article describes the internal combustion engine component
often known as a turbo. For other meanings of turbo, see
turbocharger is an exhaust gas driven compressor used in
internal-combustion engines to increase the power output of
the engine by increasing the mass of oxygen 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.
Principle of operation
turbocharger is an exhaust gas 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 wasted
energy from the exhaust gases. The compressor and turbine
spin on the same shaft, similar to a turbojet aircraft
supercharger is very often used when referring to a
mechanically driven turbocharger, which is most often driven
from the engine's crankshaft by means of a belt (otherwise,
and in many aircraft engines, by a geartrain), whereas a
turbocharger is exhaust-driven, the name turbocharger being
a contraction of the earlier "turbosupercharger". Because
the turbine of a turbocharger is in-itself a heat engine, a
turbocharger equipped engine will normally compress the
intake air more efficiently than a mechanical supercharger.
But because of "turbo lag" (see below), engines with
mechanical superchargers are typically more responsive.
compressor increases the pressure of the air entering the
engine, so a greater mass of oxygen enters the combustion
chamber in the same time interval (an increase in fuel is
required to keep the mixture the same air to fuel ratio).
This greatly improves the volumetric efficiency of the
engine, and thereby creates more power. The additional fuel
is provided by the proper tuning of the fuel injectors or
increase in pressure is called "boost" and is measured in
pascals, bars or lbf/in². The energy from the extra fuel
leads to more overall engine power. For example, at 100%
efficiency a turbocharger providing 101 kPa (14.7 lbf/in²)
of boost would effectively double the amount of air entering
the engine because the total pressure is twice atmospheric
pressure. However, there are some parasitic losses due to
heat and exhaust backpressure from the turbine, so
turbochargers are generally only about 80% efficient, at
peak efficiency, 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) and the
now-compressed intake air has been heated, reducing its
automobile use, typical boost pressure is in the general
area of 80 kPa (11.6 lbf/in²), but it can be much more.
Because it is a centrifugal pump, a typical turbocharger,
depending on design, will only start to deliver boost from a
certain rpm where the engine starts producing enough exhaust
gas to spin the turbocharger fast enough to make pressure.
This engine rpm is referred to as the boost threshold.
Another fact to observe is that the relation between boost
pressure and compressor rpm is somewhat exponential, and the
relation between compressor rpm and airflow is very small. A
turbocharger that is pushing 15 psi when the engine is at
3000 rpm will only have increased a little bit in speed when
maintaining the same pressure at 6000 engine rpm; given that
it is still within the design limits of the compressor. For
this very same reason, belt driven centrifugal superchargers
have a very narrow power band and deliver max boost only
when the engine is at max rpm.
disadvantage in gasoline engines is that the compression
ratio should be lowered (so as not to exceed maximum
compression pressure and to prevent engine knocking) which
reduces 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 turbocharging aircraft engines
considerably advantageous—and was the original reason for
development of the device.
disadvantage of high boost pressures for internal combustion
engines is that compressing the inlet air increases its
temperature. This increase in charge temperature is a
limiting factor for petrol engines that can only tolerate a
limited increase in charge temperature before detonation
occurs. The higher temperature is a volumetric efficiency
downgrade for both types of engine. The pumping-effect
heating can be alleviated by aftercooling (sometimes called
Pair of turbochargers mounted to an Inline 6 engine in a
gas is compressed, its temperature rises. It is 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, on petrol engines) cooled before it is fed into
the cylinders, using an intercooler or a charge air cooler
(a heat-exchange device).
spins very fast; most peak between 80,000 and 150,000 rpm
(using low inertia turbos, 190,000 rpm) depending on size,
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 so
most turbo-chargers use fluid bearings. These feature 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
circulates through 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 which
eliminates the need for bearing cooling or oil delivery
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 the
compressed air from turbo (the upper-deck pressure) and can
be raised by using a solenoid to regulate the pressure fed
to the wastegate membrane. This solenoid can be controlled
by Automatic Performance Control, the engine's electronic
control unit or an after market boost control computer.
Another method of raising the boost pressure is through the
use of check and bleed valves to keep the pressure at the
membrane lower than the pressure within the system.
turbochargers utilise a set of vanes in the exhaust housing
to maintain a constant gas velocity across the turbine, the
same kind of control as used on power plant turbines. These
turbochargers have minimal amount of lag, have a low boost
threshold, and are very efficient at higher engine speeds.
In many setups these turbos don't even need a wastegate. The
vanes are controlled by a membrane identical to the one on a
wastegate but the level of control required is a bit
different. The first car manufacturer to use these turbos
was the limited-production 1989 Shelby CSX-VNT. It utilised
a turbo from Garrett, called the VNT-25 because it uses the
same compressor and shaft as the more common Garrett T-25.
This type of turbine is called a Variable Nozzle Turbine (VNT).
Turbocharger manufacturer Aerocharger uses the term
'Variable Area Turbine Nozzle' (VATN) to describe this type
of turbine nozzle. Another common term is Variable Turbine
as the oil supply is clean and the exhaust gas does not
become overheated (lean mixtures or retarded spark timing on
a gasoline engine) 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. The use of
synthetic oils is recommended in turbo engines.
high speed operation of the engine it is important to let
the engine run at idle speed for one to three minutes before
turning off the engine. Saab, in its owner manuals,
recommends a period of just 30 seconds. This lets the turbo
rotating assembly cool from the lower exhaust gas
temperatures. Not doing this will also result in the
critical oil supply to the turbocharger being severed when
the engine stops while the turbine housing and exhaust
manifold are still very hot, leading to coking (burning) of
the lubricating oil trapped in the unit when the heat soaks
into the bearings 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. A
turbo timer is a device designed to keep an automotive
engine running for a pre-specified period of time, in order
to execute this cool-down period automatically.
with watercooled bearing cartridges have a protective
barrier against coking. The water boils in the cartridge
when the engine is shut off and forms a natural
recirculation to drain away the heat. It is still a good
idea to not shut the engine off while the turbo and manifold
are still glowing.
custom applications utilising tubular headers rather than
cast iron manifolds, the need for a cooldown period is
reduced because the lighter headers store much less heat
than heavy cast iron manifolds.
engines are usually 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 use.
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
positive-displacement supercharger does not suffer this
problem. (Centrifugal superchargers do not build boost at
low RPM's like a positive displacement supercharger will).
Conversely on light loads or at low rpm a turbocharger
supplies less boost and the engine is more efficient than a
be reduced by lowering the rotational inertia of the
turbine, for example by using lighter parts to allow the
spool-up to happen more quickly. Ceramic turbines are a big
help in this direction. Unfortunately, their relative
fragility limits the maximum boost they can supply. 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
common method of equalizing turbo lag, is to have the
turbine wheel "clipped", or to reduce the surface area of
the turbine wheel's rotating blades. By clipping a minute
portion off the tip of each blade of the turbine wheel, less
restriction is imposed upon the escaping exhaust gases. This
imparts less impedance onto the flow of exhaust gasses at
low rpm, allowing the vehicle to retain more of its low-end
torque, but also pushes the effective boost rpm to a
slightly higher level. The amount a turbine wheel is and can
be clipped is highly application-specific. Turbine clipping
is measured and specified in degrees.
setups, most notably in 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 "twin turbo" setup.
makers combat lag by using two small turbos (like Toyota,
Subaru, Maserati, Mazda, 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.
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 are 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 "sequential turbos". Sequential
turbochargers are usually much more complicated than single
or twin-turbocharger systems because they require what
amounts to three sets of pipes-intake and wastegate pipes
for the two turbochargers as well as valves to control the
direction of the exhaust gases. An example of this is the
current BMW E60 5-Series 535d. Many new diesel engines use
this technology to not only eliminate lag but also to reduce
fuel consumption and produce cleaner emissions. An example
of this would be the Ford Power Stroke engine.
not to be confused with the boost threshold, however many
publications still make this basic mistake. The boost
threshold 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 boost thresholds 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 engine rpm is an example of boost threshold
and not lag.
cars often utilise anti-lag to completely eliminate lag at
the cost of reduced turbocharger life.
modern diesel engines, this problem is virtually eliminated
by utilising a variable geometry turbocharger. The newly
presented Porsche 911 Turbo has eliminated this problem for
gasoline engines as well.
refers to the increased manifold pressure that is generated
by the intake side turbine. This is limited to keep the
turbo inside its design operating range by controlling the
wastegate which shunts the exhaust gases away from the
exhaust side turbine. Many diesel engines do not have any
wastegate because the amount of exhaust energy is controlled
directly by the amount of fuel injected into the engine, and
slight variations in boost pressure do not make a difference
for the engine.
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.
· Diesel engines blow nothing but air into the
cylinders during cylinder charging, squirting fuel into the
cylinder only after the intake valve has closed and
compression has begun. Gasoline/petrol engines differ from
this in that both fuel and air are introduced during the
intake cycle and both are compressed during the compression
cycle. The higher intake charge temperatures of
forced-induction engines reduces the amount of compression
that is possible with a gasoline/petrol engine, whereas
diesel engines are far less sensitive to this.
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 turbochargers 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
km/h (0-60 mph) times very close to its contemporary
non-turbo "big brother", the Porsche 928. Contemporary
examples of turbocharged performance cars include the Audi
TT, Dodge SRT-4, Subaru Impreza WRX, Mazda RX-7, Mitsubishi
Lancer Evolution, Nissan Skyline GT-R, Toyota Supra RZ, and
the Porsche 911 Turbo.
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
modern turbocharged aircraft use an adjustable wastegate.
The wastegate is controlled manually, or by a
pneumatic/hydraulic control system, or, as is becoming more
and more common, by a flight computer. In the interests of
engine longevity, the wastegate is usually kept open, or
nearly so, at sea-level to keep from overboosting the
engine. As the aircraft climbs, the wastegate is gradually
closed, maintaining the manifold pressure at or above
sea-level. In aftermarket applications, aircraft
turbochargers sometimes do not overboost the engine, but
rather compress ambient air to sea-level pressure. For this
reason, such aircraft are sometimes refered to as being
turbo-normalised. Most applications produced by the major
manufacturers (Beech, Cessna, Piper and others) increase the
maximum engine intake air pressure by as much as 35%.
Special attention to engine cooling and component strength
is required because of the increased combustion heat and
Turbo-Alternator is a form of turbocharger that generates
electricity instead of boosting engine's air flow. On
September 21, 2005, Foresight Vehicle announced the first
known implementation of such unit for automobiles, under the
name TIGERS (Turbo-generator Integrated Gas Energy Recovery
turbocharger was invented by Swiss engineer, Alfred Buchi,
who had been working on steam turbines. His patent for the
internal combustion turbocharger was applied for in 1905.
Diesel ships and locomotives with turbochargers began
appearing in the 1920s.
the first applications of a turbocharger to a non-Diesel
engine came when General Electric engineer, Sanford Moss
attached a turbo to a V12 Liberty aircraft engine. The
engine was tested at Pike's Peak in Colorado at 14,000 feet
to demonstrate that it could eliminate the power losses
usually experienced in internal combustion engines as a
result of altitude.
Turbochargers were first used in production aircraft engines
in the 1930s prior to World War II. The primary purpose
behind most aircraft-based applications was to increase the
altitude at which the airplane can fly, by compensating for
the lower atmospheric pressure present at high altitude.
Aircraft such as the Lockheed P-38 Lightning, Boeing B-17
Flying Fortress and B-29 Superfortress all used exhaust
driven "turbo-superchargers" to increase high altitude
engine power. It is important to note that turbosupercharged
aircraft engines actually utilized a gear-driven centrifugal
type supercharger in series with a turbocharger.
Turbo-Diesel trucks were produced in Europe and America
(notably by Cummins) after 1949. The turbocharger hit the
automobile world in 1952 when Fred Agabashian qualified for
pole position at the Indianapolis 500 and led for 100 miles
before tire shards disabled the blower.
first production turbocharged automobile engines came from
General Motors. The A-body Oldsmobile Cutlass Jetfire and
Chevrolet Corvair Monza Spyder 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 either a 145 in³ (2.3 L)(1962-63) or
a 164 in³ (2.7 L) (1964-66) flat-6. Both of these engines
were abandoned within a few years, and GM's next turbo
engine came more than two decades later.
Offenhauser's turbocharged engines returned to Indianapolis
in 1966, with victories coming in 1968. The Offy turbo
peaked at over 1,000 hp in 1973, while Porsche dominated the
Can-Am series with a 1100 hp 917/30. Turbocharged cars
dominated the Le Mans between 1976 and 1994.
the resurgence of the automobile turbo with the 1973 2002
Turbo, with Porsche following with the 911 Turbo, introduced
at the 1974 Paris Motor Show. Buick was the first GM
division to bring back the turbo, in the 1978 Buick Regal,
followed by the famed Mercedes-Benz 300D and Saab 99 in
1978. The worlds first production turbodiesel automobile was
also introduced in 1978 by Peugeot with the launch of the
Peugeot 604 turbodiesel. Pontiac also introduced a turbo in
1980 and Volvo Cars followed in 1981
Formula 1, in the so called "Turbo Era" of 1977 until 1989,
engines with a capacity of 1500 cc could achieve anywhere
from 1000 to 1500 hp (746 to 1119 kW) (Renault, Honda, BMW).
Renault was the first manufacturer to apply turbo technology
in the F1 field, in 1977. The project's high cost was
compensated for by its performance, and led to other engine
manufacturers following suit. The Turbo-charged engines took
over the F1 field and ended the Ford Cosworth DFV era in the
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