Numerical Simulation Of Exhaust Flow And Temperature Distribution Characteristics In The Exhaust System Of A CNG Engine

To meet stringent emission legislations, drastic reduction in emission from engines during cold start is the precondition. Therefore, the match simulation between a natural gas engine and its exhaust aftertreatment is beneficial to reduce the exhaust emissions from vehicles powered by natural gas engines.The purpose is to optimize the start strategies of natural gas engines and to reduce their emissions during cold start by coupling the one dimensional (1D) code, GT-Power and the three dimensional (3D) code, FIRE to simulate the working processes of a four cylinder Compressed Natural Gas (CNG) engine. Simulation results were validated by experiments. The exhaust flow characteristics and temperature distribution in the exhaust system during engine start were numerically studied when each cylinder was selected as the first ignition one,and when under different ignition timings and idle speeds were selected, and when a catalytic converter was installed at different positions under a high speed and high load. Results show that:The backflow from the exhaust pipe into the unignited cylinders occurs during the first operating cycle, which delays exhaust discharge into the exhaust system. After the first selected cylinder is ignited, the difference in the configuration of each exhaust manifold affects the characteristics of the exhaust flow and temperature distribution in the exhaust pipe. The optimal option of the first ignition cylinder during start can improve the homogeneity of exhaust temperature distribution at the inlet section of a catalytic converter. Among the four options of the first ignition cylinder, the highest exhaust temperature at the inlet of the catalytic converter is achieved when the second cylinder of the engine is ignited first during start, conducive to the fastest light-off of the catalytic converter. However, the effect of first ignition cylinder on the distribution of exhaust flow and temperature only happens in the first three operating cycles and disappears soon.Retarding ignition and increasing engine idle speed can increase the exhaust gas temperature during the exhaust stroke, the zone of exhaust flow with high temperature at the inlet of a catalytic converter and exhaust mass flow rate, which reduces the time needed for the catalyst to reach light-off temperature of the catalytic converter. Besides, retarding ignition also improve the uniformity of exhaust flow at the inlet of the catalytic converter, but it worsens the uniformity of exhaust flow under too late ignition timing; there is an optimal ignition timing and an idle speed to meet the uniformity of exhaust flow at the inlet of the catalytic converter. More uniform distribution of exhaust flow at the inlet of the catalytic converter can be achieved by the reasonable selection of the ignition timing and the idle speed while reaching the light-off temperature of the catalytic converter.When the catalytic converter is intalled the position close to the engine exhaust manifold, the uniformity of exhaust flow velocity distribution in the exhaust system worsens, and a stronger vortex flow appears in the zone of the inlet expansion pipe of the converter, which results in uneven distribution of exhasut flow velocity and temperature there.