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What is a Turbo?
A turbocharger, or turbo, is a gas compressor used for forced-induction of an internal combustion engine. Like a supercharger, the purpose of a turbocharger is to increase the density of air entering the engine to create more power. However, a turbocharger differs in that the compressor is powered by a turbine driven by the engine’s own exhaust gases.
Nomenclature Early manufacturers of turbochargers referred to them as “turbo superchargers”. A supercharger is an air compressor used for forced induction of an engine. Logically then, adding a turbine to turn the supercharger would yield a “turbo supercharger”. However, the term was soon shortened to “turbocharger”. This is now a source of confusion, as the term “turbo supercharged” is sometimes used to refer to an engine that uses both a crankshaft-driven supercharger and an exhaust-driven turbocharger.
Some companies such as Teledyne Continental Motors still use the term turbo supercharger in its original sense. For the purposes of this article, the more modern terms turbocharger and turbo are used.
Working principle A turbocharger is a small radial fan pump driven by the energy of the exhaust gases of an engine. A turbocharger consists of a turbine and a compressor on a shared shaft. The turbine converts heat to rotational force, which is in turn used to drive the compressor. The compressor draws in ambient air and pumps it in to the intake manifold at increased pressure, resulting in a greater mass of air entering the cylinders on each intake stroke.
The objective of a turbocharger is the same as a supercharger; to improve the engine’s volumetric efficiency 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 through the intake valves. Because the pressure in the atmosphere is no more than 1 atm (approximately 14.7 psi), there ultimately will be a limit to the pressure difference across the intake valves and thus the amount of airflow entering the combustion chamber. Because the turbocharger increases the pressure at the point where air is entering the cylinder, a greater mass of air (oxygen) will be forced in as the inlet manifold pressure increases. The additional oxygen makes it possible to add more fuel, increasing the power and torque output of the engine.
Because the pressure in the cylinder must not go too high to avoid detonation and physical damage, the intake pressure must be controlled by controlling the rotational speed of the turbocharger. The control function is performed by a waste gate, which routes some of the exhaust flow away from the exhaust turbine. This controls shaft speed and regulates air pressure in the intake manifold.
Components
The turbocharger has four main components. The turbine (almost always a radial turbine) and impeller/compressor wheels are each contained within their own folded conical housing on opposite sides of the third component, the centre 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. 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 designer of the engine system to tailor the compromises between performance, response, and efficiency to application or preference. Twin-scroll designs have two valve-operated exhaust gas inlets, a smaller sharper angled one for quick response and a larger less angled one for peak performance.
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. Variable geometry turbochargers are further developments of these ideas.
The centre hub rotating assembly (CHRA) 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. The development of air-foil bearings has removed this risk.
Pressure increase
In the automotive world, boost refers to the increase in pressure that is generated by the turbocharger in the intake manifold that exceeds normal atmospheric pressure. Atmospheric pressure is approximately 14.5 psi or 1.0 bar, and anything above this level is considered to be boost. The level of boost may be 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 volume of air that a turbo can flow.
In contrast, the instruments on aircraft engines measure absolute pressure in inches of mercury. Absolute pressure is the amount of pressure above a total vacuum. The ICAO standard atmospheric pressure is 29.92 inches (760 mm) of mercury at sea level. Most modern aviation turbochargers are not designed to increase manifold pressures above this level, as aircraft engines are commonly air-cooled and excessive pressures increase the risk of overheating, pre-ignition, and detonation. Instead, the turbo is only designed to hold a pressure in the intake manifold equal to sea-level pressure as the altitude increases and air pressure drops. This is called turbo-normalizing.
Boost pressure is limited to keep the entire engine system, including the turbo, inside its thermal and mechanical design operating range. The speed and thus the output pressure of the turbo are controlled by the waste gate, a bypass which shunts the gases from the cylinders around the turbine directly to the exhaust pipe.
The maximum possible boost depends on the fuel’s octane rating and the inherent tendency of any particular engine towards detonation. Premium gasoline or racing gasoline can be used to prevent detonation within reasonable limits. Ethanol, methanol, liquefied petroleum gas (LPG) and diesel fuels allow higher boost than gasoline, because of these fuels’ combustion characteristics. To obtain more power from higher boost levels and maintain reliability, many engine components have to be replaced or upgraded such as the fuel pump, fuel injectors, pistons, valves, head-gasket, and head bolts.
Waste gate
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. 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 waste gate is the most common mechanical speed control system, and is often further augmented by an electronic or manual boost controller. The main function of a waste gate is to allow some of the exhaust to bypass the turbine when the set intake pressure is achieved. Passenger cars have waste gates that are integral to the turbocharger.
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). This causes a surge which can raise the pressure of the air to a level which can damage the engine.
If the pressure rises high enough, a compressor stall will occur, where the stored pressurized air decompresses backwards across the impeller and out the inlet. The reverse flow back across the turbocharger causes the turbine shaft to reduce in speed quicker 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, bypass, blow-off valve (BOV) or dump valve. It is basically a pressure relief valve, and is normally operated by the excess pressure in the intake manifold.
The primary use of this valve is to maintain the turbo spinning at a high speed. The air is usually recycled back into the turbo inlet but can also be vented to the atmosphere. 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. A dump valve will also shorten the time needed to re-spool the turbo after sudden engine deceleration.
Charge cooling
Compressing air in the turbocharger increases its temperature, which can cause a number of problems. Excessive charge air temperature can lead to detonation, which is extremely destructive to engines. When a turbocharger is installed on an engine, it is common practice to fit the engine with an intercooler, a type of heat exchanger which gives up heat energy in the charge to the ambient air.
In cases where an intercooler is not a desirable solution, it is common practice to introduce extra fuel into the charge for the sole purpose of cooling. The extra fuel is not burned. Instead, it absorbs and carries away heat when it changes phase from liquid to vapour. The evaporated fuel holds this heat until it is released in the exhaust stream. This thermodynamic property allows manufacturers to achieve good power output by using extra fuel at the expense of economy and emissions.
Multiple Turbochargers
Some engines, such as V-type engines, utilize two identically-sized but smaller turbos, each fed by a separate set of exhaust streams from the engine. The two smaller turbos produce the same (or more) aggregate amount of boost as a larger single turbo, but since they are smaller they reach their optimal RPM, and thus optimal boost delivery, faster. Such an arrangement of turbos is typically referred to as a parallel twin-turbo system. The first production automobile with parallel twin turbochargers was the Maserati Biturbo of the early 1980s. Later such installations include the Mitsubishi 3000GT VR-4, the Nissan 300ZX, and the BMW twin-turbo 3.0 litre I6 cars (E90, E81, E60).
Sequential
Some car makers combat lag by using two small turbos. A typical arrangement for this is to have one turbo active across the entire rev range of the engine and one coming on-line at higher RPM. Early designs would have one turbocharger active up to a certain RPM, after which both turbochargers are active. Below this RPM, both exhaust and air inlet of the secondary turbo is closed. Being individually smaller they do not suffer from excessive lag and having the second turbo operating at a higher RPM range allows it to get to full rotational speed before it is required.
Such combinations are referred to as a sequential twin-turbo. Porsche 959 first used this technology back in 1985. Sequential twin-turbos are usually much more complicated than single or parallel twin-turbo systems because they require what amounts to three sets of pipes-intake and waste gate pipes for the two turbochargers as well as valves to control the direction of the exhaust gases. Many new diesel engines use this technology to not only eliminate lag but also to reduce fuel consumption and reduce emissions.
Remote installations
Turbochargers are sometimes mounted well away from the engine, in the tailpipe of the exhaust system. Such remote turbochargers require a smaller aspect ratio due to the slower, lower-volume, denser exhaust gas passing through them. For low-boost applications, an intercooler is not required; often the air charge will cool to near-ambient temperature en route to the engine.
A remote turbo can run 300 to 600 degrees cooler than a close-coupled turbocharger, so oil coking in the bearings is of much less concern. Remote turbo systems can incorporate multiple turbochargers in series or parallel.
