Turbocharger

"Turbo" redirects here. For the movie, see Turbo (film). For other uses, see Turbo (disambiguation).

Cut-away view of an air foil bearing-supported turbocharger.

A turbocharger, or turbo (colloquialism), from the Greek "τύρβη" ("turbulence") is a turbine driven forced induction device used to allow more power to be produced by an engine of a given size. A turbocharged engine can be more powerful and efficient than a naturally aspirated engine because the turbine forces more air, and proportionately more fuel, into the combustion chamber than atmospheric pressure alone.

Turbochargers were originally known as turbosuperchargers when all forced induction devices were classified as superchargers; nowadays the term "supercharger" is usually applied to only mechanically driven forced induction devices. The key difference between a turbocharger and a conventional supercharger is that the latter is mechanically driven from the engine, often from a belt connected to the crankshaft, whereas a turbocharger is powered by a turbine that is driven by the engine's exhaust gas. Compared to a mechanically-driven supercharger, turbochargers tend to be more efficient but less responsive. Twincharger refers to an engine which has both a supercharger and a turbocharger.

Turbos are commonly used on truck, car, train, aircraft, and construction equipment engines. Turbos are popularly used with Otto cycle and Diesel cycle internal combustion engines. They have also been found useful in automotive fuel cells.

History

Forced induction dates from 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. The turbocharger was invented by Swiss engineer Alfred Büchi (1879-1959), the head of diesel engine research at Gebruder Sulzer engine manufacturing company in Winterhur, who received a patent in 1905 for using a compressor driven by exhaust gasses to force air into an internal combustion engine to increase power output but it took another 20 years for the idea to come to fruition. During World War I French engineer Auguste Rateau fitted turbochargers to Renault engines powering various French fighters with some success. In 1918, General Electric engineer Sanford Alexander Moss attached a turbo to a V12 Liberty aircraft engine. The engine was tested at Pikes Peak in Colorado at 14,000 ft (4,300 m) to demonstrate that it could eliminate the power loss usually experienced in internal combustion engines as a result of reduced air pressure and density at high altitude. General Electric called the system turbosupercharging. At the time, all forced induction devices were known as superchargers, however more recently the term "supercharger" is usually applied to only mechanically-driven forced induction devices.

Turbochargers were first used in production aircraft engines such as the Napier Lioness in the 1920s, although they were less common than engine-driven centrifugal superchargers. Ships and locomotives equipped with turbocharged Diesel engines began appearing in the 1920s. Turbochargers were also used in aviation, most widely used by the United States. During World War II, notable examples of US aircraft with turbochargers include the B-17 Flying Fortress, B-24 Liberator, P-38 Lightning, and P-47 Thunderbolt. The technology was also used in experimental fittings by a number of other manufacturers, notably a variety of Focke-Wulf Fw 190 models, but the need for advanced high-temperature metals in the turbine kept them out of widespread use.

Turbocharging versus supercharging

Main article: Supercharger

In contrast to turbochargers, superchargers are not powered by exhaust gases but driven by the engine mechanically. Belts, chains, shafts, and gears are common methods of powering a supercharger. A supercharger places a mechanical load on the engine to drive. For example, on the single-stage single-speed supercharged Rolls-Royce Merlin engine, the supercharger uses up about 150 horsepower (110 kW). Yet the benefits outweigh the costs: For that 150 hp (110 kW), the engine generates an additional 400 horsepower, a net gain of 250 hp (190 kW). This is where the principal disadvantage of a supercharger becomes apparent: the internal hardware of the engine must withstand the net power output of the engine, plus the 150 horsepower to drive the supercharger.

Another notable disadvantage of superchargers is lower adiabatic efficiency as compared to turbochargers. Adiabatic efficiency is a measure of a compressors ability to compress air without adding excess heat to that air. The compression process always produces heat as a bi-product of that process, however, more efficient compressors produce less excess heat. The most common and widely used forms of superchargers tend to use compressors that impart significantly more heat to the air then their turbocharged counterparts. 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.

In comparison, a turbocharger does not place a direct mechanical load on the engine. It is more efficient because it uses the otherwise wasted potential and kinetic energy of the exhaust gas to drive the compressor. 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 horsepower. This lag occurs because turbochargers rely on the build up of exhaust gas pressure to spin the turbine section of the turbocharger which drives the compressor side. In variable output systems such as automobile engines the exhaust gas pressure at idle, or lower engine speeds is usually not sufficient to drive the turbine, it is only when the engine has reached sufficient speeds that the turbine section starts to "spool" up or spin fast enough to produce intake pressure above standard atmospheric pressure. When the engine has reached a sufficient speed, the turbine itself lags behind as it responds to the increase in exhaust gas pressure. To a large degree this problem has been mitigated by the use of modern ball bearing turbochargers which use higher quality production methods and materials that greatly reduce the time it takes the turbine to spool.

Other notable disadvantages of turbochargers are the inherent increase in complexity of the systems over a supercharged systems, and the cost associated with this complexity.

A combination of an exhaust-driven turbocharger and an engine-driven supercharger can mitigate the weaknesses of the other. 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 mechanical assistance is employed only for creation of charge air during starting, and the mechanical assistance is not employed thereafter. Rather, true turbocharging is employed thereafter. This, then, is a modification of a true turbocharger which employs the compressor section of the turbo-compressor only during starting, as a two-stroke engine, such as EMD's, cannot naturally aspirate, and, according to SAE definitions, a two-stroke engine which has a mechanically-assisted compressor during starting is considered to be naturally aspirated.

Operating principle

In most piston engines, intake gases are "pulled" into the engine by the downward stroke of the piston (which creates a low-pressure area), similar to drawing liquid using a syringe. The amount of air which is actually inhaled, compared with the theoretical amount if the engine could maintain atmospheric pressure, is called volumetric efficiency. The objective of a turbocharger is to improve an engine's volumetric efficiency by increasing density of the intake gas (usually air).

The turbocharger's compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. 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.

A turbocharger may also be used to increase fuel efficiency without increasing power. This is achieved by recovering waste energy in the exhaust and feeding it back into the engine 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.

The control of turbochargers is very complex and has changed dramatically over the 100-plus years of its use. Modern turbochargers can use wastegates, blow-off valves and variable geometry, as discussed in later sections.

The reduced density of intake air is often compounded 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 5,486 metres (17,999 ft), the air is at half the pressure of sea level, which means that the engine will produce less than half-power at this altitude.

Pressure increase / boost

In automotive applications, "boost" refers to the amount by which intake manifold pressure exceeds atmospheric pressure. 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.

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 will attempt to maintain a manifold pressure of 29.5 inches of mercury (100 kPa).

In all turbocharger applications, boost pressure is limited to keep the entire engine system, including the turbo, 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 (aka 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).

Turbo lag

Turbocharger applications can be categorized according to those which require changes in output power (such as automotive) and those which do not (such as marine, aircraft, commercial automotive, industrial, engine-generators, and locomotives). While important to varying degrees, turbo lag is most problematic when rapid changes in power output are required.

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 from idle 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. Inertia, friction, and compressor load are the primary contributors to turbo lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.

Lag can be reduced in a number of ways:

• lowering the rotational inertia of the turbocharger; for example by using lighter, lower radius parts to allow the spool-up to happen more quickly. Ceramic turbines are of benefit in this regard and or billet compressor wheel.
• changing the aspect ratio of the turbine.
• increasing the upper-deck air pressure (compressor discharge) and improving the wastegate response
• reducing bearing frictional losses (such as by using a foil bearing rather than a conventional oil bearing)
• using variable-nozzle or twin-scroll turbochargers (discussed below).
• decreasing the volume of the upper-deck piping.
• using multiple turbos sequentially or in parallel.
• using an Antilag system.
• using a turbo spool valve to increase exhaust gas flow speed to the (twin-scroll) turbine.

Boost threshold

Lag is not to be confused with the boost threshold. The boost threshold of a turbo system describes the lower bound of the region within which the compressor will operate. Below a certain rate of flow, a compressor will not produce significant boost. This has the effect of limiting boost at particular rpm regardless of exhaust gas pressure. Newer turbocharger and engine developments have caused boost thresholds to steadily decline.

Electrical boosting ("E-boosting") is a new technology under development; it uses an electric motor to bring the turbo up to operating speed quicker than is possible using available exhaust gases. 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 allows the compressor speed to become independent to that of the turbine. A similar system utilising a hydraulic drive system and overspeed clutch arrangement was fitted in 1981 to accelerate the turbocharger of the MV Canadian Pioneer (Doxford 76J4CR engine).

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 turbo 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 turbo. 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.

Key components of a turbocharger

The turbocharger has three main components:

1. the turbine, which is almost always a radial inflow turbine
2. the compressor, which is almost always a centrifugal compressor
3. the center housing/hub rotating assembly

Many turbocharger installations use additional technologies, such as wastegates, intercooling and blow-off valves.

Turbine

Energy provided for the turbine work is converted from the 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. The size and shape can dictate some performance characteristics of the overall turbocharger. Often the same basic turbocharger assembly will be available from the manufacturer with multiple housing choices for the turbine and sometimes the compressor cover as well. This allows the balance between performance, response, and efficiency to be tailored to the application.

The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can be flowed 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.

On the left, the brass oil drain connection. On the right are the braided oil supply line and water coolant line connections.

Compressor impeller side with the cover removed.

Turbine side housing removed.

A turbocharger’s performance is closely tied to its size. Large turbochargers take more heat and pressure to spin the turbine, creating turbo lag at low RPMs. Small turbochargers spin quickly, but may not have the same performance at high acceleration. 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

Main article: Twin-turbo

Twin-turbo or bi-turbo designs have two separate turbochargers operating in either a sequence or in parallel. 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. 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 turbo is multiplied by another, hence the name "2-stage." The distribution of exhaust gas is continuously variable, so the transition from using the small turbo to the large one can be done incrementally. Twin turbochargers are primarily used in diesel engines. For example, in Opel bi-turbo diesel, only the smaller turbocharger is active at low rpm, providing high torque at 1500-1700 rpm; both turbochargers operate together in mid range, with the larger one pre-compressing the air which is further compressed by the smaller, with bypass valve regulating the exhaust flow to each turbocharger; and at high 2500-3000 rpm, only the larger turbocharger is active, providing maximum performance.

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 2,500 CCs and in V-shape or boxer engines.

Twin-scroll

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, the exhaust valves in different cylinders can be opened 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 which can interfere with each other, so that the pulsating exhaust gasses flow through separate spirals (scrolls). This allows the engine to efficiently utilise exhaust scavenging techniques, which decreases exhaust gas temperatures and NOx emissions and improves turbine efficiency, reducing turbo lag.

Cut-out of a twin-scroll turbocharger, with two differently angled scrolls

Cut-out of a twin-scroll exhaust and turbine; the dual "scrolls" pairing cylinders 1-4 and 2-3 are clearly visible

Variable-geometry

Main article: Variable-geometry turbocharger

Variable-geometry or variable-nozzle turbochargers use moveable vanes to adjust the air-flow to the turbine, imitating a turbocharger of the optimal size throughout the power curve. 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. This variability maintains a comparable exhaust velocity and back pressure throughout the engine’s RPMs. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbo lag.

Garrett variable-geometry turbocharger on DV6TED4 engine

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.

Main article: Centrifugal compressor

The operating range of a compressor is described by the "compressor map".

Main article: Compressor map

Ported shroud

The flow range of a turbocharger compressor can be increased by allowing air to bleed from a ring of holes or a circular groove around the compressor at a point slightly downstream of the compressor inlet (but far nearer to the inlet than to the outlet).

The ported shroud is a performance enhancement that allows the compressor to operate at significantly lower flows. It achieves this by forcing a simulation of impeller stall to occur continuously. Allowing some air to escape at this location inhibits the onset of surge and widens the operating range. While peak efficiencies may decrease, high efficiency may be achieved over a greater range of engine speeds. Increases in compressor efficiency result in slightly cooler (more dense) intake air, which improves power. This is a passive structure that is constantly open (in contrast to compressor exhaust blow off valves, which are mechanically or electronically controlled). The ability of the compressor to provide high boost at low rpm may also be increased marginally (because near choke conditions the compressor draws air inward through the bleed path). Ported shrouds are used by many turbocharger manufacturers.

Center housing/hub rotating assembly

The center hub rotating assembly (CHRA) houses the shaft that connects the compressor impeller and turbine. It also must contain a bearing system to suspend the shaft, allowing it to rotate at very high speed with minimal friction. For instance, in automotive applications the CHRA typically uses a thrust bearing or ball bearing lubricated by a constant supply of pressurized engine oil. The CHRA may also be considered "water-cooled" by having an entry and exit point for engine coolant to be cycled. Water-cooled models allow engine coolant to be used to keep the lubricating oil cooler, avoiding possible oil coking (the destructive distillation of the engine oil) from the extreme heat found in the turbine. The development of air-foil bearings has removed this risk.

Ball bearings designed to support high speeds and temperatures are sometimes used instead of fluid bearings to support the turbine shaft. This helps the turbocharger accelerate more quickly and reduces turbo lag. Some variable nozzle turbochargers use a rotary electric actuator, which uses a direct stepper motor to open and close the vanes, rather than pneumatic controllers that operate based on air pressure.

Additional technologies commonly used in turbocharger installations

Intercooling

Illustration of inter-cooler location.

Even though intake temperature is reduced with the increase of pressure due to the reduction of volume per mole of oxidizer (inversely proportionate given ideal conditions), 'heat soak' results from heat transfer from the warmer exhaust gases spinning the turbine, via the turbine, increasing the air intake temperature. The warmer the intake air the less dense the oxidizer for combustion in a control volume (piston chamber) -- a reduction in thermodynamic efficiency. Extreme intake-air temperature can also contribute to the symptom of engine knock, or detonation, which is mechanically destructive to the engine.

Turbocharger units often make use of an intercooler (also known as a charge air cooler), to cool down the intake air. Intercoolers are often tested for leaks during routine servicing, particularly in trucks where a leaking intercooler can result in a 20% reduction in fuel economy.

(Note that "intercooler" is the proper term for the air cooler between successive stages of boost, whereas "charge air cooler" is the proper term for the air cooler between the boost stage(s) and the appliance that will consume the boosted air.)

Water injection

Main article:Water injection (engines)

An alternative to intercooling is injecting water into the intake air to reduce the temperature. This method has been used in automotive and aircraft applications.

Fuel-air mixture ratio

Main article: Air-fuel ratio

In addition to the use of intercoolers, it is common practice to add extra fuel to the intake air (known as "running an engine rich") for the sole purpose of cooling. The amount of extra fuel varies, but typically reduces the air-fuel ratio to between 11 and 13, instead of the stoichiometric 14.7 (in petrol engines). The extra fuel is not burned (as there is insufficient oxygen to complete the chemical reaction), instead it undergoes a phase change from vapor (liquid) to gas. This phase change absorbs heat, and the added mass of the extra fuel reduces the average kinetic energy of the charge and exhaust gas. Even when a catalytic converter is used, the practice of running an engine rich increases exhaust emissions.

Wastegate

See Main Article: Wastegate

Many turbochargers use a basic wastegate, which allows smaller turbochargers to reduce turbo lag. A wastegate regulates the exhaust gas flow that enters the exhaust-side driving turbine and therefore the air intake into the manifold and the degree of boosting. It can be controlled by a solenoid operated by the engine’s electronic control unit or a boost controller but most production vehicles use a spring loaded diaphragm.

Anti-surge/dump/blow off valves

Main article: Blowoff valve

A recirculating type anti-surge valve

Turbocharged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed, compressed air will flow to the throttle valve without an exit (i.e., the air has nowhere to go).

In this situation, the surge can raise the pressure of the air to a level that can cause damage. This is because if the pressure rises high enough, a compressor stall will occur, where the stored pressurized air decompresses backward across the impeller and out the inlet. The reverse flow back across the turbocharger causes the turbine shaft to reduce in speed more quickly than it would naturally, possibly damaging the turbocharger.

In order to prevent this from happening, a valve is fitted between the turbo and inlet, which vents off the excess air pressure. These are known as an anti-surge, diverter, bypass, blow-off valve (BOV), or dump valve. It is a pressure relief valve, and is normally operated by the vacuum in the intake manifold.

The primary use of this valve is to maintain the spinning of the turbocharger at a high speed. The air is usually recycled back into the turbo inlet (diverter or bypass valves) but can also be vented to the atmosphere (blow off valve). Recycling back into the turbocharger inlet is required on an engine that uses a mass-airflow fuel injection system, because dumping the excessive air overboard downstream of the mass airflow sensor will cause an excessively rich fuel mixture (this is because the mass-airflow sensor has already accounted for the extra air that is no longer being used). Valves that recycle the air will also shorten the time needed to re-spool the turbo after sudden engine deceleration, since the load on the turbo when the valve is active is much lower than it is if the air charge is vented to atmosphere.

Free floating

A free floating turbocharger is used in the 100 liter engine of the caterpillar mining vehicle.

A free floating turbocharger is the simplest type of turbocharger. This configuration has no wastegate and can’t control its own boost levels. They are typically designed to attain maximum boost at full throttle. Free floating turbochargers produce more horsepower because they have less backpressure but are not driveable in performance applications without an external wastegate.

Applications

Gasoline-powered cars

Main article: Turbocharged petrol engines

The first turbocharged passenger car was the Oldsmobile Jetfire option on the 1962-1963 F85/Cutlass which utilized a turbocharger mounted to a 215 cu in (3.52 L) all aluminum V8. Also in 1962 Chevrolet introduced a special run of turbocharged Corvairs called the Monza Spyder (1962-1964) and later renamed the Corsa (1965-1966) which mounted a turbocharger to its air cooled flat 6 cylinder engine. This model really popularized the turbocharger in North America and set the stage for later turbocharged models from Porsche on the 1975-up 911/930 and Saab on the 1978-1984 Saab 99 Turbo and the very popular 1978-1987 Buick Regal/T Type/Grand National. Today, turbocharging is commonly used by many manufacturers of both diesel and gasoline-powered cars. Turbocharging can be used to increase power output for a given capacity or to increase fuel efficiency by allowing a smaller displacement engine to be used. (For example, the 2013 Chevrolet Cruze is available with either a 1.8 liter non-turbocharged engine or a 1.4 liter turbocharged engine; both produce the same 138 horsepower.) Low pressure turbocharging is the optimum when driving in the city, whereas high pressure turbocharging is more for racing and driving on highways/motorways/freeways.

Diesel-powered cars

Main article: Turbodiesel

The first production turbo diesel passenger car was the Garrett-turbocharged Mercedes 300SD introduced in 1978. Today, many automotive diesels are turbocharged, since the use of turbocharging improved efficiency, driveability and performance of diesel engines, greatly increasing their popularity.

Motorcycles

Main article: Turbocharged petrol engines § Motorcycles

The first example of a turbocharged bike is the 1978 Kawasaki Z1R TC. Several Japanese companies produced turbocharged high performance motorcycles in the early 1980s, such as the CX500 Turbo from Honda- a transversely mounted, liquid cooled V-Twin also available in naturally aspirated form. Since then, few turbocharged motorcycles have been produced. This is partially due to an abundance of larger displacement, naturally aspirated engines being available that offer the torque and power benefits of a smaller displacement engine with turbocharger, but do return more linear power characteristics. The Dutch manufacturer EVA motorcycles builds a small series of turbocharged diesel motorcycle with an 800cc smart CDI engine.

Trucks

The first turbocharged diesel truck was produced by Schweizer Maschinenfabrik Saurer (Swiss Machine Works Saurer) in 1938.

Aircraft

A natural use of the turbocharger - and its earliest known use for any internal combustion engine, starting with experimental installations in the 1920s - is with aircraft engines. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off. At 5,486 m (18,000 ft), the air is at half the pressure of sea level and the airframe experiences only half the aerodynamic drag. However, since the charge in the cylinders is being pushed in by this air pressure, it means that the engine will normally produce only half-power at full throttle at this altitude. Pilots would like to take advantage of the low drag at high altitudes in order to go faster, but a naturally aspirated engine will not produce enough power at the same altitude to do so.

The table below is used to demonstrate the wide range of conditions experienced. As seen in the table below, there is significant scope for forced induction to compensate for lower density environments.

	Daytona Beach	Denver	Death Valley	Colorado State Highway 5	La Rinconada, Peru,
elevation	0 m / 0 ft	1,609 m / 5,280 ft	-86 m / -282 ft	4,347 m / 14,264 ft	5,100 m / 16,732 ft
atm	1.000	0.823	1.010	0.581	0.526
bar	1.013	0.834	1.024	0.589	0.533
psia	14.696	12.100	14.846	8.543	7.731
kPa	101.3	83.40	102.4	58.90	53.30

A turbocharger remedies this problem by compressing the air back to sea-level pressures, or even much higher, in order to produce rated power at high altitude. Since the size of the turbocharger is chosen to produce a given amount of pressure at high altitude, the turbocharger is over-sized for low altitude. The speed of the turbocharger is controlled by a wastegate. Early systems used a fixed wastegate, resulting in a turbocharger that functioned much like a supercharger. Later systems utilized an adjustable wastegate, controlled either manually by the pilot or by an automatic hydraulic or electric system. When the aircraft is at low altitude the wastegate is usually fully open, venting all the exhaust gases overboard. As the aircraft climbs and the air density drops, the wastegate must continuously close in small increments to maintain full power. The altitude at which the wastegate is fully closed and the engine is still producing full rated power is known as the critical altitude. When the aircraft climbs above the critical altitude, engine power output will decrease as altitude increases just as it would in a naturally aspirated engine.

With older supercharged aircraft, the pilot must continually adjust the throttle to maintain the required manifold pressure during ascent or descent. The pilot must also take great care to avoid overboosting the engine and causing damage, especially during emergencies such as go-arounds. In contrast, modern turbocharger systems use an automatic wastegate, which controls the manifold pressure within parameters preset by the manufacturer. For these systems, as long as the control system is working properly and the pilot's control commands are smooth and deliberate, a turbocharger will not overboost the engine and damage it.

Yet the majority of World War II engines used superchargers, because they maintained three significant manufacturing advantages over turbochargers, which were larger, involved extra piping, and required exotic high-temperature materials in the turbine and pre-turbine section of the exhaust system. The size of the piping alone is a serious issue; American fighters Vought F4U and Republic P-47 used the same engine but the huge barrel-like fuselage of the latter was, in part, needed to hold the piping to and from the turbocharger in the rear of the plane. Turbocharged piston engines are also subject to many of the same operating restrictions as gas turbine engines. Pilots must make smooth, slow throttle adjustments to avoid overshooting their target manifold pressure. The fuel/air mixture must often be adjusted far on the rich side of stoichiometric combustion needs to avoid pre-ignition or detonation in the engine when running at high power settings. In systems using a manually operated wastegate, the pilot must be careful not to exceed the turbocharger's maximum rpm. Turbocharged engines require a cooldown period after landing to prevent cracking of the turbo or exhaust system from thermal shock. Turbocharged engines require frequent inspections of the turbocharger and exhaust systems for damage due to the increased heat, increasing maintenance costs. The great majority of World War II American heavy bombers used by the USAAF - particularly the Wright R-1820 Cyclone-9 powered B-17 Flying Fortress, and Pratt & Whitney R-1830 Twin Wasp powered Consolidated B-24 Liberator four-engined bombers both used similar models of General Electric-designed turbochargers in service, as did the twin Allison V-1710-engined Lockheed P-38 Lightning American heavy fighter during the war years.

Today, most general aviation aircraft are naturally aspirated. The small number of modern aviation piston engines designed to run at high altitudes in general use a turbocharger or turbo-normalizer system rather than a supercharger. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple but fuel-hungry supercharger. As the cost of fuel has increased, the supercharger has fallen out of favor.

Turbocharged aircraft often occupy a performance range between that of normally aspirated piston-powered aircraft and turbine-powered aircraft. The increased maintenance costs of a turbocharged engine are considered worthwhile for this purpose, as a turbocharged piston engine is still far cheaper than any turbine engine.

As the turbocharged aircraft climbs, however, the pilot (or automated system) can close the wastegate, forcing more exhaust gas through the turbocharger turbine, thereby maintaining manifold pressure during the climb, at least until the critical pressure altitude is reached (when the wastegate is fully closed), after which manifold pressure will fall. With such systems, modern high-performance piston engine aircraft can cruise at altitudes above 20,000 feet, where low air density results in lower drag and higher true airspeeds. This allows flying "above the weather". In manually controlled wastegate systems, the pilot must take care not to overboost the engine, which will cause pre-ignition, leading to engine damage. Further, since most aircraft turbocharger systems do not include an intercooler, the engine is typically operated on the rich side of peak exhaust temperature in order to avoid overheating the turbocharger.

In non-high-performance turbocharged aircraft, the turbocharger is solely used to maintain sea-level manifold pressure during the climb (this is called turbo-normalizing).

Modern turbocharged aircraft usually forgo any kind of temperature compensation, because the turbochargers are in general small and the manifold pressures created by the turbocharger are not very high. Thus, the added weight, cost, and complexity of a charge cooling system are considered to be unnecessary penalties. In those cases, the turbocharger is limited by the temperature at the compressor outlet, and the turbocharger and its controls are designed to prevent a large enough temperature rise to cause detonation. Even so, in many cases the engines are designed to run rich in order to use the evaporating fuel for charge cooling.

Marine and land-based diesel turbochargers

A medium-sized six-cylinder marine Diesel-engine, with turbocharger and exhaust in the foreground

Turbocharging, which is common on diesel engines in automobiles, trucks, tractors, and boats is also common in heavy machinery such as locomotives, ships, and auxiliary power generation.

• Turbocharging can dramatically improve an engine's specific power and power-to-weight ratio, performance characteristics which are normally poor in non-turbocharged diesel engines.
• Diesel engines have no detonation because diesel fuel is injected at or towards the end of the compression stroke and is ignited solely by the heat of compression of the charge air. Because of this, diesel engines can use a much higher boost pressure than spark ignition engines, limited only by the engine's ability to withstand the additional heat and pressure.

Turbochargers are also employed in certain two-stroke cycle diesel engines, which would normally require a Roots blower for aspiration. In this specific application, mainly Electro-Motive Diesel (EMD) 567, 645, and 710 Series engines, the turbocharger is initially driven by the engine's crankshaft through a gear train and an overrunning clutch, thereby providing aspiration for combustion. After combustion has been achieved, and after the exhaust gases have reached sufficient heat energy, the overrunning clutch is disengaged, and the turbo-compressor is thereafter driven exclusively by the exhaust gases. In the EMD application, the turbocharger is used for normal aspiration during starting and low power output settings and is used for true turbocharging during medium and high power output settings. This is particularly beneficial at high altitudes, as are often encountered on western U.S. railroads.

Business and adoption

Garrett (now Honeywell) and Borg Warner are the largest manufacturers in Europe and the US. Several factors are expected to contribute to more widespread consumer adoption of turbochargers, especially in the US:

• New government fuel economy and emissions targets.
• Increasing oil prices and a consumer focus on fuel efficiency.
• Only 10 percent of light vehicles sold in the US are equipped with turbochargers, making the United States an emerging market, compared to 50 percent of vehicles in Europe that are turbo diesel and 27 percent that are gasoline boosted.
• Higher temperature tolerances for gasoline engines, ball bearings in the turbine shaft and variable geometry have reduced driveability concerns.

By 2016, 40 percent of light vehicles sold in the U.S. are expected to be turbocharged. In Europe about 65 percent of vehicles are turbocharged, which is expected to grow to 85 percent by 2015. Historically, more than 90 percent of turbochargers were diesel, however, adoption in gasoline engines is increasing. Honeywell projects the number of turbochargers in passenger vehicles in the U.S. to more than double to 23 percent by 2016.

The US Coalition for Advanced Diesel Cars is pushing for a technology neutral policy for government subsidies of environmentally friendly automotive technology. If successful, government subsidies would be based on the Corporate Average Fuel Economy (CAFE) standards rather than supporting specific technologies like electric cars. Political shifts could drastically change adoption projections. Turbocharger sales in the United States increased when the federal government boosted corporate average fuel economy targets to 35.5 mpg by 2016.

See also

• Boost gauge
• Twincharger
• Exhaust pulse pressure charging
• Hybrid turbocharger
• Twin-turbo
• Variable geometry turbocharger

External links

• Don Sherman (2006). "Happy 100th Birthday to the Turbocharger". Automobile Magazine. {{cite journal}}: Unknown parameter |month= ignored (help)
• NASA Oil Free Turbocharger
• Video showing how a turbocharger works