I was thinking of electromagnetic suspension with energy capture. Every bump in the road would contribute to your smooth sailing!
Another aspect of this, of course, is a road tracking vision system to automatically adjust the suspension for bumps that are arriving in real time. One could adjust for efficiency or comfort.
I'd also have a capture and filtration system for rainwater, to replenish that lost to the "combustion" process.
Somewhere in the engineering, I'd want to do something about vehicles sitting in the hot sun on unattended parking lots. A lot of energy going to waste there. -- Who needs oil?