| Recently, environmental improvement and energy issues have become more and more important as worldwide concerns. Natural gas is a good alternative fuel to improve these problems because of its plentiful resources and clean burning characteristics. As the increase of market demand for heavy-duty CNG (Compressed Natural Gas) engine in China and the future stringent emission legislation, it is necessary to have an in-depth study on the controlling emissions of CNG engine. Lean burn natural gas fuelled spark ignition engines are particularly attractive regarding environmental performance and well fuel ecnomy, but Exhaust emissions may not always be as low as the limits of stringent emission legislation in 21st century. In order to solving the conflict between well fuel ecnomy and low emissions, several technologies are introduced. Natuaral gas engine with lean burn operation can use an oxidizing catalyst reduce THC and carbon monoxide(CO) emissions, but NOx are still a problem.The researchers consider that one effectual technique for controlling emissions of oxides of nitrogen (NOx), used in both spark-ignition and compression-ignition engines, is exhaust gas recirculation (EGR). So EGR-techonlogy was indentified as being one of the most crucial candidate technologies for introducing the next generation of engines,which can meet the EuroⅣemission regulation.An external cooled EGR system was supplied to reduce NOx emission in CA6SE1-21N lean burn CNG engine. In this dissertation the effects of EGR on combustion performance and emissions were investigated and tested . The control Map of EGR rates had been optimized based on the objective of meeting the NOx emission limit in Europe IV legislation.Design the low pressure EGR loop system. The obtained exhaust gas from turbine export dilute the fresh inlet charge , then enter into the compressor. a magnetic valve acted as EGR valve and the magnetic valve was drived by a line step -motor. The EGR valveopening was controlled by adjusting the number of steps.of step-motor and it leaded to the changes of EGR rates. An EGR inter-cooler has been adopted to stabilize the temperature of recirculated exhaust gas varying within the range from 17 oC to 70 oC.The engine speeds from ESC 13-mod and other speeds were selected as experiment speeds. In the conditions of part loads, fixed speeds and invariable power, the effects of EGR on combustion performance and emissions were investigated and tested by changing EGR rates. Of course, the 100% loads were in rang of our study. The study showed that as EGR rates increased, mixture concentration became leaner, the flame development period and the main combustion duration became longer, the corresponding CAD of the center of heat release rate increased, maximum heat release rate and maximum cylinder pressure decreased, the combustion cycle variability was higher, the thermal efficiency was cut down, NOx decreased in an almost linear relationship, THC increased steadily. CO had a special change trace:at first, CO decreased with the increased amount of EGR, but when EGR rate exceed a limit, CO increased. Under 100% load conditions, Power diminished with the increased EGR rates. Through 1450 speed, 75% load , a detailed description were showed as follows: as EGR rates were less than 5.0%, Brake efficiencies almost keep sustained. If EGR rate exceed 5.0% , Brake efficiencies drops off slightly. As EGR rate reached 8.0%, compared with no EGR, there were a decrease in maximum cylinder pressure and maximum heat release rate by 25% and 30%, respectively. At the same time, the corresponding CAD of center of heat release rate moved afterward by 10 oCA, the relevant CAD of the timing of combustion increased 3.5 oCA and the main combustion duration increased by 30%. was constant within 3%,when EGR rate was varied in the range of 6.0%;but increased considerably with EGR rate above 6.0%. for example, rose to about 4.71%,when the engine operated with EGR rate of 7.9%.At the moment, Combustion stability deteriorated seriously. NOx reductions about 80% was observed at EGR rate of 6.0%.Because an optimal EGR rate exists at every load , The the best solution to make the NOx emissions of engine meet Europe IV legislation and keep other performances of the engine is to gain optimal EGR rates. The optimization principle of EGR rates can be described as follows. The goal of NOx less than 3.0 g/(kW·h) can be achieved for the part loads. The EGR rates corresponding to NOx emissions 3.0 g/(kW·h) are considered as the optimal EGR rates. At 100% loads , if NOx emissions can reduce to 3.0g/(kW·h) and the power losses were not excess of 5.0%,the relevant EGR rates were the optimal EGR rates, otherwise choose the corresponding EGR rates of the power losses of 5.0% as the optimal points. Testing results gained from 28 operation conditions under 7 different speeds were optimized according to the principle mentioned above and the optimal EGR control Map was established. The variation characteristics of EGR rates straightforward.were acquired, furthermore the Map can supply some data to investigate the electronically controlled system. There were some conclusions from the the optimal EGR Map as follows. The EGR rates were controlled below 8.0% in the whole operation conditions.Under the ESC 13 mod conditions , the weighted mean of the optimized EGR rate was 3.48% and fuel consumption rate with weighted coefficient was 226.5 g/(kW·h), increasing 1.3% compared with no EGR.. The BSNOx, BSTHC and BSCO was 3.33 g/(kW·h), 6.84 g/(kW·h), 3.28 g/(kW·h), respectively. NOx decrease by 47% at the cost of THC increasing by 15% and CO increasing by 1.2%.We can conclude from the above that it was a feasible solution of reducing NOx to use EGR technology in the CNG engine with lean burn operation. EGR technology not only keeped the engine the well fuel ecnomy, but also brought the NOx emissins of engine below the limit of in Europe IV legislation. It offered the engine a possibility to meet the all limits of Europe IV legislation by adding the oxidizing catalyst in next step.
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