Turbo

Turbocharger

A supercharger is an air compressor that increases the pressure or density of air supplied to an internal combustion engine. This gives each intake cycle of the engine more oxygen, letting it burn more fuel and do more work, thus increasing power. Power for the supercharger can be provided mechanically by means of a belt, gear, shaft, or chain connected to the engine’s crankshaft. When power is provided by a turbine powered by exhaust gas, a supercharger is known as a turbosupercharger – typically referred to simply as a turbocharger or just turbo.

Supercharging increases power, but turbocharging can improve power and efficiency. Turbochargers were known as turbosuperchargers when all forced induction devices were classified as superchargers. Currently, the term supercharger is only applied to mechanically driven forced induction devices.

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, the head of Diesel engine research at Gebrüder Sulzer engine manufacturing company in Winterthur, 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 turbocharger to a V12 Liberty aircraft engine. The engine was tested at Pikes Peak in Colorado at 14,000 ft 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.

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. During World War II, notable examples of U.S. 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.

Compared to superchargers, turbochargers tend to be more efficient, but less responsive. Throttle lag occurs because they rely on the build up 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.

Electrical boosting (‘E-boosting’) is a new technology under development. It uses an electric motor to bring the turbocharger up to operating speed quicker than 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 makes compressor speed independent of turbine speed. In 1981, a similar system that used a hydraulic drive system and overspeed clutch arrangement accelerated the turbocharger of the MV Canadian Pioneer (Doxford 76J4CR engine).

In most naturally aspirated 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 actually inspirated, compared to 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). 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 improves thermodynamic efficiency.

A turbocharger’s performance is closely tied to its size. Large turbochargers take more heat and pressure to spin the turbine, creating lag at low speed. 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.

Variable-geometry or variable-nozzle turbochargers use 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 rev range. The result is that the turbocharger improves fuel efficiency without a noticeable level of turbocharger lag.

The ported shroud (an air bleeding mechanism) 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 slightly downstream of the compressor inlet 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).

When the pressure of the engine’s intake air is increased, its temperature also increases. 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. (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 and the appliance that consumes the boosted air.)

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 extra fuel is not burned (as there is insufficient oxygen to complete the chemical reaction), instead it undergoes a phase change from atomized (liquid) to gas. This phase change absorbs heat, and the added mass of the extra fuel reduces the average thermal energy of the charge and exhaust gas. However, even when a catalytic converter is used, the practice of running an engine rich increases exhaust emissions.

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 gasoline-powered cars.

The first production turbocharger 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.

A natural use of the turbocharger — and its earliest known use for any internal combustion engine, starting with experimental installations in the 1920s — is with aircraft engines. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off. At 18,000 ft, the air is at half the pressure of sea level and the airframe experiences only half the aerodynamic drag. However, since the charge in the cylinders is pushed in by this air pressure, the engine normally produces only half-power at full throttle at this altitude. Pilots would like to take advantage of the low drag at high altitudes to go faster, but a naturally aspirated engine does not produce enough power at the same altitude to do so.

A turbocharger remedies this problem by compressing the air back to sea-level pressures, or even much higher, in order to produce rated power at high altitude. Since the size of the turbocharger is chosen to produce a given amount of pressure at high altitude, the turbocharger is over-sized for low altitude. The speed of the turbocharger is controlled by a wastegate (a valve that diverts exhaust gases away from the turbine). Early systems used a fixed wastegate, resulting in a turbocharger that functioned much like a supercharger. Later systems utilized an adjustable wastegate, controlled either manually by the pilot or by an automatic hydraulic or electric system. When the aircraft is at low altitude the wastegate is usually fully open, venting all the exhaust gases overboard. As the aircraft climbs and the air density drops, the wastegate must continuously close in small increments to maintain full power. The altitude at which the wastegate fully closes and the engine still produces full power is the critical altitude. When the aircraft climbs above the critical altitude, engine power output decreases as altitude increases, just as it would in a naturally aspirated engine.

With older supercharged aircraft, the pilot must continually adjust the throttle to maintain the required manifold pressure during ascent or descent. The pilot must also take care to avoid over-boosting the engine and causing damage, especially during emergencies such as go-arounds. In contrast, modern turbocharger systems use an automatic wastegate, which controls the manifold pressure within parameters preset by the manufacturer. For these systems, as long as the control system is working properly and the pilot’s control commands are smooth and deliberate, a turbocharger cannot over-boost the engine and damage it.

Yet the majority of World War II engines used superchargers, because they maintained three significant manufacturing advantages over turbochargers, which were larger, involved extra piping, and required exotic high-temperature materials in the turbine and pre-turbine section of the exhaust system. The size of the piping alone is a serious issue; American fighters Vought F4U and Republic P-47 used the same engine, but the huge barrel-like fuselage of the latter was, in part, needed to hold the piping to and from the turbocharger in the rear of the plane. Turbocharged engines also require a cooldown period after landing to prevent cracking of the turbocharger or exhaust system from thermal shock.

Today, most general aviation aircraft are naturally aspirated. The small number of modern aviation piston engines designed to run at high altitudes in general use a turbocharger or turbo-normalizer system rather than a supercharger. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple, but fuel-hungry supercharger. As the cost of fuel has increased, the supercharger has fallen out of favor. Turbocharged aircraft often occupy a performance range between that of normally aspirated piston-powered aircraft and turbine-powered aircraft. The increased maintenance costs of a turbocharged engine are considered worthwhile for this purpose, as a turbocharged piston engine is still far cheaper than any turbine engine.

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