Free «Heat Transfer» Essay Sample

Heat Transfer


Combustion engines give a similar efficiency to the engines that use diesel without the presence of high NOx, and the particulate emission of matter that are essential in characterizing the diesel engine combustion. According to (Nellis and Sanford, 2009), the mains setback that has been preventing the wide commercialization of HCCI engines are the combustion timing control and insufficient power output. The solution to control and power problems is the premixed charge compression ignition (PCCI). This is a generalization of the HCCI combustion in which there is a partial stratification of the fuel and air when they are ignited. Examples of premixed charge compression ignition are the direct injected engines that are early injected, variable valve timing engines, controlled auto ignition, and the engines that make use of residual fraction to control combustion.

Stratification is used in PCCI engines in the event of lengthening the duration of the burn and this allow and enable the engine to perform at high specific power. Engine control also makes use of the direct injection of fuel in the combustion chamber (Cox, 1976). Fluid mechanics code, when linked with chemical kinetic code is used to solve engine combustion problem. Fluid mechanics is extremely essential in the calculation of temperature distribution and mixing (Nellis and Sanford, 2009). Chemical kinetics code is useful in the calculation of the composition and the release of heat. Turbulence occurrence in the combustion engine plays a minor role in the HCCI combustion. To make sure that HCCI is analyzed accurately, a relatively coarse grid that is using millimeter size cell instead of a micron sized cell need to be used.

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Determination of the size of the cell is useful in appropriately resolution of distribution of temperature in the cylinder. To analyze HCCI better, there need to be integration between the fluid, mechanical code, and the chemical kinetic code. This would help to make the process faster and ensure high-computation capability of these activities. Another integration that need to be applied in the fluid mechanic code and the chemical kinetic code is the multi zone model where cell that have similar temperature, pressure from ten to one hundred. This helps in applying chemical kinetic solver to a smaller number of zones instead of the large amount of cells that are mostly used in the fluid mechanics.

Combustion produce pressure rise because of the aspect of uniformity of pressure which changes with time. For mass M, hb =hu. the purpose for this, is to allow the  dm is to expand against the prevailing pressure. Given that the total volume is usually constrained; the available pressure has to rise by up, and all the gas present in the cylinder is thus compressed. The rise of the temperature occurs because of the compression that happens due to the newly burned gases as noted by (Holman, 2010). The temperature relate to pressure by isentropic relationship as stated by the unburned gas. The gas pressure that is present in an engine varies all through Otto four-stroke engine cycle. During compression, the work is done by the piston, the energy is produced through the combustion process. Changes that occur in the energy and in the volume of the cylinder cause fluctuation in the pressure of the gas.

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To accurately predict the pressure, you need better understanding of various processes that usually takes place in the cylinder including the interaction that happens between the gases, the oil film, liner, and in the piston. In the expansion process, the heat that is transferred to the cylinder liner is little in comparison to the work done. The energy that is lost by the internal friction of the gas is also negligible. In the engine, the intake process occurs between exhaust valve closing (EVC) and when there is a start in compression. When the intake opens before the closing of the exhaust valve, a period of overlap by which both valves are open occurs (Holman, 2010). By this time, s- curve model is in use in describing the gradual transition that occurs between the exhaust pressure and the intake or the inlet pressure. The end of the intake process is marked by the start of compression.

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This does not essentially occur at the same time with the closing of the intake valve (IVC). The intake valve usually closes when the volume of the cylinder is decreasing and after the bottom center (BC). The Engine speed point out the part in which the fuel/air mixture halts to flow in the cylinder. During the duration, of lower revolution, the point at which compression start is closer to the IVC and at a faster speed, it is closer to BC. As the piston descends during intake, the volume of the cylinder increase, and thus draws the fuel mixture. Little resistance occurs in the flow of the liquid to the cylinder, and this ensures that the pressure in the cylinder is fairly constant also equal to the inlet pressure.

The equation PVr=constant allow the calculation of the cylinder pressure at any crank angle when there is the occurrence of compression. Knowledge of initial pressure P0 and volume V0 is useful in the determination of constant. Volume of the cylinder relates to crank angle directly also is a function of the cylinder geometries connecting rod length and crank radius. The ration that relates specific heat of the fuel to the specific heat is %u03D3; this is measured at a constant pressure and volume respectively. %u03D3 is a value that varies from compression to combustion to expansion, and it is approximately equal to 1.3.