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-supported turbocharger cutaway made by Mohawk Innovative Technology Inc. ]] A turbocharger (short for Turbine driven Supercharger ) is an Exhaust Gas driven Forced Induction Supercharger used in Internal Combustion Engines . This differentiates it from a normal supercharger (or Blower ) which uses a Prime Mover to power the compression device. WORKING PRINCIPLE A turbocharger consists of a Turbine and a Compressor linked by a shared axle. The turbine inlet receives exhaust gases from the engine exhaust manifold causing the turbine wheel to rotate. This rotation drives the compressor, compressing ambient air and delivering it to the air intake of the engine. The objective of a turbocharger is the same as a normal supercharger; to improve upon the size-to-output efficiency of an engine by solving one of its cardinal limitations. A Naturally Aspirated automobile engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder. Since the number of air and fuel molecules determine the potential energy available to force the piston down on the combustion stroke, and because of the relatively constant pressure of the atmosphere, there ultimately will be a limit to the amount of air and consequently fuel filling the Combustion Chamber . This ability to fill the cylinder with air is its Volumetric Efficiency . Since the turbocharger increases the pressure at the point where air is entering the cylinder, and the amount of air brought into the cylinder is largely a function of time and pressure, more air will be drawn in as the pressure increases. The additional air makes it possible to add more fuel, increasing the output of the engine. Also, the intake pressure can be controlled by a Wastegate , which bleeds off excess boost from the turbocharger. The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as Forced Induction . Centrifugal Supercharger s operate in the same fashion as a turbo; however, the energy to spin the compressor is taken from the rotating output energy of the engine's crankshaft as opposed to exhaust gas. For this reason turbochargers are ideally more efficient, since their turbines are actually heat engines, converting some of the Thermal Energy from the exhaust gas that would otherwise be wasted, into useful work. Contrary to popular belief, this is not totally "free energy," as it always creates some amount of exhaust backpressure which the engine must overcome. Superchargers use output energy from an engine to achieve a net gain, which must be provided from some of the engine's total output; either directly or from a separate smaller engine, perhaps electrically driven from the main engine's generator. HISTORY The turbocharger was invented by Swiss engineer Alfred Buchi, who had been working on steam turbines. His patent for the internal combustion turbocharger was applied for in 1905. Diesel ships and locomotives with turbochargers began appearing in the 1920s . One of the first applications of a turbocharger to a non-Diesel engine came when General Electric engineer, Sanford Moss attached a turbo to a V12 '' Liberty '' aircraft engine. The engine was tested at Pikes Peak in Colorado at 14,000 feet to demonstrate that it could eliminate the power losses usually experienced in internal combustion engines as a result of altitude. Turbochargers were first used in production aircraft engines in the 1930s before World War II . The primary purpose behind most aircraft-based applications was to increase the altitude at which the airplane can fly, by compensating for the lower Atmospheric Pressure present at high altitude. Aircraft such as the Messerschmitt Bf 109 , Boeing B-17 Flying Fortress and Supermarine Spitfire all used exhaust driven "turbo-superchargers" to increase high altitude engine power. It is important to note that the majority of turbosupercharged aircraft engines used both a gear-driven Centrifugal Type Supercharger and a turbocharger. Turbo-Diesel trucks were produced in Europe and America (notably by Cummins ) after 1949. The turbocharger hit the automobile world in 1952 when Fred Agabashian qualified for pole position at the Indianapolis 500 and led for 100 miles before tire shards disabled the blower. 's innovative turbocharged Flat-6 Engine ; The turbo, located at top right, feeds pressurized air into the engine through the chrome T-tube visible spanning the engine from left to right.]] The first production turbocharged automobile engines came from General Motors in 1962. The A-body Oldsmobile Cutlass Jetfire and Chevrolet Corvair Monza Spyder were both fitted with turbochargers. The Oldsmobile is often recognized as the first, since it came out a few months earlier than the Corvair. Its '' Turbo Jetfire '' was a 215 in³ (3.5 L) V8 , while the Corvair Engine was either a 145 in³ (2.3 L)(1962-63) or a 164 in³ (2.7 L) (1964-66) Flat-6 . Both of these engines were abandoned within a few years, and GM's next turbo engine came more than ten years later. Offenhauser 's turbocharged engines returned to Indianapolis in 1966, with victories coming in 1968. The Offy turbo peaked at over 1,000 hp in 1973, while Porsche dominated the Can-Am series with a 1100 hp 917/30 . Turbocharged cars dominated the Le Mans between 1976 and 1988, and then from 2000-2007. BMW led the resurgence of the automobile turbo with the 1973 2002 Turbo , with Porsche following with the 911 Turbo , introduced at the 1974 Paris Motor Show . Buick was the first GM division to bring back the turbo, in the 1978 Buick Regal , followed by the Mercedes-Benz 300D and Saab 99 in 1978. The worlds first production turbodiesel automobile was also introduced in 1978 by Peugeot with the launch of the Peugeot 604 turbodiesel. Today, nearly all automotive diesels are turbocharged. Alfa Romeo introduced the first mass-produced Italian turbocharged car, the Alfetta GTV 2000 Turbodelta in 1979. Pontiac also introduced a turbo in 1980 and Volvo Cars followed in 1981. Renault however gave another step and installed a turbocharger to the smallest and lightest car they had, the R5 , making it the first Supermini automobile with a turbocharger in year 1980. This gave the car about 160bhp in street form and up to 300+ in race setup, an exorbitant power for a 1400cc motor. When combined with its incredible lightweight chassis, it could nip at the heels of the quick Italian sports car Ferrari 308 . In Formula One , in the so called "Turbo Era" of until , engines with a capacity of 1500 cc could achieve anywhere from 1000 to 1500 hp (746 to 1119 kW) ( Renault , Honda , BMW , Ferrari ). Renault was the first manufacturer to apply turbo technology in the F1 field, in 1977. The project's high cost was compensated for by its performance, and led to other engine manufacturers following suit. The Turbo-charged engines took over the F1 field and ended the Ford Cosworth DFV era in the mid 1980s. However, the FIA decided that turbos were making the sport too dangerous and expensive, and from onwards, the maximum boost pressure was reduced before the technology was banned completely for . In Rallying , turbocharged engines of up to 2000cc have long been the preferred motive power for the Group A/ World Rally Car (top level) competitors, due to the exceptional power-to-weight ratios (and enormous torque) attainable. This combines with the use of vehicles with relatively small bodyshells for manoeuvreability and handling. As turbo outputs rose to similar levels as the F1 category (see above), the FIA , rather than banning the technology, enforced a restricted turbo inlet diameter (currently 34mm), effectively "starving" the turbo of compressible air and making high boost pressures unfeasible. The success of small, turbocharged, Four-wheel-drive vehicles in rally competition, beginning with the Audi Quattro , has led to exceptional road cars in the modern era such as the Lancia Delta Integrale , Toyota Celica GT-Four , Subaru Impreza WRX and the Mitsubishi Lancer Evolution . In the late 1970s, Ford and GM looked to the turbocharger to gain power, without sacrificing fuel consumption, during not only the emissions crunch of the federal government but also a gas shortage. GM released turbo versions of the Pontiac Firebird, Buick Regal, and Chevy Monte Carlo. Ford responded with a turbocharged Mustang in the form of the 2.3L from the Pinto. The engine design was dated, but it worked well. The bullet-proof 2.3L Turbo was used in early carburated trim as well as fuel injected and intercooled versions in the Mustang SVO and the Thunderbird Turbo Coupe until 1988. GM also liked the idea enough to evolve the 3.8L V6 used in early turbo Buicks into late '80s muscle in the form of the Buick Grand National and the pinical GNX. Although late to use turbocharging, Chrysler Corporation turned to turbochargers in 1984 and quickly churned out more turbocharged engines than any other manufacturer, using turbocharged, fuel-injected 2.2 and 2.5 litre four-cylinder engines in minivans, sedans, convertibles, and coupes. Their 2.2 litre turbocharged engines ranged from 142 hp to 225 hp, a substantial gain over the normally aspirated ratings of 86 to 93 horsepower; the 2.5 litre engines had about 150 horsepower and had no intercooler. Though the company stopped using turbochargers in 1993, they returned to turbocharged engines in 2002 with their 2.4 litre engines, boosting output by 70 horsepower. Chrysler turbocharged engines (Allpar) DESIGN DETAILS Stages Turbocharger implementations are often referred to in terms of stages, where a simple turbocharger setup may be denoted Stage I and more advanced developments as Stage II, III etc. In terms of motorcycle turbochargers, a Stage I system is usually a 'bolt-on' upgrade that requires only minimal alteration to the engine, whereas a later stage may involve increasing injector sizes and fuel pressure regulators, and so on. Components The turbocharger has four main components. The Turbine and Impeller /compressor wheels are each contained within their own folded conical housing on opposite sides of the third component, the center housing/hub rotating assembly (CHRA). The housings fitted around the compressor impeller and turbine collect and direct the gas flow through the wheels as they spin. The size and shape can dictate some performance characteristics of the overall turbocharger. The area of the cone to radius from center hub is expressed as a ratio (AR, A/R, or A:R). Often the same basic turbocharger assembly will be available from the manufacturer with multiple AR choices for the turbine housing and sometimes the compressor cover as well. This allows the designer of the engine system to tailor the compromises between performance, response, and efficiency to application or preference. Both housings resemble Snail shells, and thus turbochargers are sometimes referred to in Slang as ''snails''. 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. Generally, 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. The center hub rotating assembly houses the shaft which 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 from the extreme heat found in the turbine. Boost Boost refers to the increase in Manifold Pressure that is generated by the turbocharger in the Intake path or specifically Intake Manifold that exceeds normal Atmospheric Pressure . This is also the level of boost as shown on a Pressure Gauge , usually in Bar , Psi or possibly KPa This is representative of the extra air pressure that is achieved over what would be achieved without the Forced Induction . Manifold pressure should not be confused with the amount, or "weight" of air that a turbo can flow. Boost pressure is limited to keep the entire engine system including the turbo inside its thermal, and mechanical design operating range by controlling the Wastegate which shunts the exhaust gases away from the exhaust side turbine. The maximum possible boost depends on the fuel's Octane Rating . Gasoline engines cannot usually sustain a boost above ~12 psi without knocking, and often lower, whereas ethanol, methanol and diesel can sustain higher pressures. Due to this maximum boost pressure will often be electronically regulated using a Knock Sensor , see Automatic Performance Control (APC) Many diesel engines do not have any wastegate because the amount of exhaust energy is controlled directly by the amount of fuel injected into the engine and slight variations in boost pressure do not make a difference for the engine. Wastegate By spinning at a relatively high speed the compressor turbine draws in a large volume of air and forces it into the engine. As the turbocharger's output flow volume exceeds the engine's volumetric flow, Air Pressure in the Intake system begins to build, often called Boost . The speed at which the assembly spins is proportional to the pressure of the compressed air and total mass of air flow being moved. Since a turbo can spin to RPMs far beyond what is needed, or of what it is safely capable of, the speed must be controlled. A Wastegate is the most common mechanical speed control system, and is often further augmented by an electronic Boost Controller . The main function of a wastegate is to allow some of the exhaust to bypass the turbine when the set intake pressure is achieved. Most passenger car wastegates are integral to the turbocharger. Anti-Surge/Dump/Blow Off Valves Turbo charged 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). This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo, as this is the only path with the air can take. The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker than it would naturally. When the throttle is opened again, the turbo will have to spin-up for longer to the required speed, as turbo speed is proportional to boost/volume flow. In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess volume of air. These are known as anti-surge, dump or blow-off valves. They are normally operated by engine vacuum or by electronic control. The primary use of this valve is to prevent damage to the engine by a surge of compressed air and to maintain the turbo spinning at a high speed. They can also be used as a bypass valve to control boost in a similar fashion as a waste gate, but this is rarely seen as it is impractical. The air is usually vented to atmosphere, or can be recycled back into the turbo inlet. Recycling back into the turbo causes the venting sound to be reduced but as the excess volume of air is not removed problems may arise. Fuel efficiency Since a turbocharger increases the specific Horsepower output of an engine, the engine will also produce increased amounts of Waste Heat . This can sometimes be a problem when fitting a turbocharger to a car that was not designed to cope with high heat loads. This extra waste heat combined with the higher Compression Ratio (more specifically, expansion ratio) of turbocharged engines contributes to slightly lower Thermal Efficiency , which has a small but direct impact on overall Fuel Efficiency . It is another form of cooling that has the largest impact on fuel efficiency: charge cooling. Even with the benefits of Intercooling , the total compression in the Combustion Chamber is greater than that in a Naturally-aspirated Engine . To avoid Knock while still extracting maximum power from the engine, it is common practice to introduce extra fuel into the charge for the sole purpose of cooling. While this seems counterintuitive, this fuel is not burned. Instead, it absorbs and carries away heat when it changes phase from liquid mist to gas vapor. Also, because it is more dense than the other inert substance in the combustion chamber, Nitrogen , it has a higher specific heat and more heat capacitance. It "holds" this heat until it is released in the Exhaust stream, preventing destructive Knock . This thermodynamic property allows manufacturers to achieve good power output with common pump fuel at the expense of fuel economy and emissions. The Stoichiometric Air-to-Fuel ratio (A/F) for combustion of gasoline is 14.7:1. A common A/F in a turbocharged engine while under full design boost is approximately 12:1. Richer mixtures are sometimes run when the design of the system has flaws in it such as a catalytic converter which has limited endurance of high exhaust temperatures or the engine has a compression ratio that is too high for efficient operation with the fuel given. Lastly, the efficiency of the turbocharger itself can have an impact on fuel efficiency. Using a small turbocharger will give quick response and low lag at low to mid RPMs, but can choke the engine on the exhaust side and generate huge amounts of pumping-related heat on the intake side as RPMs rise. A large turbocharger will be very efficient at high RPMs, but is not a realistic application for a street driven automobile. Variable vane and ball bearing technologies can make a turbo more efficient across a wider operating range, however, other problems have prevented this technology from appearing in more road cars (see Variable Geometry Turbocharger ). Currently, the Porsche 911 (997) Turbo is the only gasoline car in production with this kind of turbocharger, although in Europe turbos of this type are rapidly becoming standard-fitment on Turbodiesel cars, vans and other commercial vehicles, because they can greatly enhance the Diesel Engine 's characteristic low-speed torque. One way to take advantage of the different operating regimes of the two types of supercharger is Sequential Turbocharging , which uses a small turbocharger at low RPMs and a larger one at high RPMs. The engine management systems of most modern vehicles can control Boost and fuel delivery according to charge temperature, fuel quality, and altitude, among other factors. Some systems are more sophisticated and aim to deliver fuel even more precisely based on combustion quality. For example, the Trionic-7 system from Saab Automobile provides immediate feedback on the combustion while it is occurring using an electrical charge. The new 2.0L TFSI turbo engine from Volkswagen / Audi incorporates lean burn and direct injection technology to conserve fuel under low load conditions. It is a very complex system that involves many moving parts and sensors in order to manage airflow characteristics inside the chamber itself, allowing it to use a stratified charge with excellent atomization. The direct injection also has a tremendous charge cooling effect enabling engines to use higher compression ratios and boost pressures than a typical port-injection turbo engine. Automotive design details The Ideal Gas Law states that when all other variables are held constant, if pressure is increased in a system so will temperature. Here exists one of the negative consequences of turbocharging, the increase in the temperature of air entering the engine due to compression. A turbo spins very fast; most peak between 80,000 and 200,000 RPM (using low Inertia turbos, 150,000-250,000 RPM) depending on size, weight of the rotating parts, boost pressure developed and compressor design. Such high rotation speeds would cause problems for standard Ball Bearing s leading to failure so most turbo-chargers use Fluid Bearing s. These feature a flowing layer of oil that suspends and cools the moving parts. The oil is usually taken from the engine-oil circuit. Some turbochargers use incredibly precise ball bearings that offer less friction than a fluid bearing but these are also suspended in fluid-dampened cavities. Lower friction means the turbo shaft can be made of lighter materials, reducing so-called ''turbo lag'' or ''boost lag''. Some car makers use water cooled turbochargers for added bearing life. This can also account for why many tuners upgrade their standard journal bearing turbos (such as a T25) which use a 270 degree thrust bearing and a brass journal bearing which only has 3 oil passages, to a 360 degree bearing which has a beefier thrust bearing and washer having 6 oil passages to enable better flow, response and cooling efficiency. Turbochargers with Foil Bearing s are in development which eliminates the need for bearing cooling or oil delivery systems, thereby eliminating the most common cause of failure, while also significantly reducing turbo lag. To manage the ''upper-deck'' air pressure, the turbocharger's exhaust gas flow is regulated with a Wastegate that bypasses excess exhaust gas entering the turbocharger's turbine. This regulates the rotational speed of the turbine and the output of the compressor. The wastegate is opened and closed by the compressed air from turbo (the upper-deck pressure) and can be raised by using a Solenoid to regulate the pressure fed to the wastegate membrane. This solenoid can be controlled by Automatic Performance Control , the engine's Electronic Control Unit or an after market boost control computer. Another method of raising the boost pressure is through the use of check and bleed valves to keep the pressure at the membrane lower than the pressure within the system. |
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