[Christ]

I think because hot air tends to contain a higher relative humidity? Once air gets below 30*, the relative humidity drops... the air gets very dry. If they're comparing efficiency to average relative humidity, that would kind of explain summer being worst time of the year, wouldn't it?

*I haven't looked at the papers more than a glance and skim, yet*

DCB: The first diagram in your paper shows that efficiency and NOx emissions are kind of on the level playing field in lean burn engines, and does show that as relative humidity rises, efficiency falls, as do NOx emissions. I don't think they're paying too much attention to actual temperature ranges, just the relative humidity throughout the year.

It does mention that there is no compensation for ambient humidity changes, so they're testing the same engine maps, I presume, all year 'round. (Without trying to "fix" the maps to keep the efficiency in higher humidity.)