tailieunhanh - Internal Combustion Engines Fundamentals Episode 1 part 7

Tham khảo tài liệu 'internal combustion engines fundamentals episode 1 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 212 INTERNAL COMBUSTION ENGINE FUNDAMENTALS FIGURE 6-4 Effect of exhaust to inlet pressure ratio on ideal-cycle volumetric efficiency. Combined Quasi-Static and Dynamic Effects When gas flows unsteadily through a system of pipes chambers ports and valves both friction pressure and inertial forces are present. The relative importance of these forces depends on gas velocity and the size and shape of these passages and their junctions. Both quasi-steady and dynamic effects are usually significant. While the effects of changes in engine speed and intake and exhaust manifold port and valve design are interrelated several separate phenomena which affect volumetric efficiency can be identified. FRICTIONAL LOSSES. During the intake stroke due to friction in each part of the intake system the pressure in the cylinder pc is less than the atmospheric pressure pUm by an amount dependent on the square of the speed. This total pressure drop is the sum of the pressure loss in each component of the intake system air filter carburetor and throttle manifold inlet port and inlet valve. Each loss is a few percent with the port and valve contributing the largest drop. As a result the pressure in the cylinder during the period in the intake process when the piston is moving at close to its maximum speed can be 10 to 20 percent lower than atmospheric. For each component in the intake and the exhaust system Bernoulli s equation gives tjPVj where is the resistance coefficient for that component which depends on its GAS EXCHANGE PROCESSES 213 geometric details and Vj is the local velocity. Assuming the flow is quasi-steady Vj S related to the mean piston speed Sp by SpAp where Aj and Ap are the component minimum flow area and the piston area respectively. Hence the total quasi-steady pressure loss due to friction is Palm - Pc z Pj X ỉjpvf pSị 6-6 Equation indicates the importance of large component flow areas for reducing frictional losses and the dependence of these losses on .