Cooling system for engine

Cooling device

In order for the engine to continue normal operation, it is necessary to cool heated parts such as the cylinder, the cylinder head and the piston as appropriate. However, as the combustion condition of fuel deteriorates when they are excessively cooled, the optimal temperature of the outer periphery of the cylinder is around 80 degrees Celsius.

In general, many cooling devices use the air cooling system or the water cooling system. In addition, there are the liquid cooling system using oil, the steam cooling system using heat of  vaporization generated when water evaporates and so on.

(1) Air cooling system

Cooling fins are provided in the cylinder and the cylinder head part of the engine to maximize the heat dissipation area, and heat is dissipated from the surface of the cooling fin into the air.

1) Natural air cooling system

The natural air cooling system is a system for cooling by wind while the vehicle is running, and the engine is mounted on the outside without being covered so that it is exposed to wind. Motorcycles use this system.

2) Forced air cooling system

The forced air cooling system is a system for forcibly cooling by sending air using a cooling fan, which is used for minibikes, small-sized gasoline engines and so on.

In the structure, the flywheel is equipped with a cooling fan, and a baffle plate is provided to cool the cylinder and the cylinder head evenly.

（2） Water cooling system

The water cooling system is a system for cooling by circulating cooling water in the water jacket provided around the cylinder block and the cylinder head. The system guides cooling water into the radiator and uses wind from the cooling fan for cooling. There are the natural circulation system and the forced circulation system depending on the method of circulating cooling water.

1) Natural circulation system

The system circulates cooling water by natural convection using the phenomenon that the specific gravity of water decreases as the temperature rises. Circulation occurs as water in the water jacket moves up when it gets warm and moves to the upper part of the radiator, and it gets heavy and moves down when it is cooled in the radiator, returning to the water jacket. This system is mainly used in the horizontal diesel engine.

2） Forced circulation system

The engine cooling system is the forced circulation system using the water pump. Cooling
water maintains an appropriate temperature using the thermostat. Cooling water sucked in from the lower tank of the radiator by the water pump goes through the water jackets in the crankcase and the cylinder head, cools down the cylinder and the combustion chamber, and returns to the radiator. As the thermostat is closed when the temperature of cooling water is low, cooling water goes through the return pipe and directly enters the pump, and circulates inside the engine to facilitate increase of the water temperature.

When the temperature of cooling water reaches the thermostat valve opening temperature, the thermostat opens gradually and heated cooling water is sent to the radiator.

■ Radiator

The radiator consists of the upper tank, the lower tank, and the core connecting the tanks.
The core consists of many water conduits (tubes) for sending water in the upper tank to the lower tank and the cooling fin (radiator plate), and is made of thermal conduction thinner brass sheet or the like.

The varieties of radiator fins include the plate fin type using a plate-shaped fin, the corrugate fin type using a corrugated fin and so on.

■ Radiator cap

The radiator cap is a device for maintaining the pressure inside the cooling system within
the specified pressure range, and consists of the pressure valve, the vacuum valve (negative pressure valve), the valve spring, the gasket and so on.

When the specified pressure is reached, the pressure valve compresses the valve spring and discharges cooling water into the reserve tank or the atmosphere.

On the other hand, when the temperature of cooling water decreases and the inside pressure becomes lower than the barometric pressure, the vacuum valve opens and cooling water or air is sucked into the radiator from the reserve tank.