A1 – Engine Repair

Chapter 16.
3.2. Turbocharging and Supercharging

1.1.Introduction
Turbocharging and supercharging are the two primary types of forced
induction systems used on production vehicles today, as well as the most
popular type of performance upgrade in the aftermarket. But they are not
anything new, in fact, turbo and superchargers are nearly as old as the
internal combustion engine itself, dating back to around 1885 when
Gottlieb Daimler and Rudolf Diesel were testing exhaust gas turbocharging.
In 1925, Swiss engineer Alfred Büchi was the first to successfully get a 40%
power increase out of an exhaust gas turbocharger which led to the
introduction of turbochargers into the automotive industry. In 1938 Swiss
Machine Works Saurer built the first turbocharged truck engine, and in
1963-1964 the Chevy Corvair Monza and Oldsmobile Jetfire made their
debut as the first turbocharged passenger cars. They were quickly removed
from the market due to poor reliability.
1.2.Basic Overview
Turbochargers and superchargers work by compressing air to increase its
density and forcing it into the combustion chamber. The more air forced
into the chamber, the bigger the combustion event; the bigger the
combustion, the more power the engine can output. They can also be used
to increase fuel economy such as in diesel engines or Ford’s new EcoBoost
engines. Superchargers compress air via screw type compressor wheels
that are driven by a belt system. Turbochargers, on the other hand, operate
by using the engines exhaust gas to spin a turbine wheel which is
connected by a shaft to a compressor wheel that forces air into the engine.
2. Turbochargers

2.1.Basic Operation
2.1.1. Components and function
The turbocharger is made up of two housings,
the compressor housing, which is the intake
side, and the turbine housing, which is the FIG 1: Compressor housing
exhaust side. In between these two housings is attached to intake piping
the center housing which contains two ball
bearings, the propeller shaft which connects the
compressor wheel and turbine together, as well
as oil inlets for lubrication and water inlets if the
turbo is water cooled. The turbine housing is
mounted to the exhaust manifold by the flanged
turbine inlet of the turbine housing which directs
the exhaust gas into the turbine wheel.
As the turbine wheel spins, it also spins the
compressor wheel via the propeller shaft. When
the compressor wheel spins, it forces FIG 2: Turbine housing which
pressurized air into the charge pipes. The air mounts too the exhaust manifold
then moves through an Intercooler or Charge
Air Cooler (CAC), which cools the air, increasing
its density further. The air then enters the
combustion chamber along with the fuel,
where it ignites, causing a bigger combustion
event due to the denser air. The exhaust gas
then exits the chamber and enters the exhaust,
spinning the turbine wheel again which
continues this cycle. FIG 3: Compressor wheel (LEFT) and
Turbine (RIGHT) connected by the
propeller shaft
2.1.2. Wastegate/BOV/bypass control
2.1.2.1. Wastegates
Wastegates are essential to a turbo system as they control the
amount of “boost” or pressurized air that the engine can
intake, although some diesel applications do no require them.
There are two types of wastegates, Internal and External.

Internal wastegates are built into the

turbine housing of the turbo while
External are attached to the exhaust
manifold before the turbo. Both operate
using a spring valve. Engine boost is
measure in Lbs, so for example, if the
engine is running 10 Lbs of boost, the
wastegate valve will slowly open as FIG 4: Internal waste gate
boost increases and divert pressure operated by vacuum.
away from the turbo into the exhaust to
maintain that 10 lb limit.
External wastegates operate the same way but have better
control of boost pressure and are better suited for use on
aftermarket applications with large turbo setups and for
engines making over 400hp according the manufacturer
Turbosmart©. Without a
wastegate, boost pressure
would continue to climb as
RPM increased. Stock engine
internals can only withstand a
certain amount of pressure. If
boost pressure rises above
the engine limits, catastrophic
engine failure can occur. FIG 5: External wastegate mounted
to exhaust manifold
 2.1.2.2 Blow-Off & Bypass Valves
Blow-Off valves (BOV), are another
type of pressure relief for turbo
systems, but are mounted on the
intake side between the compressor
discharge and the throttle body. The
BOV prevents the turbo system from
“surging” when the throttle plate is
suddenly closed causing pressure
fluctuations. These surges can be
heard when the BOV is actuated and
it is what gives many turbo cars that
loud distinct discharge or fluttering
FIG 6: example of a typical BOV
sound. The blow off valve operates
by monitoring manifold pressure and spring force to detect
when the throttle is closed and then vents pressure to the
atmosphere. Compressor Bypass valves serve the same
function and BOV’s but recirculate the air back into the system
rather than venting it to the atmosphere.

2.2 Types of Turbochargers

2.2.1. Single-Scroll
Single-scroll turbos are the most common type of turbocharger,
they have a single pathway or “scroll” for air to travel inside of
each housing.
2.2.2 Twin-Scroll
Twin-scroll turbos look just like single-scroll turbos, but the inside
of each housing is split in half which divides the exhaust pulses
feeding into the turbine. This is beneficial, because for example on
a four-cylinder engine with a single-scroll turbo, the #1 cylinder is
beginning its exhaust stroke at bottom dead center the exhaust
 valve will begin to open while the #2
cylinder will be entering its intake
stroke and its intake valve will be
opening, because of this valve overlap
the first cylinder’s exhaust pressure will
disrupt the air entering the second.
With a twin-scroll turbo the exhaust
pulses are divided so opposing cylinder
feed to separate scrolls, so this is not a
problem. In the case of a four-cylinder,
two cylinders feed into on scroll while
two cylinders feed into another, this FIG 7: Twin scroll design on a
turbine housing
also requires a split exhaust manifold.
2.2.4. Variable Geometry Turbo

VGT’s are a fairly new technology that is able to adjust the

amount of air entering the turbo and also the amount of gas
entering the turbine. Regular turbos have what is called an A/R
ratio, which is the area of the turbine inlet versus the radius of
the turbine wheel. Larger turbos have a greater A/R; this means
that they have what is known as turbo lag. This is time after
acceleration that it takes to turbine to begin creating boost,
because it has such a larger area it takes more air to spool the
turbine, meaning the engine is only under boost at high rpm.
Once the turbo spools it is capable of high amounts of boost.
Smaller turbos have a lower A/R ratio, meaning they spool
quickly, because of the smaller space it can only flow so much air
 meaning low boost at high
rpm. VGT’s eliminate this
problem using vanes.
Theses vanes are essential
metal blades that surround
the turbine wheel. The
vanes pivot to change the
A/R ratio of the engine. At
low RPM, the vanes pivot to
close the spaces between
them forcing the exhaust
gas into a smaller space
increasing its velocity
causing the turbine to spool
faster. As rpms increase, the
vanes slowly open to allow
the exhaust gas to enter in
greater capacity. If the
vanes did not open, the
turbo would stall due to the
FIG 8: shows the operating mechanism for operating the
increased pressure. This
vanes inside the turbine housing
system allows maximum
boost pressure at all rpms with no turbo lag. The vanes are
mounted on pivot points inside the turbo housing. The pivot
points are then connected to a ring on the outside of the turbo,
located around the center housing, via connecting links. The ring
is then connected to an actuator which pushes and pulls on the
ring to either open or close the vanes.
2.2.4. Variable-Twin-Scroll Turbos
VTST’s are the cutting edge of turbo technology and is being
developed by Borg-Warner. They are essentially the combination
of a twin-scroll turbo with variable geometry technology, giving
the advantages of both for maximum performance. The main
advantage of VTST’s are that manufacturers can use less
 expensive materials, lowering the cost of the turbos. VTST’s work
off the same principle of controlling the A/R ratio of the turbo,
instead of having vanes inside of the turbo, which are delicate
and need to be constructed of high grade exotic materials, the
VTST can switch between single and twin scroll or any increment
in between using a single valve located in the turbine inlet that is
controlled by an external actuator. At low rpm the valve is moved
all the way to one side so that only one scroll is open and all
cylinders feed to it. This also minimizes the area of the turbine
being spun by the exhaust gas causing it to spool faster. As rpms
increase, the valve will slowly open allowing more and more air
into the second scroll. At WOT, the valve will be centered,
separating the two scrolls forcing half the cylinders exhaust gas
into one scroll and the other cylinders into the second scroll
raising the A/R ratio and allowing for greater boost.

3. Superchargers
3.1. Operation
Superchargers provide the same function as turbochargers but
operate quite differently. Unlike turbochargers, which mount to
the exhaust manifold and run off of exhaust gas pressure, typical
superchargers mount to the intake and are belt driven by the crank
pulley. This direct drive means that a supercharger can provide
constant boost pressure through the entire rev range, eliminating
problems like turbo lag, but because of the belt drive system there
is a parasitic load placed on the engine. Instead of a turbine and
compressor wheel, superchargers use a lobe or screw type
compressor to increase the intake air pressure.
3.2.3. Components and function
Superchargers are simpler than turbos. On V-style engines,
superchargers usually mount in the valley of the engine on top
of the intake. A pulley on the front of the supercharger is
connected by a belt directly to the crank pulley. As the engine
 rotates, it also spins the supercharger pulley which spins the
compression lobes or screws, which are synchronized by two
drive gears, compress air between them. As they spin the force
the air down into the intake manifold then into the
combustion chamber.
3.2. Types of Superchargers
3.2.1. Roots Supercharger
The roots supercharger is the original style of supercharger
developed in the late 1800’s by the Roots brothers for
industrial engines but modified for automotive use (source).
The roots style supercharger use two tri-lobe rotors to
compress air. The rotor lobes are either straight or slightly
twisted, which reduced vibration and noise. One rotor is driven
by the pulley attached by a belt to the crankshaft. Attached to
the front of the rotors are two gears, so as the belt driven
rotor turns, it spins the other rotor. Roots superchargers are
usually identified by their large intakes which typically
protrude through the hood of many old school muscle cars.
The rotors spin in opposite directions towards each other,
compressing the air between them, allowing air to be sucked
into the intake. The rotor lobes mesh together but do not
come in contact with each other to avoid excessive heat or
binding of the supercharger. The compressed air then moves
out through the bottom of the supercharger as it passes
through a water-to-air cooler. The cooler causes the air to
become denser before it enters the engine.
3.2.2. Screw Type
 Twin screw superchargers are
found on many present-day
vehicles such as the 2015 Chevy
Corvette Z06 and 2015 Jaguar
F-Type, and they operate the
same way as Roots
superchargers. The design of
the rotors is slightly different.
Instead of the tri-lobe rotors,
twin screw superchargers use
two screw style rotors. Air
enters the top of the
supercharger usually near the
front. The air is moved
horizontally along the rotors
towards the back of the
FIG 9: shows the underside of a screw type
supercharger, because of the supercharger, the screw type rotors are
screw style rotors. The rotors visible in the highlighted area
are contoured so as the air
moves down them it is squeezed into a smaller space
increasing its pressure. The advantages of this is a greater
thermal efficiency than a Roots supercharger and a greater
boost at low rpm, but because of the precise machining of the
rotors its cost is increased.
3.2.3. Centrifugal Supercharger
A Centrifugal supercharger is a unique style of supercharger
that looks more like a belt driven turbo. Centrifugal
superchargers usually mount to the front of the engine, but
are still belt driven by the crank pulley and use centrifugal
force to propel air into the engine. The supercharger itself
looks like a turbo housing with a propeller wheel inside. The
propeller is gear driven. A larger gear is attached to the
pulley. As it spins, it turns a smaller gear attached to the
compressor wheel. This gear ratio may vary.As the propeller
 spins, it sucks in air and forces it outward and around the
inside of the housing using centrifugal force. The force
compresses the air as it moves through the coil shaped
housing. The pros of this style supercharger are its compact
size allowing it to be used in a wider variety of vehicles and
its high thermal efficiency. The cons of centrifugal
superchargers are that it has poor boost at low rpm as well
as not having a consistent amount of boost pressure. Like all
superchargers, it has parasitic engine draw. Also, unlike
Roots and Twin-screw superchargers, they often require
engine oil.
3.3. Supercharger Uses
A supercharger’s main use is to provide constant power increase
without sacrificing engine efficiency. Therefore, many
manufacturers use superchargers in vehicles with large
displacement engines to acquire greater power and fuel
economy. Superchargers are also popular in racing due to their
ability to provide off the line power and torque on the drag strip,
and fast acceleration out of corners on road courses.
4. Intercoolers
Intercoolers are a vital part of the forced induction systems, because as these
forced air systems compress incoming air they also increase the air charge
temperature. Warm air is less dense and thus does not ignite as well.
Intercoolers are placed between Turbos/Superchargers and the intake
manifold to cool the compressed air before it enters the combustion chamber.
4.1. Design
4.1.1 Air-to-Air
The first style of intercooler is known as an air-to-air intercooler. As
the name implied it uses ram air flow to cool the charged air in the
system. It looks similar to, and functions like, a radiator. The
difference being that instead of coolant flowing through the
intercooler, the pressurized air flows through the channels. There are
 two types of air-to-air intercoolers, Bar & Plate or Tube & Fin. Bar &
Plate intercoolers consist of layers of rectangular slots along the
length of the intercooler where the pressurized air flows, inside these
slots are vertical plates. In between these bars are corrugated fins
like a radiator that allows air to flow between the bars cooling the
pressurized air inside.
4.1.2. Air-to-Water
Air-to-water intercoolers use a
water cooling system similar to
an engine. As air leaves the
turbo/supercharger it enters the
intercooler which has a water
channel flowing through it. The
water in the channel is moved by
a pump through a radiator in the
front of the vehicle, the radiators
cools the water which then flows
into the intercooler. As the warm
FIG 10: a Air-to-water intercooler from a Nissan
air moves across the intercooler engine.
the heat from the air moves to
the water and the air becomes dense as it moves into the engine.
Then water then moves back to the pump where it is circulated and
cooled again. This style of intercooler is much more efficient than an
air-to-air intercooler. It also has a decreased pressure lost since the
intake is much closer, the air does not have to travel as far before
reaching the engine. The downside to this system is that it is heavier,
takes up more space in the engine bay and is also more expensive
than an air-to-air intercooler.
Sources

http://www.turbosmart.com/news/how-does-a-wastegate-work

https://whipplesuperchargers.com/index.php?dispatch=pages.view&
page_id=14

http://www.turbosmart.com/news/history-of-turbocharging

https://www.turbobygarrett.com/turbobygarrett/turbo_tech_basic

http://www.turbosmart.com/technical-articles/how-an-intercooler-
works/