ENGINE Chapter 8 Cooling System

ENGINE

Chapter 8.

Cooling System

 1. Cooling system

Of the total heat energy generated by the combustion of fuel mixture in gasoline engine, about 30% is converted into the kinetic energy to push the piston, another about 30% is wasted with exhaust gas,  another about 30% is transmitted to coolant through the piston and the other about 10% is lost by friction.

Among them, if the heat transmitted to the combustion chamber wall is not eliminated as quickly as possible, the piston or the cylinder will be deformed by this heat or the film of lubricant oil will be broken.

If this heat is cooled excessively, the much heat energy will be transmitted to the coolant so that the heat efficiency will be degraded. Therefore, cooling system should be controlled to maintain the proper temperature according to the driving situation.

The cooling system for vehicle engine is classified into the water-cooling system and the air-cooling system. The air-cooling system is hard to cool uniformly and easy to make a loud noise, so nowadays almost this system is not used in the vehicle.

According to the flow type of water cooling system, there are the U-turn flow type flowing from one side of the engine to the same side, and the cross flow type flowing from one side to the opposite side. Additionally, according to the flow direction, it can be classified into the longitudinal flowing type flowing along to the longitudinal direction of the engine, and the lateral flowing type along to the lateral direction of the engine.

In the water-cooling system, the cooling water is circulated by water pump from lower portion of the water jacket to the radiator. During running, strong winds can cool the radiator, however, when the car is stopped or slowly driven, a fan should send winds in force. The cooled water shall be returned to the water jacket by the pump. By installing a thermostat between the water jacket and the radiator for sensing the cooling water temperature, if the cooling water has too low temperature, the thermostat blocks the water passage. The most used thermostat is the wax type in which a wax is secured between capsules, and the wax opens and closes the valve using the expansion and shrinking by heat. The thermostat working temperature is about 85℃.

2. Radiator

The radiator is a body radiating heat, in this case, it is a device for radiating the heat into the atmosphere. As the heater is called also as radiator, the cooling device in a car is called as radiator; especially this is called as heat core.

To enhance the efficiency of radiator, the area should be large as possible. Generally, the radiator comprises a radiator core manufactured by welding many metal fins around a tube flowing cooling water therein and two tanks containing the cooling water at both sides of the core.

The tanks are attached at the upper portion and lower portion of the heat core to flow hot water to upper tank and cold water to lower tank using the convection principle.

Down flow type

Cross flow type

U-turn flow type

It is the most used type so called the down flow type. There is the cross flow type in which the tanks are attached at the left and right side to flow in horizontal. In the cross flow type, the height of radiator can be lowered and the design of front grill can be freely changed. However, it has larger flow resistance. For another example, there is an U-turn flow type in which the radiator core is divided in up and down at the center portion, and the cooling water flows into the one side of the upper core and returned from the lower core side.

The aluminum material is more used in both the tube flowing water and the fin colliding with wind. To make be lighter, the resin tank including nylon with glass fiber instead of brass or aluminum is more used.

High temperature

Low temperature

The radiator is equipped with a radiator cap to supplement the cooling water. Conventionally, this cap is just a cover so the cooling water can contact outer atmosphere directly. Nowadays, the cap secures the inside of the radiator that is the pressurized radiator cap. In the atmospheric pressure, the water is boiling at 100℃ and not increasing the boiling point. By pressuring the cooling water, the pressure is increased and the boiling point of water will be higher so the difference with the outer temperature is larger. Therefore, the cooling effect shall be increased.

At the pressurized radiator cap, a pressure valve and a vacuum valve are attached, When the cooling water is about 110∼120℃ and the inside pressure is high, then the pressure valve will be open to take out the cooling water; when the temperature is low and the pressure is lowered, then the vacuum valve will be open to suck the cooling water to the radiator. So the pressure of cooling water is maintained uniformly.

3. Cooling of the Cylinder Head

The most heated part of the engine is the combustion chamber. The heated cylinder and the cylinder head is cooled by the cooling water, the piston is cooled by the engine oil.

This heat affects to the temperature of fuel mixture, the combustion condition, and the temperature of exhaust gas, that is, to the engine performances. How to cool the cylinder head is the most important thing.

The most cylinder head parts are made of aluminum alloys because it has high heat conductivity and light weight. As the intake port having the similar temperature of outer atmosphere and the exhaust port taking out the so hot exhaust gas are nearly installed, they may deformed by the difference of the heat expansion. Therefore, it is very important to cool with balancing the each part temperature.

The cooling water is took into the cylinder head from the water jacket at the cylinder block, and flown around the exhaust port heated by the exhaust gas, and then took out via the intake port. Doing so, the temperature difference among the cylinder head, the exhaust port and the intake port can be minimized. It is possible to prevent the engine parts from being deformed or distorted by the difference of the heat expansion.

Additionally, the each temperature around each cylinder head in the series of cylinder should not have big difference. So the temperature of each cylinder should be similar as possible.

For example, if the cylinders are sequentially cooled from the front to the end, then the later cylinder can not be effectively cooled because the cooling water is already heated. In some cases, the cooling water is distributed to each cylinder at the beginning of the cooling to be cooled equivalently.

The oil can be used for cooling the cylinder head. However, the portions of the cooling effect by the oil is about 20%, the most cooling is performed by the cooling water.

4. Overheat

The water temperature of the engine is decided by the balance between the generated heat capacity from engine and the radiated heat capacity from radiator. If the cooling is not enough, then the cooling water can be boiled. Consequently, water vapor can be ejected from the radiator cap. This is called overheat. If the car is going with overheat state, then the performance will be degraded, at last the engine will be stuck.

If the engine is normal and the thermometer is varying, then it means that overheat will be occurred. The main reasons are four; one is that the wind amount passing the radiator is too small; another is that wind temperature is too high; the third is that amount of cooling water is too small and the last is that the engine is working in hard situation continuously.

When a car has aero parts or large fog lamp so the wind passing is hindered, or when the car runs unpaved road so the radiator covers with dirty thereon, then the cooling water can be easily overheated. In other case, if the fan belt is not tightened or broken then the cooling water can be easily overheated because the wind amount sent to the radiator is reduced.

To tune up the turbo engine, when a large intercooler is installed in front of the radiator, then, then wind amount will be reduced and the cooling water may be overheated.

If the water pipe is old so the cooling water is leaked, or the belt for driving the water pump is loosened, then the cooling water may be easily overheated because the cooling water is deficiency.