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CATEGORIES (articles) > Engine, Gearbox > Technical > Turbo charger

Turbo charger


A turbocharger is a device used in internal-combustion engines to increase the power output of the engine by increasing the mass of oxygen and fuel entering the engine. A key advantage of turbochargers is that they offer a considerable increase in engine power with only a slight increase in weight. A key disadvantage in gasoline engines is that the compression ratio must be lowered (so as not to exceed maximum compression pressure and to prevent engine knocking) which lowers engine efficiency when operating at low power. This disadvantage does not apply to specifically designed turbocharged diesel engines. However, for operation at altitude, the power recovery of a turbocharger makes a big difference to total power output of both engine types. This last factor makes turbo charging aircraft engines considerably advantageous—and was the original reason for development of the device.

Principle of operation

A turbocharger is an exhaust driven supercharger. All superchargers have a gas compressor in the intake tract of the engine which compresses the intake air above atmospheric pressure, greatly increasing the volumetric efficiency beyond that of naturally-aspirated engines. A turbocharger also has a turbine that powers the compressor using waste energy from the exhaust gases. Compressor and turbine have the same shaft, similar to a turbojet aircraft engine.

Cut away of a turbo charger

The term supercharger is very often used when referring to a mechanically driven supercharger, which is most often driven from the engine's crankshaft by means of a belt (otherwise, and in many aircraft engines, by a gear train), whereas a turbocharger is exhaust-driven.

The compressor increases the pressure of the air entering the engine, so a greater "charge" (fuel/air mixture) enters the engine in the same time interval (the increase in fuel is required to keep the mixture the same ratio). This greatly improves the volumetric efficiency of the engine.

The increase in pressure is called "boost" and is measured in Pascal, bar or PSI. The energy from the extra fuel leads to more overall engine power. For example, at 100% efficiency a turbocharger providing 100 kPa (= 1 Bar or 14.7 PSI) of boost would effectively double the power of the engine. However, there are some parasitic losses due to heat and exhaust backpressure from the turbine, so turbochargers are generally only about 80% efficient because it takes some work for the engine to push those gases through the turbocharger turbine (which is acting as a restriction in the exhaust).

For automobile use, normal maximum boost pressure is 0.8 bar, but it can be much more. Because it is a centrifugal pump, a typical turbocharger will only start to deliver boost from about 2500 engine rpm (1800 in automotive turbo-diesel engines), while a supercharger will supply some boost at most engine speeds.

A main disadvantage of high boost pressures for internal combustion engines is that pumping the inlet air increases its temperature. This increase in charge temperature is a limiting factor for gasoline engines that can only tolerate a limited increase in charge temperature before pre-ignition occurs. The higher temperature is a volumetric efficiency downgrade for both types of engine. The pumping-effect heating can be alleviated by intercooling or aftercooling, or both.

Design details

When a gas is compressed, its temperature rises. It's not uncommon for a turbocharger to be pushing out air that is 90 °C (200°F). Compressed air from a turbo may be (and most commonly is) cooled before it is fed into the cylinders, using an intercooler or a charge air cooler (a heat-exchange device).

A turbo spins very fast—10,000 to 150,000 rpm depending on size (using low inertia turbos, 190,000 rpm), weight of the rotating parts, boost pressure developed and compressor design. Such high rotation speeds would cause problems for standard ball bearings leading to failure in a turbo. Most turbo-chargers use a fluid bearing. This is a flowing layer of oil that suspends and cools the moving parts. The oil is usually taken from the engine-oil circuit and usually needs to be cooled by an oil cooler before it is returned to the engine. Some turbochargers use incredibly precise ball bearings that offer less friction than a fluid bearing but these are also suspended in fluid-dampened cavities. Lower friction means the turbo shaft can be made of lighter materials, reducing so-called turbo lag or boost lag. Some car makers use water cooled turbochargers for added bearing life.

Turbochargers with foil bearings are in development (see picture above). This design eliminates the need for bearing cooling or oil delivery systems.

To reduce the possibility of damage to the engine and to also reduce the amount of time required for the turbo to spool-up and increase the boost after the quick increase in throttle opening; turbocharged engines are usually equipped with a blow off valve or a bypass valve. This allows the upper-deck air pressure to be maintained within limits that ensure engine efficiency without danger.

To manage the upper-deck air pressure the turbocharger's exhaust gas flow is regulated with a wastegate that bypasses excess exhaust gas entering the turbocharger's turbine. This regulates the rotational speed of the turbine and the output of the compressor. The wastegate is opened and closed by an electromagnetic coil or oil pressure that is regulated by pressure of the compressed air from turbo (the upper-deck pressure), or the engine's electronic control unit.

Reliability

As long as the oil supply is clean and the exhaust gas does not get too hot (ultra-lean mixtures) a turbocharger can be very reliable but care of the unit is important. Replacing a turbo that lets go and sheds its blades will be expensive.

After high speed operation of the engine it is important to let the engine run at tick-over speed for two to three minutes before switching off. This lets the turbo rotating assembly run down in speed and to cool from the lower gas temperatures in both the exhaust and the intake tracts. Not doing this will also result in the critical oil supply to the turbocharger being severed when the engine stops while the turbine is still turning at high speed, leading to coking (burning) of the lubricating oil trapped in the unit and, later, failure of the supply of oil when the engine is next started causing rapid bearing wear and failure. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure.

Diesel engines are much kinder to turbos because their exhaust gas temperature is much lower than that of gasoline engines and because most operators allow the engine to idle and do not switch it off immediately after heavy loading.

Lag

A lag is sometimes felt by the driver of a turbocharged vehicle as a delay between pushing on the accelerator pedal and feeling the turbo kick-in. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly-driven compressor in a supercharger does not suffer this problem. Conversely on light loads or at low RPM a turbocharger supplies less boost and the engine is more efficient than a supercharged engine.

Lag can be reduced by reducing the rotational inertia of the turbine, for example by using lighter parts to allow the spin-up to happen more quickly. Ceramic turbines are a big help in this direction. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response help but there are cost increases and reliability disadvantages that car manufacturers are not happy about. Lag is also reduced by using a precision bearing rather than a fluid bearing, this reduces friction rather than rotational inertia but contributes to faster acceleration of the turbo's rotating assembly.

Some car makers combat lag by using two small turbos (like Toyota, Maserati and Audi). 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. Being such small units 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.

Lag is not to be confused with the turbo spooling up, however many publications still make this basic mistake. The spool-up time of a turbo system describes the minimum turbo RPM at which the turbo is physically able to supply the requested boost level. Newer turbocharger and engine developments have caused spool-times to steadily decline to where day-to-day use feels perfectly natural. Putting your foot down at 1200 engine rpm and having no boost until 2000 rpm is spool-up and not lag.

Applications

Turbocharging is very common on Diesel engines in conventional automobiles, in trucks, for marine and heavy machinery applications. In fact, for current automotive applications, non-turbocharged diesel engines are becoming increasingly rare. Diesels are particularly suitable for turbocharging for several reasons:

Naturally-aspirated diesels have lower power-to-weight ratios compared to gasoline engines, turbocharging will improve this P:W ratio.
Diesel engines require more robust construction because they already run at very high compression ratio and at high temperatures so they generally require little additional reinforcement to be able to cope with the addition of the turbocharger. Gasoline engines often require extensive modification for turbocharging.
Diesel engines have a narrower band of engine speeds at which they operate, thus making the operating characteristics of the turbocharger over that "rev range" less of a compromise than on a gasoline-powered engine.

The first production turbocharged engines came from an unlikely source: General Motors. The A-body Oldsmobile Cutlass and Chevrolet Corvair were both fitted with turbochargers in 1962. The Oldsmobile is often recognized as the first, since it came out a few months earlier than the Corvair. Its Turbo Jetfire was a 215 in³ (3.5 L) V8, while the Corvair engine was a 140 in³ (2.3 L) flat-6. Both of these engines were abandoned within a few years, and GM's next turbo engine came more than two decades later.

Today, turbocharging is most commonly used on two types of engines: Gasoline engines in high-performance automobiles and Diesel engines in work trucks. Small cars in particular benefit from this technology, as there is often little room to fit a larger-output (and physically larger) engine. Saab has been the leading car maker using turbo chargers in production cars, starting with the 1978 Saab 99. The Porsche 944 utilized a turbo unit in the 944 Turbo (Porsche internal model number 951), to great advantage, bringing its 0-100 speeds very close to its contemporary non-turbo big brother, the Porsche 928. Contemporary examples of turbocharged performance cars include the Subaru Impreza WRX, Mazda RX-7, Mitsubishi Lancer Evolution, and the Porsche 911 Turbo.

In Formula 1, in the so called "Turbo Era", engines with a capacity of 1500 cc could achieve 1500 to 1800 hp (1100 to 1350 kW)(Renault, Honda, BMW).

Small car turbos are increasingly being used as the basis for small jet engines used for flying model aircraft—though the conversion is a highly specialised job—one not without its dangers.


 




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