The work described in this thesis aims to conduct a systematic study of the two stage turbocharging system to improve the Diesel engine transient performance as well as NOX and CO2 emissions with a focus on the improved turbocharger matching and the control of the charging system, through the use of high fidelity engine models backed by experimental results. To perform the analytical study, commercial 1D simulation software has been used in the process of system characterisation and control strategy design. To validate the analytical results, a two stage turbocharging system was installed on a production diesel engine and tested on a transient engine test bench. The test results were then used to further calibrate the 1D engine/turbocharger model. Several other technologies were also investigated in simulation to explore their potential to further improve the system. Unlike most studies in the literature, this project focused on the system benefit of the engine and turbochargers, instead of conducting optimisation solely at the component level. The engine global parameters, such as the engine fuel consumption, emission levels and the transient response were the main parameters to be considered and were also best suited to the strengths of the 1D simulation method. The interactive use of both the analytical and experimental methods was also a strong point of this study.A novel control strategy for the system was proposed and demonstrated in the simulation. Experiments confirmed the validity of this control strategy and provided data for further model calibration. The comparison of the test results of the baseline engine to those obtained with the two stage turbocharged engine system verified the benefits of the novel turbocharging arrangement and control scheme. Transient response (T1090) was improved, with a 50% faster torque rise at 1000 rpm; the fuel consumption over the NEDC was 4% lower and NOx emissions over the NEDC were 28% lower. In the meantime, the study also revealed shortcomings of the system, such as the lack of EGR control at low speed, low load condition and a mid-speed fuel consumption deterioration of 13% on average at 3000 rpm due to excessive back pressure. With a novel 1D model corroborated using test results, exploratory simulation was done to rectify the aforementioned shortcomings and to further improve the system. Simulation results showed that by implementing VGT and ball bearing technology in the high pressure stage of the two stage system, the EGR controllability at low speed was regained and the excessive back pressure at high speed was improved. Consequently, the fuel consumption was only increased by 1.3% compared to the baseline NEDC operation and the transient response was on par with the original two stage system, with only 0.05s slower in torque rise at 1000 rpm, and still 48% faster than the baseline VGT system. Furthermore, the NOx emission can be expected to be greatly improved in the upcoming more intensive drive cycles compared to the NEDC cycle, with simulation showing NEDC NOX emissions dropped by 1%, comparing to a substantial reduction of 11% in WLTC.
- TWO STAGE TURBOCHARGING
- BALL BEARING TURBOCHARGING