Turbocharger history and overview
A turbocharger, or turbo (colloquialism), from the Greek "τύρβη" (mixing/spinning) is a forced induction device used to allow more power to be produced for 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 driven by the engine's exhaust gas turbine. 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, 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.
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, which led the world in the technology due to General Electric's early start. 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
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.
In comparison, a turbocharger does not place a direct mechanical load on the engine. It is more efficient because it uses potential and kinetic energy of the exhaust gas to drive the compressor. In contrast to supercharging, the principal disadvantages of turbocharging are back-pressure heat soak of the intake air, and the inefficiencies of the turbine versus direct-drive.
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.
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).
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, 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.
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:
- the turbine, which is almost always a radial inflow turbine
- the compressor, which is almost always a centrifugal compressor
- the center housing/hub rotating assembly
Many turbocharger installations use additional technologies, such as wastegates, intercooling and blow-off valves.
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 rpmThe 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.
A turbocharger’s performance is closely tied to its sizeLarge 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 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 turbocharges 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 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.
Variable-geometry or variable-nozzle turbochargers use nine 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.
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".
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
When the pressure of the engine's intake air is increased, its temperature will also increase. In addition, heat soak from the hot exhaust gases spinning the turbine may 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.
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.)
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
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.
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
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.
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.
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. 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.
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.