Cooling System In I.C.E

11.1 Introduction Only a part of total fuel energy supplied to the internal combustion engine is converted to useful work. The work is delivered at the crankshaft and the rest of the fuel energy is rejected as follows: Heat from the engine boundaries due to radiation, convection, and to a small extent conduction. Exhaust heat. Heat rejected to the coolant. The exact proportion of the energy supplied to the engine (as fuel) which is converted to useful work is critically influenced by many physical characteristics of the engine such as engine design, type of the fuel used, cooling system, ……etc. in general (30%) of the energy supplied is converted to useful work ; about (30%) is lost as exhaust heat and some energy is lost in friction. The rest of the energy about (30%) has to be removed to the cooling system. Fig.(11.1) shows the heat balance diagram of a typical diesel engine.  The exact amount of the heat to the coolant varies and depends on many design and operating variables of the engine. 11.2 Necessity of Engine Cooling The question remains why cooling is required at all? The cooling of the engine is necessary for the following reasons: The lubrication oil deteriorates very rapidly with temperature increase. The strength of materials used for various engine parts usually decrease with increase in temperature. High engine temperature may result in very hot exhaust valve which may be give raise to pre-ignition and knock. If the temperature is high, the volumetric efficiency and power output of the engine is reduced. Q: should we cool the engine as much as we can? No, the engine must never be overcooled. The engine must always kept sufficiently hot to assure smooth and efficient operation. Disadvantages of over cooling The engine life is greatly decreased. Sulpheric acid may be produced which increase in corrosion. Starting is difficult.

11.3 Types of cooling systems All the heat rejected from engine untimely goes to air. Nevertheless two basic system are used to cool the engine. These are: Air cooling 2. Liquid cooling   Air cooling : in the air cooling of the engine, the heat transfer area is increased by finning the cylinder and cylinder head where the air is passed over them. Since the heat is directly transferred to air, this result in a very large temperature gradient between the gases and air. Fig.(11.3) shows the two types of fins used to cover the cylinder. Note: the length of fins and spacing between them is quite important for efficient cooling. *Application of air cooling: air cooling is usually used for small engines and for engines whose application gives extreme importance to weight such as aircraft engine, army vehicles and in cold climates. Other areas of air cooled engines are industrial and agriculture engines where there can be a strong objection top use of water as coolant. *Advantage of air cooling system 1. The direct heat transfer from engine to air elements the use of water (thermal losses are small). 2. The engine design become much simpler ( no need to radiator and water pump) as reduction in weight and size of engine is small. 3. lower danger (no leakage). *Disadvantage of air cooling system 1. complex nature of fins that are required. 2. the air fan is an additional source of noise. 3. the volumetric efficiency of an air cooled engine is lower due to higher head temperature.

Liquid cooling :in case of liquid cooling engines, the cylinder head and the cylinder are closed in a water jacket. Water is circulated through this jacket to affect cooling. The local velocity of water as well as the bulk rate of water passed through the jacket are important. The cast surfaces are usually machined to improve the heat transfer coefficient. Some times drilled coolant passages are provided to cool highly stressed parts. In case of cold starting many types of anti-freeze solutions are added to the water. Commonly used anti-freeze materials as: Kerosene 2. Wood alcohol 3. Sugar solution,….. etc Various methods are used for circulating the water a rounded the cylinder head. Theses are: 1. Thermo-siphon cooling 2. Forced or pump cooling 3. Cooling with thermostat regulator 4. Pressurized water cooling 5. evaporator cooling 1. thermo-siphon cooling : fig. (11.4) shows thermo-siphon arrangement the top of radiator is connected to the top of water jacket by a pipe and bottom of radiator is connected to the bottom of water jacket. Water travels down the radiator across which air is passed to cool it. The air flow can take place due to vehicle motion or a fan can provided. The main draw backs of this system is that: 1. Cooling depends only on the temperature and its independent of the engine speed. 2. the circulation of water starts only after the engine has become hot enough to cause thermo-siphon action. 3. radiator may be above the engine for gravity flow of water to engine. 11.4 Radiators The function of radiator is to reject coolant heat to the outside air. The name of radiator is a misnomer because the heat transfer from coolant to the air by forced convection and conduction and not by radiation. There are two types of radiator in common use: Down-flow type: water flow from top to bottom Cross-flow type: the hot coolant is supplied to the top of supply tank , it flows across the radiator and is taken out from bottom of the collector tank.

The material used for radiator must have : Good corrosion resistance Good thermal conductivity Easy formable. For this reasons we use yellow brass and copper and some times and sometimes aluminum used where weight is very critical. Q: compare the quantity of cooling water required for 100 bhp petrol and diesel in which the water is raised in temperature by 30 c in passing through jackets. In petrol engine the percentage of energy going to coolant is 30 % and in diesel engine 26%. The efficiency of petrol engine is 26% and of diesel engine 31%.