HC emission characteristics of heavy vehicle equipped with lean-burn natural gas engine

Abstract: In order to provide emission performance basic data and find a method of controlling hydrocarbon (HC) emissions of the lean-burn natural gas engine, a turbocharged inter-cooled heavy vehicle natural gas engine was studied experimentally to investigate the regularity and influence factors of HC emission. The research was carried on a 6 cylinder turbocharged inter-cooled heavy vehicle natural gas engine. The cylinder diameter of the tested engine is 126 mm, the stroke is 130 mm, the compression ratio is 11 and the rated speed is 2 000 r/min. The intake mode of the engine is intake duct single point electronic controlled injection with supercharge and intercooler. The engine indicator diagram, HC emissions and coefficient of excess air were collected in three different kinds of ignition advance angle cases. The experimental results were statistically analyzed. The method of contrast test was adopted to study the effects of engine speed, engine load rate and ignition advance angle on indicator diagram, heat release rate and HC emission performance of the heavy lean-burn natural gas engine. The combustion heat release rate of external characteristic and load characteristic was analyzed, the proportion of CH4 to THC (total hydrocarbon) and the effect of running conditions on the proportion for the intake duct single point electronic controlled injection lean-burn natural gas engine were specially investigated. The experimental results show that the combustion duration period based on the calculation of crank angle is lengthened along with the increase of speed when running at external characteristic. The rapid combustion period based on the calculation of crank angle increases from 28o to 41o when the speed increases from 1 000 to 2 000 r/min. The combustion duration period shortens with the increase of load at a certain speed. The rapid combustion period shortens from 52o to 41o in the range of 0 to 100% load rate at the speed of 2 000 r/min. More than 95% of THC is CH4 when the engine running at external characteristic because of the CH4 leakage phenomenon existing in the scavenging process. The emissions of HC at middle and high speed are more than that at low speed when the engine running at external characteristic. The HC emissions rise with the increase of ignition advance angle. The effect of ignition advance angle on HC discharge at large torque speed is the most significant. The HC emissions increase with the improvement of load in the range of speed from 1 000 to 2 000 r/min, and the ratio of CH4 to THC rises with the increase of load at rated speed. For this engine a large proportion of CH4 discharge comes from CH4 leakage existing in the scavenging process, so the optimization of the valve timing and proper reduction of the valve overlap angle through designing new cam profile can be adopted to decrease the leakage of CH4 and then to control the HC emissions of the heavy vehicle natural gas engine. Simulating combustion chambers with different structural parameters by 3D numerical simulation technique and optimizing combustion chamber pit depth, diameter and the squish area can improve the in-cylinder turbulent kinetic energy and increase the burning rate, and then effectively control the engine emissions. Appropriately increasing the compression ratio and increasing the lean burn degree can control the HC emissions and ensure the economy. In addition, refining the calibration of the ignition advance angle according to real-time acquisition of emission data can reduce HC emissions. The study provides a theoretical basis to control HC emissions of the turbocharged inter-cooled single point injection vehicle lean-burn natural gas engine.