This article is about the turbine-driven air induction device. For the Philippine baseball team, see Shell Turbo Chargers.
"Turbo" redirects here. For the computer-animated comedy sports film, see Turbo (film). For other uses, see Turbo (disambiguation).
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. This improvement over a naturally aspirated engine's power output is due to the fact that 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.
Turbochargers are commonly used on 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 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 Gebrüder Sulzer (now simply called Sulzer), engine manufacturing company in Winterthur, who received a patent in 1905 for using a compressor driven by exhaust gases 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. The first use of turbocharging technology based on his design was for large marine engines, when the German Ministry of Transport commissioned the construction of the "Preussen" and "Hansestadt Danzig" passenger liners in 1923. Both ships featured twin ten-cylinder diesel engines with output boosted from 1750 to 2500 horsepower by turbochargers designed by Büchi and built under his supervision by Brown Boveri (BBC) (now ABB). 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 turbocharger 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.
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 U.S. aircraft with turbochargers—which included mass-produced ones designed by General Electric for American aviation use—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 experimental inline engine-powered Focke-Wulf Fw 190 prototype models, with some developments for their design coming from the DVL, a predecessor of today's DLR agency, but the need for advanced high-temperature metals in the turbine, that were not readily available for production purposes during wartime, kept them out of widespread use.
In contrast to turbochargers, superchargers are mechanically driven by the engine. Belts, chains, shafts, and gears are common methods of powering a supercharger, placing a mechanical load on the engine. For example, on the single-stage single-speed supercharged Rolls-Royce Merlin engine, the supercharger uses about 150 horsepower (110 kilowatts). Yet the benefits outweigh the costs; for the 150 hp (110 kW) to drive the supercharger 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 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 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).
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 (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. 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. 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.
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. 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 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. 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 inches of mercury (100 kPa).
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.
Many turbocharger installations use additional technologies, such as wastegates, intercooling and blow-off valves.
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. 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 larger one pre-compressing the air, which the smaller 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.
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 NOx emissions, improves turbine efficiency, and reduces turbo lag evident at low engine speeds.
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. Water-cooled models use engine coolant to keep lubricating oil cooler, avoiding possible oil coking (destructive distillation of engine oil) from the extreme heat in the turbine. The development of air-foil bearings 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.
When the pressure of the engine's intake air is increased, its temperature also increases. This occurrence can be explained through Gay-Lussac's law, stating that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature. With more pressure being added to the engine through the turbocharger, overall temperatures of the engine will also rise. In addition, heat soak from the hot exhaust gases spinning the turbine will also heat the intake air. The warmer the intake air, the less dense, and the less oxygen available for the combustion event, which reduces volumetric efficiency. Not only does excessive intake-air temperature reduce efficiency, it also leads to engine knock, or detonation, which is destructive to engines.
To compensate for the increase in temperature, turbocharger units often make use of an intercooler between successive stages of boost to cool down the intake air. A charge air cooler is an air cooler between the boost stage(s) and the appliance that consumes the boosted air.
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 atomized (liquid) to gas. This phase change absorbs heat, and the added mass of the extra fuel reduces the average thermal energy of the charge and exhaust gas. Even when a catalytic converter is used, the practice of running an engine rich increases exhaust emissions.
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 occurs—stored pressurized air decompresses backward across the impeller and out the inlet. The reverse flow back across the turbocharger makes the turbine shaft reduce in speed more quickly than it would naturally, possibly damaging the turbocharger.
To prevent this from happening, a valve is fitted between the turbocharger and inlet, which vents off the excess air pressure. These are known as an anti-surge, diverter, bypass, turbo-relief valve, blow-off valve (BOV), or dump valve. It is a pressure relief valve, and is normally operated by the vacuum from the intake manifold.
The first turbocharged passenger car was the Oldsmobile Jetfire option on the 1962–1963 F85/Cutlass, which used 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, initially called the Monza Spyder (1962–1964) and later renamed the Corsa (1965–1966), which mounted a turbocharger to its air cooled flat six cylinder engine. This model popularized the turbocharger in North America—and set the stage for later turbocharged models from Porsche on the 1975-up 911/930, Saab on the 1978–1984 Saab 99 Turbo, and the very popular 1978–1987 Buick Regal/T Type/Grand National. Today, turbocharging is common on both diesel and gasoline-powered cars. Turbocharging can increase power output for a given capacity or increase fuel efficiency by allowing a smaller displacement engine. The 'Engine of the year 2011' is an engine used in a Fiat 500 equipped with an MHI turbocharger. This engine lost 10% weight, saving up to 30% in fuel consumption while delivering the same HP (105) as a 1.4 litre engine.
The first production turbocharger diesel passenger car was the Garrett-turbocharged Mercedes 300SD introduced in 1978. Today, most automotive diesels are turbocharged, since the use of turbocharging improved efficiency, driveability and performance of diesel engines, greatly increasing their popularity. The Audi R10 with a diesel engine even won the 24 hours race of Le Mans in 2006, 2007 and 2008.
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.
The first turbocharged diesel truck was produced by Schweizer Maschinenfabrik Saurer (Swiss Machine Works Saurer) in 1938.
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 pushed in by this air pressure, the engine normally produces only half-power at full throttle at this altitude. Pilots would like to take advantage of the low drag at high altitudes to go faster, but a naturally aspirated engine does 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.
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. The additional systems and piping increase an aircraft engine's size, weight, complexity and cost. A turbocharged aircraft engine costs more to maintain than a comparable normally aspirated engine. 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-engine 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 piston engine powered aircraft are naturally aspirated.[citation needed] Modern aviation piston engines designed to run at high altitudes typically include a turbocharger (either high pressure or turbonormalized) rather than a supercharger.[citation needed] 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.
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 that are normally poor in non-turbocharged diesel engines.
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 automatically disengaged, and the turbo-compressor is thereafter driven exclusively by the exhaust gases. In the EMD application, the turbocharger acts as a compressor 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. It is possible for the turbocharger to revert to compressor mode momentarily during commands for large increases in engine power.
The U.S. 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.
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