Aircraft Engines And Their Types

AIRCRAFT  ENGINE

An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. An aviation engine is an engine specifically designed for the needs of the airborne environment.

These engines advanced rapidly through the 20th century, driven by the never ending need to fly a heavier, higher and faster aircraft with cargo, troops, passengers and/ or cargo to longer distances more economically.

“Aircraft is basically an engine-driven vehicle that can fly through the air supported by the action of air against its wings”. By this definition of air plane we can see the importance of an engine of the aircraft.

Thrust is the forward force that pushes the engine and, therefore, the airplane forward.

Sir Isaac Newton discovered that for "every action there is an equal and opposite reaction." An engine uses this principle. The engine takes in a large volume of air. The air is heated and compressed and slowed down. The air is forced through many spinning blades. By mixing this air with jet fuel, the temperature of the air can be as high as three thousand degrees. The power of the air is used to turn the turbine. Finally, when the air leaves, it pushes backward out of the engine. This causes the plane to move forward.

There are two main types of aircraft engines: Jet and Reciprocating/Piston engines.

There are many different kinds of each. Most engine types will be presented concisely here.

INTERNAL  COMBUSTION  ENGINES / PISTON ENGINE OR JET ENGINE

A jet engine is simply a reaction engine that develops thrust by the rapid discharge of a gas. Jet engines depend on the surrounding atmosphere for oxygen to support combustion and therefore can only operate in regions where an atmosphere exists.

TYPES

There are several types of jet engines: turbojet, turboprop, turbofan, ramjet, pulse-jet, and scram-jet.

  WANKEL ENGINE    

The Wankel Engine is a type of internal combustion engine using an eccentric rotary design to convert pressure into a rotating motion instead of using reciprocating pistons.

FELIX WANKEL and his rotary engine

The Wankel engine is about one half the weight and size of a traditional four-stroke cycle piston engine of equal power output, and much lower in complexity. In aviation, the power to weight ratio is very important, making the Wankel engine a good choice. This is a very cost effective alternative to certified aircraft engines, with power output from 120 to 380 hp at a fraction of the cost of a conventional engine.

Wankel engines are becoming increasingly popular in home-built experimental air-crafts.

USE

This engine is mostly used in small gliders and two seated light aircrafts

ARV Super2 with MidWest AE110 twin-rotor Wankel engine

It has also been used in the famous sports car, Mazda RX-8 and some motor bikes.

   RAM-JET

A ramjet engine is the simplest type of jet engine since it has no moving parts.

WORKING

The engine is basically a specially-shaped duct open at both ends, with the air necessary for combustion being compressed by the forward motion of the engine. Fuel is sprayed into the air stream and the mixture is ignited. The high-pressure air coming into the combustion chamber keeps the reaction from going back toward the inlet.

USE

Ramjet engines cannot operate under static conditions. In order to function, they have to already be traveling through the air at slightly over the speed of sound (somewhat over 740 miles per hour at sea level). This means that the aircraft using them must first get up to the required speed using some other type of propulsion, then start the ramjets.

 They can operate at up to five times the speed of sound.

The USAF Bomarc missile cruised with ramjet engines.

  PULSE-JET

Pulse jet engines, or Pulsejets are a very simple family of internal combustion engine that have few or no moving parts.

They were invented in the early 1900′s. Pulsejets got abandoned, many feel long before they even reached their prime. They went from being the main source of power for tens of thousands of cruise missiles, to something that a skilled hobbyist could build in their garage.

WORKING

A pulsejet engine uses the principle of "intermittent combustion." The inlet duct has a series of shutters which are spring-loaded to the open position. Air is drawn in through these open shutters and fuel is injected and ignited inside the combustion chamber. The increased pressure caused by this combustion forces the inlet shutters to close, forcing all the combustion gases to be expelled through the outlet duct at the rear of the engine. When the exhaust gases are expelled, the internal pressure subsides, allowing the inlet shutters to spring open again and the cycle repeats itself.

TYPES

There are two basic types of pulse jet engines, valved engines, and valveless engines. Both types operate on the same principles, and can run on a wide range of fuels, can be scaled to very small or very large sizes, and are surprisingly powerful for their simplicity.

1.     Valved Pulse Jet Engines:

A valved pulse-jet engine has a mechanical valve inside it with which to control the flow of air in and out of the engine.

This means that it does have a moving part that can wear out, and also more mechanical complexity.

Valved engines are much like a two stroke engine carburetor system, and an exhaust pipe, but no piston!

WORKING

Air flows in through the intake, lowering the pressure inside the carburetor section, drawing in fuel without need of a fuel pump. The fuel vaporizes and passes through the valve system, into the tail pipe, where it burns, causing a pressure rise and slamming the valve system shut. Exhaust blasts out of the back of the engine at great speed, so fast that it causes a vacuum to form inside the tail pipe. More fuel and air gets drawn in through the intake system, into the tail pipe, where it builds up until it is again ignited by remaining burning gas.

Valved Pulse Jet Benefit:

■High power to weight ratio

Valved Pulse Jet Drawback:

■temperamental to start

■valves wear out

■more difficult to build

2. Valveless Pulse Jet Engines:

Valveless pulse jet engines operate on the same principles as a valved engine, except with no valve, and no moving parts.

WORKING

The shape of the engine entirely controls the flow of air in and out, this usually means that the intake of the engine also faces backwards. When the engine fires, exhaust shoots out both the intake and the exhaust pipe. There are a very few Valveless Pulse Jet Engines engine designs that approach the efficiency of even turbofan engines.

Valveless Pulse Jet Benefits:

■Easy to start

■Can run on almost anything that burns including waste material

■Extremely large throttle range

Valveless Pulse Jet Drawbacks:

■Difficult to design one with really good performance

USE

The German V-1 operated with a pulse-jet engine.

  SCRAM-JET

Scramjets are ramjets that operate at supersonic speeds.

A scramjet, or "supersonic combustion ramjet" engine is similar to a ramjet, but is designed to operate at well over five times the speed of sound i.e. hyper-sonic velocities.

As with ramjets, aircraft powered by scramjets must first be brought up to required speed by some other means of propulsion. Unlike ramjets, which slow the supersonic air stream entering the inlet to subsonic speeds before combustion, a scramjet combusts the supersonic air stream without slowing it.

The National Aerospace Plane (NASP) will operate with scramjet engines.

  Afterburner

Modern turbine engines are extremely efficient, but there is still a lot of oxygen available in the exhaust stream. An apparatus called an afterburner can be built onto a turbine engine to inject fuel directly into the exhaust stream and burn it using up the remaining oxygen. This heats and expands the exhaust gases further, and can increase the thrust of a jet engine by 50% or more.

ADVANTAGE

The big advantage of an afterburner is that you can significantly increase the thrust of an engine without adding much weight or complexity to it.

An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable exhaust nozzle. A jet engine with an afterburner needs an adjustable nozzle so that it can work both with the afterburners on and with them off.

DISADVANTAGE

The big disadvantage of an afterburner is that it uses a LOT of fuel for the power it generates. Therefore, most planes use afterburners sparingly. For example, a military jet would use its afterburners when taking off from a short runway or from an aircraft carrier's deck, or during a high-speed maneuver in a dogfight.

US Navy F/A-18 fighter with afterburning turbofan engines.

   TURBO-PROP ENGINES

A turboprop engine is a turbojet engine in which a portion of the exhaust energy is used to drive a propeller.

With their ability to perform well at slow airspeeds and fuel efficiency, turboprop engines are often used in small, commuter aircraft and agricultural applications due to their greater reliability offsetting their higher initial cost.

Compared with a turbojet engine, the turboprop has better propulsion efficiency at flight speeds below about 500 miles per hour. Modern turboprop engines are equipped with propellers that have a smaller diameter but a larger number of blades for efficient operation at much higher flight speeds.

The engine's thrust is therefore generated by a combination of the propeller's thrust and the jet exhaust from the engine. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller.

Approximately 90% of thrust comes from propeller and about only 10% comes from exhaust gas.

WORKING

In its simplest form a turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure.

Turboprops are very efficient at flight speeds below 450 mph.

USE

Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required.

The most common application of turboprop engines in civilian aviation are the Turboprop airliners which now operate at near the same speed as small turbofan-powered aircraft but burn two-thirds of the fuel per passenger.

Turboprop engines are generally used on small subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kt (926 km/h, 575 mph). Large military and civil aircraft, such as the Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop power. Hence Turboprops remain popular on very small or slow aircraft, such as small commuter airliners, for their fuel efficiency at lower speeds, as well as on medium military transports and patrol planes, such as the C-130 Hercules and P-3 Orion, for their high take-off performance and mission endurance benefits.

The USAF C-130 transport operates with turboprop engines.

  PROP-FAN ENGINE

This engine is not to be confused with turboprop or turbofan.

The propfan concept was developed to deliver 35% better fuel efficiency than contemporary turbofans.

The big advantage of the Propfan is that it offers significant fuel savings and therefore lower carbon emissions, compared to existing aeroengines. There is therefore a real prospect that Propfan-powered aircraft will be in widespread use in the future. However, the noise generation is not yet fully understood, and needs to be addressed if the Propfan is to be a success.

A propfan was first defined as a small diameter, highly loaded multiple bladed variable pitch propulsor having swept blades with thin advanced airfoil sections, integrated with a nacelle (Casing of Engine) contoured to retard the airflow through the blades thereby reducing compressibility losses and designed to operate with a turbine engine and using a single stage reduction gear resulting in high performance.

USE

• §    Antonov An-70
• §    Beriev A-40
• §     McDonnell Douglas MD-94X

Antonov An-70 with Propfan

   TURBO-FAN ENGINES

Turbofan jet engines were designed to merge the best features of the turbojet and turboprop.

The turbofan or fanjet is a type of air-breathing jet engine that is widely used for aircraft propulsion. Turbofan engines are significantly quieter than a pure-jet of the same thrust.

These engines are quite reliable, providing years of trouble- free service.

Most of today's airliners are powered by turbofans.

Turbofans are thus the most efficient engines in the range of speeds from about 500 to 1000 km/h (310 to 620 mph), the speed at which most commercial aircraft operate. Turbofans retain an efficiency edge over pure jets at low supersonic speeds up to roughly Mach 1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above.

The air accelerated by the fan in a turbofan engine contributes significantly to the thrust produced by the engine, particularly at low forward speeds and low altitudes. In large engines, such as the engines that power the B747, B757, B767, A300, A310, etc., as much as three-quarters of the thrust delivered by the engine is developed by the fan.

The fan is not like a propeller. On a propeller, each blade acts like an airplane wing, developing lift as it rotates. The "lift" on a propeller blade pulls the engine and airplane forward through the air.

WORKING

A turbofan engine is a turbojet engine in which additional thrust is gained by extending a portion of the compressor or turbine blades outside the inner engine casing. These extended blades propel bypass air around the engine core, between the inner and outer engine casings. This air is not combusted but does provide additional thrust since it is compressed by the blades.

Approximately more than 75% of thrust comes from fan and less than 25% comes from exhaust gas.

TYPES

Turbofans are coarsely split into low-bypass and high-bypass categories. Bypass air flows through the fan, but around the jet core, not mixing with fuel and burning. The ratio of this air to the amount of air flowing through the engine core is the bypass ratio. Low-bypass engines are preferred for military applications such as fighters due to high thrust-to-weight ratio, while high-bypass engines are preferred for civil use for good fuel efficiency and low noise. High-bypass turbofans are usually most efficient when the aircraft is traveling at 500 to 550 miles per hour (800 to 885 km/h), the cruise speed of most large airliners. Low-bypass turbofans can reach supersonic speeds, though normally only when fitted with afterburners.

USE

The USAF C-17 transport operates with turbofan engines.

   TURBO-SHAFT ENGINE

Turboshaft engines are used primarily for helicopters and auxiliary power units.

The first turboshaft engine was built by the French firm, Turbomeca in 1949.

A turboshaft engine is very similar to a turboprop but in a turboprop the propeller is supported by the engine, and the engine is bolted to the airframe. In a turboshaft, the engine does not provide any direct physical support to the helicopter's rotors. The rotor is connected to a transmission, which itself is bolted to the airframe, and the turboshaft engine simply feeds the transmission via a rotating shaft.

USE

The X3 is equipped with two turboshaft engines that power a five-blade main rotor system and two propellers installed on short-span fixed wings, creating an advanced transportation system offering the speed of a turboprop-powered aircraft and the full hover flight capabilities of a helicopter

  Turbo-jet engines

A turbojet is a type of gas turbine engine that was originally developed for military fighters during World War II.

A turbojet is the simplest of all aircraft gas turbines.

Turbojet engines are used on many aircraft. These engines are limited on range and endurance and today are mostly used in military aviation. They are known for being slow to respond to throttle applications at slow compressor speeds.

Working

A turbojet engine has a turbine-driven compressor to take in and compress air for the combustion of fuel. The exhaust from the combustion drives the turbine and creates the thrust-producing jet.

USE

When turbojets were introduced, the top speed of fighter aircraft equipped with them was at least 100 miles per hour faster than competing piston-driven aircraft. The relative simplicity of turbojet designs lent them to wartime production. In the years after the war, the drawbacks of the turbojet gradually became apparent. Below about Mach 2, turbojets are very fuel inefficient and create tremendous amounts of noise. The early designs also respond very slowly to power changes, a fact which killed many experienced pilots when they attempted the transition to jets. These drawbacks eventually led to the downfall of the pure turbojet, and only a handful of types are still in production

The last airliner that used turbojets was the Concorde, whose Mach 2 airspeed permitted the engine to be highly efficient

The USAF T-43 trainer operates with turbojet engines.

   ELECTRIC MOTOR POWERED AIRCRAFTS

An electric aircraft is an aircraft that runs on electric motors rather than internal combustion engines, with electricity coming from fuel cells, solar cells, ultra-capacitors, power beaming, or batteries.

The solar-electric NASA-Helios Prototype 

Currently flying electric aircraft are mostly experimental demonstrators, including manned and unmanned aerial vehicles. About 60 electrically powered aircraft have been designed since the 1960s. Some are used as military drones.

In France in late 2007, a conventional light aircraft powered by an 18 kW electric motor using lithium polymer batteries was flown, covering more than 50 kilometers, the first electric airplane to receive a certificate of airworthiness.

Limited experiments with solar electric propulsion have been performed by NASA.