A turbocharger (short for turbine driven supercharger) is an exhaust gas driven forced induction supercharger used in internal combustion engines. This differentiates it from a normal supercharger (or
blower) which uses a prime mover to power the
compression device.
Working Principle
A turbocharger consists of a turbine and a compressor
linked by a shared axle. The turbine inlet receives exhaust gases from the engine exhaust manifold causing the turbine wheel to
rotate. This rotation drives the compressor, compressing ambient air and delivering it to the air intake of the engine; this
allows more fuel to enter the cylinder because the air is compressed.
The objective of a turbocharger is the same as a normal supercharger; to improve upon the size-to-output efficiency of an
engine by solving one of its cardinal limitations. A naturally aspirated
automobile engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the
cylinder. Because the number of air and fuel molecules determine the potential energy available to force the piston down on the
combustion stroke, and because of the relatively constant pressure of the atmosphere, there ultimately will be a limit to the
amount of air and consequently fuel filling the combustion chamber. This ability to
fill the cylinder with air is its volumetric efficiency. Because the turbocharger
increases the pressure at the point where air is entering the cylinder, and the amount of air brought into the cylinder is
largely a function of time and pressure, more air will be drawn in as the pressure increases. The additional air makes it
possible to add more fuel, increasing the output of the engine. Also, the intake pressure can be controlled by a wastegate, which bleeds off excess boost from the turbocharger.
The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as
forced induction. Centrifugal
superchargers operate in the same fashion as a turbo; however, the energy to spin the compressor is taken from the
rotating output energy of the engine's crankshaft as opposed to exhaust gas. For this reason turbochargers are ideally more
efficient, since their turbines are actually heat engines, converting some of the thermal
energy from the exhaust gas that would otherwise be wasted, into useful work. Contrary to popular belief, this is not
totally "free energy," as it always creates some amount of exhaust backpressure which the engine must overcome. Superchargers use
output energy from an engine to achieve a net gain, which must be provided from some of the engine's total output; either
directly or from a separate smaller engine, perhaps electrically driven from the main engine's generator.
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 Pikes 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 before 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, Boeing B-17
Flying Fortress and Republic P-47 all used exhaust driven "turbo-superchargers"
to increase high altitude engine power. It is important to note that the majority of turbosupercharged aircraft engines used both
a gear-driven second stage centrifugal type supercharger and a first stage
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
Corvair's innovative turbocharged
flat-6
engine; The turbo, located at top right, feeds pressurized air into the engine
through the chrome T-tube visible spanning the engine from left to right.
The first production turbocharged automobile engines came from General Motors in 1962.
The A-body Oldsmobile Cutlass Jetfire and
Chevrolet Corvair Monza Spyder were both fitted with turbochargers. 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 ten years 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 1988, and then from 2000-2007.
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 Mercedes-Benz 300D, Saab 99 in 1978.
Japanese manufacturers followed suit, with Mitsubishi Lancer in 1978, Toyota
Supra in 1980, Nissan 280ZX in 1981 and Mazda RX-7 in 1984.
The worlds first production turbodiesel automobile was also introduced in 1978 by Peugeot
with the launch of the Peugeot 604 turbodiesel. Today, nearly all automotive diesels are
turbocharged.
Alfa Romeo introduced the first mass-produced Italian turbocharged car, the Alfetta GTV
2000 Turbodelta in 1979. Pontiac also introduced a turbo in 1980 and
Volvo Cars followed in 1981. Renault however gave another
step and installed a turbocharger to the smallest and lightest car they had, the R5, making it
the first Supermini automobile with a turbocharger in year 1980. This gave the car about
160bhp in street form and up to 300+ in race setup, an exorbitant power for a 1400cc motor. When combined with its incredible
lightweight chassis, it could nip at the heels of the quick Italian sports car Ferrari
308.
In Formula One, 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, Ferrari).
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. However, the FIA decided that turbos were making the sport too dangerous and expensive, and
from 1987 onwards, the maximum boost pressure was reduced before the technology
was banned completely for 1989.
In Rallying, turbocharged engines of up to 2000cc have long been the preferred motive power
for the Group A/World Rally Car (top level) competitors, due to the exceptional power-to-weight
ratios (and enormous torque) attainable. This combines with the use of vehicles with relatively small bodyshells for
manoeuvreability and handling. As turbo outputs rose to similar levels as the F1 category (see above), the FIA, rather than banning the technology, enforced a restricted turbo inlet
diameter (currently 34mm), effectively "starving" the turbo of compressible air and making high boost pressures unfeasible. The
success of small, turbocharged, four-wheel-drive vehicles
in rally competition, beginning with the Audi Quattro, has led to exceptional road cars in
the modern era such as the Lancia Delta Integrale, Toyota Celica GT-Four, Subaru Impreza WRX and the
Mitsubishi Lancer Evolution.
In the late 1970s, Ford and GM looked to the turbocharger to gain power, without sacrificing fuel consumption, during not only
the emissions crunch of the federal government but also a gas shortage. GM released turbo versions of the Pontiac Firebird, Buick
Regal, and Chevy Monte Carlo. Ford responded with a turbocharged Mustang in the form of the 2.3L from the Pinto. The engine
design was dated, but it worked well. The bullet-proof 2.3L Turbo was used in early carburated trim as well as fuel injected and
intercooled versions in the Mustang SVO and the Thunderbird Turbo Coupe until 1988. GM also liked the idea enough to evolve the
3.8L V6 used in early turbo Buicks into late '80s muscle in the form of the Buick Grand National and it's pinnacle (and final)
form, the GNX.
Although late to use turbocharging, Chrysler Corporation turned to turbochargers in 1984 and
quickly churned out more turbocharged engines than any other manufacturer, using turbocharged, fuel-injected 2.2 and 2.5 litre
four-cylinder engines in minivans, sedans, convertibles, and coupes. Their 2.2 litre turbocharged engines ranged from 142 hp to
225 hp, a substantial gain over the normally aspirated ratings of 86 to 93 horsepower; the 2.5 litre engines had about 150
horsepower and had no intercooler. Though the company stopped using turbochargers in 1993,
they returned to turbocharged engines in 2002 with their 2.4 litre engines, boosting output by 70 horsepower. [1]
Design details
Components
On the left, the brass oil drain connection. On the right are the braided oil supply line and water coolant line
connections.
Compressor impeller side with the cover removed
Turbine side housing removed.
A wastegate installed next to the turbocharger.
The turbocharger has four main components. The turbine and impeller/compressor wheels are each contained within their own folded conical housing on opposite sides of the
third component, the center housing/hub rotating assembly (CHRA).
The housings fitted around the compressor impeller and turbine collect and direct the gas flow through the wheels as they
spin. The size and shape can dictate some performance characteristics of the overall turbocharger. The area of the cone to radius
from center hub is expressed as a ratio (AR, A/R, or A:R). Often the same basic turbocharger assembly will be available from the
manufacturer with multiple AR choices for the turbine housing and sometimes the compressor cover as well. This allows the
designer of the engine system to tailor the compromises between performance, response, and efficiency to application or
preference. Both housings resemble snail shells, and thus turbochargers are sometimes referred to
in slang as snails.
The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can be flowed through the system, and the
relative efficiency at which they operate. Generally, the larger the turbine wheel and compressor wheel, the larger the flow
capacity. Measurements and shapes can vary, as well as curvature and number of blades on the wheels.
The center hub rotating assembly houses the shaft which connects the compressor impeller and turbine. It also must contain a
bearing system to suspend the shaft, allowing it to rotate at very high speed with minimal friction. For instance, in automotive
applications the CHRA typically uses a thrust bearing or ball bearing lubricated by a constant supply of pressurized engine oil.
The CHRA may also be considered "water cooled" by having an entry and exit point for engine coolant to be cycled. Water cooled
models allow engine coolant to be used to keep the lubricating oil cooler, avoiding possible oil coking from the extreme heat found in the turbine.
Boost
Boost refers to the increase in manifold pressure that is
generated by the turbocharger in the intake path or specifically intake manifold that exceeds normal atmospheric
pressure. This is also the level of boost as shown on a pressure gauge,
usually in bar, psi or possibly
kPa This is representative of the extra air pressure that is achieved over what would be
achieved without the forced induction. Manifold pressure should not be confused with
the amount, or "weight" of air that a turbo can flow.
Boost pressure is limited to keep the entire engine system including the turbo inside its thermal, and mechanical design
operating range by controlling the wastegate which shunts the exhaust gases away from the
exhaust side turbine.
The maximum possible boost depends on the fuel's octane rating. Also, Depending on the
engine you may be able to run more or less boost than other cars. To run higher boost you need to have a source to cool the
charging air. With proper tuning and efficient charge cooling, you can run upwards to 15 PSI of boost pressure on a stock motor.
Ethanol, methanol and diesel can naturally allow for higher boost than a normal gasoline engine.
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 engines.
Wastegate
By spinning at a relatively high speed the compressor turbine draws in a large volume of air and forces it into the engine. As
the turbocharger's output flow volume exceeds the engine's volumetric flow, air pressure in
the intake system begins to build, often called boost. The speed
at which the assembly spins is proportional to the pressure of the compressed air and total mass of air flow being moved. Since a
turbo can spin to RPMs far beyond what is needed, or of what it is safely capable of, the speed must be controlled. A
wastegate is the most common mechanical speed control system, and is often further augmented
by an electronic boost controller. The main function of a wastegate is to allow some of
the exhaust to bypass the turbine when the set intake pressure is achieved. Most passenger car wastegates are integral to the
turbocharger.
Anti-Surge/Dump/Blow Off Valves
Turbo charged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and
the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air
has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can be destructive to the engine e.g. damage may
occur to the throttle plate, induction pipes may burst. The surge will also decompress back across the turbo, as this is the only
path with the air can take.
The reverse flow back across the turbo acts on the compressor wheel and causes the turbine shaft to reduce in speed quicker
than it would naturally. When the throttle is opened again, the turbo will have to spin-up for longer to the required speed, as
turbo speed is proportional to boost/volume flow. In order to prevent this from happening, a valve is fitted between the turbo
and inlet which vents off the excess volume of air. These are known as a anti-surge, blow-off or dump valve. They are normally operated by engine vacuum or by electronic control.
The primary use of this valve is to prevent damage to the engine by a surge of compressed air and to maintain the turbo
spinning at a high speed. They can also be used as a bypass valve to control boost in a similar fashion as a waste gate, but this
is rarely seen as it is impractical. The air is usually vented to atmosphere, or can be recycled back into the turbo inlet.
Recycling back into the turbo causes the venting sound to be reduced but as the excess volume of air is not removed problems may
arise.
Fuel efficiency
Since a turbocharger increases the specific horsepower output of an engine, the engine
will also produce increased amounts of waste heat. This can sometimes be a problem when
fitting a turbocharger to a car that was not designed to cope with high heat loads. However, the higher compression ratios
attained generally contribute to greater fuel efficiency.
It is another form of cooling that has the largest impact on fuel efficiency: charge cooling. Even with the benefits of
intercooling, the total compression in the combustion
chamber is greater than that in a naturally-aspirated engine. To avoid
knock while still extracting maximum power from the engine, it is common practice to
introduce extra fuel into the charge for the sole purpose of cooling. While this seems counterintuitive, this fuel is not burned.
Instead, it absorbs and carries away heat when it changes phase from liquid mist to gas vapor. Also, because it is more dense
than the other inert substance in the combustion chamber, nitrogen, it has a higher specific
heat and more heat capacitance. It "holds" this heat until it is released in the exhaust
stream, preventing destructive knock. This thermodynamic property allows manufacturers
to achieve good power output with common pump fuel at the expense of fuel economy and emissions. The stoichiometric Air-to-Fuel ratio (A/F) for combustion of gasoline is 14.7:1. A common A/F in a
turbocharged engine while under full design boost is approximately 12:1. Richer mixtures are sometimes run when the design of the
system has flaws in it such as a catalytic converter which has limited endurance of high exhaust temperatures or the engine has a
compression ratio that is too high for efficient operation with the fuel given.
Lastly, the efficiency of the turbocharger itself can have an impact on fuel efficiency. Using a small turbocharger will give
quick response and low lag at low to mid RPMs, but can choke the engine on the exhaust side and generate huge amounts of
pumping-related heat on the intake side as RPMs rise. A large turbocharger will be very efficient at high RPMs, but is not a
realistic application for a street driven automobile. Variable vane and ball bearing technologies can make a turbo more efficient
across a wider operating range, however, other problems have prevented this technology from appearing in more road cars (see
Variable geometry turbocharger). Currently, the Porsche 911 (997) Turbo is the only gasoline car in production with this kind of turbocharger, although in
Europe turbos of this type are rapidly becoming standard-fitment on turbodiesel cars, vans and other commercial vehicles, because they can greatly enhance the diesel engine's characteristic low-speed torque. One way to take advantage of the different operating
regimes of the two types of supercharger is sequential turbocharging, which uses a small
turbocharger at low RPMs and a larger one at high RPMs.
The engine management systems of most modern vehicles can control boost and fuel delivery
according to charge temperature, fuel quality, and altitude, among other factors. Some systems are more sophisticated and aim to
deliver fuel even more precisely based on combustion quality. For example, the Trionic-7 system from Saab Automobile provides immediate feedback on the combustion while it is occurring by using the spark
plug to measure the cylinder pressure via the ionization voltage over the spark plug gap.
The new 2.0L TFSI turbo engine from Volkswagen/Audi incorporates lean burn and direct injection technology to
conserve fuel under low load conditions. It is a very complex system that involves many moving parts and sensors in order to
manage airflow characteristics inside the chamber itself, allowing it to use a stratified charge with excellent atomization. The
direct injection also has a tremendous charge cooling effect enabling engines to use higher compression ratios and boost
pressures than a typical port-injection turbo engine.
Automotive design details
The ideal gas law states that when all other variables are held constant, if pressure
is increased in a system so will temperature. Here exists one of the negative consequences of turbocharging, the increase in the
temperature of air entering the engine due to compression.
A turbo spins very fast; most peak between 80,000 and 200,000 RPM (using low inertia
turbos, 150,000-250,000 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. 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. This can also account for why many tuners upgrade their
standard journal bearing turbos (such as a T25) which use a 270 degree thrust bearing and a brass journal bearing which only has
3 oil passages, to a 360 degree bearing which has a beefier thrust bearing and washer having 6 oil passages to enable better
flow, response and cooling efficiency. Turbochargers with foil bearings are in development
which eliminates the need for bearing cooling or oil delivery systems, thereby eliminating the most common cause of failure,
while also significantly reducing turbo lag.
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 (normally called variable geometry turbochargers)
use 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 (with full boost as low
as 1,500 rpm), and are efficient at higher engine speeds; they are also used in diesel engines. [2] 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 production car to use these turbos was the limited-production 1989 Shelby CSX-VNT, in essence a Dodge Shadow equipped with a 2.2L
petrol engine. The Shelby CSX-VNT uses 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. Other common terms include Variable Turbine
Geometry (VTG), Variable Geometry Turbo (VGT) and Variable Vane Turbine (VVT). A number of other Chrysler Corporation vehicles used this turbocharger in 1990, including the Dodge Daytona and Dodge Shadow. These engines produced 174
horsepower and 225 pound-feet of torque, the same horsepower as the standard intercooled 2.2 liter engines but with 25 more
pound-feet of torque and a faster onset (less turbo lag). However, the Turbo III engine, without a VATN or VNT, produced 224
horsepower. The reasons for Chrysler's not continuing to use variable geometry turbochargers are unknown, but the main reason was
probably public desire for V6 engines coupled with increased availability of Chrysler-engineered V6 engines. [3]
The 2006 Porsche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbos
used are BorgWarner's Variable Geometry Turbos (VGTs). This is significant because although
VGTs have been used on advanced diesel engines for a few years and on the Shelby CSX-VNT, this is the first time the technology
has been implemented on a production petrol car since the 1,250 Dodge engines were produced in 1989-90. Some have argued this is
because in petrol cars exhaust temperatures are much higher (than in diesel cars), and this can have adverse effects on the
delicate, moveable vanes of the turbocharger; these units are also more expensive than conventional turbochargers. Porsche
engineers claim to have managed this problem with the new 911 Turbo.
Motorcycles
Using turbochargers to gain performance without a large gain in weight was very appealing to the Japanese factories in the
1980s. Suzuki, Yamaha and Kawasaki chose the route of an inline four with a turbo unit.
Properties and applications
Reliability
Turbochargers can be damaged by dirty or ineffective oil, and most manufacturers recommend more frequent oil changes for
turbocharged engines; many owners and some companies recommend using synthetic oils, which
tend to flow more readily when cold and do not break down as quickly as conventional oils. Because the turbocharger will get hot
when running, many recommend letting the engine idle for one to three minutes before shutting off the engine if the turbocharger
was used shortly before stopping (most manufacturers specify a 10-second period of idling before switching off to ensure the
turbocharger is running at its idle speed to prevent damage to the bearings when the oil supply is cut off). This lets the turbo
rotating assembly cool from the lower exhaust gas temperatures, and ensures that oil is supplied to the turbocharger while the
turbine housing and exhaust manifold are still very hot; otherwise coking of the lubricating
oil trapped in the unit may occur when the heat soaks into the bearings, causing rapid bearing wear and failure when the car is
restarted. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure. This problem is less
pronounced in diesel engines, due to the lower exhaust temperatures and generally slower
engine speeds.
A turbo timer can keep an engine running for a pre-specified period of time, to
automatically provide this cool-down period. Oil coking is also eliminated by foil
bearings. A more complex and problematic protective barrier against oil coking is the use of watercooled bearing
cartridges. 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 utilizing 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.
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. (Centrifugal superchargers do not build boost at low RPMs like a
positive displacement supercharger will). Conversely on light loads or at low RPM a turbocharger supplies less boost and the
engine is less 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 helps but there are cost increases and reliability
disadvantages that car manufacturers are not happy about. Lag is also reduced by using a foil
bearing rather than a conventional oil bearing. This reduces friction and contributes to faster acceleration of the
turbo's rotating assembly. Variable-nozzle turbochargers (discussed above) also reduce lag.
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 gases 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 parallel
twin-turbo system.
Some car makers combat lag by using two small turbos (such as Nissan,
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 a sequential twin-turbo. Sequential twin-turbos are usually much more complicated than
a single or parallel twin-turbo 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. Another well-known example is the
1993-2002 Mazda RX-7. Many new diesel engines use this technology to not only eliminate lag but also to reduce fuel consumption
and produce cleaner emissions.
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 engine RPM at which there is sufficient exhaust flow to the turbo to allow it
to generate significant amounts of boost[citation needed]. 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. If lag was experienced in this situation, the RPM
would either not start to rise for a short period of time after the throttle was increased, or increase slowly for a few seconds
and then suddenly build up at a greater rate as the turbo become effective. However, the term lag is used for boost threshold by
many manufacturers themselves so as not to confuse common man with many words.
Electrical boosting ("E-boosting") is a new technology under development; it uses a high speed electrical motor to drive the
turbocharger to speed before exhaust gases are available, e.g. from a stop-light. The electric motor is about an inch long.
[4]
Race cars often utilize an Anti-Lag System to
completely eliminate lag at the cost of reduced turbocharger life.
On modern diesel engines, this problem is virtually eliminated by utilizing a
variable geometry turbocharger.
Boost Threshold
Turbochargers start producing boost only above a certain rpm (depending on the size of the turbo) because they are powered by
the movement exhaust gases; without an appropriate exhaust gas velocity, they logically cannot force air into the engine. The
point at which the airflow in the exhaust is strong enough to force air into the engine is known as the boost threshold rpm.
Engineers have, in some cases, been able to reduce the boost threshold rpm to idle speed to allow for instant response.
[citation needed].
Both Lag and Threshold characteristics can be acquired through the use of a compressor map using a compressor map and a
mathematical equation. Performance shops have the maps on hand and/or can walk you through the process of mapping a turbo for
your particular vehicle and the type of racing you wish to do.
Automotive Applications
Turbocharging is very common on diesel engines in conventional automobiles, in
trucks, locomotives, 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 will develop less power than a gasoline
engine of the same size, and will weigh significantly more because diesel engines require heavier, stronger components. This
gives such engines a poor power-to-weight ratio; turbocharging can dramatically
improve this P:W ratio, with large power gains for a very small (if any) increase in weight.
- 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 transportation and other industrial equipment. Small cars in particular benefit from this technology, as there
is often little room to fit a larger-output (and physically larger) engine. Saab is a
leader in production car turbochargers, starting with the 1978 Saab
99; all current Saab models are turbocharged with the exception of the 9-7X. 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.
In the 1980s, when turbocharged production cars became common, they gained a reputation for being difficult to handle. The
tuned engines fitted to the cars, and the often primitive turbocharger technology meant that power delivery was unpredictable and
the engine often suddenly delivered a huge boost in power at certain speeds. Some drivers said this made cars such as the
BMW 2002 and the Porsche 911 exciting to drive,
requiring high levels of skill. Others said the cars were difficult and often dangerous. As turbocharger technology improved, it
became possible to produce turbocharged engines with a smoother, more predictable but just as effective power delivery.
Chrysler Corporation was an innovator of turbocharger use in the 1980s. Many of their production vehicles, for example the Chrysler
LeBaron, Dodge Daytona, Dodge
Shadow/Plymouth Sundance twins, and the Dodge
Spirit/Plymouth Acclaim twins were available with turbochargers, and they proved
very popular with the public. They are still considered competitive vehicles today, and the experience Chrysler obtained in
observing turbochargers in real-world conditions has allowed them to further turbocharger technology with the PT Cruiser Turbo, the Dodge SRT-4 and the Dodge Caliber SRT-4.
Aircraft Applications
Turbochargers are used in reciprocating aircraft engines which are designed for high altitude use. As an aircraft climbs in
altitude, the density of the air surrounding it decreases. As the density of the air decreases, so does the drag on the airframe,
but so does the power of the engine. With this in mind, turbochargers were developed for aircraft to keep the pressure of the air
entering the engine equivalent to a normally aspirated engine at sea level. In this case the system is called a
turbo-normalizer. Other systems use the turbocharger to boost the engine manifold pressure to much higher than sea level
pressures; in the area of 35 to 45 inches of mercury; and this is called
turbo-boosting. In either case, an automatic or manually-controlled wastegate is used
to vary the turbocharger output according to operating conditions.
Relationship to Gas Turbine Engines
Prior to World War II, Sir Frank Whittle started his experiments on early
turbojet engines. Due to a lack of sufficient materials as well as funding, initial progress
was slow. However, turbochargers were used extensively in military aircraft during World War II to enable them to fly very fast
at very high altitudes. The demands of the war led to constant advances in turbocharger technology, particularly in the area of
materials. This area of study eventually crossed over in to the development of early gas turbine
engines. Those early turbine engines were little more than a very large turbocharger with the compressor and turbine
connected by a number of combustion chambers. The cross over between the two has been
shown in an episode of the TV show Scrapheap Challenge where contestants were able
to build a functioning Jet Engine using a ex-automotive turbocharger as a compressor.
Consider also, for example, that General Electric manufactured turbochargers for
military aircraft and held several patents on their electric turbo controls during the war, then used that expertise to very
quickly carve out a dominant share of the gas turbine market which they have held ever since.
Advantages and Disadvantages
Advantages
- More specific power over naturally aspirated engine. This means a turbocharged engine can achieve more power from same engine
volume.
- Better thermal efficiency over both naturally aspirated and supercharged engine when under full load (i.e. on boost). This is
because the excess exhaust heat and pressure, which would normally be wasted, contributes some of the work required to compress
the air.
- Weight/Packaging. Smaller and lighter than alternative forced induction systems and may be more easily fitted in an engine
bay.
Disadvantages
- Lack of responsiveness if an incorrectly sized turbocharger is used. If a turbocharger that is too large is used it reduces
throttle response as it builds up boost slowly. However, doing this may result in more peak power.
- Boost threshold. Turbocharger starts producing boost only above a certain rpm due to a lack of exhaust gas volume to overcome
inertia of rest of turbo propeller. This results in a rapid and unlinear rise in torque, and
will reduce the usable power band of the engine. Also may result in sudden oversteer of rear
wheel drive cars during cornering [citation needed].
- Cost. Turbocharger parts are costly to add to naturally aspirated engines.
- Complexity. Further to cost, turbochargers require numerous additional systems if they are not to damage an engine. Even an
engine under only light boost requires a system for cooling the lubricating oil and upgraded piston crowns, exhaust valves and
valve seats. Intercooled turbo engines require additional plumbing, whilst highly tuned turbocharged engines will require
extensive upgrades to their lubrication, cooling and breathing systems.
- Lag can be disadvantageous in racing. If throttle is applied in a turn, power may unexpectedly increase when the turbo winds
up, which can induce wheelspin.
See also
References
- ^ Chrysler turbocharged engines (Allpar)
- ^ Parkhurst, Terry. Turbochargers: an interview with
Garrett’s Martin Verschoor. Allpar, LLC. Retrieved on [[12 December 2006]].
- ^ Allpar turbo engine history
- ^ Parkhurst, Terry. Turbochargers: an interview with
Garrett’s Martin Verschoor. Allpar, LLC. Retrieved on 12/12/2006.
External links
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