Heat Transfer In IC Engines

Heat Transfer In IC Engines

Heat transfer in IC engines is a very serious problem since you need high temperatures to combust the fuel but you also need to keep the temperature at a controllable level in order to operate the engine safely. Once the temparature in the engine has reached intolerable values the engine block and components may su er damage. Therefore it is essential to have a heat removal process which will maintain the engine at a safe operating condition. A water jacket or air through nns are two ways that reduce the temperature in the engine.

Engine Temperatures 

There are components in an engine that su er more from high temperatures than others.The components with the highest temperatures are the spark plug, the piston face, and exhaust valve and port. The problem wirth these components are that they are not only the hottest components but they are also very di cult to cool. They are very di cult to cool because their location in the engine does not permit them for proper heat transfer. The design of the engine is such that there is not much space for a water jet to cool of the components. The spark plug is located in the middle and fastened to the chamber. There is really no water ow that will aid in the reduction of the temperature of the hot spot. The exhaust valve and port carry the steady state ow of hot exhaust gases. The exhaust gases heat the valve through convective heat transfer. Because of the geometry created by the valve mechanism it is very di cult to have a water jet or even a nned surface to provide the necessary cooling. The third part of the chamber where heat is a problem is the piston face. The geometry of the piston face gives very little opportunity for cooling. Figure 8.2 shows the relationship between temperature and time. At about 60 seconds a steady temperature is reached.

Heat Transfer In Intake System

The heat transfer involved in the intake system occurs when air or an airfuel mixture comes into the manifold. The intake manifold is hotter than the air-fuel mixture because of its proximity to the engine components or the design of the manifold. The intake manifold can be designed to heat the air-fuel mixture, so that the mixture can start to vaporize once it has entered the combustion chamber. One way of heating the manifold is to put it in close proximity with other hot components. The manifold will heat through convective heat transfer. Electricity and hot coolant ow are other ways in which the manifold can also be heated. After the manifold is heated then the air-fuel mixture is heated through convective heat transfer. Equation 8.1 shows the heat transfer problem associated with the air-fuel mixture and the manifold walls.

Q_ = hA(Twall -Tgas) 

where  T=temperature

            h=convective heat transfer coe cient

            A=inside surface area of intake manifold The advantage of heating the intake manifold is that the fuel vaporizes sooner. By making the fuel vaporize at an earlier time, there is more time available for the fuel and the air to mix, therefore providing a homogeneous mixture that is ready for combustion in the combustion chamber.

Heat Transfer In Combustion Chamber

The heat transfer involved in the combustion chamber involves all three modes of heat transfer. Conduction, convection, and radiation all play very important roles when de ning the heat transfer characteristics of the combustion process in an IC engine. During the intake stroke the air-fuel mixture is cooler than the cylinder walls and heat transfer to the mixture occurs. This process further vaporizes the rest of the fuel until all the fuel has been vaporized. At this same time evaporative cooling takes place, which lowers the heat in the compressive stroke. In evaporative cooling the substance, in this case the mixture, is evaporated because of an applied heat source. In order to vaporize the fuel energy has to be used in the form of heat from the hot cylinder walls. Since energy was used, in the form of heat, then there is less energy in the cylinder walls, and hence the cylinder walls are cooled. After the compression stroke and during combustion there is heat transfer to the sorroundings from the hot gas through the cylinder walls. The peak gas temperatures of combustion are on the order of 3000 K and that is why the cylinder walls of the chamber overheat. The only way in which energy can be transferred away from the combustion chamber is through convection and conduction. This form of heat transfer will keep the walls of the cylinder from melting. Figure 8.3 shows the two two types of cylinder walls used for heat transfer purposes. The gure shows an air and liquid cooled engine. 

Since the process of combustion in an IC engine is a cyclic process some of the coe cients and temperatures will no be constant during the cycle. The gas temperature in the combustion chamber will not be constant over the cycle. As a matter of fact, the gas temperature in the combustion chamber will be greater than the cylinder walls' temperature during the expansion stroke and lower during most of the compression stroke. The coolant temperature will be fairly constant and any changes will occur over a longer period of time. The convective heat transfer coe cient on the gas side will vary greatly within a cycle because of changes in gas motion, swirl, turbulance, and velocity. The convection heat transfer coe cient on the coolant side will be fairly constant throughout the cycle. The thermal conductivity of the cylinder wall will also be fairly constant because the cylinder wall temperature is maintained at an overall constant temperature.

Heat transfer to and from the cylinder walls occur throughout the four strokes of the combustion process. During the intake stroke the cylinder walls are hotter than the fuel, which causes vaporation of the fuel. Heat transfer from the cylinder walls to the fuel is through convection. During the compression stroke the gases become warmer than the cylinder walls and during the expansion stroke the greatest amount of heat transfer from the gases to the cylinder wall occurs. In the exhaust stroke the gases have cooled down so there is not a signi cant amount of heat transfer. Since the process is cyclic the amount of heat transfer at a particular point in the combustion process should renain fairly constant. Also the heat transfer at any particualr point can be either positive or negative, which means thatthe heat is travelling either away or to the cylinder walls. Figure 8.4 demonstrates the cycle-to-cycle variation in heat transfer for a particular point. Even though heat transfer is fairly constant there are still small variations.

The heat transfer from cycle to cycle varies for di erent points in an engine. Figure 8.5 shows the heat transfer at three di erent locations in the combustion chamber for a single cylinder during one cycle. There is signi cant variation in the heat transfer for these points.