They say 226g/kWh, so 16MJ/kg, and diesel is ~48MJ/kg, so 16/48 gives ~33% total thermodynamic efficiency. At full throttle, a smaller turbodiesel gives ~39% (from memory), but them are weasel words, of course.
From the interwebs:
Gasoline engines are about 15%-25% efficiency. New cars are at the 25% end. Diesel engines are 30%-49% efficient. Diesel semi trucks are about 35%. Large multimegawatt diesel power plant engines are up to 49%. (See Wartsila engines).
This may still be a useful engine, because it's part throttle efficiency may be better than gasoline / small turbo diesel, bu tI suspect it'd be in production if the advantages were that compelling. I'd heard Prius get a similar effect by only compressing part of the compression stroke (valve timing jiggery), but using the full expansion stroke. Easier to manufacture, (no odd sized pistons, bores, counterweights), just needs cam change.