# Matching and Control Optimisation of Variable-Geometry Turbochargers for Hydrogen Fuel Cell Systems
## 4 Discussion  
The aim of this study was to assess whether a well-matched VGT was able to outperform an FGT over the full range of current densities of a modern fuel cell stack. Our results show that VGTs deliver negligible benefit compared to FGTs when used with fuel cell powertrains under optimised operating conditions, resulting in a less than 0.25% increase in system efficiency across the range of fuel cell loads. The results suggest that the additional degree of control freedom offered by the VGT, the nozzle gap, does not inherently result in enhanced system efficiency at any point across the fuel cell stack operating range, and that both turbines were capable of operating at high efficiency across the full range of conditions. The similarity in FGT and VGT performance observed is explained by the control actuations used to meet the optimised operating conditions. For the VGT, these are achieved using a nozzle gap between 48 and 59% across the full range of testing. A VGT operating with only a small range of rack positions is operating somewhat similar to an FGT, so this results in similar VGT and FGT performance. The minimal range of rack positions is also centred around 51%, which for the VGT in this study represents the 'flush gap'. The flush gap typically provides the best turbine performance and, in this study, also represents the FGT. Similarly, the FGT controller used minimal intervention from either a wastegate (<1% actuation) or back pressure (not required at any operating point) in order to achieve its optimised operating conditions. The minimal intervention of fuel cell pressure control, including the nozzle gap, wastegate, and back pressure valve, indicates that the matching exercise was effective. Little variance in rack position around the flush gap suggests that the VGT remains comfortably in an efficient part of its operating window, whilst meeting the air system requirements. The FGT similarly requires little use of the wastegate and back pressure valve to meet its operating requirements, the use of which would result in system efficiency losses. This shows that the air flow characteristics such as mass flow rate and pressure do not need to be modified significantly, and hence the turbines are well matched and sized for this fuel cell system. It is debated in the literature whether either kind of turbine can meet the full range of fuel cell air system requirements; however, this study indicates they can when the component sizing is well matched to the application and the operating conditions are optimised. This verifies the importance of matching (and air system design generally) in conjunction with optimal control. These two distinct, but coupled, areas of the literature were identified to construct the aim of this work. Despite the various similarities in the performance of the FGT and VGT, the key difference between the two architectures is their robustness to sub-optimal boundary conditions. The operating point optimisation exercise illustrated how the fuel cell system efficiency quickly reduces when the FGT is operated away from its optimal operating points, compared to the use of the VGT, where the system efficiency is much more stable. A small deviation from the optimal inlet conditions would result in a smaller penalty in vehicle efficiency when using a VGT than that when using an FGT. Therefore, the choice of a VGT for an FCEV may not provide a performance benefit over an FGT but appears to be more resilient to sub-optimal air conditions such as altitude. These results correlate to those of [PERSON] et al. [PERSON] et al. (2017); however, in this work, we have also shown that the VGT is resilient to deviations in pressure as well as in air flow rate. In contrast to [PERSON] et al. [PERSON] et al. (2015), this study showed that both the FGT and VGT were able to operate at high efficiency across the full range of current densities.