Turbocharger
In an internal combustion engine, a turbocharger is a forced induction device that compresses the intake air, forcing more air into the engine in order to produce more power for a given displacement.
Turbochargers are distinguished from superchargers in that a turbocharger is powered by the kinetic energy of the exhaust gases, whereas a supercharger is mechanically powered, usually by a belt from the engine's crankshaft. However, up until the mid-20th century, a turbocharger was called a "turbosupercharger" and was considered a type of supercharger.
History
Prior to the invention of the turbocharger, forced induction was only possible using mechanically-powered superchargers. Use of superchargers began in 1878, when several supercharged two-stroke gas engines were built using a design by Scottish engineer Dugald Clerk. Then in 1885, Gottlieb Daimler patented the technique of using a gear-driven pump to force air into an internal combustion engine.The 1905 patent by Alfred Büchi, a Swiss engineer working at Sulzer is often considered the birth of the turbocharger. This patent was for a compound radial engine with an exhaust-driven axial flow turbine and compressor mounted on a common shaft. The first prototype was finished in 1915 with the aim of overcoming the power loss experienced by aircraft engines due to the decreased density of air at high altitudes. However, the prototype was not reliable and did not reach production. Another early patent for turbochargers was applied for in 1916 by French steam turbine inventor Auguste Rateau, for their intended use on the Renault engines used by French fighter planes. Separately, testing in 1917 by the National Advisory Committee for Aeronautics and Sanford Alexander Moss showed that a turbocharger could enable an engine to avoid any power loss at an altitude of up to above sea level. The testing was conducted at Pikes Peak in the United States using the Liberty L-12 aircraft engine.
The first commercial application of a turbocharger was in June 1924 when the first heavy duty turbocharger, model VT402, was delivered from the Baden works of Brown, Boveri & Cie, under the supervision of Alfred Büchi, to SLM, Swiss Locomotive and Machine Works in Winterthur. This was followed very closely in 1925, when Alfred Büchi successfully installed turbochargers on ten-cylinder diesel engines, increasing the power output from. This engine was used by the German Ministry of Transport for two large passenger ships called the Preussen and. The design was licensed to several manufacturers and turbochargers began to be used in marine, railcar and large stationary applications.
Turbochargers were used on several aircraft engines during World War II, beginning with the Boeing B-17 Flying Fortress in 1938, which used turbochargers produced by General Electric. Other early turbocharged airplanes included the Consolidated B-24 Liberator, Lockheed P-38 Lightning, Republic P-47 Thunderbolt and experimental variants of the Focke-Wulf Fw 190.
The first practical application for trucks was realized by Swiss truck manufacturing company Saurer in the 1930s. BXD and BZD engines were manufactured with optional turbocharging from 1931 onwards. The Swiss industry played a pioneering role with turbocharging engines as witnessed by Sulzer, Saurer and Brown, Boveri & Cie.
Automobile manufacturers began research into turbocharged engines during the 1950s; however, the problems of "turbo lag" and the bulky size of the turbocharger were not able to be solved at the time. The first turbocharged cars were the short-lived Chevrolet Corvair Monza and the Oldsmobile Jetfire, both introduced in 1962.
The turbo found early success in motorsports. The 1968 Indianapolis 500 was the first recorded and large scale race to be won using a turbocharged engine, with turbos continuing to hold the title for the track in future years. Porsche had pioneered turbos in mass production engines, like the engines deriving from the 1963 Porsche 911. The air-cooled flat six engine, similar to the earlier Chevrolet Corvair's engine, was later turbocharged in 1973. The Porsche 935 and Porsche 936 won both types of Sportcars World Championships in 1976, as well as winning the 24 Hours of Le Mans, proving their reliability. In Formula One, the first race victories happened using turbos in the late 1970s, due to turbos excelling at giving small displacement engines power. It had also won the first F1 World Championship in 1983, with a BMW M10-based 4-cylinder engine that dates back to 1961.
Turbodiesel passenger cars entered the global market in the 1970s, with the Mercedes 300 D. Greater adoption of turbocharging in passenger cars began in the 1980s, as a way to increase the performance of smaller displacement engines.
Design
Like other forced induction devices, a compressor in the turbocharger pressurises the intake air before it enters the inlet manifold. In the case of a turbocharger, the compressor is powered by the kinetic energy of the engine's exhaust gases, which is extracted by the turbocharger's turbine.The main components of the turbocharger are:
- Turbine – usually a radial turbine design
- Compressor – usually a centrifugal compressor
- Center housing hub rotating assembly
Turbine
The turbine uses a series of blades to convert kinetic energy from the flow of exhaust gases to mechanical energy of a rotating shaft. The turbine housings direct the gas flow through the turbine section, and the turbine itself can spin at speeds of up to 250,000 rpm. Some turbocharger designs are available with multiple turbine housing options, allowing a housing to be selected to best suit the engine's characteristics and the performance requirements.
A turbocharger's performance is closely tied to its size, and the relative sizes of the turbine wheel and the compressor wheel. Large turbines typically require higher exhaust gas flow rates, therefore increasing turbo lag and increasing the boost threshold. Small turbines can produce boost quickly and at lower flow rates, since it has lower rotational inertia, but can be a limiting factor in the peak power produced by the engine. Various technologies, as described in the following sections, are often aimed at combining the benefits of both small turbines and large turbines.
Large diesel engines often use a single-stage axial inflow turbine instead of a radial turbine.
Twin-scroll
A twin-scroll turbocharger uses two separate exhaust gas inlets, to make use of the pulses in the flow of the exhaust gasses from each cylinder. In a standard turbocharger, the exhaust gas from all cylinders is combined and enters the turbocharger via a single intake, which causes the gas pulses from each cylinder to interfere with each other. For a twin-scroll turbocharger, the cylinders are split into two groups in order to maximize the pulses. The exhaust manifold keeps the gases from these two groups of cylinders separated, then they travel through two separate spiral chambers before entering the turbine housing via two separate nozzles. The scavenging effect of these gas pulses recovers more energy from the exhaust gases, minimizes parasitic back losses and improves responsiveness at low engine speeds.Another common feature of twin-scroll turbochargers is that the two nozzles are different sizes: the smaller nozzle is installed at a steeper angle and is used for low-rpm response, while the larger nozzle is less angled and optimised for times when high outputs are required.
Variable-geometry
Variable-geometry turbochargers are used to alter the effective aspect ratio of the turbocharger as operating conditions change. This is done with the use of adjustable vanes located inside the turbine housing between the inlet and turbine, which affect flow of gases towards the turbine. Some variable-geometry turbochargers use a rotary electric actuator to open and close the vanes, while others use a pneumatic actuator.If the turbine's aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, variable-geometry turbochargers often have reduced lag, a lower boost threshold, and greater efficiency at higher engine speeds. The benefit of variable-geometry turbochargers is that the optimum aspect ratio at low engine speeds is very different from that at high engine speeds.