IC Engine Cooling System

Introduction:

Generally, for safe working of the engine, it is necessary to carry 30% of the total heat generated by the combustion of fuel through cooling of the engine. The temperature of the gases in a reciprocating IC engines varies from 40⁰C to 2500⁰C during the cycle. In addition to this, the heat is also carried by lubricating oil which also accounts for 1 to 1.5%.

If the engine is not cooled, then the cylinder and piston temperatures may exceed 1500⁰C. At such higher temperatures, the metals will lose their properties and expansion of piston will be considerable and seize the liner. The lubrication of the engine will be badly affected if the engine cylinder exceeds 80⁰C because the lubricating oil will start evaporating and piston and cylinder will be badly damaged. In addition to this, high stresses will be induced and damage many parts of the engine. Therefore, it is essential to maintain the temperature of engine parts below some limit by proper cooling.

The cooling is provided to avoid the bad effects of overheating as listed below:

a.      The high temperature reduces the strength of the piston and piston rings and uneven expansion of cylinder and piston may cause the seizer of the piston,

b.      The high temperature may cause the decomposition of the lubricating oil and lubrication between the cylinder wall and piston and may break down resulting in a scuffing of the piston,

c.      If the temperature around the valve exceeds 250⁰C, the overheating of the valves may cause the scuff of the valve guides due to lubrication break down,

d.      The tendency of the detonation increases with an increase in temperature of the cylinder body,

e.      The pre-ignition of the charge is possible in spark ignition engines if the ignition parts initially are at higher temperature.

To avoid all the adverse effects mentioned above, it is necessary to cool the engine. The cooling system used for IC engine generally carries 30 to 35% of the total heat generated in the cylinder due to the combustion of air fuel mixture. It is also necessary that the temperature of engine should be maintained above a particular temperature. This is essential for easy running and better evaporation of the fuel.

Factors affecting the temperature of the cylinder and piston and Engine Heat Transfer:

The factors which affect the temperature distribution in the different parts of the engine are discussed below:

1.      Air-Fuel Ratio:

The burned gases temperature and indirectly the temperatures in the piston and the cylinder depend upon the A:F ratio. The maximum gas temperature occurs at A:F ratio of 13.5:1 in SI engine. Therefore the heat transfer from the gas to the engine parts will be maximum. At equivalence ratio of one, the increase in piston temperature is small but increase in gas temperature and hence, the valve temperature is significant. This is due to slow burning of the fuel.

2.      Compression ratio:

The gas temperature increases as the compression ratio increases. But higher pressure ratio allows the gas to expand more and therefore gas temperature becomes minimum at the end of expansion and therefore, the heat rejected during blow down will also be less.

3.      Engine speed:

At higher speed, the gas velocity increases and heat transfer coefficient from the gas to the engine parts increases and also the temperatures. If the load on the engine is same, but the speed is increases, the heat input per cycle increases and therefore, the gas temperature also increases.

4.      Engine output

The fuel supply per cycle increases with increase in load on diesel engine and mixture mass in petrol engine. Therefore, heat released and indirectly heat transferred increases with load on the engine. This increases the temperature of piston and liner temperatures. The common trend observed in diesel engine, the piston temperature is always lower for supercharged engine than naturally aspirated engine at all speeds.

5.      Ignition timing:

The engine efficiency becomes maximum when the heat release occurs at TDC. As combustion takes finite time, the ignition has to start a few degrees before TDC. Higher the angle of advance, the pressure and the temperature in the cylinder increase. This increase the piston as well as cylinder temperature.

6.      Effect of coolant:

Mostly water and air are used as coolant for petrol as well as diesel engines as per the capacity of the engine. Small capacity engines are air cooled where higher capacity engines are water cooled. The heat carrying capacity of water is much higher than air, the temperatures of the ports are lower in water cooled engine compared with air cooled engines.

7.      Turbulence:

The heat transfer for different combustion systems may differ by a factor of 10. This is mostly because of high turbulence created in these combustion chambers. In pre-combustion chambers of diesel engine, high turbulence occurs simultaneously with high pressure and temperature. This increases the instantaneously heat transfer.

8.      Bore-Stroke ratio:

With increased speed of the engine, the duration of all events, during each cycle, is decreased but the increased piston speeds create higher turbulence and as a result, heat rejected to the jacket remains more or less same or a marginal increase.