Editing Afterburner (section)

==Efficiency==
{{main|Propulsive efficiency}}
In heat engines such as jet engines, efficiency is highest when combustion occurs at the highest pressure and temperature possible, and expanded down to ambient pressure (see [[Carnot cycle]]).

Since the exhaust gas already has a reduced [[oxygen]] content, owing to previous combustion, and since the fuel is not burning in a highly compressed air column, the afterburner is generally inefficient in comparison to the main combustion process. Afterburner efficiency also declines significantly if, as is usually the case, the inlet and tailpipe pressure decreases with increasing altitude.{{Citation needed|reason=Efficiency is governed by pressure ratios, not absolute pressure, so reducing ambient pressure should not affect efficiency.|date=May 2016}}

This limitation applies only to turbojets. In a military turbofan combat engine, the bypass air is added into the exhaust, thereby increasing the core and afterburner efficiency. In turbojets the gain is limited to 50%, whereas in a turbofan it depends on the bypass ratio and can be as much as 70%.<ref>"Basic Study of the Afterburner" Yoshiyuki Ohya, NASA TT F-13,657</ref>

However, as a counterexample, the [[SR-71]] had reasonable efficiency at high altitude in afterburning ("wet") mode owing to its high speed ([[Mach number|Mach]] 3.2) and correspondingly high pressure due to [[Ram-air intake|ram intake]].