Journal of Transportation Technologies, 2011, 1, 83-93
doi:10.4236/jtts.2011.14011 Published Online October 2011 (
Copyright © 2011 SciRes. JTTS
Lethe@-UDR1 Passenger Sedan Final
Proposed Configuration
Roberto Capata
Department Of Mechani cal a nd Aeros pace Engineering, University of Roma “Sapienza”
Received June 29, 2011; revised August 9, 2011; accepted August 27, 2011
For years the interest of the UDR1 researcher group has been focused on the development of a Hybrid Series
(HS) vehicle, different from the standard one, using a Gas Turbine (GT) set as a thermal engine. The reason
of this choice resides in the opportunity to reduce weight and dimension, in comparison to a traditional In-
ternal Combustion Engine (ICE). It’s not possible to use the GT engine directly for the vehicle traction, so
the UDR1 HS configuration shows the GT set connected with the electric generator only. The result is that
the traction is pure electric. Many efforts are spent in the definition of a generic scientific method to define
the correct ratio HD (the Hybridization Degree) between the installed power of the battery pack and that of
the GT electric generator which simultaneously guarantee the main life for the battery package and the ca-
pacity of the vehicle to complete a common mission (about 15 km - 30 km) without lack of energy or stop.
This paper reports all studies carried out and finally proposes a possible configuration (weight distribution,
VMU logic control, GT dimensions and power rate, battery package characteristics and so on) for mid term
solution in the Italian transport system.
Keywords: Hybrid Vehicle, Passenger Sedan, Components Distribution, Degree of Hybridization, VMU
1. Introduction
1.1. Brief Review of Existing Hybrid Vehicle
Concepts and of The Present Market
In the last decade, governmental incentives and the ever
stricter emissions regulations have prompted some of the
largest world automakers to dedicate resources to the
study, design, development and production of hybrid
vehicles, which offer undisputed advantages in terms of
emissions and fuel consumption with respect to tradi-
tional, reciprocating internal combustion engines. In fact,
hybrid engines are substantially smaller than conven-
tional ICE, because they are designed to cover the vehi-
cle’s “average” power demand, which ensures proper
traction for about 99% of the actual driving time, and is
exceeded only for prolonged mountain drives and in-
stantaneous accelerations. When excess power is needed
above this average, the hybrid vehicle relies on the en-
ergy stored in its battery pack. Hybrid cars are often
equipped with braking energy recovery systems that col-
lect the kinetic energy lost in braking, which would be
dissipated into heat otherwise, and use it to recharge the
battery. Smaller sizes and an (almost) constant opera-
tional curve lead to lower emissions. Moreover, a hybrid
vehicle can shut down completely its gasoline engine and
run off its electric motor and battery only, at least for a
certain range: this “mixed operation” increases the net
mileage and releases a substantially lower amount of
pollutants over the vehicle lifetime. The best known hy-
brid vehicles (HV) that recently have enjoyed the lime-
light of headlines and of technical reviews are all hybrid
cars equipped with a traditional ICE and an electric mo-
tor coupled in parallel. The thermal engine is sized, with
some exceptions, for the average power, and the surplus
power needed during rapid acceleration phases is sup-
plied by the electric motor.
1.2. Hybrid Configurations
There are several hybrid concepts, differing by the type
of coupling between the electric and the thermal engine,
and by the control logic that supervises the energy flows.
The main distinction is between series and parallel hy-
brids (Figure 1), though parallel designs are most com-
mon today.
In a series design, the internal combustion engine is
not mechanically connected to the drive-train, but drives
an electrical generator instead. The electricity produced
by the generator powers the motor or motors (one per
axle or one per wheel) that actually propel the car, and
the excess energy is used to charge the battery package.
When the power demand exceeds the thermal engine
capabilities, electrical energy is extracted instantaneously
from the battery pack to restore the power balance. Be-
cause electrical motors can operate quite efficiently over
a wide range, this design removes or reduces the com-
plexity of the mechanical transmission. The ICE can also
operate most of the time in the immediate neighbourhood
of its peak efficiency. The main advantage of the series
hybrid is the design flexibility afforded by the lack of a
mechanical connection between the internal combustion
engine and the wheels.
Figure 1. Scheme of the parallel (a) and series (b) hybrid
The components can be literally positioned at the de-
signer’s whim, offering important advantages regarding
weight balance and useful interior space. Series hybrids
are best suited for driving missions that foresee continu-
ously repeated stop-and-go, such as for urban delivery
vehicles. Parallel systems, which are more popular at
present, connect both the electrical and internal combus-
tion systems to a mechanical transmission, and can be
further classified depending on their hybridization ratio
(thermal engine power divided total supplied power: PICE
/ (PICE + PB)). In all commercial applications to date, the
ICE is by far the dominant power supplier and is used for
primary operation, with the electrical motor switching in
only when a boost is needed. Most present designs can
run for a very limited range in an all-electric mode. Par-
allel designs combine a large electrical generator and a
motor into one unit, often physically located between the
ICE and the transmission, in place of the ICE flywheel:
they replace both the conventional starter motor and the
generator or alternator. A rather large battery pack is
required, providing a higher voltage than the normal
automotive 12 volts [1]. A full hybrid, sometimes also
called a strong hybrid, is a vehicle that can operate in all
three modes: only ICE, only electric, or a combination of
both. Actual hybrids, for instance, are capable of elec-
tric-only operation. A large, high-capacity battery pack is
indeed needed for battery-only operation. These vehicles
have a power-splitting device that allows for more flexi-
bility in the drive-train by inter-converting mechanical
and electrical power, at some cost in complexity. Thus a
smaller, less flexible engine may be used, which is de-
signed for maximum efficiency (and often enacts a
modified indicated cycle, such as the so-called Miller or
Atkinson cycle). This significantly contributes to the
higher overall efficiency of the vehicle, with regenerative
braking playing a much smaller role.
2.The Lethe@ Concept
The series hybrid configuration developed by this re-
search group [1,2], nicknamed LETHE, is a vehicle in
which two small turbogas sets, fuelled with natural gas,
are coupled to high speed electrical generators and a
lead-acid battery package: the vehicle can operate in
electric-only mode if requested, or in hybrid mode,
where the gas turbine and the battery package operate
together to satisfy the power demand [3]. In the hybrid
vehicle scheme discussed in this paper, the electronic
vehicle management unit (“VMU”) controls ignition and
on-off switching under a Load Following logic. The
VMU determines at each instant time how much of the
energy produced by the GT reaches the battery package
or the electric engine directly. In addition, the electric
Copyright © 2011 SciRes. JTTS
motors can also act as brakes, recovering much of the
energy that is otherwise lost. In order to maximize the
recovered energy and to avoid possible battery over-
loading, an additional dynamic storage unit has been
included: a relatively small flywheel capable of storing
the excess power from the regenerative braking and of
releasing it at a later time according to the instantaneous
power demands. A general scheme of the LETHE@ is
illustrated in Figure 2. The VMU performs its en-
ergy-management task on the basis of a certain number
of instantaneous mission parameters: the batteries may
thus provide or absorb the difference between the energy
requirements of the vehicle and the GT energy produc-
tion. The generator acts as a starter for the GT as well. A
continuous GT control can be enforced via fuel flow
control and/or with a variable geometry GT. Since GT
power modulation is affected by a substantial efficiency
penalty at off-design conditions, the fuel flow control is
coupled with a variable-stator turbine and the inlet
guided vanes (IGV) blades for the compressor. Any con-
siderations about fuel storage have not been studied at
the present level of research. We are waiting for gov-
ernment grants to realize a prototype and test it.
The GT-hybrid propulsion system (GTHV) has ad-
vantages and drawbacks.
The following parameters ought to be considered
when selecting/designing such a system:
The GTHV has a small number of moving parts;
It is of compact size and can be mounted within strict
space limitations in the engine compartment of a se-
Both the micro turbines and the electric engine have a
very high power-to-weight ratio;
The GTHV attains a very high fuel economy;
The GTHV has a lower emission level, with effective
multi-fuel capability;
There is the possibility of improving the overall vehi-
cle design due to weight and size savings;
All components have a high reliability;
Figure 2. Scheme of the LETHE propulsion system.
The battery package has a very high total gross
weight per installed kW;
The state of charge (SOC) trend during any mission
must be monitored to avoid overcharge and excessive
discharge of the battery pack;
The GT may be subjected to several ignitions during
a mission, which affects its mean-time-between-fail-
ure (MTBF);
There is the necessity of monitoring and satisfying the
instantaneous vehicle total power demand.
3. Definition of The Degree of Hybridization
The mechanical power in an HV vehicle is typically sup-
plied by one electric motor (EM) and from the vehicle
traction point of view, it is considered an electric vehicle.
The choice of the EM is a direct function of the required
performance. Once the maximum required electric power
is fixed, then the total power source, supplied by the ICE
and battery package, has been calculated. Finally we
evaluate the Hybridization Degree HD (our design tar-
get), that means to calculate the correct ratio between the
GT power and the total installed power (GT and battery
HD = PGT / PGT+BP (1)
4. Design Procedure, Optimal Configuration
To find the correct HD we have adopted a method based
on an energy balance evaluation [1-3,5]. To perform a
driving mission any vehicle needs an energy supply; in a
traditional ICE vehicle this energy is guaranteed by the
thermal unit that, instant by instant, delivers the power
for the traction. The optimum solution is a system where,
for each assigned mission, the sum of GT and KERS
produced energy, supplies exactly the total energy con-
sumption. This assumption, in our code, represents the
energy balance. A positive result, in the calculations,
indicates that the battery package state of charge (SOC),
at the end of the mission, is higher than the initial one;
obviously a negative result indicates a lower SOC. To
perform the analysis we have started defining the vehicle,
with the identification of typical physical characteristics:
mass, frontal area, Cx, rolling resistance [1,4]. A second
step is the definition of the characteristic environmental
parameter, typically, standard values for air density and
gravity acceleration. To consider the presence on board
of KERS dispositive we have been defined and set some
operative coefficients like breaking recovery coefficient
(BRC) and minimum recovery velocity (MRV). The last
operative parameter is the degree of hybridization (HD).
Each operative parameters set represents a vehicle con-
Copyright © 2011 SciRes. JTTS
figuration. At the end of this preliminary phase we have
been selected several representative missions (ex. urban
routes for a city car or bus).
5. Preliminary Calculation Method
The code, implemented on Matlab™ environment, for
any assigned mission and vehicle configuration, calcu-
lates, second by second, the required power for traction,
breaking and available to KERS [5]. As the first calcula-
tion step, it evaluates the needed power for acceleration,
deceleration, inertia, rolling resistance and aerodynamic
resistance. This operation has to be repeated for every
different vehicle configuration and for all selected mis-
sions, so the number of data and diagrams increase
quickly. During this procedure step, some vehicle pa-
rameters have been considered as constant and for some
others a range of variability has been set (i.e. mass vehi-
cle, KERS efficiency coefficient, etc.). The typical code
analysis simulates every selected mission, considering
the maximum and minimum vehicle pay load. In addition
to these two options for the vehicle set up, also KERS
minimum and maximum efficiency is considered, so the
total combination of these variable vehicle parameters
have as a consequence the generation of four simulating
cases for each selected mission. The second calculation
step analyzes all power requests among all four cases for
a chosen mission, and the maximum value of the power
is considered as the vehicle total required power. The
second part of the code performs the optimization of the
vehicle configuration, considering a variable GT nominal
power into a range within 0 kW to the maximum power
request for the analyzed mission, this is repeated for each
configuration combination. This mean that we analyze
the mission for a variable degree of hybridization into a
range within 0% to 100%. The choice to adopt an
“on-off” logic derives from experimental considerations
and by the experience in the HS vehicle design, espe-
cially in the urban cycles, where it is mandatory to
minimize the “working time” of the GT device. The
adopted electronic logic is described here follow.
6. The Vehicle Management Unit [5]
The control logic implemented via a specially designed
microprocessor, manages the energy flow through the
different components, aiming to optimize them in terms
of fuel consumption, emission levels, and battery life. An
important aspect of the control logic is that the efficiency
of the regenerative braking, measured as the ratio of the
recovered energy to the total energy available in a decal-
eration, is closely correlated with the electric characteris-
tics of the batteries. For the Pb-acid batteries considered
in this work, the limit during the recharge phase is set by
the allowable current that can flow through the battery,
which in turn is a function of the battery capacity for a
certain state of charge. When the SOC increases, the
maximum admissible current decreases and thus, above a
certain threshold (SOC = 0.8 in this case), the kinetic
energy recovery is no longer possible and the traditional
friction brakes must be fully activated. To counter this
effect, another energy storage system has been added
(both compact flywheels and ultra-capacitors have been
considered), to absorb the “instantaneous” energy excess
and the intervention of the traditional braking system to a
minimum. Both compact flywheels and ultra-capacitors
have been considered, and the the flywheel was selected.
This means that for example, when selecting the size of
the battery pack, not only the maximum power requested
in the electric-only mode, but also the global amount of
energy recovered by the regenerative braking should be
considered. In practice, to proceed with instantaneous
system optimization using the Vehicle Management
Logic Unit, it is necessary to provide that all the instan-
taneous values of thermal, mechanical and electrical sys-
tem parameters be available to processor in acceptable
These parameters include:
The instantaneous power demand;
The instantaneous energy balance;
The type of batteries;
The SOC trend during the mission (dictated by the
type of batteries and their service life);
The number of ignitions of the GT set;
The allocation of power between the two GT sets.
A closed-loop logic was selected. This type of energy
flow management has the advantage of being mis-
sion-independent within broad limits. The power demand
at the wheels is a fundamental parameter both for system
design and VMU definition. The VMU computes the
instantaneous power demand, compares it with the
available power and keeps track of the history of the en-
ergy flows. To achieve correct system operation in all
possible conditions, the GT unit must be designed using
the maximum mission power demand criterion. The type
and model of battery package are the other fundamental
parameters. The VMU continuously controls electrical
parameters of the battery package to optimize its use and
to avoid harmful overloads that would reduce its life and
efficiency. For this reason the current flow through the
battery pack is constantly monitored. The current is usu-
ally correlated to the capacity C of the battery (in Ah), i.e.
to the quantity of current it can provide per hour. Most
manufactures recommend to not exceed 5C during dis-
charge, i.e. five times the value of the maximum per-
missible current per hour’s discharge. In a electric-only
Copyright © 2011 SciRes. JTTS
traction it is sufficient to size the battery package based
on the peak power. In the charging phase, the suggested
limit is 2C; the constraint is correlated to the maximum
absorbable power, indicated as Pbt,max,in. These constrains
are an integrating part of the VMU software. A continu-
ous monitoring of the SOC curve is necessary for a com-
plete and valid Logic definition. For any application, the
number of “complete charge and discharge” cycles is a
fundamental parameter: in fact, batteries tolerate only a
certain number of charge/recharge cycles before their
electrical characteristic deteriorate to unacceptable val-
ues. In the present application, the VMU allows the SOC
to vary only between the values 0.6 (for lower values the
GT are ignited to recharge the batteries) and 0.8 (over
this values the GT are switched off). It should be noted
that the design specifications adopted here do not pro-
vide any grid-recharging of the batteries, and the only
way to recharge the batteries is by turning on the
turbo-generators until a SOC = 80% is reached. First, in
order to provide at any time the proper vehicle opera-
tional mode, the GT installed power must be equal to the
peak power request, because when the battery pack
reaches its minimal SOC level, the GT set may be acti-
vated to cover all the power demand. Second, during
sizing of the GT, a proper attention should be given to
the Pbt,max,in, because at the end of each mission the GT
has to fully recharge the batteries. The Pbt,max,in depends
on both the size of the battery package and the SOC; its
value is generally lower than the peak value, in particular
at the final stage of the recharge phase (SOC = 80%).
Another strong design requirement is the GT to operation
in a possibly extended power range with an acceptable
efficiency. Since the GT is relatively rigid in this respect
in comparison with traditional I.C.E. (the GT efficiency
rapidly decreases with the operation conditions shifting
away from the design ones), this problem can be solved
only by installing two GT units: a smaller one (GEN1),
with operational yield corresponding to the Pbt,max,in
variation (i.e. between SOC = 60% and SOC = 80%),
and a larger one (GEN2). The sum of PGEN1max and
PGEN2max is equal to the maximum power demand of the
vehicle. The first condition that the VMU should verify is
that the energy flows (GT-to-motors, GT-to-batteries,
brake-to-batteries, battery-to-motors, brakes-to-auxiliary
storage, auxiliary storage-to-batteries, auxiliary stor-
age-to-motors) are balanced at each moment (ΔE = 0).
Since one of the aims of the project is to reduce the fuel
consumption, the preference during the development of
the VMU has been given to the electric-only operational
mode. It should be noted that the GT ignites only when
either one of two conditions is satisfied: a) the batteries
are not able instantly to satisfy the energy demand and b)
the VMU enforces a battery recharge. Two different
paradigms for the control logic have been developed:
they differ by the role and presence of the auxiliary stor-
age unit. In the first version (Logic A) the auxiliary stor-
age unit absorbs only the excess energy from the regen-
erative braking that the batteries are not able to absorb,
while in the second version (Logic B) it may also absorb
the excess power supplied by the thermal engine.
6.1. Logic A
The logic of the VMU calculates the power demand at
each moment. Then based on the value of the SOC, the
amount of stored energy in the auxiliary storage unit, and
the power setting of the two turbo-gas units, assigns the
fulfilment of the power demand among the system com-
ponents, leaving to the batteries the task of closing the
instantaneous energy balance. As a first step, the VMU
controls the net power (Pnet) supplied or absorbed by the
electric motor (during braking and driving phases re-
spectively). This power is calculated from the Pwheels, that
accounts for the motor, transmission, and regenerative
braking efficiency. The turbo-gas sets supply power
(PGEN ) to ensure proper traction or recharge the battery
package. The logic performs control on the SOC (both
present value and trend) to determine which element of
the system was supplying energy in the previous instant
of time. The logic gives preference to the pure electric
traction. Therefore, until the batteries have an energy
reserve, expressed by the SOC, which is high enough to
cover the requested power, the GT will not be turned on.
For a higher power demand or if the batteries have been
discharging at the previous time step, the hybrid mode of
operation is activated. If the demand power is higher
than the maximum GT power, both units will work at
110% and the recharge of the battery is delayed. Table 1
illustrates the power-check test on the above-mentioned
situation. This procedure provides operation of each tur-
bine within high-efficiency range without the risk of
supplying excess power. In fact the power-check test (PC)
calculates PGEN and determines the ignition of the appro-
priate GT, with the logic shown in Table 1.
Table 1. Power check.
Power check GT1 GT2
PGEN < PGEN1,min Off Off
PGEN < PGEN2,min On Off
PGEN2,min PGEN < PGEN21,max Off On
PGEN2,max < PGEN On On
Copyright © 2011 SciRes. JTTS
Within the values of the GT operational field limits,
the two turbo-generators supply in a load-following
mode the exact amount of power for traction and battery
recharge. If power demand is less than PGEN1min, the first
turbo-generator (GT1) is turned off, because the excess
power produced would overload the batteries. If power
demand exceeds PGEN2max , GT2 operates at 110% of its
nominal condition and the remaining demand is sup-
ported by GT1. Between PGEN1max and PGEN2min the hybrid
traction is activated and the power demand is supplied by
both GT units and batteries. The operational power range
of the traction system under Logic A is illustrated in
Figure 3.
Thus, the VMU monitors the energy flow at each in-
stant in time, and also records historical data to avoid
electrical overloads and excessive efficiency derating.
An additional setting provides that during any braking
recovery phase the turbo-gas sets are switched off. In
case of a “dead” battery (SOC < 0.6), the requested
power is supplied only by the two GT groups. As men-
tioned above, the auxiliary storage unit absorbs the
power that the batteries are not able to store during the
braking recovery. The logic calculates the recovered
power input and maximum permissible power of the bat-
tery package, and directs the excess power, if any, to the
auxiliary storage unit. The VMU designates the auxiliary
storage unit as the primary source of power supply mak-
ing this device as dynamic storage unit with very low
inertia (i.e. short response time). This component, as well
as the GT sets are not allowed to supply energy during
braking. The auxiliary storage unit supplies power to the
electric motors during any acceleration or driving at con-
stant speed, and is also used to recharge the battery if the
vehicle is stopped. Whenever the power is supplied by
the auxiliary storage unit, a balance of its residual energy,
of SOC and the maximum power that the batteries can
absorb is also performed. In case of the SOC increase,
another test called Forced Battery Charge (FBC), verifies
if this recharging operation is a result of deceleration or
forced recharge. This test is necessary to avoid continu-
ous recharging of the battery. Without this control, the
turbo-generators would supply current even if the SOC is
increasing, producing an oscillation of the SOC around
the SOCMAX. This condition must be avoided, because it
might induce repeated and useless GT ignitions that
would decrease the expected GT life and increase fuel
6.2. Logic B
The basic concept of the “Logic B” is that the auxiliary
storage unit is designed not only to improve the braking
energy recovery but also to buffer the GT power output.
Figure 4 shows that there is a range PGEN2,min-PGEN1,max ,
when both GT are off and the traction is only electric
regardless of the actual on-the-road driving conditions.
This “gap” is reduced if the auxiliary storage unit ab-
sorbs the excess power PGEN2,min-PGEN1,max and GT2 de-
livers PGEN2min. Besides increasing the flexibility of the
traction system, the adoption of Logic B allows the de-
signer more freedom in the selection of GT1, which is no
longer dictated by the condition PGEN1min = Pbt, max, in (at
SOC=80%). The turbogas will be switched “off” (idling
conditions) either when the maximum capacity of the
auxiliary storage unit (EAUXmax) has been reached, or
when it is sufficient for a complete battery recharge
(Figure 4).
7. System Simulation
Several numerical tests have been carried out to compute
the vehicle performance, in two different driving mis-
sions: a combination of 10 consecutive urban cycles
ECE15* and a “complex driving mission” composed of 4
consecutive extra-urban cycles EUDC* and 72 minutes
of continuous highway drive at 120 km/hr. Each mission
has been simulated for each of the two concept cars
studied here: a “city-car” and a standard passenger sedan.
The simulation computes the power balance on the basis
of the imposed wheel speed and vehicle characteristics
[1,2], and determines the power supplied by each system
GEN 1 ,min
GEN 1 ,ma x
GEN 2 ,min
GEN 2 ,ma x
GT OffOperational
Power Range
GEN 1 ,min
GEN 1 ,ma x
GEN 2 ,min
GEN 2 ,ma x
GT OffOperational
Power Range
Figure 3. Operational power range of the traction system
under Logic A.
Power Range
GEN1 , min
GEN1 , max
GEN2 , max
GT Off
Power Range
GEN1 , min
GEN1 , max
GEN2 , max
GT Off
*EEC Directive 90/C81/01: this is a series of Regulations that pre-
scribe both the emissions limits (adjusted every year) and the methods
for testing and qualifying passenger and commercial vehicles. The test
driving are in one urban cycle (European Cycle Emission) and an extra
urban driving mission (Extra Urban Driving Cycle) Figure 4. Operational power range of the traction system
with the Logic B.
Copyright © 2011 SciRes. JTTS
component. This process is repeated with a 1 s interval,
assuming that within every time interval, the power, the
speed, and all other significant parameters remain con-
stant. As mentioned above, the GT set is switched on
when the SOC is lower than a set point (0.6), and
switches to idling or partial load mode when the SOC
reaches the maximum set point (0.8). In real operation, a
manual override must also be provided, but this was not
considered in the calculations. The GT load management
protocol is based on the assumption that the GT sets can
operate, without substantial efficiency loss, between 70%
and 110% of their nominal power. Each simulation, con-
sists of assigning first the number of modules in the bat-
tery package, then the installed power, and finally the GT
power: these three values must satisfy the limitations
imposed by the abovementioned criteria of maximum
power demand and maximum absorbable battery power
[6]. The GT nameplate power was adjusted by iteration
until the minimal fuel consumption was obtained. This
heuristic procedure was also iterated by increasing the
number of battery modules, with a consequent correction
of the total vehicle weight. The vehicle design specifica-
tions (Table 2 ) are the same as those adopted in previous
papers [1,5,6].
7.1. The power management protocol
In the final model, the load following operation mode
was selected [1]. This selection brings two significant
advantages with respect to a simple on/off GT operation:
a) maintaining the GT within a sufficiently flat portion
of the efficiency/load diagram;
b) it allows for a “gentler” modulation of the charging
and discharging of the battery by decreasing the slope
of the current/time charging curve and thus increasing
battery life.
The logic of this “load following” GT management
can be described as follows:
1) The energy flows (GT-to-battery; brake-to-battery;
battery-to-motor; GT-to-motor) must be exactly bal-
anced at each in time instant;
2) The battery SOC is allowed to vary only between 0.6
(below which the GT is turned on to recharge the
battery, irrespective of the vehicle power demand)
and 0.9 (above which the GT is turned off, irrespec-
tive of the vehicle power demand).
3) In the SOC range between 0.6 and 0.9, the GT is
modulated to provide the battery with the exact
amount of instantaneous power extracted from the
battery by the motor.
The logic matrix is shown in Table 3.
8. Results of the Simulations
Eight different computer simulations have been per-
formed (2 types of mission respectively simulated with 2
types of logic, and 2 types of battery recharge limit
BRL).(Figure 5)
The choice of the optimal configurations, within those
several simulations, is a heuristic balance between the
relative advantages and drawbacks of the following pa-
Table 2. GT Hybrid Vehicle (GTHV) Design Specifications.
Wheel rolling radius R = 0.265 m
Vehicle width b = 1.7 m
Vehicle height H = 1.4 m
Net front area Sf = 2.142 m2
Area ratio (Sf/Stot) = 0.9
Aerodynamic drag coefficient cx = 0.25
Tire rolling friction coefficient fr = 0.015
Vehicle mass m = 1200 kg
Equivalent mass me = 1240 kg
Air density = 1.18 kg/m3
Air intake temperature T = 300 K
Minimum SOC 0.6
Maximum SOC 0.8
Table 3. LOGIC table for the GT “load following” mode.
Soc, % (Pv – Pbt)/PGTnom GT load factor, %
(= PGT/PGTnom)
< 0.6 100
> 0.8 0
< 0
0 (Pv – Pbt)/PGTnom 0.7
0.7 (Pv – Pbt)/PGTnom 1.1
(Pv – Pbt)/PGTN > 1.1
(Pv – Pbt)/PGTnom
Logic A
Logic B
Logic B
Logic A
Logic B
Logic B
Figure 5. Scheme of the performed simulations
Copyright © 2011 SciRes. JTTS
Copyright © 2011 SciRes. JTTS
barycentre on the central vehicle plane; when this is not
possible, the elements that can be housed more freely,
such as the inverter and the flywheel, that are positioned
on the side opposite the driver, in order to partly balance
the weight if the driver is alone in the vehicle and to
leave the required space for the steering wheel and ped-
als. The components were also positioned taking into
account the size of the air vents and the exhaust pipes.
Finally, although safety was not an issue involved in this
work, the battery pack was placed on the main frame,
under the rear seats, in order to respect “crash protect-
tion” conditions and to be easily accessed for mainte-
nance or battery-pack replacement.
Total gross weight of the battery package;
SOC trend during the mission;
Number of GT ignitions during the mission;
Instantaneous coverage of the total demand power of
the vehicle;
Size of the several devices (GT, battery package,
9. Components Distribution
The distribution of the components has been studied on
the frame of an Audi A2 (Figure 6, Figure 7, Figure 8).
The frame was rebuilt, once its structure [7] and sizes
were known (Table 4), using the SOLID EDGE@ pro-
gramme and was used to check the size of the compo-
nents for the chosen configurations. Of these configure-
tions, a version with a traditional motor (LETHE@ City
car or LETHE@ Sedan [3,4,5]) which power to the front
wheels (another configuration, named “packaged” was
also studied, with a motor-wheel directly housed in the
rear wheels) will be shown. When positioning the rotat-
ing parts, the respective gyroscopic movements were
considered, although they generally appear to be less
important than the one due to wheels. However, it is
necessary to consider the fact that a mass rotating around
a vertical axis can link rolling and pitching, while a ro-
tating mass around a longitudinal axis can coupled the
pitch to the yaw. All these situations have to be avoided
[8]. For this reason, all the rotating parts are placed with
a rotation axis that is parallel to the wheel axis, whose
gyroscopic effect can be contrasted by appropriate bal-
ancing of suspensions. The components have been ar-
ranged while trying to maintain the traction system
10. LETHE@ Sedan
The weights and overall dimensions of the components
are summarised in Table 5. The weight of the battery
pack is 150 kg (10 modules) that are positioned under-
neath the rear seats. The gas tank is placed in the rear
section while all the other components are housed in the
front section (Figures 6 and 7). This distribution gives
an overall weight of 181 kg on the fore-carriage and 187
kg on the rear carriage (Figure 8). I n spite of the pres-
ence of the second GT unit, all the parts can still be
comfortably installed in the area available under the
Audi A2 bonnet. The two turbo-gas units were placed
centrally and staggered in order to leave enough space
for the air vents and the various connection pipes. The
weights were equally distributed on the two wheel axles.
In spite of the fact that the parts were closer than in the
previous solutions, the motor area is still easily accessed
and ventilated.
Figure 6. Components distribution for the LETHE@ Sedan configuration.
Figure 7. Views and main dimensions (in mm) of the LETHE@ Sedan configuration.
Table 4. Components dimensions.
Gas Turbine 130 mm 500 mm 130 mm
Electric motor 205 mm 170 mm 205 mm
Regenarator 120 mm 400 mm 130 mm
Battery package (city car) 600 mm 500 mm 170 mm
Battery package (sedan) 340 mm 1000 mm 170 mm
Copyright © 2011 SciRes. JTTS
Copyright © 2011 SciRes. JTTS
181 kg187 kg
Figure 8. Weights distribution in the LETHE@ Sedan configuration.
In the following tables (Table 6, 7, 8, 9), it is possible to
analyze some comparison with actual vehicle fleet (city
car, passenger sedan and transporters)
Table 6. Calculated emissions for typical vehicles fleet [9].
g/km CO2,
g/km Particulate
PM10, g/km
Gasoline ICE vehicles 0.06 102 n.a.
Diesel ICE vehicles 0.20 99.6 0.021
Transporters 3.5 129 0.02
Table 7. EURO 6 normative [9].
Normative NOX
[g/km] HC
[g/km] CO [g/km] PM
EURO 6 (2013) 0.02 n.a. 0.15 0.01
Table 8. Levels totals of emission of champion fleet com-
posed by 500 gasoline ICE vehicles, 500 diesel ICE vehicles,
500 Diesel transporters. Specific emissions for distances:
10000-30000 km/year for the ICE vehicle and 50000 km/year
for the transporters.
Total Emis-
sions NOx,
kg/year CO2, t/year Particulate
Gasoline ICE
vehicles 300 510 n.a.
Diesel ICE
vehicles 1000 498 105
Transporters 87500 3225 500
Table 9. Total emissions of a fleet composed by 500 gasoline
cars, cars, 500 diesels engine and 500 transporters with
annual distance respectively of 10000 km - 30000 km (for
cars) and 50000 km (for transporters) and the comparison
with composed Lethe® fleet by 500 city cars (distance
10000 km), 500 Lethe-transporters (distance 50000 km) and
500 Lethe-passenger sedans (distance 30000 km).
Gasoline ICE vehicles 300 510 n.a.
Diesel ICE vehicles 1000 498 105
Transporters 87500 3225 500
TOTAL EMISSIONS88800 4223 605
City Cars 30.9 375 450
Sedan 180 451 135
Transporters 12500 2625 100
TOTAL EMISSIONS12530 3000 635
Table 10. Lethe@ Sedan data in detail.
[km/l] [l/100 km] [g/kWh]
28 3.5
Pollutants emissions Comparison with
EURO 5 directive
[g/kWh] [g/km]
NOx 0.0680 0.0120 –80%
HC 0.1099 0.0194 –80%
CO 0.1072 0.0190 –90%
PM 0.0011 0.0002 –90%
11. Conclusions
The energy flows management logic (VMU) for a
gas-turbine-driven hybrid propulsion system has been
described in detail. It provides proper operational mode
in all driving conditions. The application to possible con-
figurations has been studied, the configurations being
characterized by the presence or absence of the dynamic
storage unit (the flywheel) and by different recharge
modes of batteries. All simulations confirm that the
LETHE vehicle is a competitive solution with respect to
traditional I.C.E. vehicles and also to other standard hy-
brid vehicles: the fuel efficiency is (at moment we use
Methane or GPL fuel) is 29 km/l for urban cycles and
over 18 km/l for complex missions (compared respec-
tively on 20 km/l and 25 km/l for actual diesel vehicle,
and 16 km/l and 22 km/l for gasoline sedan), since dur-
ing highway operation the car is constantly at high speed
and the absence of braking doesn’t allow for the energy
recovery. Through a preliminary series of tests, it was
possible to identify an optimal configuration for both
types of mission. For a city car, the optimal performance
is combination of 75 kg battery pack and two 5 kW GT,
managed by Logic B+2C, while for the passenger sedan
the best performance was provided by a combination of a
150 kg battery pack and two GT sets, rated 10 and 15
kW respectively. It is, therefore, clear that we are not
talking about a complete “revolution” in the concept of
cars as we know it today, but about a simple reorganisa-
tion of the components. The most important innovation
in this project are the advantages offered by a GT device
in replacement of the traditional thermal motor
(ICE).Advantages that can be summed up in terms of
weight, size and the opportunity of utilization of a
multi-fuel motor that can work with all types of fuel that
are currently available on the market, thus reducing the
economic effects of price fluctuations for the various
types of fuel. By returning to all the analyses carried out
in this article, we can summarise everything in the Table
10 below, which shows the consumption and emissions
of a hybridised vehicle according to the described pro-
cedure. The table contains data obtained by previous
simulations and various assumptions and calculations
made [1-5]. It remarks a fuel consumption reductions of
about 30%, for a hybrid city car powered by methane
compared to current commercial production. With re-
gards to emissions, we have highlighted the drastic re-
duction in all the main pollutants emitted from the motor
compared to the values permitted by the current rules,
made possible due to optimisation of fuel for a thermal
motor that operates at a nominal point. The HS configu-
ration makes the use of GT device possible. We have
been proposing hybridisation studies for commercial
vehicles applied to cargo and passenger transport for
several years, studies that have involved trains, trucks
and cars and that have emphasised the feasibility of such
conversions in both energy and economic terms. In a
global context of reduction: greenhouse gas emissions,
consumption of fossil fuels, of city pollution, it is clear
that the benefits introduced by the HS vehicle would
provide an immediate reply to the most mandatory envi-
ronmental subjects.
12. References
[1] R. Capata, E. Cioffarelli and E. Sciubba, “A Gas Tur-
bine-Based Hybrid Vehicle-Part II: Technological and
Configuration Issues,” Journal of Engineering for Gas
Turbines and Power, Vol. 125, No. 2, July 2003, pp.
[2] R. Capata and E. Sciubba, “An Innovative Solution for
Suburban Railroad Transportation: The Gas Turbine Hy-
brid Train,” International Journal of Thermodynamics,
Vol. 8, No. 1, March 2005, pp. 55-66.
[3] R. Capata and E. Sciubba, “The Concept of the Gas Tur-
bine-Based Hybrid Vehicle: System, Design and Con-
figuration Issues,” International Journal of Environ-
mental Research, Vol. 30, 2006, pp. 671-684.
[4] R. Capata and A. Coccia, “Procedure for the Design of a
Hybrid-Series Vehicle at UDR1 and the Hybridization
Degree Choice,” Energies, Vol. 3, 2010, pp. 450-461.
[5] R. Capata and M. Lora, “The Comparative Assessment
and Selection of an “Optimal” Configuration for a Gas
Turbine-Based Hybrid City Car,” Journal of Engineering
for Gas Turbines and Power, Vol. 129, No. 2, 2008, pp.
[6] E. Cioffarelli and E Sciubba, “A New Type of Gas Tur-
bine Based-Hybrid Propulsion System-Part I: Concept
Development, Definition of Mission Parameters and Pre-
liminary Sizing,” Proceedings of AES/ASME Winter
Meeting,” Orlando FL, 2000.
[7] Audi site, 2011
[8] G. Pede, ENEA (National Agency of Energy and Envi-
ronment), Personal Communication, 2009.
[9] Automobile Club Italy, 2011
Copyright © 2011 SciRes. JTTS