A turbocharger, colloquially known as a turbo, is a turbine-driven, forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra compressed air into the combustion chamber.[1][2] This improvement over a naturally aspirated engine's power output is because the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure (and for that matter, ram air intakes) alone.
Turbochargers were originally known as turbosuperchargers when all forced induction devices were classified as superchargers. Today, the term "supercharger" is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. Twincharger refers to an engine with both a supercharger and a turbocharger.
Manufacturers commonly use turbochargers in truck, car, train, aircraft, and construction-equipment engines. They are most often used with Otto cycle and Diesel cycle internal combustion engines.
Contents
- 1 History
- 2 Turbocharging versus supercharging
- 3 Operating principle
- 4 Key components
- 5 Additional technologies commonly used in turbocharger installations
- 5.1 Intercooling
- turbine-driven, forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra compressed air into the combustion chamber.[1][2]
This improvement over a naturally aspirated engine's power output is because the compressor can force more air—and proportionately more fuel—into the combustion chamber than atmospheric pressure (and for that matter, ram air intakes) alone.
Turbochargers were originally known as turbosuperchargers when all forced induction devices were classified as superchargers. Today, the term "supercharger" is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. Twincharger refers to an engine with both a supercharger and a turbocharger.
Manufacturers commonly use turbochargers in truck, car, train, aircraft, and construction-equipment engines. They are most often used with Otto cycle and forced induction devices were classified as superchargers. Today, the term "supercharger" is typically applied only to mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that a supercharger is mechanically driven by the engine, often through a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine driven by the engine's exhaust gas. Compared with a mechanically driven supercharger, turbochargers tend to be more efficient, but less responsive. Twincharger refers to an engine with both a supercharger and a turbocharger.
Manufacturers commonly use turbochargers in truck, car, train, aircraft, and construction-equipment engines. They are most often used with Otto cycle and Diesel cycle internal combustion engines.
Forced induction dates back to the late 19th century, when Gottlieb Daimler patented the technique of using a gear-driven pump to force air into an internal combustion engine in 1885.[3]
The 1905[4] patent by Alfred Büchi, a Swiss engineer working at Gebrüder Sulzer (now simply called Sulzer) is often considered the birth of the turbocharger.[5][6] This patent was for a compound radial engine with an exhaust-driven axial flow turbine and compressor mounted on a common shaft.[7][8] The first prototype was finished in 1915 with the aim of overcoming the power loss experienced by aircraft engines due to the decreased density of air at high altitudes.[9][10] However, the prototype was not reliable and did not reach production.[9] Another early patent for turbochargers was applied for in 1916 by French steam turbine inventor Auguste Rateau, for their intended use on the Renault engines used by French fighter planes.[7][11] Separately, 1917 testing by the American National Advisory Committee for Aeronautics and Sanford Alexander Moss showed that a turbocharger could enable an engine to avoid any power loss (compared with the power produced at sea level) at an altitude of up to 4,250 m (13,944 ft) above sea level.[7] The testing was conducted at Pikes Peak in the United States using the V12 Liberty aircraft engine.[11]
The first commercial application of a turbocharger was in 1925, when Alfred Büchi successfully installed turbochargers on ten-cylinder diesel engines, increasing the power output from 1,300 to 1,860 kilowatts (1,750 to 2,500 hp).[12][13][14] This engine was used by the German Ministry of Transport for two large passenger ships called the "Preussen" and "Hansestadt Danzig". The design was licensed to several manufacturers and turbochargers began to be used in marine, railcar and large stationary applications.[10]
Turbochargers were used on several aircraft engines during World War II, beginning with the Boeing B-17 Flying Fortress in 1938, which used turbochargers produced by General Electric.[7][15] Other early turbocharged airplanes included the B-24 Liberator, P-38 Lightning, P-47 Thunderbolt and the experimental Focke-Wulf Fw 190 prototypes.
Automobile and truck manufacturers began research into turbocharged engines during the 1950s, however the problems of "turbo lag" and the bulky size of the turbocharger were not able to be solved at the time.[5][10] The first turbocharged cars were the short-lived Chevrolet Corvair Monza and the Oldsmobile Jetfire, introduced in 1962.[16] Following the 1973 oil crisis and the 1977 Clean Air Act amendments, turbocharging became more common in automobiles, as a method to reduce fuel consumption and exhaust emissions.[7]
T
The 1905[4] patent by Alfred Büchi, a Swiss engineer working at Gebrüder Sulzer (now simply called Sulzer) is often considered the birth of the turbocharger.[5][6] This patent was for a compound radial engine with an exhaust-driven axial flow turbine and compressor mounted on a common shaft.[7][8] The first prototype was finished in 1915 with the aim of overcoming the power loss experienced by aircraft engines due to the decreased density of air at high altitudes.[9][10] However, the prototype was not reliable and did not reach production.[9] Another early patent for turbochargers was applied for in 1916 by French steam turbine inventor Auguste Rateau, for their intended use on the Renault engines used by French fighter planes.[7][11] Separately, 1917 testing by the American National Advisory Committee for Aeronautics and Sanford Alexander Moss showed that a turbocharger could enable an engine to avoid any power loss (compared with the power produced at sea level) at an altitude of up to 4,250 m (13,944 ft) above sea level.[7] The testing was conducted at Pikes Peak in the United States using the V12 Liberty aircraft engine.[11]
The first commercial application of a turbocharger was in 1925, when Alfred Büchi successfully installed turbochargers on ten-cylinder diesel engines, increasing the power output from 1,300 to 1,860 kilowatts (1,750 to 2,500 hp).[12][13][14] This engine was used by the German Ministry of Transport for two large passenger ships called the "Preussen" and "Hansestadt Danzig". The design was licensed to several manufacturers and turbochargers began to be used in marine, railcar and large stationary applications.[10]
Turbochargers were used on several aircraft engines during World War II, beginning with the Boeing B-17 Flying Fortress in 1938, which used turbochargers produced by General Electric.[7][15] Other early turbocharged airplanes included the B-24 Liberator, P-38 Lightning, P-47 Thunderbolt and the experimental Focke-Wulf Fw 190 prototypes.
Automobile and truck manufacturers began research into turbocharged engines during the 1950s, however the problems of "turbo lag" and the bulky size of the turbocharger were not able to be solved at the time.[5][10] The first turbocharged cars were the short-lived Chevrolet Corvair Monza and the Oldsmobile Jetfire, introduced in 1962.[16] Following the 1973 oil crisis and the 1977 Clean Air Act amendments, turbocharging became more common in automobiles, as a method to reduce fuel consumption and exhaust emissions.[7]
In contrast to turbochargers, superchargers are mechanically driven by the engine.[17] Belts, chains, shafts, and gears are common methods of powering a supercharger, placing a mechanical load on the engine.[18][19] For example, on the single-stage single-speed supercharged Rolls-Royce Merlin engine, the supercharger uses about 150 hp (110 kW). Yet the benefits outweigh the costs; for the 150 hp (110 kW) to drive the supercharger the engine generates an additional 400 hp (300 kW), a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent; the engine must withstand the net power output of the engine plus the power to drive the supercharger.
Another disadvantage of some superchargers is lower adiabatic efficiency when compared with turbochargers (especially Roots-type superchargers). Adiabatic efficiency is a measure of a compressor's ability to compress air without adding excess heat to that air. Even under ideal conditions, the compression process always results in elevated output temperature; however, more efficient
Another disadvantage of some superchargers is lower adiabatic efficiency when compared with turbochargers (especially Roots-type superchargers). Adiabatic efficiency is a measure of a compressor's ability to compress air without adding excess heat to that air. Even under ideal conditions, the compression process always results in elevated output temperature; however, more efficient compressors produce less excess heat. Roots superchargers impart significantly more heat to the air than turbochargers. Thus, for a given volume and pressure of air, the turbocharged air is cooler, and as a result denser, containing more oxygen molecules, and therefore more potential power than the supercharged air. In practical application the disparity between the two can be dramatic, with turbochargers often producing 15% to 30% more power based solely on the differences in adiabatic efficiency (however, due to heat transfer from the hot exhaust, considerable heating does occur).
By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses.[17] This is more efficient because while the increased back pressure taxes the piston exhaust stroke, much of the energy driving the turbine is provided by the still-expanding exhaust gas that would otherwise be wasted as heat through the tailpipe. In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as "lag" or "spool time". This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power.
Throttle lag occurs because turbochargers rely on the buildup of exhaust gas pressure to drive the turbine. In variable output systems such as automobile engines, exhaust gas pressure at idle, low engine speeds, or low throttle is usually insufficient to drive the turbine. Only when the engine reaches sufficient speed does the turbine section start to spool up, or spin fast enough to produce intake pressure above atmospheric pressure.
A combination of an exhaust-driven turbocharger and an engine-driven supercharger can mitigate the weaknesses of both.[20] This technique is called twincharging.
In the case of Electro-Motive Diesel's two-stroke engines, the mechanically assisted turbocharger is not specifically a twincharger, as the engine uses the mechanical assistance to charge air only at lower engine speeds and startup. Once above notch # 5, the engine uses true turbocharging. This differs from a turbocharger that uses the compressor section of the turbo-compressor only during starting and, as a two-stroke engines cannot naturally aspirate, and, according to SAE definitions, a two-stroke engine with a mechanically assisted compressor during idle and low throttle is considered naturally aspirated.
In naturally aspirated piston engines, intake gases are drawn or "pushed" into the engine by atmospheric pressure filling the volumetric void caused by the downward stroke of the piston[21][22] (which creates a low-pressure area), similar to drawing liquid using a syringe. The amount of air actually inspired, compared with the theoretical amount if the engine could maintain atmospheric pressure, is called volumetric efficiency.[23] The objective of a turbocharger is to improve an engine's volumetric efficiency by increasing density of the intake gas (usually air) allowing more power per engine cycle.
The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure.[24] This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine's exhaust gases.[25]
In automotive applications, 'boost' refers to the amount by which intake manifold pressure exceeds atmospheric pressure at sea level. This is representative of the extra air pressure that is achieved over what would be achieved without the forced induction. The level of boost may be shown on a pressure gauge, usually in bar, psi or possibly kPa.[26] The control of turbocharger boost has changed dramatically
The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure.[24] This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the kinetic energy of the engine's exhaust gases.[25]
In automotive applications, 'boost' refers to the amount by which intake manifold pressure exceeds atmospheric pressure at sea level. This is representative of the extra air pressure that is achieved over what would be achieved without the forced induction. The level of boost may be shown on a pressure gauge, usually in bar, psi or possibly kPa.[26] The control of turbocharger boost has changed dramatically over the 100-plus years of their use. Modern turbochargers can use wastegates, blow-off valves and variable geometry, as discussed in later sections.
In petrol engine turbocharger applications, boost pressure is limited to keep the entire engine system, including the turbocharger, inside its thermal and mechanical design operating range. Over-boosting an engine frequently causes damage to the engine in a variety of ways including pre-ignition, overheating, and over-stressing the engine's internal hardware. For example, to avoid engine knocking (also known as detonation) and the related physical damage to the engine, the intake manifold pressure must not get too high, thus the pressure at the intake manifold of the engine must be controlled by some means. Opening the wastegate allows the excess energy destined for the turbine to bypass it and pass directly to the exhaust pipe, thus reducing boost pressure. The wastegate can be either controlled manually (frequently seen in aircraft) or by an actuator (in automotive applications, it is often controlled by the engine control unit).
A turbocharger may also be used to increase fuel efficiency without increasing power.[27] This is achieved by diverting exhaust waste energy, from the combustion process, and feeding it back into the turbo's "hot" intake side that spins the turbine. As the hot turbine side is being driven by the exhaust energy, the cold intake turbine (the other side of the turbo) compresses fresh intake air and drives it into the engine's intake. By using this otherwise wasted energy to increase the mass of air, it becomes easier to ensure that all fuel is burned before being vented at the start of the exhaust stage. The increased temperature from the higher pressure gives a higher Carnot efficiency.
A reduced density of intake air is caused by the loss of atmospheric density seen with elevated altitudes. Thus, a natural use of the turbocharger is with aircraft engines. As an aircraft climbs to higher altitudes, the pressure of the surrounding air quickly falls off. At 18,000 feet (5,500 m), the air is at half the pressure of sea level, which means that the engine produces less
A reduced density of intake air is caused by the loss of atmospheric density seen with elevated altitudes. Thus, a natural use of the turbocharger is with aircraft engines. As an aircraft climbs to higher altitudes, the pressure of the surrounding air quickly falls off. At 18,000 feet (5,500 m), the air is at half the pressure of sea level, which means that the engine produces less than half-power at this altitude.[26] In aircraft engines, turbocharging is commonly used to maintain manifold pressure as altitude increases (i.e. to compensate for lower-density air at higher altitudes). Since atmospheric pressure reduces as the aircraft climbs, power drops as a function of altitude in normally aspirated engines. Systems that use a turbocharger to maintain an engine's sea-level power output are called turbo-normalized systems. Generally, a turbo-normalized system attempts to maintain a manifold pressure of 29.5 inHg (100 kPa).[26]
Turbocharger lag (turbo lag) is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost which can also be referred to as spooling. Inertia, friction, and compressor load are the primary contributors to turbocharger lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.
Turbocharger applications can be categorized into those that require changes in output power (such as automotive) and those that do not (such as marine, aircraft, commercial automotive, industrial, engine-generators, and locomotives). While important to varying degrees, turbocharger lag is most problematic in applications that require rapid changes in power output. Engine designs reduce lag in a number of ways:
- Lowering the rotational inertia of the turbocharger by using lower radius parts and ceramic and other lighter materials
- Changing the turbine's aspect ratio
- Increasing upper-deck air pressure (compressor discharge) and improving wastegate response
- Reducing bearing frictional losses, e.g., using a foil bearing rather than a conventional oil bearing
- Using variable-nozzle or twin-scroll turbochargers
- Decreasing the volume of the upper-deck piping
- Using multiple turbochargers sequentially or in parallel
- Using an antilag system
- Using a turbocharger spool valve to increase exhaust gas flow speed to the (twin-scroll) turbine
Sometimes turbo lag is mistaken for engine speeds that are below boost threshold. If engine speed is below a turbocharger's boost threshold rpm then the time needed for the vehicle to build speed and rpm could be considerable, maybe even tens of seconds for a heavy vehicle starting at low vehicle speed in a high gear. This wait for vehicle speed increase is not turbo lag, it is improper gear selection for boost demand. Once the vehicle reaches sufficient speed to provide the required rpm to reach boost threshold, there will be a far shorter delay while the turbo itself builds rotational energy and transitions to positive boost, only this
Turbocharger applications can be categorized into those that require changes in output power (such as automotive) and those that do not (such as marine, aircraft, commercial automotive, industrial, engine-generators, and locomotives). While important to varying degrees, turbocharger lag is most problematic in applications that require rapid changes in power output. Engine designs reduce lag in a number of ways:
Sometimes turbo lag is mistaken for engine speeds that are below boost threshold. If engine speed is below a turbocharger's boost threshold rpm then the time needed for the vehicle to build speed and rpm could be considerable, maybe even tens of seconds for a heavy vehicle starting at low vehicle speed in a high gear. This wait for vehicle speed increase is not turbo lag, it is improper gear selection for boost demand. Once the vehicle reaches sufficient speed to provide the required rpm to reach boost threshold, there will be a far shorter delay while the turbo itself builds rotational energy and transitions to positive boost, only this last part of the delay in achieving positive boost is the turbo lag.
Boost threshold
The boost threshold of a turbocharger system is the lower bound of the region within which the compressor operates. Below a certain rate of flow, a compressor produces insignificant boost. This limits boost at a particular RPM, regardless of exhaust gas pressure. Newer turbocharger and engine developments have steadily reduced boost thresholds.
Electrical boosting ("E-boosting") is a new technology under development. It uses an electric motor to bring the turbocharger up to operating speed quicker than possible using available exhaust gases.[28] An alternati
Electrical boosting ("E-boosting") is a new technology under development. It uses an electric motor to bring the turbocharger up to operating speed quicker than possible using available exhaust gases.[28] An alternative to e-boosting is to completely separate the turbine and compressor into a turbine-generator and electric-compressor as in the hybrid turbocharger. This makes compressor speed independent of turbine speed.
Turbochargers start producing boost only when a certain amount of kinetic energy is present in the exhaust gasses. Without adequate exhaust gas flow to spin the turbine blades, the turbocharger cannot produce the necessary force needed to compress the air going into the engine. The boost threshold is determined by the engine displacement, engine rpm, throttle opening, and the size of the turbocharger. The operating speed (rpm) at which there is enough exhaust gas momentum to compress the air going into the engine is called the "boost threshold rpm". Reducing the "boost threshold rpm" can improve throttle response.
The turbocharger has three main components:
- The turbine, which is almost always a radial inflow turbine (but is almost always a single-stage axial inflow turbine in large Diesel engines)
- The compressor, which is almost always a Many turbocharger installations use additional technologies, such as wastegates, intercooling and blow-off valves.
Turbine
enthalpy and kinetic energy of the gas. The turbine housings direct the gas flow through the turbine as it spins at up to 250,000 rpm.[29][30] The size and shape can dictate some performance characteristics of the overall turbocharger. Often the same basic turbocharger assembly is available from the manufacturer with multiple housing choices for the turbine, and sometimes the compressor cover as well. This lets the balance between performance, response, and efficiency be tailored to the application.
The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can flow through the system, and the relative efficiency at which they operate. In general, the larger the turbine wheel and compressor wheel the larger the flow capacity. Measurements and shapes can vary, as well as curvature and number of blades on the wheels.
A turbocharger's performance is closely tied to its size.[31] Large turbochargers take more heat and pressure to spin the turbine, creating lag at low speed. Small turbochargers spin quickly, but may not have the same performance at high acceleration.[32][33] To efficiently combine the benefits of large and small wheels, advanced schemes are used such as twin-turbochargers, twin-scroll turbochargers, or variable-geometry turbochargers.
Twin-turbo
Twin-turbo or bi-turbo designs have two separate turbochargers operating in either a sequence or in parallel.[34] In a parallel configuration, both turbochargers are fed one-half of the engine's exhaust. In a sequential setup one turbocharger runs at low speeds and the second turns on at a predetermined engine speed or load.[34] Sequential turbochargers further reduce turbo lag, but require an intricate set of pipes to properly feed both turbochargers.
Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series so that boost pressure from one turbocharger is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger t
The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can flow through the system, and the relative efficiency at which they operate. In general, the larger the turbine wheel and compressor wheel the larger the flow capacity. Measurements and shapes can vary, as well as curvature and number of blades on the wheels.
A turbocharger's performance is closely tied to its size.[31] Large turbochargers take more heat and pressure to spin the turbine, creating lag at low speed. Small turbochargers spin quickly, but may not have the same performance at high acceleration.[32][33] To efficiently combine the benefits of large and small wheels, advanced schemes are used such as twin-turbochargers, twin-scroll turbochargers, or variable-geometry turbochargers.
Twin-turbo or bi-turbo designs have two separate turbochargers operating in either a sequence or in parallel.[34] In a parallel configuration, both turbochargers are fed one-half of the engine's exhaust. In a sequential setup one turbocharger runs at low speeds and the second turns on at a predetermined engine speed or load.[34] Sequential turbochargers further reduce turbo lag, but require an intricate set of pipes to properly feed both turbochargers.
Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series
Two-stage variable twin-turbos employ a small turbocharger at low speeds and a large one at higher speeds. They are connected in a series so that boost pressure from one turbocharger is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbocharger to the large one can be done incrementally. Twin turbochargers are primarily used in Diesel engines.[34] For example, in Opel bi-turbo Diesel, only the smaller turbocharger works at low speed, providing high torque at 1,500–1,700 rpm. Both turbochargers operate together in mid range, with the smaller one pre-compressing the air, which the larger one further compresses. A bypass valve regulates the exhaust flow to each turbocharger. At higher speed (2,500 to 3,000 RPM) only the larger turbocharger runs.[35]
Smaller turbochargers have less turbo lag than larger ones, so often two small turbochargers are used instead of one large one. This configuration is popular in engines over 2.5-litres and in V-shape or boxer engines.
Twin-scroll or divided turbochargers have two exhaust gas inlets and two nozzles, a smaller sharper angled one for quick response and a larger less angled one for peak performance.
With high-performance camshaft timing, exhaust valves in different cylinders can be open at the same time, overlapping at the end of the power stroke in one cylinder and the end of exhaust stroke in another. In twin-scroll designs, the exhaust manifold
With high-performance camshaft timing, exhaust valves in different cylinders can be open at the same time, overlapping at the end of the power stroke in one cylinder and the end of exhaust stroke in another. In twin-scroll designs, the exhaust manifold physically separates the channels for cylinders that can interfere with each other, so that the pulsating exhaust gasses flow through separate spirals (scrolls). With common firing order 1–3–4–2, two scrolls of unequal length pair cylinders 1 and 4, and 3 and 2. This lets the engine efficiently use exhaust scavenging techniques, which decreases exhaust gas temperatures and NO
x emissions, improves turbine efficiency, and reduces turbo lag evident at low engine speeds.[36]Cut-out of a twin-scroll turbocharger, with two differently angled nozzles
Cut-out of a twin-scroll exhaust and turbine; the du
Cut-out of a twin-scroll exhaust and turbine; the dual "scrolls" pairing cylinders 1 and 4, and 2 and 3 are clearly visible
Variable-geometry
[31][32] The vanes are placed just in front of the turbine like a set of slightly overlapping walls. Their angle is adjusted by an actuator to block or increase air flow to the turbine.[32][33] This variability maintains a comparable exhaust velocity and back pressure throughout the engine's rev range. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbocharger lag.[31]
Compressor
The compressor increases the mass of intake air entering the combustion chamber. The compressor is made up of an impeller, a diffuser and a volute housing.
The operating range of a compressor is described by the "compressor map".
Ported shroud
The operating range of a compressor is described by the "compressor map".
enthalpy
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