NOx
G'day Mike S, Thanks for the correction and comment.
Of course you are correct about NOx. That opens up a whole new can of worms on engine design to reduce the effects of temperature and pressure. As usual, we're stuck between a rock and a hard place, because generally, higher temperature and pressure inside the engine lead to increased efficiency, and also increased NOx production. Same problem with piston engines. The automotive guys have to use catalytic converters etc to reduce NOx, but so far, I don't think anything like that is practical on turbine engines.
Thanks again, you've prompted me to do some more reading.