A spark-ignition engine with the efficiency of a diesel.
It’s rather like taking the Atkinson engine to the next level. A great deal more expansion than compression.
Back in the late 40s, aircraft engine designers played with turbocompounding – using expansion turbines to wring every last bit of work out of a pound of hot exhaust gas. And it worked, sorta. The engines of Shackleton bombers and B-29s were marvels of thermal efficiency. But they weren’t reliable. They relied on gear reduction to match turbine RPM with engine RPM. Those stressed out gear trains were more of a danger to B-29 crews than Zeros.
Even earlier steam locomotives used compounding. The exhaust steam from one cylinder was ducted into another (larger diameter) cylinder and allowed to expand some more. This worked but they were tricky to operate using 1920s controls.
But this uses the older idea to make an engine that does not have a stressed out system.
Ilmor is highly respected in the auto world.