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Renewable and Sustainable Energy Reviews
journal homepage:
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Variable Geometry Turbocharger Technologies for Exhaust Energy
Recovery and Boosting‐A Review
Adam J. Feneley
a
, Apostolos Pesiridis
a,
⁎
, Amin Mahmoudzadeh Andwari
a,b
a
Centre for Advanced Powertrain and Fuels Research (CAPF), Department of Mechanical, Aerospace and Civil Engineering, Brunel University London, UB8
3PH, UK
b
Vehicle, Fuel and Environment Research Institute, School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran
ARTICLE INFO
Keywords:
Turbocharging
Variable geometry turbine
Variable geometry compressor
Variable nozzle turbine
Variable geometry turbocharger
Automotive turbocharging
ABSTRACT
As emissions regulations become increasingly demanding, higher power density engine (downsized/down-
speeded and increasingly right-sized) requirements are driving the development of turbocharging systems.
Variable geometry turbocharging (VGT) at its most basic level is the first step up from standard fixed geometry
turbocharger systems. Currently, VGTs offer significant alternative options or complementarity vis-à-vis more
advanced turbocharging options. This review details the range of prominent variable geometry technologies that
are commercially available or openly under development, for both turbines and compressors and discusses the
relative merits of each. Along with prominent diesel-engine boosting systems, attention is given to the control
schemes employed and the actuation systems required to operate variable geometry devices, and the specific
challenges associated with turbines designed for gasoline engines.
1. Introduction
In response to increasing emissions regulations, engine manufac-
turers around the world have adopted a wide array of turbocharging
technologies in order to maintain performance when downsizing their
engines. Variable geometry turbocharging represents a large portion of
the technology present in today’s vehicles. VGT technology (also known
as VNT-Variable Nozzle Turbocharger) is employed in a huge range of
applications, such as in commercial on- and o ff-highway, passenger,
marine and rail internal combustion engine applications. Aside from
the emissions and engine downsizing components, other key develop-
mental drivers include increased transient response, improved torque
characteristics, over-boosting prevention and better fuel economy.
Turbocharger growth has been substantial in the last two decades
and has experienced particular growth in areas where naturally-
aspirated engine domination was until recently, still viable (USA and
China in particular). Substantial growth figures are posted in recent
years with a significant proportion of the realized as well projected
market share being taken up by VGTs. VGTs are predicted to account
for 63.3% of the global turbocharging market by volume by the year
2020. In the Asia/Oceania region, the adoption of VGTs is growing
rapidly, and is projected to grow at a high compound annual growth
rate of 14.61% from 2015 to 2020, when calculated by volume
[1].
VGTs are therefore important not only due to the market share and
value that they represent in standalone, single stage boosting terms but
increasingly as cost-effective boosting devices compared to more recent
and advanced technologies such as electric turbocharging and super-
charging. In addition, and for the same cost-effectiveness reasons they
are being increasingly encountered, as part of advanced, multi-stage
(two- and three-stage) architectures.
In addition, the other part of the Variable Geometry (VG) equation,
the compressor has seen little implementation but is also of significant
interest especially in view of the persistent requirement for maximized
boost per stage. In addition, the compressor is being asked to operate
across an increasingly expanding operating envelope and this is seen as
a potential enabler for advanced engine cycle (Miller/Atkinson for
example).
The objective of this paper is to present the first complete review of
variable geometry technologies that are available commercially, as well
as those currently under development and to highlight the merits of the
increasing more complex options now available to powertrain devel-
opers where VG turbochargers are encountered as components of a
more complex boosting architecture. The operating principles of
variable geometry are covered, initially, followed by details of the
range of different VG systems for both the turbine and compressor. A
summary of current control systems and strategies, actuation methods
and VG efforts specific to the gasoline engine are covered before
concluding with a discussion on future trends for variable geometry
http://dx.doi.org/10.1016/j.rser.2016.12.125
Received 8 September 2015; Received in revised form 19 October 2016; Accepted 26 December 2016
⁎
Corresponding author.
E-mail address:
apostolos.pesiridis@brunel.ac.uk (A. Pesiridis).
Renewable and Sustainable Energy Reviews 71 (2017) 959–975
Available online 29 December 2016
1364-0321/ Crown Copyright © 2016 Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
MARK
turbochargers development and implementation.
2. Turbocharger systems
The modern day turbocharger market is diverse, as manufacturers
strive to provide the improved technologies to lower exhaust emissions.
There are numerous technology variants available on the commercial
market, as well as under development. The most basic technology is the
conventional, fixed geometry turbocharger, which consists of turbine
and compressor wheels connected by a common shaft. Electrically
assisted turbocharging systems use electrical machines in motoring
mode to impart additional power onto the common shaft during low
load operation to improve upon the performance of the fixed geometry
variant. VG devices are employ different designs and/or are employed
in different ways to alter the cross sectional area of the housing or inlet
which guides the exhaust gas into the turbine rotor; these devices can
also be coupled with di ffusers to effect variable geometry for the
compressor
[2].
Even though not directly linked to boosting (but only to energy
recovery) one additional system that can be included here is turbo-
compounding. This is a waste-heat energy recovery technology using an
additional power turbine to recover energy in two forms: mechanical or
electrical. In electrical turbo-compounding, the energy is transferred as
electrical power and transmitted to the engine or to vehicle auxiliaries
through the battery; the mechanical variant feeds kinetic energy back
into the engine using a high ratio transmission.
Sequential turbocharging is an additional option that involves using
two (typically) or more turbochargers of different sizes operating
entirely or partially in sequence. A small turbocharger is used at low
speeds due to its low rotating inertia, and a second larger turbocharger
is used at higher engine speeds, usually with an intermediate stage
where both may be in operation. Despite clear weight, cost and thermal
inertia disadvantages this technology is becoming increasingly impor-
tant in meeting the increased power density demand from engines of
Nomenclature
AFR Air to Fuel Ratio
ANNs Artificial Neural Networks
AR Aspect Ratio
BSFC Break Specific Fuel Consumption
CFD Computational Fluid Dynamics
CI Compression Ignition
CTT Cummins Turbo Technologies
EAT Electrically Assisted Turbocharger
ECU Engine Control Unit
EGR Exhaust Gas Recirculation
FEA Finite Element Analysis
FGT Fixed Geometry Turbocharger
HTT Honeywell Turbo Technologies
MAS Multi-Agent Systems
MHI Mitsubishi Heavy Industries
MVEM Mean-Value Engine Models
NA Naturally Aspirated
NOx Mono-Nitrogen Oxides
PID Proportional-Integral-Derivative
PWM Pulse Width Modulation
SI Spark Ignition
VFT Variable Flow Turbocharger
VGT Variable Geometry
VGT Variable Geometry Turbocharger
VST Variable Sliding Ring Turbocharger
VNT Variable Nozzle Turbocharger
VVT Variable Volute Turbocharge
Variables
A Area
ṁ Mass flow rate
M Mach number
T Temperature
p Pressure
γ ratio of specific heats
Subscript notation
* Critical value
in Inlet
Fig. 1. A presentation of the major contribution to the system delay during transient response of a turbocharged engine [4].
A.J. Feneley et al.
Renewable and Sustainable Energy Reviews 71 (2017) 959–975
960
the future.
3. Limitations of fixed geometry turbochargers
Downsizing engines may mean lighter, smaller and more compact
powertrains, but there are limitations for turbocharging in these cases.
To date, turbocharging has been far more commonly used in compres-
sion ignition engines (CI). Spark ignition (SI) engines are difficult to
match with turbochargers due to the wider speed range and need to
carefully control ignition timing to avoid knock. SI engines often
operate at reduced compression ratios in order to prevent pre-ignition
and limit knock; this makes fuel efficiency savings harder to achieve
using a turbocharging. CI engines also face difficulties in matching
turbochargers and engines, particularly for transient response
[3,4].
The most widely recognised problem with fixed geometry devices is
turbocharger lag;
[5] the poor transient response of the turbocharger at
low engine loads.
Fig. 1 shows the major contributors to turbocharger
lag for a SI engine. The biggest contributor is the rotating inertia of the
turbine; this is due to the airflow not being sufficient to spool up the
turbine rotor to higher speeds, a problem that is directly addressed by
variable geometry systems. Analysis of Newton’s second law of motion
for rotational systems suggests reducing the rotor size and mass will
reduce turbocharger lag
[4].
In addition to the rotor size, another important parameter of
turbocharger design that affects turbocharger lag and over-boosting
is the aspect ratio (AR). This is the ratio of cross sectional area of the
volute divided by the distance from the centre of this cross sectional
area to the geometric centre of the volute. A small AR means that the
velocity of the exhaust gas is increased and, therefore, a greater kinetic
energy is available to the turbine rotor. Variable geometry devices in
essence manipulate the AR value by altering the cross sectional area of
the volute in order to increase air velocity at low engine speeds
[6].
Fig. 2 shows a typical curve of turbine pressure ratio versus mass
flow; the ideal relationship between these variables would be linear, but
this is not possible with a fixed geometry turbocharger (fixed AR). To
achieve a more linear relationship the cross sectional area of the
turbine can be altered with a VGT for different load conditions. In
summary, fixed geometry turbochargers are optimised with a fixed AR
for a specific engine condition; for other engine conditions the system’s
efficiency is limited. VGT technology allows the performance of the
turbocharger to be optimised across the whole engine range.
4. Operating principles of VGTs
VGT devices are designed to increase boost pressure at low speeds,
reduce response times, increase available torque, decrease the boost at
high engine speeds to prevent over-boosting, reduce engine emissions,
improve fuel economy and increase the overall turbocharger operating
range [7,8].
There are a number of different mechanical systems that are used to
manipulate the AR value, and these are discussed in
Sections 5 and 6 of
this review. All technologies however share the common goal of using a
nozzle-like system, or other movable components, to provide a variable
cross sectional area. At low engine speeds the basic principle of most
turbine systems is to narrow the inlet area to the rotor (reduced AR)
such that air velocity is increased. Conversely, the passage is opened at
higher loads. These positions are controlled by the ECU (Engine
Control Unit) which is programmed to alter the nozzle geometry to
achieve optimal performance at any given engine condition
[9].In
simple terms, VGT systems (with the exception of a variable outlet
turbine) have the ability to adjust flow conditions upstream of the
turbine without altering the moment of inertia
[4,10]. Early studies
such as those by Lundstrom and Gall [11] highlighted the significant
differences between early variable geometry devices and fixed geometry
alternatives, particularly with regards to improved acceleration and
response times.
The performance of a turbocharger is commonly described by non-
dimensional mass flow rate and speed, which can be plotted against
expansion ratio in the case of the turbine. The flow range of a radial
flow turbine (
mTp
/
) is limited at high pressure ratios by the choking
of flow. The minimum area possible (A*) for the nozzle section of the
turbine can be defined (assuming an isentropic process with a perfect
gas) as shown in Eq.
(1) [2].
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
A
AM
γM
γ
*
=
1
1+ ( −1)*
(+1)
*
in
γ
γ
1
2
2
1
2
(0.5) ( +1)
−1
(1)
The area of the nozzle throat is a limiting factor in the performance
of a turbocharger; many variable geometry turbocharger concepts
allows for the alteration of this area. The e ffective area depends on
the height of the passage (which can be altered in a sliding vane
system) and the angle of the vanes (which can be altered in a pivoting
vane system). In a vaneless system, the effective area depends on the
exducer area and gas angle, this can be manipulated by changing the
cross sectional area of the scroll.
Fig. 3 shows the effect of a VGT in comparison to a fixed geometry
device during acceleration in second gear of a 6-cylinder, 11 L turbo-
diesel engine. The solid lines on the graphs indicate a steeper curve in
all three cases; VGT offers improved turbocharger rotational speed,
engine speed and boost pressure than a regular turbocharger. It can
also been seen at around 3 s that the nozzle is opened to reduce boost
pressure and therefore prevent over-boosting; a wastegate is not
needed and therefore there is no associated throttling loss.
The peak efficiency of a VGT is often lower than a FGT equivalent,
partially due to leakage in the turbine casing and around the mountings
of moving components
[10,12]. The peak efficiency drops significantly
when the nozzle is moved from its optimal position, refer to
Fig. 4.
Despite this the overall efficiency of a VGT is greater than that of a FGT
due to the larger operating range
[13].
5. Variable geometry systems for turbines
There are two main types of turbine design available on the market:
radial and axial turbines. In a radial turbine, the exhaust gas enters the
rotor perpendicular to the rotor blades (radially), and is redirected 90°
by the rotor before exiting the housing in the axial direction. Axial
variants work in the opposite manner, with exhaust gases entering the
rotor axially and exiting in the radial direction. In an axial turbine the
gas flow enters the turbine at zero angle, which minimises mechanical
stress on the blades.
An example of an axial turbine for automotive use in the Honeywell
Turbo Technologies (HTT) DualBoost™, this utilises zero-reaction
aerodynamics, no nozzles and tall-bladed design to achieve a high-
speed axial turbocharger. Using this technology HTT were able to
reduce the mass of the turbine wheel, therefore reducing inertia by up
to 40%. [15] In addition, axial turbines hold the advantage of better
Fig. 2. Typical pressure ratio vs. mass flow curve for a FGT [4].
A.J. Feneley et al.
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961
efficiency at lower blade speed ratios than radial equivalents. This
DualBoost™ turbocharger was tested against a conventional radial
device.
[16] Results showed that both were capable of achieving the
target full load steady state torque and power. However the
Dualboost™ device responded much faster to increasing engine load,
reaching maximum torque at just 1200 rpm, the radial device didn’t
peak until 5000 rpm, and failed to reach the torque level of the
Dualboost™ turbocharger. The results were replicated in both steady
state and transient tests, with the Dualboost™ curves steeper in all
instances.
Fig. 5a and b shows a comparison of radial and axial types from a
study by K.H. Bauer et al.
[16] for HTT. Fig. 5a indicates the efficiency
curves for both rotor types, with axial devices excelling at lower
normalised blade speeds and radial peaking higher in terms of
efficiency and speed.
Fig. 5b shows the reduced inertia of axial devices
when compared with radial counterparts.
Early attempts to compare different methods of variable area
devices for turbines, such as that by Flaxington and Szczupak,
[17]
concluded that not one VG method existed that was superior for all
applications. However, the authors did note that VG methods in
general did improve engine torque, widen the speed range and improve
the transient response.
5.1. Sliding nozzle
A common method of variable geometry in radial turbines is the use
of a sliding vane ring. This simple and robust method is most
commonly found in the turbochargers of trucks and buses due to its
suitability to larger engines. The sliding nozzle method allows for
higher boost at lower engine speeds, and is the best fuel-efficient means
of driving EGR (Exhaust Gas Recirculation).
Sliding nozzle devices comprise of a series of vanes that are rigidly
mounted on a ring, which is positioned around the rotor, as shown in
Fig. 6. The purpose of the vanes is to direct the radial flow onto the
rotor, and the sliding mechanism is used to narrow, or widen, the
passage for the exhaust gas flow to suit the engine conditions. Since the
vane ring slides axially into the flow, packaging is relatively compact. A
minimal number of wear sites equates to improved durability.
Franklin
[19] documented the development of Holset’s VGT
system, highlighting the benefits of the robust sliding vane technology
at its conception. Other attempts have been made at having multiple
sets of sliding vanes at different angles; a design from the Nippon
Institute of Technology [20] used two sets of vanes, one with a hollow
space to accommodate the other. This meant a smaller vane with a
different angle setting could be used at higher speeds. At low speeds a
larger second vane would slide out (with a hollow space to accom-
modate the initial high speed vane) to provide a greater nozzle effect.
5.2. Pivoting vanes
Similarly to sliding vane devices, pivoting vane turbochargers have
a ring of vanes mounted on a flat plate. In this case however the vanes
Fig. 3. Comparison of FGT and VGT [4].
Fig. 4. Turbine pressure ratio, mass flow and efficiency for different nozzle positions
[14].
Fig. 5. a. Comparison of radial and axial turbine efficiency (a) and inertia (b), red
indicates axial and black indicates radial
[16]. b. Comparison of radial and axial turbine
efficiency (a) and inertia (b), red indicates axial and black indicates radial
[16]. (For
interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
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are mounted on pins that allow them to rotate axially. These vanes
remain permanently in the gas flow with no sliding motion to narrow
the flow passage. The nozzle effect here is provided by the rotation of
the vanes; they can be opened and closed to allow varying amounts of
air onto the rotor (refer to
Fig. 7). Vanes are closed during low engine
loads to accelerate the airflow. As the engine revolutions increase, the
vanes open to prevent choke. The pivoting vane system has a higher
overall efficiency than sliding vane variants
[21].
Axially moving vanes are a well-established technology, with much
of the performance development already undertaken in previous
decades, such as the study from Shao et al
[24].
Like sliding vane, pivoting vane mechanisms and exhaust gas
recirculation (EGR) systems are a good match. The pivoting vanes
provide the improved flow conditions needed for successful EGR. By
pumping some exhaust gases back into the cylinder NOx emissions are
reduced owing to a smaller proportion of O2. High-pressure EGR
systems
[25] are most common for turbines, whereby exhaust gas is
drawn from upstream of the turbocharger. In VGT devices, the aspect
ratio will determine the EGR flow, since it governs the pressure
difference between the inlet manifold and exhaust manifold
[4]. EGR
is more commonly found on turbocharged diesel engines than petrol
variants, since the exhaust gas temperatures are significantly lower;
around 850 °C for diesel engines and 1000 °C for petrol engines
[12,26].
Whilst the pivoting vane system is the most common for VGT
devices, it is not without its drawbacks. Durability problems exist,
particularly in higher temperature applications such as gasoline
engines. At elevated temperatures metal-to-metal friction becomes a
problem, which can cause the pivoting mechanism to stick. This will
drastically reduce performance, and if over-speed occurs can lead to
turbine failure.
Mitsubishi Heavy Industries (MHI) conducted research into the
design of VGT vanes for their own turbochargers, designed for diesel
engines
[12]. Along with the shape of the vanes themselves, the issue
Fig. 6. Cross section view of a sliding ring turbine mechanism [18].
Fig. 7. Pivoting vane turbocharger in fully closed (upper) and fully open (lower) positions [22,23].
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