COMPLETE guide to Airplane Engine Types (Turbojet, Turboprop, Turbofan & Turboshaft) : How Do They Work?

A few decades ago, humans successfully discovered the secret behind flight. But no matter how long we fly, many still marvel at the mystery of how such big metal bodies can sustain motion in the air. All of the “magic” that keeps airplanes flying is powered by engines. However, not all airplane engines are the same. Most of the airplane engines in use today are jet engines/gas turbines. You’ll, therefore, find that the terms “airplane engines”, “jet engines” and “gas turbines” are used interchangeably in this article.

Gas turbines have seen years and years of innovation and modernization and now come in all sizes. In general, there are 4 types of airplane engines namely:

1. Turbojet engines;
2. Turboprop engines;
3. Turbofan engines and;
4. Turboshaft engines.

This particular classification of airplane engines is based on how these engines generate power. In reality, all the other three types of jet engines are mere modifications of the turbojet engine.

Under this classification, another airplane engine type worthy of note is the ramjet engine. Though the ramjet engine was loved by some for its simplicity, its application is rather limited. This is as a result of the fact that its compression ratio depends largely on the speed of the aircraft. What this means is that it cannot independently generate thrust (move the airplane) from a position of rest. When used, the ramjet is boosted by other vehicles until the airplane reaches a certain velocity.

History and Advancement of Airplane Engines

It is common knowledge that the Wright brothers are credited with the first sustained, controlled flight in 1903. But even by the standards of the decade’s technology, the Flyer’s 4-cylinder engine was relatively rudimentary. The jet engine, as we know it today, was first patented by the English engineer, Sir Frank Whittle in January 1930. Nevertheless, it wasn’t until during the Second World War that jet engines seized to be ordinary subjects of lab experiments. For further comprehension, here’s an abridged timeline that portrays how different airplane engine types have improved over the years:

• 1791: English inventor, John Barber is granted a patent for the first gas turbine engine. It was a rotary motion engine powered by burning wood, coal, and oil. The invention, however, failed mainly due to a lack of quality materials (metals) at the time.
• 1930: Sir Frank Whittle of England patents the first-ever turbojet engine design.
• 1936: German inventors, Hans von Ohian and Max Han patent their own design.
• 1939: German company, Ernst Heinkel Aircraft flies Ohian’s model beating Whittle to become the first operational gas turbine engine.
• 1941: Sir Frank Whittle and his company, Power Jets Ltd. design the first true turbojet engine. The engine later powered the Gloster Meteor, Britain’s first jet fighter and the Allies’ only jet-powered aircraft with combat capacity during WW II.
• 1943: The first turbofan engine is test-run by the Nazi Ministry of Aviation. The model was abandoned as the German government ran into graver problems.
• 1945: Rolls-Royce develops the first turboprop engine and uses it to power a modified Gloster Meteor. The model flew but never really went into production.
• 1948: French firm Turbomeca builds the first turboshaft engine, the 100-hp 782.
• 1949: Turbojet engines are used for commercial purpose for the first time.
• 1953: North American Aviation product, YF-100 Super Sabre becomes the first airplane powered by a turbojet to break the sound barrier.
• 1964: AVCO-Lycoming runs the first experimental high-bypass turbofan engine.

How Do Turbojet Engines Work?

The turbojet engine is the most basic and simplest model of all airplane engines. Sir Frank Whittle’s original patent was a turbojet and most turbojets available today are made based on that design. The turbojet’s working principle involves passing air through the 5 major components of the engine:

• Inlet: The inlet is all about sucking air into the engine. Positioned right in front of the engine, the inlet is a highly essential part of the engine. Though it may look rather basic, the engine cannot do without the air that it strategically directs into the compressor blades. The inlet primarily ensures that airflow speed into the engine is always subsonic (below Mach 1) no matter the airplane’s speed.
• Compressor: The turbojet compressor’s main task is to increase the pressure of the air that is received from the inlet. The compressor is powered by the turbine—another component in the engine’s rear.
• Combustion chamber: In the combustion chamber, the high-pressure air from the compressor is burnt to release a lot of energy to power the turbine. This is achieved by combining compressed air with fuel to produce a high-velocity gas. The combustion chamber works at very high temperatures.
• Turbine: Just like the compressor, the turbine is a series of blades that absorbs energy from the fast-moving air released from the combustion chamber. Unlike the compressor, the turbine expands the air that passes through it. Generally, the overall efficiency of the engine is largely determined by the turbine.
• Exhaust nozzle: Here, all the high-speed expanded air escapes from the engine producing thrust.

How Do Turboprop Engines Work?

In essence, a turboprop engine is a turbojet with additional components. So, a turboprop has all the parts of the turbojet with a gearbox and propeller attached before the inlet. These additional components are connected to the rest of the engine through a shaft. But how do the propeller and gearbox impact the work of the turboprop system?

1. In the turbojet, some of the energy generated by the turbine is expended on turning the compressor blades. However, the turboprop does things a little differently. A great percentage of the turbine’s energy is channeled towards rotating the propeller through the connecting shaft. Only a very small percentage (about 10%) of the thrust comes from the exhaust gas.
2. The speed at which the propeller spins is controlled by the gearbox. The gear with the lowest speed connects to the propeller in front.
3. The propeller generates most of the engine’s thrust by its rotational movement.

The efficiency of this turboprop system begins to reduce as the speed of the aircraft increases. It is therefore used mostly in low-speed aircraft like cargo planes.

How Do Turbofan Engines Work?

The turbofan engine is the most advanced modification of the simple turbojet engine. It is a brilliant concept that combines the best aspects of the turbojet and the turboprop. In the turbofan engine, a large fan is added at the front with an additional low-pressure compressor and turbine. This fan essentially generates more thrust and helps to cool the engine. Here are some notable differences between how turbofans and turbojets work:

1. In the turbojet, all the air entering the engine goes straight to the compressor and into the combustion chamber. On the other hand, in the turbofan, only a fraction of the air passing through the fan reaches the core components of the engine—the main compressor, the combustion chamber, and the main turbine.
2. The other portion of the air is passed around the engine and accelerated by the fan. This stream of air is called bypass air.
3. Bypass air is ejected either directly from the engine (called “cold” jet) or mixed with the engine’s exhaust (called “hot” jet).
4. In the end, thrust is generated by both the gas generator (engine’s core) and the fan’s accelerated air.

The ratio of the air that passes around the gas generator to the air that passes through the gas generator is termed “bypass ratio”. The bypass ratio (BPR) is the most important parameter of turbofan engines.

How Do Turboshaft Engines Work?

Turboshaft engines are basically helicopter engines. A turboshaft engine is more like a simple turbojet with a large shaft attached in the rear. Instead of connecting the shaft to a propeller as in the case of turboprops, the shaft here is connected to a rotor blade transmission system. Here are the key principles of how the turboshaft operates:

1. For the most part, the turboshaft functions like a turbojet engine—compressing air in the compressor, burning it in the combustion chamber and expanding it in the turbine.
2. But unlike the turbojet, the turboshaft engine’s turbine generates a relatively low amount of thrust. Most of the turbine’s energy is captured by the shaft connected to it.
3. The power shaft in turn powers the transmission which together with one or more gearboxes spins the rotor blades.
4. The rotor blades produce shaft power to drive the airplane instead of thrust like in the other engine types.

The application of turboshafts is most especially suited for constructions requiring a small yet powerful engine. In turboshafts, the rotor blades’ speed does not depend on the rotating speed of the gas generator. This means that the rotor speed can be kept constant even when the generator’s speed varies.

Which Type of Engine Is Used in Most Commercial Airplanes?

Because of their high bypass ratio and high fuel efficiency, turbofan engines are the most commonly used engines in commercial aviation. The majority of commercial airplanes are large carriers, require high speed and travel long distances. Considering these factors, turbofan engines are the perfect fit for the job.

However, turbofans weren’t always major airliners’ favorites. Considering that the first operational turbofan engine flew in 1964, it took quite some time before the aviation world finally embraced the technology in the early 1980s. Boeing became the first company in the world to fully adopt turbofan engines in their aircraft. The Boeing 737-300 was powered by the French CFM56-3 engine.

Interestingly, at that time, the CFM factory was at the brink of shutting down its jet engine program and closing the factory. When Boeing chose the CFM56-3 as the main engine for the Boeing 737 Classic series in 1981, the company was saved and a new era for commercial aviation engines was born. This choice was largely based on the need to meet the new noise regulations at the time.

However, prior to ordering the engines, CFM had to agree to implement certain adjustments to the CFM56. The CFM56 engine was previously designed for the Douglas DC-8 aircraft with DC-8-61 using CFM56-2. But the Boeing 737 has wings closer to the ground than those of the DC-8. As a result, the diameter of the fan was reduced leading to a decrease in the bypass ratio.

Today, the Boeing 737 fleet has recorded over 88 million hours of flight. Manufacturers like General Electric, Pratt & Whitney and Rolls-Royce are also major producers of turbofan engines. In fact, the world’s largest and most powerful engine, General Electric’s GE9X is a turbofan engine.

Most Common Type of Engine Used in Military Airplanes

Turbofan engines ousted the simple turbojets in the military aviation industry long before they were ever used for commercial transportation. While commercial airliners focus mainly on fuel efficiency in their choice of engines, jet fighters are all about thrust and response time. Afterburning turbofans are a perfect response to that need. When the first afterburning turbofan, the TF30, was used to power a multirole fighter, it became clear that this was a great solution for achieving superb fuel efficiency at subsonic speeds and high thrust for supersonic flight. This is why turbofans still dominate the fighter category to date.

Most modern combat aircraft are powered by low bypass turbofan engines as against the high bypass turbofans present in commercial airplanes. This is mainly because low bypass engines possess a higher thrust-to-weight ratio. And with the help of the afterburner, these engines can generate maximum thrust even at takeoff. Unfortunately, this often leads to an increase in fuel consumption.

The biggest manufacturers of fighter jet engines today are Pratt & Whitney, General Electric, and Rolls-Royce. And a good fraction of their most delivered military engines—General Electric F404, F414, and Pratt & Whitney F119—are turbofan engines.

Classification of Airplane Engines Based on Compressor Types

Apart from power generation, airplane engines can also be classified based on the type of compressors they use. As earlier mentioned, the compressor is a very important component of the engine and largely influences its performance. There are 2 main types of compressors used in airplane engines namely: axial compressors and centrifugal compressors. Here, the compressors are classified according to the direction of airflow.

The first jet engines made use of centrifugal compressors. But while an average single-stage centrifugal compressor can increase air pressure by a factor of 4 (very good figure), modern airplanes require higher compression ratios. To achieve higher compression ratios, it becomes imperative to make use of multi-stage compressors. But losses between stages of centrifugal compressors make them a less practical option. Enter axial compressors!

Axial compressors

Axial compressors are named so because air flows through them in a direction parallel to their axis of rotation. The axial compressor is made up of a rotor and a stator. Put simply, the rotor is the moving part of the compressor while the stator is the stationary part of the compressor.

Most axial compressors consist of multiple stages with rotor blades followed by stator blades on each stage. The rotor blades turn the air at a high speed and impel it towards the fixed stator blades converting the high velocity of the moving air to high pressure. As the air moves through each stage, its pressure is further increased. Therefore, the number of stages in a compressor depends on the required compression ratio.

Advantages

• High peak efficiency due to the straight-flow design.
• Higher compression ratios are achievable by adding multiple stages.
• Small frontal area for a given airflow rate.

Disadvantages

• Relatively heavy.
• The high cost of production.

Centrifugal Compressors

In centrifugal compressors, air pressure is increased using centrifugal action. Centrifugal compressors also require a rotor (impeller) – stator (diffuser) pair to perform their job. The rotor picks up the air and accelerates it outwardly in the direction of the stator increasing air pressure in the process.

Advantages

• Low weight.
• Easier to produce.
• High compression ratio per stage.

Disadvantages

• Large frontal area for a given airflow rate.
• Losses between stages make multi-stage designs (above 2 stages) impractical.

Some manufacturers combine the axial and centrifugal flow designs to make mixed-flow compressors. A very good example is the Pratt & Whitney PW600 which was featured in a number of light jet aircraft.

How Much Do Airplane Engines Cost On Average?

Even for the average layman, it wouldn’t be shocking to imagine that airplanes are expensive. They fly hundreds of people over such long distances after all! But what many may not realize is that the airplane engine is arguably the most expensive part of the plane. The next question then is: how much does an airplane engine actually cost?

This is a rather difficult question to answer. Airplane engines are hardly ever bought individually. Most airlines purchase aircraft with engines already fitted. Asides, there are different types of jet engines available in the market with widely varying prices. But if we were to give a rough estimate, airplane engine prices can range from US$1.5 million to US$41.4 million per unit.

That’s huge, right? But if we took into consideration all the contributing factors, then it will become clear that this cost is justified. To begin with, it generally costs billions of dollars (program costs) to design and develop an average airplane engine. Jet engine manufacturers often put in huge amounts of resources into innovation to ensure that these complex machines not just meet but also exceed weight, efficiency, safety, and durability standards.

The more modern turbofan engines with higher thrust ratings are always the most expensive airplane engines. For example, a single unit of the American GE9X which first flew on March 13, 2018 costs above US$40 million.

Uses of Each Engine Type

It has already been stated earlier that turbofans are the most widely used engine type. But different types of gas turbines excel in different departments making them more suited for certain applications.

Turbojets

Though turbojet engines may have been replaced in many industries by its other improved modifications, they are still an ideal option for supersonic flight. This is why they were largely used on the Concorde which was made to travel long distances at supersonic speeds.

Advantages

• Relatively simple design.
• Less number of connecting parts reducing losses due to friction.
• Relatively low cost of production and maintenance.
• Absence of unbalanced forces. This means they are free from vibrations which are a great source of losses.
• Capable of reaching very high speeds.

Disadvantages

• High levels of noise pollution.
• High fuel consumption.
• Poor performance (low thrust) at takeoff and low speeds.

Turboprops

 Due to their propellers’ low jet velocity, turboprops are mostly used for flights below 725 km/hr (450 mph). They are also known to have a lower ceiling (maximum flight altitude).

Advantages

• Highly efficient at lower altitudes (in dense air).
• Require shorter runways (about 3,200 ft) meaning they can operate in most airports. For some perspective, turbojets often need a 5,000 ft runway.
• Propellers can be feathered to minimize drag in the event of engine failure. This is impossible with the other three types of engines.
• High fuel efficiency.

Disadvantages

• Propellers lose efficiency at high altitudes.
• Gearing systems are heavy and prone to breakdown.
• High vibration levels which may lead to passenger discomfort.

Turbofans

Thanks to the fact that turbofans are now a market favorite, these engines have seen a lot of innovation. Built to avoid the turbojet’s inefficiency at subsonic speeds, turbofan engines create a good balance between high thrust ratings and fuel efficiency.

Advantages

• High fuel efficiency.
• Fans ensure more air is sucked into the engine generating more thrust in the process.
• By far the quietest engine type.

Disadvantages

• Relatively heavy due to increased engine diameter.
• Largely inefficient at very high altitudes
• More complex design.

Turboshafts

Being modifications of turboprop engines, turboshafts generally share a good number of characteristics with turboprops.

Advantages

• High power to weight ratio.
• Relatively lightweight and smaller.
• Mostly require no runway.

Disadvantages

• Loudest engine type.
• Just like turboprops, complex gear systems aren’t very durable.

How Many Engines Do Planes Normally Need?

While a lot of commercial flyers are powered by two engines, there are many airplanes that are not twinjet. Generally, airplanes can use from 1 to 6 engines for flight. Due to the fact that some pilots have successfully landed planes with one engine, people often assume that a second engine is needed solely for safety. But this is not necessarily true. Because of their cost, airplane manufacturers always strive to minimize the number of engines they put in an airplane.

The number of engines an airplane needs is determined by how much thrust is required for takeoff and to sustain motion in the air. If a plane carries only the right amount of engines it needs to sustain flight, why then do planes still land successfully even after one engine fails? The simple answer to this is that transport regulations demand that twinjets must be able to produce 200% of the minimum thrust necessary for climb.

The most common configurations of aircraft are those carrying either two or four jet engines. Essentially twinjet airplanes have higher fuel efficiency—a high priority, especially in commercial aviation. However, in May 2017, a colossal aircraft named Stratolaunch was rolled out with 6 Pratt & Whitney turbofan engines! (Seen in the image above)

Biggest Manufacturers Driving Airplane Engine Innovation

New-generation airplane engines continue to push the jet technology beyond the limit in terms of designs and materials. In the face of challenges with temperature limits like in the case of the Trent 1000, one may be forced to think that the technology is beginning to reach its peak. But the competition between these big players continues to assure us of more development ahead.

CFM International

After its brilliant success with the CFM56 series, the Safran-GE joint venture gradually created a name for itself in the jet engine manufacturing industry. Today, with its LEAP high-bypass turbofan engine series, the company continues to set the pace for aviation innovation. The LEAP is advertised to have 15% fuel consumption improvement over the older CFM56. Built for 99.98% dispatch reliability, this new-generation engine promises more flight hours and less maintenance time.

General Electric Aviation

Boasting the most powerful engine in the world, General Electric has already shown great dedication to technological advancement over the years. But even the GE9X is not perfect and the firm recognizes this. This is why the digital transformation of the aviation industry through avionics has been at the heart of its research. Apart from design and production, GE is also investing heavily in engine maintenance, repair, and overhaul.

Pratt & Whitney

The Pratt & Whitney JT8D remains one of the most delivered commercial engines to date. But one of the ways the American company has shown most promise is with its Geared Turbofan (GTF) which adds a gearbox between the fan and the low-pressure compressor. Having taking about 20 years of research and $10 billion in expenses, the relatively novel unit has been gaining wide acceptance in the industry. In view of the continually increasing orders, it’s only a matter of time before the model undergoes significant alterations.

Other manufacturers worthy of recognition are Rolls-Royce, China’s Comac and Russia’s Aviadvigatel.

Final Notes

Having examined the different types of aircraft engines in detail, it becomes clear that irrespective of the peculiar differences in design, all jet engines run based on a similar principle. In general, turbojets, turboprops, turbofans, and turboshafts alike rely on the process of intake – compression – combustion – exhaust to generate power.