Component and system design of a mild hybrid 48 V powertrain for a light vehicle

Doctoral thesis, 2020

This thesis presents contributions in three areas relevant for the development of 48 V mild hybrid electric powertrains for cars.

The first part comprises methodologies and extensive testing of lithium-ion battery cells in order to establish the electric and thermal performance using equivalent circuit models.  Empirical, lumped-parameter models are used to ensure fast simulation execution using only linear circuit elements. Both electrochemical impedance spectroscopy and high-current pulse discharge testing is used to extract model parameters. Plenty of parameter results are published for various cells, temperatures and SOC levels. Further on, the model accuracy in voltage response is also evaluated. It is found that an R+2RC equivalent circuit offers the lowest error, 11 mV RMSE in a 1.5 h drive cycle, which is among the lowest numbers found in the literature for similar models.

In the second part, electric machines with tooth-coil windings are explored as a viable candidate for mild hybrids. First, a method of analytically calculating the high-level electro-magnetic properties for all possible combinations of three-phase, dual layer tooth-coil winding machines is established and presented in a graphically appealing manner.  Then, a pair of pseudo-6-phase 50 kW PMSMs are designed, constructed and validated in a custom designed calorimetric dynamo test stand. These machines feature in-stator and in-slot forced oil cooling, enabling very high current densities of 25 A/mm² continuous and 35 A/mm² peak. A high net power density (19 kW/l) and a large area of high peak efficiency (95%) is shown numerically and validated by calorimetric measurements.

Finally, low-level design, construction and evaluation of 48 V inverter hardware is explored. By using high-performance, extra-low-voltage silicon-based MOSFETs with custom designed metal substrate printed circuit boards, custom made gate drivers, and water cooling, 3x220 A RMS is reached experimentally on a 154 cm² area and an efficiency of 95.6%.