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INVESTIGATION OF THE POTENTIAL OF
GAS TURBINES FOR VEHICULAR APPLICATIONS
Henrique Cunha
Department of Mechanical Engineering – IDMEC, Instituto Superior Técnico,
Technical University of Portugal (TULisbon), Av. Rovisco Pais, 1049-001 Lisboa, Portugal;
e-mail: [email protected]
ABSTRACT
Nowadays, the reduction of fuel consumption and
pollutant emissions has become a top priority for society and
economy. In the past decades, some of the environmental
advantages of the gas turbine have led some car
manufacturers to evaluate the potential of this type of engine
as prime mover. This paper suggests a strategy to assess the
fuel consumption of gas turbines applied in road vehicles.
Based on a quasistatic approach, a model was crated that
can simulate road vehicles powered by gas turbines, and
thereafter a comparison was established with reciprocating
engines. The system developed also allows the simulation of
hybrid configurations using gas turbines as range extenders,
a solution that some car manufacturers consider to be the
most promising in the coming years.
Keywords: Gas Turbine, Road Vehicle, Fuel Consumption,
Computational Simulation
NOMENCLATURE
CO2 Carbon Dioxide
CVT Continuously Variable Transmission
ECE Urban Driving Cycle (European)
EUDC Extra-urban Driving Cycle (European)
HC Hydrocarbon
IC Internal Combustion
NEDC New European Driving Cycle
NOx Nitrogen Oxide
OOL Optimal Operation Line
SUV Sport Utility Vehicle
1. INTRODUCTION
The increasing number of passenger cars worldwide and
the increasing rate of global oil consumption, urgently
demands for the development of more efficient power
generation systems for the transportation sector. For the first
time ever, the number of vehicles in operation worldwide
surpassed the 1 billion-unit mark in 2010 [1], a number that is
expected to be 2.5 billion by 2050. This trend will further
increase the pressure on fuel prices and cause serious
problems to the environment. For these and more reasons, the
reduction of fuel consumption and pollutant emissions has
become a top priority for society and economy.
In the past decades, some of the environmental advantages of
the gas turbine, such as low concentration of hydrocarbon and
carbon monoxide emissions, have led some car manufactures
to evaluate the potential of this type of engine as prime
mover.
The development of new powertrain systems involves high
costs. However, these costs can be mitigated by utilizing,
during design, simulation tools that can predict the
performance of the vehicle and its on-board subsystems under
a variety of driving conditions.
The main purpose of this paper is focused on the assessment
of the potential of gas turbine engines for vehicular
applications, in terms of fuel economy. A review of relevant
literature has been performed, regarding different gas turbine
configurations, the past efforts with respect to this type of
engine applied in road vehicles, as well as the simulation
approaches used to predict a vehicle’s fuel consumption. The
methodology adopted in the simulation of both gas turbine
and vehicle models are also described, the results from the
simulations are reported, the conclusions are discussed, and
some future work is proposed.
Previous research has shown the utility of simulation tools in
the prediction of fuel consumption of conventional vehicles
powered by reciprocating engine [2]. This study pretends to
allow this same assessment but for gas turbine engines (in
multiple configurations), operating as prime movers in road
vehicles, in a wide range of simulation conditions.
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2. LITERATURE REVIEW
2.1 Gas Turbine Engine
Since the successful introduction as power plants for
airplanes during the Second World War, gas turbines have
had an immeasurable impact upon society. The favourable
power-output-to-weight ratio makes this type of engine
suitable for several applications, ranging from electric power
generation, mechanical drive systems and jet propulsion.
2.1.1 Single and Twin-Shaft Configurations Gas turbines can be arranged either in single-shaft or
twin-shaft configurations. The single-shaft arrangement
requires the turbine to provide power to drive both the
compressor and the load, which means that the compressor is
influenced by the load, as illustrated in fig. 1. By its turn, in a
twin-shaft configuration the sole function of the first-stage
turbine is to drive the compressor at steady speed without
being influenced by the load, while the net power of the gas
turbine is produced by a free turbine, as it is shown in fig. 2.
2.1.2 Road Vehicle Applications Since road vehicle engines operate at continuously
variable loads and speeds, the twin-shaft configuration, due to
its operating mode, seems to be the most suitable gas turbine
arrangement for this type of application. In fact, considering
its characteristics, a twin-shaft configuration offers excellent
torque at low engine output speeds, which better matches the
vehicle requirements, especially for hill climb and
acceleration conditions. Contrarily to the twin-shaft gas
turbine, the reciprocating engine (the most common
powertrain used for vehicular purposes) has a torque profile
that builds up to a maximum in the mid-speed range and then
declines.
A comparison between the torque-speed profiles of three
different types of engine - a single-shaft gas turbine, a twin-
shaft gas turbine and a reciprocating engine, also called
piston engine - is shown in fig. 3.
2.1.3 Gas Turbine Advantages Beyond its better torque capability (twin-shaft
configuration), the main benefits of the gas turbine,
considering vehicular applications, are its light weight,
compactness and reliability. Its high reliability results from
the reduced number of moving parts (approximately 80% less
than a piston engine), which means a vibration free engine
operation with few balancing problems (absence of
reciprocating and rubbing), low lubricating oil consumption
and lower on-going costs due to the reduced maintenance.
Another advantage of a gas turbine engine, over the spark and
compression-ignition engines, is its capability of operating
satisfactorily with a variety of fuels, including kerosene,
diesel fuel, natural gas, hydrogen and others [4].
2.1.4 Gas Turbine Disadvantages The major disadvantage of the gas turbine is its
extremely low efficiency under no load or partial load, which
constitutes a considerable fraction of the actual operation in
an automotive application. Even at idle conditions, turbines
turn at very high rotational speeds (around 20000 rpm) and
thus consume considerable amounts of fuel. Another
drawback that can be pointed out is the relatively long time
that gas turbine spools require for accelerating from idle to
full load. This slow throttle response, usually called
acceleration lag, is even more pronounced in a twin-shaft
configuration, where the acceleration time depends upon the
free turbine inertia, as well as the inertia and loading
characteristics of the driven equipment [3]. A final problem of
employing gas turbines as automotive power plants is their
high production costs. An estimate from the 70's reveals that
a production volume of 400 gas turbines per day would cost
two and a half times more than an equivalent piston engine
production [5]. Ingersoll [6] points out a cost of $125 per hp
for a mass production of automotive gas turbines, a cost that
is still five or six times higher than the cost of a good spark
ignition engine.
Figure 3. Torque vs. Output Speed (based on data from [3])
Figure 1. Single-Shaft Gas Turbine
Figure 2. Twin-Shaft Gas Turbine
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2.1.5 Previous Projects In the late 1960's, research scientists in California
managed to relate the smog in the air of certain U.S. cities
with some chemical compounds found in automotive exhaust
gases (NOx and HC). This scientific discovery led to strict
national legislation in terms of pollutant emissions, affecting
significantly the automotive industry. Due to the
environmental advantages of the gas turbine, a considerable
amount of work was done in this field with a great support
from the US Government. Chrysler Corporation was the
American car manufacturer that conducted the largest number
of experiences with gas turbine-powered cars [4]. As result,
several prototypes were developed, including the Chrysler
Gas Turbine, as shown in fig. 4.
Interest in gas turbines was also high among truck
manufacturers. The 1965 Chevrolet Turbo Titan III and the
1972 GMC Astro Gas Turbine are some of the many
operational prototypes, aiming at a possible future production
[7]. However, the high manufacturing costs, the continuing
technology refinement of gas turbine engines and the
Government regulations concerning NOx emissions, which
could not be complied, determined the end of these projects.
By the end of the 70’s, almost all gas turbine vehicle
programs were cancelled [8].
The gas turbine also left its mark on the racing track. The first
racing turbine car, the Rover-BRM, participated in 1963 at the
24 hours of Le Mans, achieving a top speed of 235 km/h.
Some years later, this type of engine also made its appearance
in Formula 1 (1971), through the Lotus Team (Lotus 56). In
this car, the gas turbine used was smaller and lighter than a
regular piston engine, the drivetrain was simpler and there
was no need for cooling system; however, the acceleration lag
associated to gas turbines operation determined the end of
this project two years after its first race [9].
During the last two decades, however, the interest in gas
turbines has been revived, with the use of this type of engine
associated with an electric drive system, in a hybrid
configuration. This solution poses as a compromise between
reducing environmental pollution and the limited range
capability of today’s purely electric vehicles. In this
configuration, the gas turbine is only used for maximum
power and for charging the vehicle’s propulsion batteries.
This configuration solves many of the drawbacks previously
attributed to this type of engine, such as the slow throttle
response or the low efficiency under partial load. The most
recent study presented by the automotive industry is the
Jaguar C-X75, as illustrated in fig. 5. This concept car uses a
pair of twin-shaft micro-turbines acting as range extenders, a
solution that Jaguar claims to be even cleaner than the current
hybrids, with just 28g of CO2 emitted per km [10].
2.2 Vehicle Energy and Fuel Consumption
2.2.1 Vehicle Performance Analysis To move a vehicle forward, a propulsive force must be
delivered by the engine. This force is also known as tractive
force (Ft) and it must be such to overcome the resisting
forces, which can be described as the sum of the following:
Aerodynamic Drag Force (Fd)
Rolling Resistance Force (Fr)
Gravitational Force (Fg)
Acceleration Force (Fa)
A schematic representation of these forces is given in fig. 6.
2.2.2 Methods for the Prediction of Fuel
Consumption In order to analyse the efficiency of the propulsion
system for a road vehicle, and consequently its fuel
consumption, there are three possible approaches [11]:
Average Operating Point Approach
Quasistatic Approach
Dynamic Approach
From these three methods, the most used ones are the
Quasistatic and the Dynamic approach. The Average
Operating Point method uses one single representative
average engine operating point (based on a specific test
Figure 4. Chrysler Gas Turbine (1964) [4]
Figure 5. Jaguar C-X75 (2011) [10]
Figure 6. Schematic representation of forces acting on a vehicle in motion
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Figure 9. Diagram of Chrysler Gas Turbine [4]
cycle) in order to assess the mean fuel consumption, a
procedure that makes this a limited model, only suitable for
simple powertrains. The two other methods use a calculation
scheme to generate outputs of the vehicle’s velocity and
energy use, in every time step of a given simulation.
A Quasistatic approach is driven by the required vehicle
velocity, which means that the force required to accelerate the
vehicle through the time step is calculated directly from the
speed trace of the driving cycle. The Quasistatic method may
also be designated as backward-facing approach since the
calculation proceeds backwards, i.e. from the tire/road
interface through the drivetrain, ending with the energy
source (engine and fuel tank), as it is shown in figure 7. Since
this backwards formulation assumes that the vehicle meets
the required speed trace, this method is not suited to analyse
best-effort performance, such as acceleration tests.
In contrast to the Quasistatic approach, the Dynamic
approach begins with a driver model that inputs the throttle
position and braking based on the desired speed. The
computation proceeds forward from the engine, through the
transmission till the wheels, resulting in the calculation of a
tractive force at the tire/road interface, as illustrated in fig. 8.
The versatility of this method, which can also be called
forward-facing approach, makes it desirable in detailed
control simulations and hardware development.
However, the relatively high computational demand
associated to this method turns it into a too much time-
consuming solution for preliminary design. For that reason,
the Quasistatic approach is the most used method for
predicting vehicles fuel consumption and is also the method
utilized within this study [11].
3. METHODOLOGY
3.1 Gas Turbine Modelling
3.1.1 Proposed Models In this paper an attempt is made to assess the
applicability of gas turbines in road vehicles. As a starting
point, an engine model (model 1) was developed based on a
spec-sheet of a gas turbine developed by Chrysler
Corporation for automotive applications, during the 60’s [4].
This model, illustrated in figure 9, uses a regenerator (heat-
exchanger) that recovers much of the heat from the exhaust
gases, enabling the achievement of acceptable exhaust
temperatures and fuel consumption, at least for that period.
To assess the influence of heat-exchangers in the
thermodynamic cycle of a gas turbine when used as an
automotive power plant, a similar gas turbine was also
modelled, but without regenerator (model 2).
Although relevant to this study, the Chrysler gas turbine is an
“antique” model with component efficiencies substantially
lower to what can be achieved with the latest technology. In
order to establish a fair comparison with modern
reciprocating engines, a third engine was modelled
representing the modern gas turbine (model 3). The major
change carried out, compared with the Chrysler model, was
in terms of turbine inlet temperature, the parameter with the
most pronounced effect on the overall gas turbine
performance [12]. The change of the turbine inlet
temperature, at nominal conditions, implies the selection of
overall pressure ratio and the re-calculation of
compressor/turbines efficiency. A parametric study was
performed, in which the variation of the specific fuel
consumption was evaluated in a range with different
compressor pressure ratios.
The thermodynamic cycle of the three different gas turbine
models was assessed using an in-house performance code, for
both design and off-design point operating conditions.
However, due to some limitations associated with the version
of this software it was not possible to assess engine behaviour
in the full range of output speed and power. In order to get
the full performance spectrum, the results generated by the
performance code were post-processed.
3.1.2 Post-Processing Data An algorithm was developed with the purpose of
conceiving engine maps for any gas turbine configurations,
and thus to assess its fuel consumption when operating as a
road vehicle powertrain. The first part of this algorithm
consists of reading the data generated by the performance
code, and then extrapolate (based on several assumptions) in
order to get the output torque and shaft speed values for the
entire spectrum of operating conditions, as shown in fig. 10.
Figure 7. Quasistatic Approach Diagram
Figure 8. Dynamic Approach Diagram
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The second part consists of filling the sparse data matrixes,
through the use of linear interpolations, to enable the
assessment of the fuel consumption necessary to sustain any
torque/speed combination, as illustrated in fig. 11.
3.1.3 Gas Turbine Engine Map In figure 11, the engine map is represented in terms of
fuel mass flow (kg/s). This map is useful for assessing the
engine fuel consumption. Nevertheless it is inappropriate for
comparing fuel efficiency between different engines.
Therefore, the engine performance is represented in terms of
specific fuel consumption (g/kWh). In figure 12, the
efficiency map of gas turbine model 1 is illustrated, in which
it is possible to verify that this type of engine achieves its
highest efficiency at maximum output speed and power. At
lower rotational speeds, the pressure of the compressed air
decreases, resulting in a dramatic drop in thermal efficiency.
On the other hand, reciprocating engines are typically more
efficient at maximum torque.
3.2 Vehicle Modelling The purpose of this study is to attempt the evaluation and
comparison of the performance, in terms of fuel consumption,
of several road vehicles powered with different internal
combustion engines, with a special focus on gas turbines. To
do so, an existing simulation tool was used, known as QSS
[13]. For creating a simulation model that can describe all
aspects of the vehicle significant for estimating fuel
consumption. The QSS toolbox was developed by ETH
(Zurich) and consists of a collection of Simulink blocks and
m-files. This tool permits the design of powertrain systems in
a flexible manner and allows a fast and simple estimation of
the fuel consumption of such systems. There are several
parameters that can be set in these simulation blocks,
concerning the driving pattern, the chassis of the vehicle, its
type of engine, the gear system used or the type of fuel, as
illustrated in fig. 13.
3.2.1 Vehicle Model powered by Gas
Turbine The sub-models described above were used to model a
vehicle powered by a gas turbine, as illustrated in fig. 14.
Figure 14. Simulink model of a vehicle
powered by a gas turbine
Figure 13. QSS toolbox library [13]
Figure 11. Algorithm Diagram – part 2
Figure 10. Algorithm Diagram – part 1
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Driving Cycle
It is of interest to simulate the vehicle under different driving
cycles; different driving conditions imply different demands
and consequently distinct results. Therefore, three default
driving cycles were chosen: an urban, extra-urban and
combined driving. Furthermore, a custom driving cycle was
created in order to analyse the vehicle model at constant
speed conditions. Regarding the default driving cycles, these
were the three European regulated cycles: ECE (urban),
EUDC (extra-urban) and NEDC (combined).
Vehicle Category
With the aim of making a meaningful and relevant study,
three different vehicle categories were chosen: a small, a
medium and a large sized car, with the respective
designations of city car, family car and SUV. Most of the
parameters input, as detailed in table 1, were taken from
spec-sheets of vehicles that are currently in production.
Gas Turbine Engine
One of the main limitations of the QSS toolbox is the lack of
a block simulating a gas turbine engine. Significant changes
were performed in the Combustion Engine block in order to
simulate the gas turbine operation, and thus assess its
performance and fuel consumption. The default fuel
consumption map corresponding to piston engines was
replaced by an executable code that generates the fuel
consumption of a gas turbine; an efficiency map is presented
in figure 12. The developed code is generic in the sense that
it is capable of simulating the fuel consumption of any gas
turbine modelled with a performance code.
It is important to note that there are some important
differences between reciprocating engines and twin-shaft gas
turbines that need to be taken in consideration. For instance,
while a piston engine only has one rotating output shaft, a
twin-shaft gas turbine has two different sets of rotating parts,
the high and low pressure spools. Since there is no
mechanical connection between these two spools, their speed
can be considered independent from each other. This fact
makes the assessment of the inertia force of the engine
difficult. This is an important parameter used in the
determination of the vehicle performance. In this model,
although the inertia forces are exclusively calculated based on
the low pressure spool acceleration, a somehow higher inertia
was used as an input in an attempt to account for the inertial
forces generated when varying high pressure spool speed.
Gear System
The drivetrain is an essential system in the whole vehicle
model, since it is responsible for the transmission of the
mechanical work between the gas turbine and the wheels. The
great influence of this system in the engine operation, and
thus in the vehicle fuel economy, requires special attention to
its modelling. Considering vehicular applications, the ideal
power plant should have a performance characteristic with a
constant output power over the entire vehicle speed range.
This constant power characteristic provides the vehicle with a
high tractive effort at low speeds, as illustrated in figure 15,
i.e. in situations where demands for acceleration or grade
climbing capacity are high [14].
In the case of reciprocating engines, the torque-speed profile
is almost flat compared to an ideal power plant, as previously
illustrated in figure 3. Therefore, a multi-gear transmission is
usually coupled to piston engines in order to convert its
torque characteristic close to the ideal one, as illustrated in
figure 16.
Figure 12. Efficiency Map – g.t. model 1
Table 1. Vehicle Parameters
Figure 15. Ideal performance characteristics for a vehicle traction power plant [14]
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As described in section 2.1.2, a twin-shaft gas turbine offers
excellent torque at low speeds for hill climb and vehicle
acceleration. Since its speed-torque profile is much closer to
the ideal one, twin-shaft gas turbines only require a small
number of gears, as shown in fig. 17.
Rather than a manual gearbox, a Continuously Variable
Transmission (CVT) was chosen for the gas turbine.
Although CVTs typically offer lower efficiencies than manual
gearboxes, their operation depends less on the driving style,
i.e. driver behaviour. A CVT system is mechanically designed
to provide an infinite number of gear ratios, which enables
the engine to operate close to the called Optimal Operation
Points1, resulting in an increased fuel economy. The CVT
operation requires the use of a controller containing an
algorithm that determines the ideal gear ratio depending on
the power demanded [15].
Additional to that, due to the extremely high rotational speeds
of the power turbine, a reduction gear needs to be
implemented in the transmission, in order to reduce the
output shaft speed.
CVT Controller
The Continuous Variable Transmission makes part of a group
of gear systems characterized by an automatic operating
mode. This mode requires the use of a controller that replaces
the driver in the selection of the most adequate gear ratio
during the drive. In this model, a completely new CVT
controller was developed, with the purpose of: i) allowing its
“interface” with the gas turbine data generated by the
performance code and ii) ensuring a more accurate
calculation of the power required to drive the vehicle.
The main function of the controller is to determine the
optimal CVT gear ratio. This procedure starts with the
1 The Optimal Operation Points are the engine torque-speed
combinations in the entire engine power range, with the minimum specific
fuel consumption.
assessment of the power required by the vehicle to perform
the speed trace pre-defined in the driving cycle. This
assessment is based on the resisting forces actuating on the
vehicle in motion (section 2.2.1), the losses associated to the
drivetrain system, and inertia forces due to the vehicle
acceleration. After calculating the power required to drive the
vehicle, the CVT controller determines at which speed the
engine should be operating. This assessment is performed by
a code that defines the Optimal Operation Points of each gas
turbine engine being analysed, based on the correspondent
engine map, as illustrated in fig. 18. The connection of all
these points sets the Optimal Operation Line (OOL), a quite
irregular curve that results from the very small fluctuation of
fuel consumption levels over a large operating region.
As previously mentioned, the transient performance of a gas
turbine is characterized by a slow throttle response. With this
in mind, it was decided to implement in this new CVT
controller, a function that allows the operator to input the
adequate acceleration lag, i.e. the time that gas turbine
spools take from idle to full load, after the accelerator pedal
is depressed. This improvement aims to simulate the gas
turbine operation more realistically.
Figure 16. Tractive effort characteristics of a 96 kW reciprocating engine
Figure 18. Efficiency Map of g.t. model 1 with the correspondent OOL
Figure 17. Tractive effort characteristics of a 102 kW twin-shaft gas turbine
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Figure 21. Operation points of g.t. model 3 powering the city car in the NEDC
3.2.2 Vehicle Model powered by
Reciprocating Engine A model simulating conventional vehicles, i.e.
powered by a reciprocating engine and coupled to a manual
gearbox, was implemented, as illustrated in fig.19.
Thereafter, the results obtained were compared with results
for the gas turbine, in terms of fuel consumption, for several
driving conditions.
4. RESULTS
The first simulations were performed using the default
driving cycles earlier mentioned: ECE, EUDC and NEDC.
The results are illustrated in figure 20.
According to the results presented, the fuel consumption is
always higher in an urban drive (ECE). In this driving cycle,
the start-stop traffic conditions, found in the congested urban
networks, are simulated through the immobilization of the
vehicle in several occasions over time. These idling situations
have a severe effect on the average fuel consumption, since
the engine is working, i.e. consuming fuel, for a distance
covered equal to zero.
The results for the two Chrysler gas turbine versions (model
1 and 2) allow as also to assess the importance of the heat-
exchanger in the overall efficiency of a gas turbine. For a
combined driving cycle (NEDC), model 2 (without heat-
exchanger) consumes 178% more fuel than model 1 that
recovers part of the energy stored in exhaust gases through a
regenerator. An even higher fuel economy can be achieved
with the gas turbine model 3 that has components
(compressor and turbines) with higher efficiencies and
operates at an inlet turbine temperature 150º higher than the
one used in the Chrysler models. With this model the fuel
economy improves 34% when compared with model 1 in the
NEDC.
Since model 3 is the gas turbine with the best performance, in
terms of fuel consumption, a detailed comparison was carried
out between this model and the results corresponding to the
vehicles powered by a reciprocating engine, as shown in table
2.
Although model 3 has the best fuel economy among all gas
turbine models simulated, it still has a fairly poor
performance compared to the reciprocating engine. For the
city car, model 3 demonstrates fuel consumption values about
118% higher than the corresponding piston engine model. In
turn, for the SUV this difference is reduced to 75%. As a
matter of fact, it can be observed that the difference between
the gas turbine and piston engine reduces as the vehicle
weight increases. This trend can be explained by the fact that,
in heavier vehicles, more power is required from the engine.
In other words, for heavy vehicles, like an SUV, the gas
turbine operates closer to its nominal conditions, i.e. the
operating conditions for which it was designed. Basically, it is
appropriate to conclude that the gas turbine has a better
relative fuel consumption in the SUV, due to the higher
operational efficiencies achieved in the simulations (lower
overall specific fuel consumption). This statement can be
confirmed by figures 21 and 22, where one can observe how
the gas turbine operates at substantially better efficiency as
engine load increases.
Figure 19. Simulink model of a vehicle powered by a reciprocating engine
- 34%
+ 178%
Figure 20. Simulation results for the default driving cycles
Table 2. Fuel consumption results correspondent to reciprocating eng. models and g.t. model 3
9
From these last two figures it is also possible to verify the
increased proximity of the engine operation points (white
dots) to the Optimal Operation Line, in the case of the city
car for a larger part of the driving cycle. This is a
consequence of the higher acceleration lag of the heavier
vehicle. In situations where the free power turbine cannot
operate at its ideal speed for a given power requirement,
represented by the OOL, the high pressure spool increases its
speed, in order to increase the output power delivered, and
thus achieve the requirements imposed by the driving cycle.
It is important to note that in both figures there are the same
number of dots. However, because the operation points are
closer to the OOL in the city car, they are overlapped, and
thus less visible.
To get a more detailed comparison between the reciprocating
engine and model 3, the evolution of the fuel consumption of
both engines powering the SUV along the NEDC is plotted in
fig. 23. The extremely high fuel consumption found for both
engines, at the early stages of the driving cycle are attributed
to idle operating conditions, which represent those situations
when the engine is running but the vehicle is immobilized.
From figure 23 it is also possible to observe that model 3 has
a higher idle fuel consumption than the reciprocating engine.
This means that there are little benefits in using gas turbines
in urban driving conditions.
In order to understand some of the uncertainties in these
results it is important to assess the percentage of operation
points that were calculated based on either the extrapolation
or linear interpolation of the data generated by the
performance code. This assessment is relevant since it can
address two different sources of error in the gas turbine data.
On one hand, one has the data generated by the linear
interpolation of the original data, which have a higher level of
certainty. On the other hand, one also has the data obtained
by extrapolation (assumptions) and whose accuracy is rather
difficult to assess. Depending on the type of vehicle in
analysis, the percentage of operation points found inside the
“original data” area (the most reliable data) varies. Since
heavier vehicles require more power from the engine, the
SUV has a higher percentage of operation points inside this
area than the city or family car, as it is shown in table 3. In
this same table, the percentage of fuel consumed in the
NEDC, that is associated to this area, is also given. The
percentages achieved reflect the better level of uncertainty
associated with the simulation of those vehicle and driving
cycle combinations that force the engine to operate closer to
its design point, i.e. in demanding situations.
In order to provide a better understanding of these two
different groups of results, the operation points associated to
gas turbine model 3 powering the SUV in the NEDC are
illustrated in figure 24. In this figure, the operation points
located inside the “original data” area, i.e. the data generated
by interpolation, are represented by dark points, while the
remaining points (outside this area) have a white coloration.
Figure 22. Operation points of g.t. model 3 powering the SUV in the NEDC
Table 3. Percentage of operation points (NEDC) located inside the “original data” area of model 3
Figure 23. Fuel consumption evolution of SUV along the NEDC
Figure 24. Operation points of g.t. model 3 powering the SUV (NEDC)
10
Apart from the three default driving cycles used, a
customized one was also created with the purpose of
simulating a vehicle running at constant speed (100km/h).
The fuel consumption of the SUV when powered by either
the reciprocating engine or the gas turbine (model 3) was
assessed. At this driving condition, it was verified that the gas
turbine is less efficient, requiring some 3.6 litres more of fuel
per 100km.
It is also important to note that the predictions for the
vehicles powered by piston engines are quite similar to the
fuel consumption values provided by some car
manufacturers.
5. CONCLUSIONS & FUTURE WORK
The simulations performed generated quite distinct
results for the different vehicles categories, driving patterns
and engine models simulated. From these results it is possible
to identify different trends, and hence draw some interesting
conclusions:
The absence of heat-exchanger in gas turbines
applied in road vehicles increases the fuel
consumption in 178%, on average.
A modern gas turbine can be 34% more efficient
than the Chrysler gas turbine model from 1964.
Vehicles equipped with a gas turbine have higher
fuel consumption than the ones with reciprocating
engines, independently of the driving conditions
simulated: urban, extra-urban or combined driving.
For a given power, gas turbines have a better relative
fuel consumption when applied in heavier vehicles,
since the engine operates closer to its design point,
resulting in a higher overall efficiency.
At idle conditions, gas turbines have a substantially
higher fuel consumption compared to reciprocating
engines. This fact indicates a limited benefit from
the use of gas turbines in urban driving conditions
where the vehicle immobilization occurs frequently.
Exception to this could be a hybrid configuration.
Operating at a constant regime, when the SUV is
moving at a constant speed of 100km/h, the modern
gas turbine requires some 3.6 litres more of fuel per
100km than the reciprocating engine.
It would be of added value to compare the fuel consumption
between gas turbines and reciprocating engines, in
demanding driving conditions, such as acceleration tests.
However, in order to proceed with an accurate analysis, a
dynamic approach would need to be implemented in the
simulation model compared to the quasistatic approach
utilized within this study.
The methodology developed in this work sets the framework
for performing system assessments and providing first
estimates for the fuel economy of different powertrain
topologies. Some car manufacturers argue that the use of a
gas turbine operating as range extender in a series hybrid
configuration is the most promising solution in the coming
years. Looking at potential future work, the developed
platform could be used for modelling and simulating hybrid
powertrains, combining electric propulsion with gas turbines.
Furthermore, innovative cycles could also be investigated,
such as: i) an intercooled-recuperated cycle with variable
geometry vanes in the power turbine, and ii) a combined
cycle configuration, i.e. a gas turbine with a heat-exchanger
on the hot side recovering part of waste heat of exhaust gas to
a steam cycle.
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