everal terms redirect here. For other uses, see Turbo (disambiguation).
Not to be confused with Supercharger.
Cut-away view of an air foil bearing-supported turbocharger
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.
Manufacturers commonly use turbochargers in truck, car, train, aircraft, and construction-equipment engines. They are most often used with Otto cycle and Diesel cycle internal combustion engines.
History
Forced induction dates back to 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 1905patent by Alfred Büchi, a Swiss engineer working at Gebrüder Sulzer (now simply called 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, 1917 testing by the American National Advisory Committee for Aeronautics and Sanford Alexander Moss showed that a turbocharger could enable an engine to avoid any power loss (compared with the power produced at sea level) at an altitude of up to 4,250 m (13,944 ft) above sea level.The testing was conducted at Pikes Peak in the United States using the V12 Liberty aircraft engine.
The first commercial application of a turbocharger was in 1925, when Alfred Büchi successfully installed turbochargers on ten-cylinder diesel engines, increasing the power output from 1,300 to 1,860 kilowatts (1,750 to 2,500 hp).This engine was used by the German Ministry of Transport for two large passenger ships called the “Preussen” and “Hansestadt Danzig”. 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 B-24 Liberator, P-38 Lightning, P-47 Thunderbolt and the experimental Focke-Wulf Fw 190 prototypes.
Automobile and truck 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, introduced in 1962.Following the 1973 oil crisis and the 1977 Clean Air Act amendments, turbocharging became more common in automobiles, as a method to reduce fuel consumption and exhaust emissions.
Turbocharging versus supercharging
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 hp (110 kW). Yet the benefits outweigh the costs; for the 150 hp (110 kW) to drive the supercharger the engine generates an additional 400 hp (300 kW), 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).
By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses.This is more efficient because while the increased back pressure taxes the piston exhaust stroke, much of the energy driving the turbine is provided by the still-expanding exhaust gas that would otherwise be wasted as heat through the tailpipe. In contrast to supercharging, the primary disadvantage of turbocharging is what is referred to as “lag” or “spool time”. This is the time between the demand for an increase in power (the throttle being opened) and the turbocharger(s) providing increased intake pressure, and hence increased power.
Throttle lag occurs because turbochargers rely on the buildup of exhaust gas pressure to drive the turbine. In variable output systems such as automobile engines, exhaust gas pressure at idle, low engine speeds, or low throttle is usually insufficient to drive the turbine. Only when the engine reaches sufficient speed does the turbine section start to spool up, or spin fast enough to produce intake pressure above atmospheric pressure.
A combination of an exhaust-driven turbocharger and an engine-driven supercharger can mitigate the weaknesses of both.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 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.
Operating principle
Variable geometry turbocharger
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 at sea level. 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).
Pressure increase (or boost)
A turbocharger may also be used to increase fuel efficiency without increasing power.This is achieved by diverting exhaust waste energy, from the combustion process, and feeding it back into the turbo’s “hot” intake side that spins the turbine. As the hot turbine side is being driven by the exhaust energy, the cold intake turbine (the other side of the turbo) compresses fresh intake air and drives it into the engine’s 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.
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 inHg (100 kPa).
Applications
Petrol-powered cars
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 petrol-powered cars. Turbocharging can increase power output for a given capacity[44] 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 peak horsepower (105) as a 1.4-litre engine.
Diesel-powered cars
The first production turbocharger diesel passenger car was the Garrett-turbocharged[45] Mercedes 300SD introduced in 1978.[46][47] Today, most automotive diesels are turbocharged, since the use of turbocharging improved efficiency, driveability and performance of diesel engines,[46][47] 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.
Motorcycles
The first example of a turbocharged bike is the 1978 Kawasaki Z1R TC.[48] 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.
