Off-Road C4_thermoCycles

Off-Road C4_thermoCycles - Chapter 4 Thermodynamics and...

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1 Chapter 4 Thermodynamics and Engine Cycles Brief Engine History ± Internal combustion (IC) engines produce power through a combustion process occurring within the piston chambers. In contrast, combustion occurs outside the piston chambers in a steam engine – the combustion heats a boiler to produce steam and the steam is delivered to the piston chambers to produce power. Brief History con’t ± In 1862, Beau de Rochas set forth his famous four principles of operation for an efficient IC engine: 1. The combustion chamber should have the smallest possible surface-to-volume ratio 2. The expansion should take place as rapidly as possible 3. The piston should have the longest possible stroke 4. The pressure should be as high as possible at the start of expansion
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2 Brief History con’t ± The first two principles were aimed at reducing the heat loss to a minimum, while the second two were aimed at obtaining the maximum possible work from each power stroke. Brief History con’t ± Nikolaus Otto built on Beau de Rochas’ principles, patenting his famous Otto-cyle engine in 1876. Two years later he built a successful IC engine. Otto was the first to use the four-stroke cycle (intake, compression, power, and exhaust strokes) that is still used in most IC engines today. With the expiration of the Otto patent in 1890, there was a spurt in development and commercialization of IC engines. Brief History con’t ± Dr. Rudolph Diesel patented his compression-ignition engine in 1892. The first diesel engine in the United States was built for the Busch brewery in St. Louis under license from Rudolph Diesel.
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Four-Cycle Engine Analysis ± The intake, compression, expansion, and exhaust processes can be carried out in two or four piston strokes. Thus, there are two types of engines, those with two-stroke cycles and those with four-stroke cycles. ± Four-cycle engines have become much more popular than two-stroke engines. Basic Thermodynamic Equations ± Two thermodynamic laws are used in cycle analysis, the ideal gas law and the polytropic compression law. The ideal gas law is: pV = MRT ± Where p = absolute gas pressure, kPa ± V = gas volume, m 3 ± M = mass of trapped gas, kg ± T = absolute temperature of gas, K ± R = ideal gas constant = 8.314/mole wt of gas ± Note: the gas constant drops out of the analysis Basic Thermodynamic Equations con’t ± The mass of gas in a combustion chamber remains essentially constant during the compression process. If the temperature also remained constant, the next equation indicates that the pressure would vary inversely with the volume. That is, the pressure would double if the volume was halved. Such a process is called an isothermal process. In an engine, the temperature increases considerably during the
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Off-Road C4_thermoCycles - Chapter 4 Thermodynamics and...

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