Listed below are the theoretical thermal efficiencies (as calculated using the formula above) associated with various pressure ratios, ignoring all losses due to compression not happening isentropically, viscous drag, as well as the process not taking place perfectly adiabatically.
This means that more of the heat energy is converted to jet speed, and energetic efficiency improves.
[disputed – discuss][2] One of the primary limiting factors on pressure ratio in modern designs is that the air heats up as it is compressed.
[citation needed] Military engines are often forced to work under conditions that maximize the heating load.
For instance, the General Dynamics F-111 Aardvark was required to operate at speeds of Mach 1.1 at sea level.
As a side-effect of these wide operating conditions, and generally older technology in most cases, military engines typically have lower overall pressure ratios.
A higher compression ratio implies a heavier engine, which in turn costs fuel to carry around.
Thus, for a particular construction technology and set of flight plans an optimal overall pressure ratio can be determined.
Improvements in materials, compressor blades, and especially the introduction of multi-spool engines with several different rotational speeds, led to the much higher pressure ratios common today.
In the case of the Otto cycle reciprocating engine, the maximum expansion of the charge is limited by the mechanical movement of the pistons (or rotor), and so the compression can be measured by simply comparing the volume of the cylinder with the piston at the top and bottom of its motion.
The same is not true of the "open ended" gas turbine, where operational and structural considerations are the limiting factors.
Nevertheless, the two terms are similar in that they both offer a quick way of determining overall efficiency relative to other engines of the same class.