Here’s something Citroën should be paying attention to.  The Duke engine from New Zealand.  Quite the revolutionary redesign of the internal combustion engine using an axial-radial approach:

https://pixelbark.com/13045/how-the-duke-engines-increases-the-efficiency-of-the-internal-combustion-engine

[Ed Note:  We can’t help but wonder what fitting one of these in a DS might do for performance!]