可变喷嘴涡轮增压及废气再循环系统改善柴油机排放性能

针对一台废气旁通阀式（wastegate，WG）增压柴油机，提出可变喷嘴涡轮增压器（variable nozzle turbine，VNT）+文丘里管废气再循环系统（venturi exhaust gas recirculation，vEGR）的 VNT-vEGR 系统设计，并进行匹配优化。搭建了发动机台架试验系统，在保证原机经济性与颗粒物（particulate matter，PM）排放性能基本不变的前提下，对氮氧化物（NOx）排放进行重点优化。研究结果表明：优化后的 VNT+vEGR 柴油机的欧洲稳态测试循环（European steady state cycle，ESC）试验加权的有效燃油消耗率、NOx 排放和 PM 排放分别为229.4、3.53和0.055 g/(kW·h)，相比于原 WG 柴油机的变化率分别为0.04%、?48.2%和14.6%，油耗和 PM 排放略有升高，而 NOx 排放大幅降低，且动力性整体提高约5%～10%。匹配新系统的柴油机能够满足现行的排放法规并具有满足未来排放法规的潜力。该研究为改善柴油机的排放性能提供了参考。

Variable nozzle turbine combined with Venturi exhaust gas recirculation system improving emission performance of diesel engines

Currently, the primary problem for diesel engine is NOx (nitric oxide) and PM (particulate matter) emissions. Study on a diesel engine was completed in this paper in order that the emission performance of the diesel engine could be improved under the premise of unchanged engine economy. An engine test bench, which could measure engine power performance, economy performance and exhaust performance, was established. The study object, an inline four-cylinder four-stroke WG (wastegate turbocharging with intercooler) engine, was tested, refitted, matched and optimized, which was under the target of reducing NOx emission through adding vEGR (Venturi exhaust gas recirculation) system and VNT (variable nozzle turbine) turbocharger system. Firstly, the selected original WG engine was tested on the engine test bench under the ESC (European steady state cycle) test condition, for the sake of obtaining the data of its power performance, economy performance and emission performance. Secondly, an EGR system with a Venturi tube was designed and added to the diesel engine, and combined into a WG-vEGR diesel engine system. Engine bench test, which was the same as the way of the original engine test, was conducted, and its result showed that the BSFC (brake specific fuel consumption), NOx emission and PM emission of the WG-vEGR diesel engine reached 234.5, 4.92 and 0.058 g/(kW·h), whose rates of change were 2.3%, -27.8% and 20.8%respectively, compared with the data of the original WG diesel engine which were 229.3, 6.81 and 0.048 g/(kW·h). Next, for the sake of achieving dynamic regulation according to the demand, the WG turbocharger was replaced by a matched VNT turbocharger, and then an original VNT-vEGR diesel engine system was established. At the same time, a virtual original VNT-vEGR diesel engine model was set up through GT-power software, which was employed to find the optimal vane position of the VNT turbocharger in different operating conditions. In the Optimizer module of GT-power software, the original VNT-vEGR engine was regulated under the target of the best BSFC through changing the control parameter of vane position of the VNT turbocharger. The test result showed that the BSFC, NOx emission and PM emission of the original VNT-vEGR diesel engine system were decreased by 3.0%, 14.4% and 5.2% respectively, compared with the WG-vEGR diesel engine. Finally, in the Optimus software, DoE (design of experiment) optimization method was employed for the original VNT-vEGR engine system, in which EGR rate was a control parameter, and NOx emission, PM emission and BSFC were optimizing targets, so a single-factor multi-objective optimization problem was established. After optimization and engine bench test, the changing rates of the optimized VNT-vEGR diesel engine were 0.8%, -16.2% and 0 respectively, compared with the original VNT-vEGR diesel engine. The overall results showed that the BSFC, NOx emission and PM emission of the optimized VNT-vEGR diesel engine under the ESC test condition were 229.4, 3.53 and 0.055 g/(kW·h), whose rates of change were 0.04%, -48.2% and 14.6% respectively, compared with the original WG diesel engine; and at the same time, the engine power was improved by 5%-10% on the whole. It means that the NOx emission performance is improved from the national emission stage of II to the national emission stage of IV, and the PM emission performance is declined a little, which is stayed in the national III emission stage and can be removed through aftertreatment device easily. This study can improve the emission performance under the premise of unchanged fuel consumption of the diesel engine, and provide a reference for the design and optimization of the diesel engine system to some extent.