A more direct source:
https://www.hondarandd.jp/point.php?pid=1271&lang=en
Quote:
A 2.0 L gasoline engine for which the development target was the realization of thermal efficiency of 40% or higher (a first for Honda) has been developed for the new Accord hybrid. As in the case of the previous model, the developed engine employs the Atkinson cycle and the cooled EGR system. In order to further increase theoretical thermal efficiency, knocking has been reduced through the application of mirror finishing to the intake valves and the use of sodium-filled exhaust valves, and the higher compression ratio than the previous model. To reduce heat loss, high-speed combustion has been realized by increasing EGR amount and employing high-tumble-flow ports. Technologies for the reduction of mechanical loss, including low-friction plateau honing and a low-friction balance shaft, have been employed in the other engine framework, helping to enable the realization of a minimum fuel consumption rate of 208.1 g/kWh (thermal efficiency: 40.6%) in the engine unit in isolation.
The verification results of the fuel efficiency performance achieved in an actual vehicle through the use of the newly-developed engine demonstrated a reduction of 1.96 mpg in fuel consumption in the US Combined Mode as an effect of the engine unit in isolation.
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For comparison, the 2000 Insight's engine has a zone from ~1500-2500rpm where it can achieve 215g/kwh, or approximately 39.5% thermal efficiency. Most driving occurs in the region of 225g/kwh, or ~37.7% thermally efficient.
If Mazda can break 50%, that will, of course, be revolutionary.