09 Gas exchange process for IC engines

Page 1

Gas exchange process for IC-engines: poppet valves, valve timing and variable valve actuation Topics Analysis of the main parameters influencing the volumetric efficiency in IC engines: - Valves and valve timing - Dynamic effects - Turbocharging Introduction At the end of each cycle, in an IC engine the burned gases have to be replaced by new charge of air/fuel and (often) recycled exhaust gases. The gas exchange process is quite complex since it involves many time-dependent phenomena, such as: unsteady and compressible flows, frictional losses, heat transfers, area changes and complex geometrical paths. During the gas exchange process, because of the cyclic operation of cylinders, the flows through each component are pulsating and strongly influenced by inertia effects and wave actions, and hence difficult to be analyzed. Layout of the intake and exhaust systems for a 4- stroke, TC, SI engine. Volumetric efficiency or filling coefficient Volumetric efficiency (ηv) or filling coefficient (λv), defined as: ma: trapped air mass per cycle mt: theoretical amount of mass that can fill the cylinder displaced volume ρa: air density at reference conditions V: cylinder displaced volume The filling coefficient measures the engine capability to fill its displacement with air. For this reason, it is defined as the ratio between the trapped air mass per cycle and the theoretical amount of mass that can fill the engine displacement at reference conditions. The actual air mass, ma, aspirated inside the cylinder at the end of the single cycle, is different from the ideal one mt, for different reasons: 1) Burned gases inside the combustion chamber Vc at the end of the exhaust process, have a pressure pr which is higher than the atmospheric pressure pa. Therefore, burned gases expand,…