolants used to remove the heat from the engine block and head: air, and water. With air as a coolant, the heat is removed through the use of fins attached to the cylinder wall. With water as a coolant, the heat is removed through the use of fluid filled internal cooling passages.

    Figure 17.  Liquid Coolant

    Both have various advantages and disadvantages. Air systems are much noisier, since there are no water jackets to absorb the combustion sound. Air systems use fins to increase the heat transfer rate. An analysis of the heat transfer from a fin is performed with the Fin Performance Applet.

Water systems can freeze unless additives are used. The water cooling system is usually a single loop where a water pump sends coolant to the engine block, and then to the head. The coolant will then flow to a radiator or heat exchanger and back to the pump.

Figure 18.  Cooling System Loop

During engine warmup, a thermostatically controlled valve will recycle the coolant flow through the engine block, bypassing the heat exchanger. As the engine heats up, the valve will open up, and allow the coolant to flow to the radiator.

Nucleate Boiling of Coolant

The heat fluxes and surface temperatures near the exhaust manifold and port are high enough so that nucleate boiling can occur in the coolant at those locations. The boiling heat transfer coefficients are much larger than single phase forced convection, so that the metal temperatures are lower.

For heat fluxes of the order of 1.5 MW/m2 and above, the resulting surface temperature of the cooling jacket will be about 20 to 30 C above the saturation temperature , typically, 130 C (400 K). The convection boiling process is very complex, as bubbles formed on the jacket surface are swept downstream and condense in cooler fluid. The surface temperature of the jacket throughout the engine block will be fairly uniform. The saturation or boiling temperature can be raised by increasing the pressure or by adding an additive with a high boiling point, such as ethylene glycol.

The design of the cooling passages in the engine block and head is done empirically. The primary design consideration is to provide for sufficient coolant flow at the high heat flux regimes, such as the exhaust valves.