aeronautical engineering Aerospace engineering is the primary field of engineering concerned with the development of aircraft and spacecraft. It has two major and overlapping branches: aeronautical engineering and astronautical engineering. Avionics engineering is s ...

, overall pressure ratio, or overall compression ratio, is the amount of times the pressure increases due to ram compression and the work done by the compressor stages. The compressor pressure ratio is the ratio of the

stagnation pressure In fluid dynamics, stagnation pressure, also referred to as total pressure, is what the pressure would be if all the kinetic energy of the fluid were to be converted into pressure in a reversable manner.; it is defined as the sum of the free-strea ...

s at the front and rear of the compressor of a

gas turbine A gas turbine or gas turbine engine is a type of Internal combustion engine#Continuous combustion, continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas gene ...

. Overall pressure ratio in a

high-bypass turbofan A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the additional fan stag ...

is a function of inlet pressure ratio and compressor pressure ratio:

OPR = IPR \times CPR

The terms ''compression ratio'' and ''pressure ratio'' are used interchangeably.

Advantages of high overall pressure ratios

As can be seen in the formula for maximum theoretical thermal efficiency in an ideal

Brayton cycle The Brayton cycle, also known as the Joule cycle, is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. It is characterized by isentropic process, isentropic compre ...

engine, a high pressure ratio leads to higher thermal efficiency:

\eta = 1-\left(\frac\right)

where PR is the pressure ratio and gamma the

heat capacity ratio In thermal physics and thermodynamics, the heat capacity ratio, also known as the adiabatic index, the ratio of specific heats, or Laplace's coefficient, is the ratio of the heat capacity at constant pressure () to heat capacity at constant vol ...

of the fluid, 1.4 for air. Keep in mind that pressure ratio scales exponentially with the number of compressor stages. Imagine a gas turbine with compressor stages, each one of which compresses the air by a factor . The pressure ratio would therefore equal . 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.

Disadvantages of high overall pressure ratios

One of the primary limiting factors on pressure ratio in modern designs is that the air heats up as it is compressed. As the air travels through the compressor stages it can reach temperatures that pose a material failure risk for the compressor blades. This is especially true for the last compressor stage, and the outlet temperature from this stage is a common

figure of merit A figure of merit (FOM) is a performance metric that characterizes the performance of a device, system, or method, relative to its alternatives. Examples *Absolute alcohol content per currency unit in an alcoholic beverage *accurizing, Accuracy o ...

for engine designs. Military engines are often forced to work under conditions that maximize the heating load. For instance, the

General Dynamics F-111 Aardvark The General Dynamics F-111 Aardvark is a retired supersonic, medium-range, multirole combat aircraft. Production models of the F-111 had roles that included attack (e.g. interdiction), strategic bombing (including nuclear weapons capabiliti ...

was required to operate at speeds of Mach 1.1 at

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

. As a side-effect of these wide operating conditions, and generally older technology in most cases, military engines typically have lower overall pressure ratios. The

Pratt & Whitney TF30 The Pratt & Whitney TF30 (company designation JTF10A) is a military low-bypass turbofan engine originally designed by Pratt & Whitney for the subsonic F6D Missileer fleet defense fighter, but this project was cancelled. It was later adapted with ...

used on the F-111 had a pressure ratio of about 20:1, while newer engines like the

General Electric F110 The General Electric F110 is an afterburning turbofan jet engine produced by GE Aerospace (formerly GE Aviation). It was derived from the General Electric F101 as an alternative engine to the Pratt & Whitney F100 for powering tactical fighter a ...

and

Pratt & Whitney F135 The Pratt & Whitney F135 is an afterburning turbofan developed for the Lockheed Martin F-35 Lightning II, a single-engine strike fighter. It has two variants; a Conventional Take-Off and Landing ( CTOL) variant used in the F-35A and F-35C, and a ...

have improved this to about 30:1. An additional concern is weight. 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.

History of overall pressure ratios

Early jet engines had limited pressure ratios due to construction inaccuracies of the compressors and various material limits. For instance, the

Junkers Jumo 004 The Junkers Jumo 004 was the world's first production turbojet engine in operational use, and the first successful axial compressor turbojet engine. Some 8,000 units were manufactured by Junkers in Germany late in World War II, powering the Mess ...

from

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

had an overall pressure ratio 3.14:1. The immediate post-war

Snecma Atar The Snecma Atar is a French axial-flow turbojet engine built by Snecma. It was derived from the German World War II BMW 018 design, and developed by ex-BMW engineers through a progression of more powerful models. The name is derived from it ...

improved this marginally to 5.2:1. 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. Modern civilian engines generally operate between 40 and 55:1. The highest in-service is the

General Electric GEnx The General Electric GEnx ("General Electric Next-generation") is an advanced dual rotor, axial flow, high-bypass turbofan jet engine in production by GE Aerospace for the Boeing 747-8 and Boeing 787, 787. The GEnx succeeded the General Electri ...

-1B/75 with an OPR of 58 at the end of the climb to cruise altitude (Top of Climb) and 47 for

takeoff Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff. For aircraft that take off horizontally, this usually involves starting with a tr ...

Examples

Differences from other similar terms

The term should not be confused with the more familiar term

compression ratio The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine. A fundamental specification for such engines, it can be measured in two different ways. Th ...

applied to

reciprocating engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of al ...

s. Compression ratio is a ratio of volumes. In the case of the

Otto cycle An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile engines. The Otto cycle is a description of what happ ...

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. Engine pressure ratio (EPR) differs from OPR in that OPR compares the intake pressure to the pressure of the air as it exits the compressor, and is always greater than 1 (often very much so), whereas EPR compares the intake pressure to the pressure at the engine's tailpipe (i.e., after the air has been used for combustion and given up energy to the engine's turbine wheel(s)), and is often less than 1 at low power settings. The broadly equivalent measure of

rocket engine A rocket engine is a reaction engine, producing thrust in accordance with Newton's third law by ejecting reaction mass rearward, usually a high-speed Jet (fluid), jet of high-temperature gas produced by the combustion of rocket propellants stor ...

efficiency is chamber pressure/exit pressure, and this ratio can be over 2000 for the

Space Shuttle Main Engine The RS-25, also known as the Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System. Designed and manufactured in the United States by Rocketd ...

References