Optimization of engine air path with hybrid boosting systems

  • Yang Liu

Student thesis: Doctoral ThesisPhD


The electrification of powertrains is now the accepted roadmap for automotive vehicles. The next big step in this area will be the adoption of 48V systems, which will facilitate the use of technologies such as electric boosting and integrated starter-generators. The introduction of these technologies gives new opportunities for engine air path design as an electrical energy source may now be used in addition to the conventional mechanical and exhaust thermal power used in super- and turbo-chargers. This thesis aims to investigate the design of a mild hybrid boosting system, to determine if electrified boosting can improve overall system performance, with respect to efficiency, power and transient response. The study covers both gasoline and diesel engines, based on a series of engine air path case studies, discusses the benefits of a hybrid boost system through both an experiment and a simulation:
• The study of diesel engine with electrically driven compressor.
• The study of petrol engine with mechanically decoupled electric turbocharger.
• The study of inner-insulated turbocharger.
• The study of a T-PIECE junction in the two-stage air path.

A comprehensive literature review has been carried out for turbocharger technologies and mild hybrid system status, this details the challenges related to turbo-matching, the development of an electric boost system, and the difficulties in air path interaction in more comprehensive engine systems. The performance of the two-stage system with an electrically driven compressor (EDC) has been investigated in both experiment and simulation. A two-stage engine gas stand test rig has been constructed based on a prototype 2.2L diesel engine, with the intention to undertake steady flow and transient characterization for a complex air path system and investigate potential future system layouts and control strategies, in order to undertake the steady and transient study of two-stage electric boost system. In simulation phase, it has been demonstrated that by operating the EDC, variable geometry turbine (VGT) properly can reduce the engine output response time by 0.1s to 2s; however, these methods will reduce the system efficiency. Besides, EDC and VGT can also affect the EGR mixing, e.g., high speed EDC and lower positioned VGT could help to quantify the EGR transient flow and mixing speed (according to the O2 rate changes). The intercooler and HP EGR have also been discussed under different air path route design. EDC operation will be affected by the hot flow from EGR if it was located downstream of the HP EGR, as it can boost the inlet flow which will affect the mixed flow rate, temperature and pressure, thus changed the EGR ratio. Intercooler mainly affects either engine inlet temperature or EDC inlet temperature as its location decides which flow will be cooled down.

The decoupled electric turbocharger (DET) engine system in both the single-stage and two-stage situation was investigated. With different size of turbocharger based on the scaling method, by the introducing of engine mean value model, the DET can generate up to 0.38kW average power in an RDE simulation which can significantly increase the engine fuel economy. In the NEDC and WLTC cycles, the e-turbo system can always generate energy and store it in a battery (0.21kW and 0.23kW average power over the whole cycle, respectively). In a transient study, the DET was mounted together with a single turbocharger. After optimizing the controlling methodologies, the E-turbine energy generated changed from 237 kJ to 244 kJ; the EDC energy consumed underwent a big difference, dropping from 954.37 kJ to 47.22 kJ, in WLTC cycle.

Further, a thermally insulated turbocharger turbine is proposed to increase the aftertreatment system inlet temperature. Steady state tests showed that the insulation could increase the T4 for the turbocharger turbine under different engine speed and load conditions. In the transient study, a small (1.5K) temperature benefit can be observed in the experiment, which is different from simulation due to the inaccuracy of the model. In engine warm up tests, the T4 enhancement was also low, but a 2kRPM turbo speed benefit was achieved at this process due to the increased turbine inner gas temperature.

Finally, the optimisation of the design of a T-piece junction in the engine intake side has been achieved by different methods. The T-piece junction between the EDC and the turbocharger compressor has been optimised for lower pressure drop and reduced swirl compared to the baseline design. The pressure drop was reduced by 0.3 kPa (15 %) and the turbocharger compressor efficiency increased by up to 2 % (compressor efficiency rises from 68 % to 70 %).
Date of Award24 Jun 2020
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorRichard Burke (Supervisor) & Sam Akehurst (Supervisor)


  • Optimization
  • Turbocharging
  • mild hybrid
  • Internal combustion engines

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