What general effect does altitude above the tropopause have on engine efficiency?

As aircraft ascend to higher altitudes, particularly above the tropopause, the air density decreases significantly. This lower air density means that engines receive less oxygen for combustion, which can lead to suboptimal combustion conditions. Although jet engine designs can be optimized for high-altitude flight, the reduction in air pressure impacts overall performance, often resulting in a deterioration of overall engine efficiency.

While specific engine designs might compensate to some extent for these changes, generally, the physical laws governing combustion and aerodynamics mean that engines operating at these altitudes struggle more to maintain their designed performance metrics. Additionally, the effects of lower temperatures at high altitudes can also change the way engines behave, further complicating their efficiency. Thus, while some aspects of engine performance can improve under certain conditions (like thrust at optimal design altitude), the overall engine efficiency typically declines when considering factors such as combustion effectiveness and specific fuel consumption above the tropopause.