Turbocharger
This article describes the internal
combustion engine component often
known as a turbo. For other meanings
of turbo, see turbo
(disambiguation).
Turbocharger cutaway
A 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
A 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 engine.
The term 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.
The 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 carburetor.
The 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 density.
For 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.
A 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.
A main 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 intercooling).
A
Pair of turbochargers mounted to an
Inline 6 engine in a dragster.
Design details
When 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).
A turbo 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 systems.
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 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.
Some 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 Geometry.
Reliability
As long 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.
After 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.
Turbos 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.
In 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.
Diesel 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.
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
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 supercharged
engine.
Lag can 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 assembly.
Another 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.
Other 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.
Some car 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.
Lag is 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.
Race cars often utilise anti-lag to
completely eliminate lag at the cost
of reduced turbocharger life.
On 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.
Boost
Boost 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.
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.
· 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.
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
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.
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.
Most 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 power.
Turbo-Alternator[1] 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
System).[2]
History
The 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.
One of 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.
The 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.
BMW led 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
In 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 mid 1980s.
References
Article From
Wikipedia, the free encyclopedia
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