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| Fig. 1: Schematic of the jet engine components. There are three major components: the compressor, combustion chamber, and turbine. (Image source: I. Chen, after Saravanamuttoo et al. [3]) |
The jet turbine engine revolutionized aircraft flight propulsion. As a solution to meeting the needs for superiority in speed and altitude of military aircrafts and war dominance, World War II accelerated the discovery and development of jet turbines. The modern concept of the jet turbine engines was conceived independently by Frank Whittle from England and Hans von Ohain from Germany, both proposing a brilliant concept utilizing a compressor, combustion chamber, and a turbine that allowed jets to travel faster than its piston-engine precedents. The design by Hans von Ohain brought the world's first jet airplane flight in 1939, powered by the HeS 3b engine using diesel oil. The Whittle design led to the first British jet flight with the Gloster E.28/39 in 1941, which was ordered into production by the British Air Ministry to power the Gloster Meteor twin-jet fighter. After World War II, technology transfer helped engineers recognize the superiority of jet engines, which spurred the development of civil jet airliners to bring air travel to the public. [1] It was estimated that air travel revenue grew up to $31.7 billion before the COVID-19 pandemic in 2020. Aviation revenue has since recovered and is projected to grow even more than before. Therefore, with the substantial increase in air travel, anticipated pollution from the emission of CO2 and other pollutants from the gas turbine using the petroleum-based fuels could contribute tremendously to global warming and the greenhouse effect. [2]
The primary goal of the open-cycle gas turbine engine is to expand gas through rotating blades, converting kinetic and thermal energy to mechanical work. A schematic of the jet engine is displayed in Fig. 1. A gas turbine engine draws ambient air through a compressor to achieve the pressure ratio for expansion in the turbine. After the compression stage, fuel mixed with the air combusts in the combustion chamber brings additional energy to increase the air temperature. The heated gases expand through the turbine which turns blades to drive the compressor on a shared spool to perform the shaft power cycle again. Exhaust gases also spin a generator for electrical power and exhaust out of a nozzle at a high velocity to create thrust. [3]
Fig. 1 illustrates the intertwined roles between the compressor, combustion chamber, and turbine. To increase the performance of the engine, engineers have identified important factors to seek after and maximize, including 1) the compression ratio, 2) the inlet gas temperature to the turbine, and 3) the combustion efficiency. A comparison of engines in the 1940s and 1950s to modern engines provides a snapshot of the mechanical strides and triumphs in the last six decades. The compression ratio improved from 4:1 in the 1940s to 42:1 as attained in the Rolls-Royce Trent 900. Inlet temperature for the turbines improved from 1,000 °C in the 1940s to 1,700 °C in the modern engines. Combustion efficiency is measured by specific fuel consumption (sfc), a ratio of kilograms of fuel consumed per hour per Newton of thrust. The engines of 1950s hovered around 1.0 sfc, whereas the modern Trent 800 achieved almost twice the efficiency with a 0.56 cruise sfc. [4]
The power output of the turbine engines enables an aircraft to take off, cruise, climb, descend, and land. During flight, four main forces acting on an aircraft are the upward lift, downward weight, forward thrust, and backward drag. Stable flight requires these forces to be balanced. The turbine engine produces additional thrust to accelerate the aircraft during takeoff, reaching speeds up to 140 knots for jet fighters and 180 knots for civil aircraft. Separate from the main engines, gas turbine engines power the auxiliary power units (APUs), which are used to start the main engines and provide electrical power on the ground. For major aircraft classes, APUs generate between 10 kW and 300 kW of power. [5]
To quantify the superiority of gas turbine jet engines to piston-engines, the Rolls-Royce Merlin XX, a 12 cylinder piston-engine has a power-to-weight ratio of 1.68 kW/kg. [6] Meanwhile, the Rolls-Royce W.2B/23 turbojet called the Welland, powered the Meteor with a power-to-weight ratio of 3.38 kW/kg. [7]
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| Fig. 2: Plot comparing the relationship of sfc and thrust to Vjet. Image source: I. Chen, after R. Royce. [4] |
The net momentum thrust, F, of an engine is calculated by [4]
where W is the mass flow, Vflight is the speed of the engine in flight, and Vjet is the speed of the ejected gas from the rear nozzle of the engine. The equation holds as long as the nozzle is not choked. At a set altitude and flight speed, thrust can increase by increasing mass flow or increasing Vjet. Evaluating fuel energy is related to the jet kinetic energy (KE) equation
The KE relation shows that fuel consumption is proportional to the square of Vjet, while thrust is proportional to Vjet. Fig. 2 compares the relationship between the sfc and Vjet, displaying the linearly increasing thrust versus the sharper increase of sfc.
Less energy is required to accelerate a larger mass of air by a small velocity difference, compared to accelerating a smaller mass of air by a larger velocity difference. Modern turbofans utilize high bypass ratios to accelerate greater mass flow than mass flow from turbojets. Therefore, a turbofan generates higher thrust for the same energy input compared with a turbojet.
Massive aviation emissions come from the need to use high-energy-density liquid fuels to power large airplanes for long-distance flights. Nowadays, fossil kerosene-based Jet A/A-1 are the most used jet fuels for commercial gas turbine airplanes. Jet A-1 has low freezing-points (-47 °C or below) property with additives to enhance performance and safety. [8] These fuels are inexpensive, easy to store, and convenient to transport. However, gas turbine engine emissions contribute to environmental pollution. The combustion chamber burns hydrocarbon fuels with air to maintain high turbine inlet gas temperatures, while emitting pollutants into the environment. A general stoichiometric combustion equation is
where
Since the supplied oxygen comes from ambient air entering the gas turbine engine, the nitrogen component of the air may form nitrogen oxides (NOX) during combustion. [3] Incomplete combustion also forms exhaust CO and unburned hydrocarbons. The emission of these pollutants exacerbates the greenhouse effect due to high-altitude effects despite aviation currently contributing only a few percent of global CO2 emissions. Therefore it is urgently necessary to seek alternative energy resources for gas turbine engines. [9]
For decarbonization we will focus on two alternatives: sustainable aviation fuels (SAFs) and hydrogen. It was projected that to reach the 2050 net-zero emissions aviation decarbonization goal, aircraft efficiency must be improved to avoid upwards of 27% of projected emissions, and replacing fossil jet fuel with upwards of 2.5 to 19.8 EJ of SAFs. [10]
SAFs have so far shown to be a main contributor in the decarbonization effort due the fact that it can serve as drop-in fuels. Drop-in fuels can be blended with kerosene, used on kerosene-powered airplanes, and be compatible with current aircraft technologies and infrastructure. SAFs include bio-waste based from feedstock and synthetic fuels of power-to-liquid, e-fuels, solar-to- liquid from hydrogen and CO2. [11] The American Society of Testing Materials (ASTM) mandates up to 50% blend limit due to the low aromatic content of SAFs. Bio-based jet fuel has been slowly integrated for gas turbine engines to decrease CO2 emissions since 2008, and 100% biofuels have been used by some airplanes since 2021. The use of SAFs accounted for less than 0.1% of jet kerosene consumption by 2022 due to SAFs being more expensive than jet kerosene, and to limited capacity and production volume. [12]
Compared to Sustainable Aviation Fuels, hydrogen is abundant and offers exceptionally high gravimetric energy density (120 MJ/kg to 142 MJ/kg) compared to Jet-A (43 MJ/kg). Having a high flame speed and wider flammability range brings higher efficiency and cleaner combustion, leading to decreased formation of NO2. Water vapor is the only byproduct of hydrogen and has the potential to cool the environment. In 2012, Boeing demonstrated the unmanned aerial vehicle Phantom Eye, an aircraft flown by hydrogen-powered combustion engines that reached altitudes of 65,000 feet and remained in flight up to several days. [13] Hydrogen is an advantageous renewable fuel alternative to traditional hydrocarbon fuels to address environmental concerns regarding aviation gas turbine engines.
The unique properties of hydrogen cause some technical challenges for incorporation into the gas turbine engines. Hydrogen gas turbines need to be redesigned to accommodate the following properties of hydrogen: low density and energy content, high flame temperature and speed. Although hydrogen is abundant, it is difficult to store due to its extremely low volumetric energy density (energy per volume). Specialized tanks are required for storage on an aircraft. To use hydrogen as an aviation fuel, aircraft and engines must be engineered and modified to accommodate the new jet fuel source. [13]
Jet turbine engines enable faster and more efficient aviation. From World War II prototypes to the current engines, advancements in compressor pressure ratios, turbine inlet temperatures, and combustion efficiency have achieved swift and convenient global travel. However, the byproducts of engine emissions contribute to hazardous environmental pollution and climate impact. Ongoing research on alternative fuels and cleaner combustion shows promising progress toward sustainable aviation. High performance and reduced environmental impact can be achieved as the jet engine industry continues.
© Ivana Chen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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