In diesels the overhead cam driven common rail injection at exceedingly high pressures paired with variable geometry turbochargers has vastly improved efficiency in the last few years. I’m of the opinion that the systems would be more reliable if fewer electronics and sensors were used though. In my thirty years I’ve seen engines go from the 2 million mile no problems CAT 3406B to the 1 million mile no problem Detroit Series 60 to the modern $40k rebuild at 500,000 mile, thousands per month in diagnostic bullshit we have today. I have no problem with emissions controls, modern emissions systems are solid, but the electronics are a constant, expensive problem. I’m excited about the development of single stroke combustion engines, they look promising, this “liquid” piston engine looks like a rendition of the old oil burning, compression losing rotary engines from the 1970’s though. All that having been said though, the key to transportation efficiency is energy scavenging. It’s physics, potential energy to kinetic energy and back again with as little heat as possible. Whether you use a battery or a pressure vessel or a spinning mass in a vacuum to recover and store energy, as long as any vehicle has friction brakes that are engaged outside of an emergency we’re not doing as well as we could. I get what you’re saying though, maybe a relatively simple car getting 40 mpg and lasting a long time is good enough for now.