They really cheat with that number. They are using engine-to-wheels efficiency, but then talk about the wave-disk in a hybrid presumably with no drive-train losses. A gasoline engine running over a much smaller RPM range will run much more efficient than 15%.
An efficient car may run at about 18% on average (hence the 15% after external losses), but quite possibly have a peak efficiency of 30%
Also, they compare to a gasoline Otto cycle engine. The diesel cycle is more efficient than the Otto cycle. A cars turbocharged diesel may run nearly 30% average efficiency, and peak efficiency in the 40s. (The ratio of peak/average eefficiency in diesel engines tends to be closer to unity than in gasoline engines).
There are actual operating now diesel engines operating at over 50% efficiency, but they are not the sort of thing you would put in your car (more the sort of thing where your car would fit inside the engine).
[edit]I wanted to find info on the prius before posting this, but was unable to. I finally did, and the complexity of the drivetrain is to (among other things) maximize the amount of the time that the engine is in it's peak efficiency area of 230g/kWh which is roughly an efficiency of 33%. Since the article specifically talks about running the engine in a hybrid comparison, this is apples-to-apples, and it is less than a 2x improvement, not 3.5x
[edit2]The Prius is not a vanilla Otto cycle, but it's pretty close. I really think of it as more of a Miller cycle than an Atkinson, but in any event it's a 4-stroke spark-ignited reciprocating piston engine.
