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Kaustubh P. Ghodke Int. Journal of Engineering Research and Application www.ijera.com
Vol. 3, Issue 5, Sep-Oct 2013, pp.394-400
www.ijera.com 394 | P a g e
Introduction To Turbochargers For Modern Diesel Engines
Kaustubh P. Ghodke
(Department of Mechanical Engineering, Rajarshi Shahu College of Engineering, Pune-33
ABSTRACT
CO2 emissions reduction for vehicles and demand for more driving comfort are the challenges for the
automobile industry today. One approach to this problem is the reduction of the displacement of combustion
engine while maintaining the characteristics of large displacement engine. This method is often referred to using
the term” downsizing” and requires the engine to be turbocharged to improve its performance and torque. It has
been demonstrated that a simple charging unit alone is not enough and more complex charging systems are
required when emission norms are stringent. The goals of a developed engine in terms of the thermodynamics
and operation of future passenger car are increase in the power density of the engine, highest possible maximum
torque at low engine speeds across the widest possible range of speeds, improvement of the driving response in
transient operating conditions like start up response and elasticity response, reduction of the primary energy
consumption during testing and driving on the road, observances of the future exhaust gas thresholds which
mean a drastic reduction in the current emission levels. The latter goals can be reached through the use of
smaller displacement engines. Engines with low engine displacement yield significant advantages in the test
cycles with respect to fuel consumption and emissions, but the torque produced by small engine is pronouncedly
less than that of a large displacement, naturally aspirated engine attained in terms of the steady state response
and of the transient response. This paper summarizes review of advancements in turbocharger technology to
meet the demand of high performance and low emission of passenger car vehicle application.
Keywords- Emission, performance, turbocharger, VNT, VTG
I. INTRODUCTION
The thermodynamic demands placed on
modern charging systems for diesel engines in
passenger cars result mainly from the target values of
the steady-state and transient characteristics of the
charged engine. A higher power and torque density
result in shift in the direction of the steady-state power
and intermediate pressure generally require an
additional injection of a large amount of fuel in the
cylinder and therefore a correspondingly higher air
mass flow rate i.e. higher charging pressure. The initial
approximation of the increase in the air mass flow rate
required across the entire operating range of the diesel
engine is equal to the realizable increase in
performance and intermediate pressure. An important
demand placed on the charging system is therefore to
make a constant high pressure level available across
the widest possible range of engine speeds. High
charging pressure at low engine speeds permits high
intermediate pressures (Steady-state) to be attained
relatively quickly, which in turn contribute to the
amount of surplus torque available and therefore the
acceleration response of the engine.
The airflow required for the rated output
range can only be produced by charging device of a
corresponding large design. The design for a high
charging pressure even at low engine speeds means
that the compressor and turbine designs need to be
relatively smaller. An ideal yet impossible to realize
reduction to this conflict is charging device for the
compressor and turbine that is continuously variable
on the housing and rotor sides. A real possibility that
is already the state of the art is the well known method
of designing the charging device with variable
elements. Another significant advance will be made
possible by corresponding optimized multistage
charging systems. Selected multistage systems with
different performance features will be prepared in
addition to single stage units to fulfill the future
demands on diesel engines for the passenger cars.
II. MODERN TURBOCHARGER
DESIGN FOR PASSENGER CAR
DIESEL ENGINE
2.1 Single stage charging systems.
In general, modern diesel engines place
particularly high demands on the charging system in
terms of the air flow required at various engine
operating conditions. The map width of flow
compressor however becomes increasingly smaller as
the pressure ratio increases so that the problem of
having enough useable map width arises, especially for
high performance turbocharged engines. In the single
stage charging method, the selection of the compressor
must therefore be based on the torque or rated output
of the engine. In the former case, the selected
compressor can only deliver a high rated output over
limited range, which in the latter case there will not be
RESEARCH ARTICLE OPEN ACCESS

enough steady state torque available at low engine
speeds. The limited useable compressor map width
therefore represents an important and decisive
restriction for an improvement across the entire
intermediate pressure curve. This means that an
entirely vertical shift of the intermediate pressure
curve of the diesel engine to higher values is not
possible with these simple units.
Figure No 1: Single stage charging system
2.2 Variable turbine geometry charging system
In single stage charging units, turbochargers
with variable turbine geometry are primarily used for
passenger car diesel engines in addition to modern
and economical turbochargers with boost pressure
control valves. The waste gate turbocharger was
developed especially for charging small combustion
engines and designed to meet the requirements of
variable turbine geometries. The turbochargers in this
type can also be equipped with a boost pressure
control valve, depending on the requirements of
performance and emissions. Fig.2 shows a
turbocharger with rotary blades (VTG) known for its
use in the production of passenger car diesel engines.
Figure 2: Turbocharger with Variable Blades (VTG)
An interesting alternative to the VTG with
rotary blades, especially for small displacement
passenger car diesel engines, is the VST turbocharger
with variable slider ring turbine. In this design there
is a bypass valve integrated into the turbine housing
for the upper operating range of the engine. At low
engine speeds only the left duct of the double-flow
turbine housing conducts exhaust gas. The efficiency
of single flow, unregulated housing is reached in this
manner. As the exhaust gas mass flow increases, the
right channel is constantly actuated by a regulating
valve that moves axially. At the highest range of
engines speeds, one control edge of the regulating
valve opens a bypass from the right channel of the
turbine housing to the housing outlet. The yoke of a
control rod converts rotational motion introduced
from outside of the housing to the axial motion of the
regulating valve. The control rod can be controlled by

a pneumatic control box supplied with pressure. To
provide electrical motor drive power to the
turbocharger a suitable electric motor is integrated
into the turbocharger shaft, for example between the
turbine wheel and compressor impeller shown in
figure 4. This integrated system results in a noticeably
improved transient response at operating points where
there is not much exhaust gas available in spite of the
increase in the mass moment of inertia of the rotating
blades. The potential for improvement in the transient
operating characteristics depends primarily on the
amount of electric power available and the electrical
infrastructure of the vehicle. There are limits however
to this approach since improvements in the steady-
state torque curve can only be achieved within the
given compressor map limits due to the single stage
processing of the system. The potential for
improvement with this concept is therefore limited
when compared to other concepts.
2.3 Electrically driven flow compressors in
combination with turbochargers
This charging system was specially developed to
improve the transient response at low engine speeds
and that, in addition to other goals, makes a
significant contribution to the development of the
future small displacement engines with a transient
torque response that approaches that of large
displacement, naturally aspirated engines. Fig. 6 & 7
show the basic design. The e-Booster can be placed
before or after the turbocharger, although placing it
before the turbocharger compressor shown in figure 6
provides more flexibility in terms of the mounting
position, while placing it after the exhaust
turbocharger compressor shown in figure 7 allows for
shorter cable lengths.
Figure 6: Basic Schematic of e-Booster before the Exhaust Turbocharger

The concept is based on a regulated, two
compressions in which an electrically driven
compressor (e-Booster) is connected in series with
an exhaust turbocharger. At operating points where
there is little exhaust gas available, the two stage
compression reaches a higher overall charging
pressure level faster.
Significant advantages result from the
separation of the turbocharger and electrically
driven charger when compared to other approaches.
Thanks to its electric drive, the e-Booster is
completely independent from the turbocharger and
the thermal energy of the exhaust gases. The
electrical system of the vehicle is the only
component that determines the maximum amount of
energy available. The advantages that can be gained
using the e-Booster system in terms of the transient
response are only limited by the amount of
electrical power made available. An increase in the
steady-state torque at low engine speeds is possible,
in contrast to the electrically driven turbocharger, if
the electrical power required can be made available
permanently by the vehicle electrical system. While
the e-Booster and turbocharger combination supply
the necessary charging pressure at the engine
speeds below 2000 rpm, the turbocharger alone
supplies the charging pressure at the engine speeds
above this value.
Since two centrifugal compressors are
combined in this system, their maps can be
combined, which then substantially increases the
total map width. The separation of the turbocharger
and electric motor results in a significant
improvement in the transient response since only
the compressor impeller and the rotor need to be
accelerated with the e-Booster, and a turbine wheel
with a comparatively high density and therefore a
high mass inertia does not have to be accelerated.
Numerical simulations show that the power
consumption of the e-Booster system is about 30%
lower than that if the electrically driven
turbocharger. The e-Booster can also be mounted at
location where it is not subject to a thermal load
from the turbocharger turbine or a mechanical load
from the operation of the combustion engine.
2.4 Two-stage regulated Turbocharging
The main advantage of two stage charging
systems over the single stage units is that two
different sized compressors can be connected in
series, parallel & mixed way so that an optimized
map is available for each airflow rate range. This then
circumvents the restriction of the limited useable
compressor map.
The two stage regulated charger consists of
two exhaust turbochargers of different sizes
connected in series shown in figure 9 .The advantage
of this type of charging system over the single stage
version is the increase in the rated output
while simultaneously improving the steady state
torque at low engine speeds and the acceleration
response of the passenger car diesel engine by
quickly building up charging pressure.
The entire fresh air flow is compressed first
by the low pressure stage in this design. In the high
pressure stage, the charging air is compressed further
and then cooled. Due to the pre-compression process,
the relatively small high pressure compressor can
reach a high pressure level so that it can force the
required amount of air flow through the system.

Structural design of a two stag regulated
charging system on a passenger car diesel engine is
shown in figure 10.
Figure 10: Two-stage regulated charging system
With the use of a bypass on the exhaust side, for
example a waste gate valve (German Abbreviation
is LRK), it is possible to expand the entire exhaust
mass flow using the high pressure turbine or to
redirect some of the mass flow to the low pressure
turbine located downstream.
At low engine speeds, meaning when the
exhaust mass flow rate is low, the bypass remains
closed and the entire exhaust mass flow is fully
expanded by the small high pressure turbine. This
yields a high charging pressure that is built up
quickly. As the exhaust gas mass flow increases, the
work of expanding the mass flow is constantly
transferred to the low pressure turbine by
increasingly opening the bypass. Two stage
regulated charging therefore allows for continues,
variable adaptation of the turbine and compressor
to the actual requirements of the operating engine.
Fig-11 shows a comparison of the measured
intermediate pressure curve of the passenger car
diesel engine with two stage regulated charging to
that of passenger car diesel engine charged by a
turbocharger with the variable geometry
turbocharger (VTG) with the same rated output.
The potential of downsizing with two stage
regulated charging can be seen in figure 11.The
plot of a standardized torque curve of two
passenger car diesel engines with VTG
turbochargers is compared to that with two stage
regulated charging .

III. EVALUATING THE CHARGING
SYSTEM FOR PERFORMANCE
An evaluation scheme for the various
charging concepts can be created from the
thermodynamic requirements. The requirements result
primarily from the steady state response and the
transient response of the turbocharged passenger car
diesel engine.
With respect to the steady state response, the
higher output density of the engine targeted requires a
parallel vertical shift of the entire power
curve/intermediate pressure curve to higher values.
Consequently, an important evaluation criterion is
whether or not a charging system can shift the entire
intermediate pressure curve upwards and if so, how
far it can be shifted upwards.
There are some other important parameters
from transient operations to be used in the
comparison. These parameters are the amount of
startup (low speed) torque the engine supplies at 1000
rpm, the acceleration performance from 0-100 km/h
and the acceleration performance when in the upper
gears (acceleration from 60-100 km/h and from 80-
120 km/h).
3.1 Startup torque at an engine speed of 1000 rpm
There is no increase in the startup torque to
be expected when a VTG or VST exhaust
turbocharger is used when compared to the basic
version, but a satisfactory potential is offered by two
stage regulated charging when the high pressure
turbine is designed accordingly.
The electrically driven designs have significant
advantages over charging system driven only by
exhaust gas in terms of a high start up torque. The eu-
ATL permits an improvement within the limits of its
compressor map. The greatest potential for
improvement can be seen in the e-Booster concept.
Realize charging pressure depends mainly on the
amount of electrical power supplied.
3.2 Acceleration from 0 to 100 km/h
The perforrmance of a vehcile accelerating
from 0 to 100 km/h primarily depends on the rated
output of the motor, which in turn depends on the
amount of air availble in the motor and therefore on
the map of the turbocharger compressor.
Assuming the compressor impellers are the same in
the VTG/VST exhaust turbocharger, in the eu-ATL
and in the exhaust turbocharger of the e-Booster
system, the same rated motor output is obtained in an
initial approximation for all engines charged in this
manner. This means that all of the designs stated also
have about the same potential in terms of the
acceleration from 0 to 100 km/h. However, it must be
kept in mind that the current design of the e-Booster
unit is only active at engine speeds below about 2000
rpm. Since the limitation of a single compressor map
is not present in two-stage regulated charging, this
charging system offers the greatest potential for
increasing the rated output and therefore for
improving the acceleration from 0 to 100 km/h.
3.3 Acceleration from 60 to 100 km/h and from 80
to 120 km/h
A very important criterion used to evaluate
an engine is its elasticity, which is usually assessed
using the acceleration performance from 60 to 100
km/h and or 80 to 120 km/h. In this case,
turbochargers with variable turbine geometries (VTG
or VST) achieve perfectly satisfactory improvements
in comparison to the turbocharger with the waste gate
(LRK). However, a substantially greater potential for
increasing the elasticity driven systems and two-stage
regulated charging, although the advantage of the

electric system increases as the amount of readily
available extra power from the vehicle electrical
system increases.
IV. SUMMARY AND CONCLUSION
The thermodynamic demands placed on
modern charging systems for diesel engines in
passenger cars result mainly from the criteria relating
to the steady state and transient characteristics of the
charged engine. The increasing demands placed in the
future on exhaust emission values must also be met,
even when charged. A higher output and torque
density initially leads to the demand to shift the entire
power curve/intermediate pressure curve of the
charged passenger car diesel engine to higher values.
Higher rated output and higher effective intermediate
pressure require the injection of a large amount of
fuel into the cylinder and a corresponding high air
mass flow rate, which is achieved through a higher
charging pressure. An important demand placed on
future charging systems is therefore to make a high
pressure level available continuously across the
widest possible range of engine speeds. A higher air
flow for the rated output range always requires a
correspondingly large turbocharger in order to handle
high mass flow rates at high efficiency levels. Desire
for a high charging pressure even at low engine
speeds means that the compressor and turbine designs
need to be relatively small. In order to fulfill both
these contradictory demands, turbo suppliers are
refining the designs of existing charging concepts
with variable turbine geometries with the goal of
designing a single stage unit that operates at the limits
of what is technically possible.
Other leading modern charging concepts for
passenger car diesel engines result in systems that are
too complex according to the current state of art.
Expansion is possible with two stage systems (i.e. an
e-Booster in combination with an exhaust
turbocharger and two stage regulated charging, the
known limits of single stage turbochargers, which
results from a compressor map from just one single
compressor). In connection with the increase in the
pressure ratio, these systems provide important
characteristics that can be used to meet the high
demands placed on the future diesel engines.
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Technologies to Meet Critical performance
Demands of Ultra Low Emissions Diesel
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