Turbojet vs Turboprop A turbojet is an air breathing gas turbine engine executing an internal combustion cycle during the operation. It also belongs to the reaction engine type of the aircraft propulsion engines. Sir Frank Whittle of United Kingdom and Hans von Ohain of Germany independently developed the practical engines concept during the late 1930s, but only after the WWII, the jet engine became a widely used propulsion method. Turboprop engines are another variant built on the turbojet engine, and use the turbine to produce shaft work to drive a propeller. They are a hybrid of early reciprocating engine propulsion and newer gas turbine powered propulsion. Also, turboprop engines can be seen as a turboshaft engine with propeller connected to the shaft through a reduction gear mechanism. More about Turbojet Engine Cold air entering through the intake is compressed to high pressure in the successive stages of an axial flow compressor. In a common jet engine, air flow undergoes several compression stages, and at each stage, raising the pressure to a higher level. Modern turbojet engines can produce pressure ratios as high as 10:1 due to advanced compressor stages designed with aerodynamic improvements and variable compressor geometry to produce optimal compression at each stage.
The pressurization of the air also increases the temperature, and when mixed with the fuel produces a combustible gas mixture. Combustion of this gas increases the pressure and temperature to a very high level (1200 oC and 1000 kPa) and the gas pushes through the blades of the turbine. In the turbine section, gas exerts force on the turbine blades and rotates the turbine shaft; in a common jet engine, this shaft work drives the compressor of the engine. Then the gas is directed through a nozzle, and this produce a large amount of thrust, which can be used to power an aircraft. At the exhaust, the speed of the gas can be well above the speed of sound. The operation of the Jet engine is ideally modeled by the Brayton cycle. The turbojets are inefficient at low speed flight, and optimal performance lies beyond Mach 2. Another disadvantage of the turbojets is that the turbojets are extremely noisy. However, they are still used in the mid-range cruise missiles because of the simplicity of production and low speed More about Turboprop Engine Turboprop engine is an advanced version of the turbojet engine, where the shaft work is used to drive a propeller through a reduction gear mechanism attached to the turbine shaft. In this form of jet engines, majority thrust is generated by the propeller reaction and the exhaust generates a negligible amount of usable energy; hence mostly not used for thrust.
The propellers in turboprop engines are usually a constant speed (variable pitch) type, similar to propellers used in larger reciprocating aircraft engines. While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression. Propellers lose efficiency as aircraft speed increases, but very efficient at flight speeds below 725 km/h. Hence turboprops are normally not used on high-speed aircraft and are used to power small subsonic aircraft. Some exceptions exist, such as Airbus A400M and Lockheed Martin C130, which are large military freighters, and turboprops are used for high-performance short-takeoff and landing requirements of these aircrafts. What is the difference between Turbojet and Turboprop Engine? • Turbojets were the first air breathing gas turbine engine for aircraft, while turboprop is an advanced variant of turbojet, using the gas turbine to drive a propeller to generate thrust. • Turbojets show good performance at supersonic speed, while turboprops show good performance at subsonic speeds. • Turbojets are used in specific military applications at present, but turboprops are widely used in both military and commercial aircraft. Steam Engine vs Steam Turbine While, steam engine and steam turbine use the large latent heat of vaporization of steam for the power, the main difference is the maximum revolution per minute of the power cycles that both could provide. There is a limit for the number of cycles per minute that could provide with a steam driven reciprocating piston, inherent in its design. Steam engines in locomotives, normally have double acting pistons run with steam accumulated at both faces alternatively. The piston is supported with piston rod connected with a cross head. Cross head is further attached to the valve control rod by a linkage. The valves are for supply of the steam, as well as, for exhausting the used steam. The engine power generated with the reciprocating piston is converted to a rotary motion and transferred to the drive rods and the coupling rods that drive the wheels. In turbines, there are vanes designs with steels to give a rotary movement with the steam flow. It is possible to identify three major technological advancements, which make the steam turbines more efficient to steam engines. They are steam flow direction, the properties of the steel that is used to manufacture the turbine vanes, and the method of producing “supercritical steam”. The modern technology used for steam flow direction and flow pattern is more sophisticated compared to the old technology of peripheral flow. The introduction of direct hit of steam with blades at an angle that produces a little or almost no back resistant gives the maximum energy of the steam to the rotary movement of the turbine blades. The supercritical steam is produced by pressurizing the normal steam such that, the water molecules of the steam are forced to a point that it becomes more like a liquid again, while retaining the gas properties; this has excellent energy efficiency compared to the normal hot steam. These two technological advancements were realized through the use of high quality steels to manufacture the vanes. So, it was possible to run the turbines at much high speeds withstanding the high
pressure of the supercritical steam for the same amount of energy as traditional steam power without breaking or even damaging the blades. The disadvantages of the turbines are: small turndown ratios, which are the degradation of performance with the reduction of steam pressure or flow rates, slow start up times, which is to avoid thermal shocks in thin steel blades, large capital cost, and the high quality of steam demanding feed water treatment. The main disadvantage of steam engine is its limitation of the speed and the low efficiency. Normal steam engine efficiency is around 10 – 15 % and newest engines are capable of operating at much higher efficiency, around 35% with the introduction of compact steam generators and by keeping the engine in an oil free condition thus, increasing the fluid life. For small systems, the steam engine is preferred to steam turbines since the efficiency of turbines depend on the steam quality and the high speed. The exhaust of the steam turbines is at very high temperature and thus, low thermal efficiency too. With the high cost of the fuel used for internal combustion engines, the rebirth of steam engines is visible at present. Steam engines are very good in recapturing the waste energy from many sources including steam turbines exhaust. The waste heat from steam turbine is used in combined cycle power plants. It further allows discharging the waste steam as exhaust in much low temperatures. Two vs Four Strokes The internal combustion (IC) engines are classified as two and four strokes engines. The difference between the two is the number of times the piston moves up and down in the cylinder to complete one combustion cycle, named as Otto Cycle (Suck, Squeeze, Bang and Blow of the air and fuel mixer). In two strokes engine, there is one upward and downward stroke, whereas in four strokes it has two each giving a total of four strokes in its combustion cycle. Two Strokes The two strokes of the two strokes engine are named as compression stroke and return stroke. During the compression stroke, the compression of the sucked air-fuel-oil mixture (with petrol engine) or air (with diesel engines) is compressed and then followed by the explosion of the fuel. In the return stroke, the exhaust is forced out through the bypass port using the passage formed with the piston slots and simultaneously a new mixture is sucked into the cylinder. The presence of only two strokes to complete the combustion cycle and the absence of valves to control suction and discharge of fuel mix give a simple engine construction. So, it is easier and less costly to manufacture. It also has a power stroke for each revolution of the crankshaft producing twice the power of a four stroke engine with the same size. The small size of the engine to a given power has given a wide range of applications such as in chain saws, lawn movers, motor bikes and large, high power marine ships and electric – diesel trains etc. With the simple construction of the two strokes engine, it does not have a separate lubricating system. So, its spare parts could wear out much faster compared to the four strokes. The addition of oil to fuel and its combustion makes the two strokes engine produce much more pollution. Four Strokes In four strokes engines, there is one compression and one exhaust stroke, and they are followed by return stroke to complete the combustion cycle. Compression stroke compress the fuel mixture, and at the TDC (Top Dead Centre), the combustion takes place. Piston returns with the power and starts moving up again. The exhaust valve gets open during this second upward movement (Exhaust stroke) and allows the burnt fuel to exhaust from the cylinder. During the next return stroke of the engine with exhaust valve closed and intake valve open, the mixture is sucked into the cylinder. With this combustion system, four strokes engine has to have a separate mechanism for controlling the valves and a proper lubricating mechanism. It also produces one power stroke for two revolutions of the crankshaft. So, for a given power, the construction of the engine is costly compared to two strokes engines. Four strokes engines can have much higher compression ratios compared to two strokes engines, and thus, much more fuel efficient. It means, four strokes engines can do more mileage per a gallon of fuel. The four strokes to complete one combustion cycle give a smoother operation of the engine. Addition of no oil with fuel gives a much cleaner exhaust and less pollution of the environment. Difference between two strokes and four strokes
The number of available strokes to complete a combustion cycle in an engine distinguishes it as a two or four stroke engine. With the main similarity of the two engines as the “internal combustion�, they have distinct differences in its construction as well as advantages and disadvantages by having two strokes and four strokes. The main advantages of two strokes engines are less costly, simple construction together with high cycle (engine) efficiency. However, the fuel efficiency is bit lower compared to four strokes engine. While the four strokes engine is complicated in its construction with the addition of puppet valves and a separate mechanism for lubrication, it gives a smoother, less polluted operation with high fuel efficiency. The above advantages of four strokes engines and the longer lasting of the engines have attracted the use of them in automobiles. Gas Turbine vs Steam Turbine Turbines are a class of turbo machinery used to convert the energy in a flowing fluid into mechanical energy by the use of rotor mechanisms. Turbines, in general, convert either thermal or kinetic energy of the fluid into work. Gas turbines and steam turbines are thermal turbo machinery, where the work is generated from the enthalpy change of the working fluid; i.e. The potential energy of the fluid in the form of pressure is converted into mechanical energy. Based on the direction of the fluid flow turbines are categorized into axial flow turbines and radial flow turbines. Technically a turbine is an expander, which delivers mechanical work output by the decrease in pressure, which is the opposite operation of the compressor. This article focuses on the axial flow turbine type, which is more common in many engineering applications. The basic structure of an axial flow turbine is designed to allow a continuous flow of fluid while extracting the energy. In thermal turbines, the working fluid, at a high temperature and a pressure is directed through a series of rotors consisting of angled blades mounted on a rotating disk attached to the shaft. In between each rotor disks stationary blades are mounted, which act as nozzles and guides to the fluid flow. More about Steam Turbine Even though the concept of using steam to do mechanical work was used for a long time, the modern steam turbine was designed by English engineer Sir Charles Parsons in 1884. The steam turbine uses pressurized steam from a boiler as the working fluid. The superheated steam entering the turbine loses its pressure (enthalpy) moving through the blades of the rotors, and the rotors move the shaft to which they are connected. Steam turbines deliver power at a smooth, constant rate, and the thermal efficiency of a steam turbine is higher than that of a reciprocating engine. The operation of steam turbine is optimal at higher RPM states. Strictly, the turbine is only a single component of the cyclic operation used for power generation, which is ideally modelled by the Rankine cycle. The boilers, heat exchangers, pumps, and condensers are also components of the operation but are not parts of the turbine. In modern days, primary use of the steam turbines is for the electrical power generation, but at the early 20th century steam turbines was used as the power plant for ships and locomotive engines. As an exception, in some marine propulsion systems where the diesel engines are impractical, such as aircraft carriers and submarines, the steam engines are still used. More about Gas Turbine Gas turbine engine or simply a gas turbine is an internal combustion engine, using gases such as air as the working fluid. Thermodynamic aspect of the operation of the gas turbine is ideally modelled by the Brayton cycle. Gas turbine engine, unlike the steam turbine, consists of several key components; those are the compressor, combustion chamber, and turbine, which are assembled along a rotating shaft, to perform different tasks of an internal combustion engine. Gas intake from the inlet is first compressed using an axial compressor; which performs the exact opposite of a simple turbine. The pressurized gas is then directed through a diffuser (a diverging nozzle) stage, in which the gas loses its velocity, but increases the temperature and the pressure further. In the next stage, gas enters the combustion chamber where a fuel is mixed with the gas and ignited. As a result of the combustion, the temperature and pressure of the gas rise to an incredibly high level. This gas then passes through the turbine section, and when passing through produces rotational motion to the
shaft. An average size gas turbine produces shaft rotation rates as high as 10,000 RPM, while smaller turbines may produce 5 times as much. Gas turbines can be used to produce torque (by the rotating shaft), thrust (by high speed gas exhaust), or both in combination. In the first case, as in the steam turbine, the mechanical work delivered by the shaft is merely a transformation of enthalpy (pressure) of the high temperature and pressure gas. Part of the shaft work is used to drive the compressor through an internal mechanism. This form of the gas turbine is used mainly for electric power generation and as power plants for vehicles such as tanks and even cars. The US M1 Abrams tank uses a gas turbine engine as the power plant. In the second case, the high pressure gas is directed through a converging nozzle to increase velocity, and the thrust is generated by the exhaust gas. This type of gas turbine is often called Jet engine or turbojet engine, which powers the military fighter aircraft. The turbofan is an advanced variant of above, and the combination of both thrust and work generation is used in turboprop engines, where shaft work is used to drive a propeller. There exist many variants of the gas turbines designed for specific tasks. They are preferred over other engines (mainly reciprocating engines) due to their high power to weight ratio, less vibration, high operation speeds, and reliability. The waste heat is dissipated almost entirely as the exhaust. In electrical power generation, this waste thermal energy is used to boil water to run a steam turbine. The process is known as combined cycle power generation. What is the difference between Steam Turbine and Gas Turbine? • Steam turbine uses high pressure steam as the working fluid, while the gas turbine uses air or some other gas as the working fluid. • Steam turbine is basically an expander delivering torque as the work output, while a gas turbine is a combined device of compressor, combustion chamber, and turbine executing a cyclic operation to deliver work as either torque or thrust. • Steam turbine is only a component executing one step of the Rankine cycle, while gas turbine engine executes the whole Brayton cycle. • Gas turbines can deliver either torque or thrust as the work output, while steam turbines almost all the time delivers torque as the work output. • The efficiency of the gas turbines is much higher than the steam turbine due to higher operating temperatures of the gas turbines. (Gas turbines ~1500 0C and steam turbines ~550 0C) • The space required for the gas turbines is much less than steam turbine operation, because steam turbine requires boilers and heat exchangers, which should be connected externally for heat addition. • Gas turbines are more versatile, because many fuels can be used and working fluid, which has to be fed continuously, is readily available everywhere (air). Steam turbines, on the other hand, require large amounts of water for the operation and tend to cause problems in lower temperatures due to icing. Jet Engine vs Rocket Engine The jet and rocket engines are reaction engines based on Newton’s third law. Rocket engine is also a jet engine with few specific variations between the two. The thrust of the two is from the speed of the exhaust of the engine. The exhaust of a rocket engine reaches the speed of sonic near the throat of the nozzle, and the expansion in the nozzle further multiplies the speed, giving hypersonic exhaust jet. The jet engine uses air and fuel for combustion, and operates at subsonic or sonic speeds. Jet engine only works in atmosphere, whereas rockets can operate in vacuum and in atmosphere. Jet engines take oxygen for combustion from the atmosphere but rockets have their own oxygen. Rocket Engine A rocket engine, or simply “rocket,” is a kind of jet engine that uses only propellant mass, which produces pressurized gas for forming its high speed propulsive jet that is directed through a nozzle to produce thrust in Rocket engines. Most of them are internal combustion engines, and instead of using external materials to form the jet they use the exhaust from the IC engines. The highest exhaust velocities of jets are from the rocket engines. The principal of operation of the rocket engine is divided into three main components, and differ slightly with the type of propellant used. First is the propellant combustion or heating, which produces exhaust gas, second is, passing it through a supersonic propelling nozzle, which helps to accelerate the exhaust gas to high speeds using the heat energy of gas itself. Then the engine is pushed in the opposite direction, as the reaction to the exhaust flow. This gives better thermodynamic efficiency based on high
temperatures and pressures. It is because at high temperatures the sonic speed too is very high. Sonic velocity is roughly proportional to the square of the temperature of the exhaust. The construction of the rocket engine depends on the type of propellant use. Many engines are internal combustion engines, which use propellant masses of mixture of fuel and oxidizing components, or a combination of solid and liquid, or gaseous propellants. The other type is heating the chemically inert reaction mass using a high energy power source via a heat exchanger. Jet Engine Jet engine consists of many parts such as a fan, compressor, combustor, turbine, mixer, and nozzle. The availability and the arrangement of these parts together with the drive mechanism give different types of jet engines. Engine sucks the air and compress it in the compressor. Then the compressed and heated air is sent to the combustor and mix with fuel and burn. The exhaust is sent to the turbine to produce the thrust to drive the engine. The available types of the jet engines are: ramjet, turbojet, turbofan, turboprop and turbo shaft. The principal of operation of all the engines are similar with the following exceptions. In turbofan, a portion of compressed air is directly fed to the turbine. Although it is not heated as the exhaust from the combustor, it carries a high mass of air and thus, contributes to a larger portion to the total thrust. In turboprop and turbofan, the thrust is produced by a propeller too. In turbo fan, the total thrust is produced by a propeller as we can see it in the helicopters. Jet Engine vs Rocket Engine - Rockets are used for spacecrafts and missiles. - The use of jet is mainly in the transport industry and can be found with military aircrafts, aircrafts, high speed cars, boats and ships too. Other uses are in cruise missiles and unmanned aerial vehicles (UAV). - Rocket engine is least energy efficient to jet. - The Noise pollution is higher with rocket engines compared with jet engines. - The jet engines are more complex to rocket engines.
Turbojet vs Turbofan A turbojet is an air breathing gas turbine engine executing an internal combustion cycle during the operation. It also belongs to the reaction engine type of the aircraft propulsion engines. Sir Frank Whittle of United Kingdom and Hans von Ohain of Germany, independently developed the practical engines concept during the late 1930s, but only after the WWII, the jet engine became a widely used propulsion method. A turbojet poses several disadvantages in performance at subsonic speeds, such as efficiency and noise; therefore, advanced variants were built based on the turbojet engines to minimize those problems. Turbofans were developed as early as 1940s, but not used due to less efficiency until 1960’s when RollsRoyce RB.80 Conway became the first production turbofan engine. More about Turbojet Engine Cold air entering through the intake is compressed to high pressure in the successive stages of an axial flow compressor. In a common jet engine, air flow undergoes several compression stages, and at each stage, raising the pressure to a higher level. Modern turbojet engines can produce pressure ratios as high as 20:1 due to advanced compressor stages designed with aerodynamic improvements and variable compressor geometry to produce optimal compression at each stage.
The pressurization of the air also increases the temperature, and when mixed with the fuel produces a combustible gas mixture. Combustion of this gas increases the pressure and temperature to a very high level (1200 oC and 1000 kPa) and the gas pushes through the blades of the turbine. In the turbine section, gas exerts force on the turbine blades and rotates the turbine shaft; in a common jet engine, this shaft work drives the compressor of the engine. Then the gas is directed through a nozzle, and this produce a large amount of thrust, which can be used to power an aircraft. At the exhaust, the speed of the gas can be well above the speed of sound. The operation of the Jet engine is ideally modeled by the Brayton cycle. The turbojets are inefficient at low speed flight, and optimal performance lies beyond Mach 2. Another disadvantage of the turbojets is that the turbojets are extremely noisy. However, they are still used in the mid-range cruise missiles because of the simplicity of production and low speed. More about Turbofan Engine Turbofan engine is an advanced version of the turbojet engine, where the shaft work is used to drive a fan to take in large amounts of air, compress, and direct through the exhaust, to generate thrust. Part of the air intake is used to drive the jet engine in the core, while the other portion is directed separately through a series of compressors and directed through the nozzle without undergoing combustion. Because of this ingenious mechanism the turbofan engines are less noisy and deliver more thrust.
High Bypass Engine The bypass ratio of air is defined as the ratio between the mass flow rates of air drawn through a fan disk that bypasses the engine core without undergoing combustion, to the mass flow rate passing through the engine core that is involved in combustion, to produce mechanical energy to drive the fan and produce thrust. In a high bypass design, most of the thrust is developed from the bypass flow, and in the low
bypass, it is from the flow through the engine core. High bypass engines are usually used for commercial applications for their less noise and fuel efficiency, and low bypass engines are used where higher power to weight ratios are required, such as military fighter aircraft. What is the difference between Turbojet and Turbofan Engines? • Turbojets were the first air breathing gas turbine engine for the aircrafts, while turbofan is an advanced variant of turbojet using a jet engine to drive a fan to generate thrust (turbofan has a gas turbine at the core). • Turbojets are efficient at higher speeds (supersonic) and produce a large noise, while turbofans are efficient at both subsonic speeds and transonic speeds and produces less noise. • Turbojets are used in specific military applications at present, but turbofan remains the most preferable choice of propulsion for both military and commercial aircraft. • In turbojet, thrust is purely generated by the exhaust from the gas turbine while, in turbofan engines, a portion of the thrust is generated by the bypass flow.