Investigation On Steady And Transient Performance Of 3-Phase Sequential Turbocharging System Diesel Engine With Two Unequal-size Turbochargers

With the continuous development of high power density and wide speed range diesel engines, the requirements of the turbocharging systems matching with the diesel engines are increasing. Sequential turbocharging (STC) system is an effective measure to improve the fuel economy performance and the transient responsive performance, and to reduce the smoke emission at low speed operation. The current STC system is generally the 2-phase STC with 2 equal or unequal-size turbochargers (TCs). When diesel engines work with 2-phase STC, they also need to adopt measures like the intake and exhaust bypass, the wastegate or the bypass and complementary combustion in order to take into account the whole running range performance. The more number of the phases of STC, the performaces of the turbocharging system are closer to the needs of diesel engines. However, the existing 3-phase STC systems must have three TCs at least. In this case, a novel 3-phase STC system with parallel two unequal-size TCs has been studied in this paper. This STC system achieves 3-phase adjustable function by the method that only a small TC works at low-speed operation of the diesel engine, only a large TC works at medium-speed operation and the two TCs work in parallel at high-speed operation, which is better to solve the matching contradiction between diesel engines and turbocharging systems. In this paper, the critical components and control systems of the STC are designed, its steady-state matching rule and transient switching strategy are mastered and the performance of its transient speed-up and load-up problems are discussed.On the basis of the D6114 diesel engine, the intake and exhaust switching valves, the pneumatic actuator system and the control acquisition system were designed. Then, the experimental prototype engine with the 3-phase STC system was established. The devised exhaust switching valve can endure temperature up to 750℃and it can be switched from fully open to fully close in less than 0.1s. Moreover, the internal and external leakage level can meet the STC's requirements.On the prototype engine, the steady-state proformance experiments of vehicle and propeller characteristics with different TCs matching were carried out. Then, based on the principle of optimum fuel economy of each characteristic, the steady-state matching rule of the 3-phase STC system with two unequal size TCs was determined. Comparative experimental results between the original general turbocharging system and the STC system showed that the diesel engine performances were greatly improved with the 3-phase STC system, especially in the low-speed and high-load operations: the brake specific fuel consumption dropped by 7.1%, the smoke emission reduced by 70.2% and the exhaust temperature before the turbine decreased by 12.6%.In transient switching experiments of the 3-phase STC, the relative smooth switching process was realized by controlling the opening and closing sequence and the delay time of intake and exhaust switching valves, and thus obtained the system's transient switching control strategy. The experimental results of different switching speeds of the diesel engine showed that in the same type of switching process, the higher the switching speed was, the less the transient intake pressure dropped and the more the engine speed dropped, but the shorter the time returning to target was. Moreover, in the same switching speed, the intake pressure and the speed's dropping extent and recovery time of the switching from the large TC to the small TC were less than that of the switch from the small TC to the large TC.The experimental results of upwards and downwards switching processes showed that the fluctuation range of engine speed increased as the load increased in the valve switching process. In the condition that the engine load was constant before and after switching, contrastive experiments of two switching processes were carried out. One process was that engine speeds were equal before and after switching and the other was that fuel injection amounts were equal before and after switching. The results showed that the speed difference between upwards switching line and downwards of the former were more than that of the latter.The transient speed-up and load-up experiments of the STC system were executed to determine the optimal switching scheme of speed-up. The test results showed that in the process of increasing torque at constant speed, the increase in pme of this STC system was fast than that of the original engine. But the higher the constant speed, the less the difference between the two. And in the process of increasing speed at constant torque, in the same adjustment time of speed-up, the rising time of the engine speeds was increased as the engine torque rised, and, the reduction value of the engine speed in the switching process from the small TC to the large TC was increased as the load rised. The engine performances were optimal only with the small TC when it is in the allowed running range of small TC and in the process of the engine speed-up and load-constant, Thus, in the engine speed-up process, when the initial condition was that the diesel engine used the small TC only, the optimal STC switching boundary was the full-load boundary of the engine with the small TC only.