SYMBIOSIS OF ENERGY RECOVERY AND DOWNSIZING

C OVER STORY Innovativ e Engine Systems
SYMBIOSIS OF ENERGY
RECOVERY AND DOWNSIZING
Recovering exhaust gas energy is a promising approach to reducing the fuel consumption of future vehicles
powered by internal combustion engine. For application in passenger cars, IAV is pursuing a systematic approach
to integrating a Rankine cycle that involves a close-coupled heat exchanger upstream of a turbine, a main heat
exchanger downstream of a catalytic converter and a single-cylinder reciprocating piston expander. The entire
cycle has been examined on the engine test bench using ethanol as the working medium.
4
AUTHORS
DR.-ING. HEIKO NEUKIRCHNER
is Senior Vice President for Advance
Powertrain Development at the
IAV GmbH in Chemnitz (Germany).
TORSTEN SEMPER
is Project Manager for Waste Heat
Recovery, Advance Powertrain
D evelopment Division, at the
IAV GmbH in Chemnitz (Germany).
DANIEL LÜDERITZ
is responsible for Waste Heat
Recovery, Energy Management
Department, Advance Powertrain
Development Division, at the
IAV GmbH in Chemnitz (Germany).
HOLISTIC ENERGY CONSIDERATION
Besides reducing exhaust emissions, the key focus in developing today’s vehicle propulsion concepts is on improving the use
of primary energy. Achieving this objective not only involves
enhancing the efficiency of each individual powertrain component but also intelligent energy management to avoid unnecessary losses. Many measures inside the engine are already being
used today to increase the gasoline engine’s thermodynamic efficiency. In addition to this, recent years have witnessed the trend
towards low-displacement supercharged engines, or downsizing.
Irrespective of displacement, however, internal combustion
engines convert less than 40 % of the fuel energy used into
mechanical energy. In low-load urban driving cycles, efficiency
in many cases even falls to levels below 20 %. Here, the lost
energy is mainly divided between the flow of exhaust gas and
coolant. In the case of substoichiometric combustion, losses
also occur as a result of fuel burning incompletely.
Making at least partial use of these previously wasted energy
components therefore promises a further significant reduction
in fuel consumption. However, in homologation cycles, such as
the NEDC, automobile recuperation systems can only help to
cut fuel consumption on a very minor scale as a result of cold
starting, short cycle duration and low engine loads. However,
several other approaches to reducing consumption are produced by viewing combustion engine and waste heat not as
separate entities but as an overall, two-stage energy conversion
system with its mutual interactions. The following article
describes how configuring and integrating a Rankine cycle in a
specific way can reduce the fuel consumption of a vehicle with
supercharged gasoline engine under all operating conditions.
USING THE RANKINE CYCLE TO RECOVER WASTE HEAT
OLIVER DINGEL
is Director of Energy Management,
Advance Powertrain Development
Division, at the IAV GmbH
in C hemnitz (Germany).
In past years various approaches to using heat lost in vehicles
have been examined in depth and evaluated in terms of their
potential to reduce fuel consumption and CO2 emissions [1-4].
The Rankine cycle is shown to promise good results. This is a
process by which steam-operated engines commonly found in
thermal power plants generate heat. In its simplest form, the
steam cycle comprises a steam generator, a steam turbine, a
condenser and a feed pump, ❶. Instead of a steam turbine it is
also possible to use other expansion machines, such as reciprocating, axial piston or rotary piston engines. In cases where
waste heat is used, the medium is vaporised in the exhaust gas
heat exchanger.
From a thermodynamic perspective the ideal Rankine cycle
is a clockwise process that runs through the state points in the
T/s diagram in a clockwise manner. The individual changes of
state are described below for the ideal cycle:
:: 1 – 2: isentropic compression
:: 2 – 3: isobaric heat input
:: 3 – 4: isentropic expansion
:: 4 – 1: isobaric heat output.
The efficiency of the Rankine cycle is calculated from the ratio
of usable energy to the input of heat:
EQ. 1
|wuseful|
in
09I2014 Volume 75
|wout – win |
Δhexp –Δhfp
ηRankine=  ______
  
 
= ________
  =
 
  ________
 
Δh    
|q |
|q |
in
HE
5
C OVER STORY Innovativ e Engine Systems
RANKINE SYSTEM WITH
TWO HEAT EXCHANGERS
Gaseous
3
Steam
turbine
A supercharged 1.4-l direct-injection
gasoline engine was chosen as the base
engine for the measurements and simulations described below. Delivering
maximum torque of 200 Nm at 1500 rpm
and a maximum power output of 90 kW
at 5000 rpm, it is a typical example of
today’s downsized engines and suitable
both for vehicles of the subcompact-size
category as well as the compact-size and
standard-size category.
As mentioned in the introduction, a
Rankine cycle was configured at IAV by
considering the overall system made up
of engine and use of waste heat. Applying the Rankine cycle was to achieve the
following objectives:
:: the highest possible heat transfer rate
even at low exhaust gas mass flows
and temperatures
:: provision of component protection
for turbine and catalyst with lowest
possible enrichment at full engine load
:: use of mechanical expander power for
further downsizing the combustion
engine.
Proceeding from these demands, a system layout was defined with an initial
heat exchanger downstream of the
catalyst and a second heat exchanger
upstream of the turbine.
components. At the rated power output,
it was possible to feed approximately
80 kW of thermal power from the exhaust
gas into the steam cycle.
Serving as the test engine for thermodynamic and tribological studies, a single-cylinder reciprocating engine was
designed and constructed at IAV as the
expansion machine. Different material
pairings and sealing versions also permitted conclusions on wear behavior and
blow-by [6].
EXPERIMENTAL STUDIES
SIMULATION MODELS
➋ shows the schematic configuration of
The above-mentioned measurement
results from the test bench provided the
basis for generating models for simulat-
the engine with the waste heat recovery
components on the engine test bench.
In this setup for measuring power output, the expander was connected to a
brake. The power recovered was not fed
back to the combustion engine at this
stage. Both of the first-generation heat
exchangers had the basic purpose of
verifying that the concept shown in ②
actually works. It does not yet satisfy
the demands made on weight, package
or transient behavior. These are only
considered in a second generation of
components. The setup with the two
heat exchangers was able to prove that
more heat can be recuperated in the
part-load range than with just one heat
exchanger downstream of the catalyst
on account of the higher exhaust gas
temperature upstream of the turbine.
Enrichment at full load could also be
significantly reduced to protect the
6
Steam
generator
Generator
Liquid
2
4
Gaseous
Condenser
Feed water
❶Schematic diagram
Liquid
of a Rankine cycle [5]
1
ing each component with the GT-Suite
program system and for validating them
under steady-state and dynamic conditions. Using the simulation models, it
was now also possible to change and
optimise the dimensions and characteristic values of individual components on
a physical basis. Finally, all models were
interlinked to produce an overall vehicle
model with the aim of calculating fuel
consumption in driving cycles.
Given the difference in the dynamic
behavior of combustion engine and
steam cycle, the expander was not coupled with the vehicle’s powertrain in a
mechanically rigid way in the model.
Instead it was connected with a generator capable of delivering its electrical
Main heat
exchanger
Manifold heat
exchanger
Catalyst
EGT
Feed pump
G
Expander
❷Schematic configuration of
the engine with Rankine cycle
Condenser
Fluid tank
30
25
20
15
10
Electric power output [kW]
5
0
4.0
Thermal power output [kW]
❸Thermal and electric power output in the NEDC
3.0
2.0
1.0
0
0
200
400
600
800
1000
1200
POTENTIAL FOR REDUCING
CONSUMPTION FROM
FURTHER DOWNSIZING
Time [s]
❹Change in effective efficiency from increasing the load point [%]
20
ηe [%]
OP 2000 rpm / 44.2 Nm - base
OP 2000 rpm / 44.2 Nm - VD = 70 %
18
Mean effective pressure pme [bar]
16
14
12
35
10
8
33
6
25
2
0
Shift in operating point
through downsizing
30
4
20
0
1000
2000
15
3000
4000
Engine speed neng [rpm]
energy either directly to an electric
motor integrated in the powertrain or to
a buffer battery. In this way, energy
recuperated during deceleration or stationary phases can also be stored and
used later on. Efficiency levels were
applied in every step to account for the
energy conversion losses occurring here.
Finally, in simulating the driving cycle,
the torque required by the combustion
engine was reduced by the level of
torque delivered from the electric motor.
SIMULATION RESULTS
The overall vehicle model was used to
simulate fuel consumption in the New
European Driving Cycle (NEDC) with and
without the Rankine cycle described. In
09I2014 Volume 75
5000
6000
0.8 % rise in consumption in the simulation. By contrast, fuel consumption can
be lowered by 7.4 % in the highway driving part of the cycle. In the overall cycle
this lowers consumption by 3.8 %.
However, to reduce consumption further in the NEDC, there is still the option
of using the expander power output available at full load to reduce cubic capacity
rather than increase system output. This
way, the load spectrum can be moved up
in the engine map to increase efficiency
and therefore also reduce fuel consumption in the urban part of the NEDC. This
capability is described in the following.
7000
both cases, allowance was made for cold
starting at 20 °C. ❸ shows the thermal
power fed into the working medium and
the electric power produced by the generator as a function of cycle time. It can
be seen that the generator only delivers
electric power after approximately 600 s
in spite of thermal power flowing into
the medium shortly after starting. This
delay is attributable to the time taken for
the components and medium to heat up.
Obtained from a maximum thermal
power output of approximately 28 kW, a
peak output of approximately 4 kW is
then reached.
In the urban part of the NEDC, the
additional weight of the exhaust gas
energy recovery (EGER) components and
longer catalyst heating phase lead to a
In downsizing, the losses inside the
engine are reduced by shifting the spectrum of operating points. ❹ shows the
effective efficiency of the 1.4-l base
engine (described above) as a function of
engine speed and mean effective pressure. Assuming combustion properties
remain unchanged, effective efficiency
can be seen to increase by shifting the
operating point while reducing displacement by 30 %. This results in lower losses
occurring, consequently suggesting that
there is less lost heat available too. This
is examined in greater detail below.
❺ initially shows that 28.4 % of the
fuel energy flow set at 100 % is converted
into mechanical power at the 2000 rpm /
44.2 Nm operating point. The losses are
divided among cooling water heat flow
(23.9 %), exhaust gas heat flow (22.1 %)
and remaining losses, like oil heat flow
and head radiation/convection. If we
now look at the use of exhaust gas heat
with exhaust gas assumed to cool to
90 °C (temperature of working medium
upstream of HE) and an assumed heat
exchanger quality of 90 % (pWM/pEG), it
can be seen that 17.4 % of the fuel
energy flow used can be passed to the
working medium. Assuming a cycle efficiency of 15 %, this means that 2.6 %
more of the fuel energy flow used can go
into producing mechanical power.
➏ shows that if engine displacement is
now reduced to 70 % for the same level
of drive torque (44.2 Nm at 2000 rpm),
only 92.1 % of the fuel mass flow
used in the base engine is needed for
the same power output. Despite fuel and,
with this, exhaust gas mass flow being
7
C OVER STORY Innovativ e Engine Systems
Fuel energy flow
100 %
OUTLOOK: DEVELOPMENT OF
THE SECOND GENERATION
OF COMPONENTS
How fuel consumption could be significantly reduced by optimising the overall
8
Exhaust gas heat flow
22.1 %
Radiation / convection / oil
Technically usable work
Heat flow
Losses /
14.8 % condenser
25.6 %
28.4 % Mechanical power
Exhaust gas heat used
with Rankine cycle
η = 15 %
2.6 %
➎Sankey diagram of base engine with EGER: operating point 2000 rpm / 44.2 Nm
Fuel energy flow
92.1%
4.4 %
Cooling water
heat flow
Exhaust gas heat flow
21,8 %
Exhaust gas heat used
with Rankine cycle
η = 15 %
20.6 %
Radiation / convection / oil
Technically usable work
Heat flow
2.6 %
17.4 %
21.3 %
Exhaust gas heat transfer
Cooling to 90 °C
ηQHE = 90 %
14,8 % Losses /
condenser
POTENTIAL FOR DOWNSIZING AND
REDUCING CONSUMPTION FROM
INTEGRATING THE RANKINE CYCLE
Based on the results from expander simulation, the combustion engine’s cubic
capacity could now be reduced to 1.22 l.
System power output from downsized
engine and expander corresponds to the
power output from the 1.4-l base engine,
❼. The expander power output needed
for downsizing the engine demands an
expansion machine with a level of efficiency that must be as high as possible
over a very wide output range. The latter
could be achieved in the expander model
by altering timing and adjusting the
working medium to pressures and temperatures that optimise efficiency in relation to the load point.
Simulation of fuel consumption in this
concept revealed that downsizing produces a reduction of 4.7 % even in the
urban part of the NEDC. In the highway
driving part, the reduction was as much
as 9.8 %, resulting in a fuel saving of
7.6 % over the entire cycle. As shown
in ❽, this means that the potential for
reducing fuel consumption is twice as
high as it is for the base engine with 1.4 l
displacement. Compared with the NEDC,
higher fuel savings can be expected for
both engines in the forthcoming WLTC
on account of longer cycle duration as
well as loads and exhaust gas temperatures which are higher from the outset.
4.7 %
Cooling water
heat flow
17.4 %
23.9 %
28.4 % Mechanical power
lower, thermal exhaust gas power
output remains virtually unchanged
(21.8 instead of 22.1 %). On account
of exhaust gas temperature rising from
550 to 595 °C, the level of thermal power
transferred to the working medium is
even the same as it is with the base
engine (17.4 %). This means that a
downsized engine is also capable of
recuperating the same mechanical
power (2.6 %) as the base engine is.
The example shows that there is no
contradiction in combining exhaust gas
energy recovery and downsizing. On the
contrary, both technologies complement
each other in a positive way.
Exhaust gas heat transfer
Cooling to 90 °C
ηQHE = 90 %
❻Sankey diagram of downsized engine (VD=70 %) with EGER: operating point 2000 rpm /
44.2 Nm (percentages given in the diagram relate to 100 % fuel energy flow for the base engine)
system on the basis of downsizing and
waste heat recovery is described above.
However, the concept presented also
involves a number of problematical
elements:
:: The close-coupled heat exchanger
leads to a delay in catalyst heating
after cold start.
:: Maximum torque is only reached at
higher engine speeds owing to the partial withdrawal of exhaust gas enthalpy,
and transient boost pressure buildup is
slowed down in the lower rpm range.
:: Transient expander power buildup is
slower than for the combustion engine.
Simulations have shown that the first
two problems can largely be avoided by
integrating the heat exchanger into the
exhaust manifold upstream of the turbine. Converting expander energy into
electrical energy, buffering it and then
feeding it back to the electric motor can
even overcompensate the drawback of
point three.
Based on the findings from the first
generation, IAV will be developing and
bench-testing a second generation of
components during the course of this
year. All components will be configured
in a way that, despite being highly efficient, minimises their complexity and
allows them to fit into the package of a
vehicle from the compact-size category.
Alongside the optimised reciprocal
piston expander, a constant-pressure
turbine developed in-house will also be
investigated as an alternative machine.
SUMMARY
Recovering exhaust gas energy is a
promising approach to reducing the fuel
220
ABBREVIATIONS
ABBREVIATION
MEANING
C
Compressor
°C
Degrees Celsius
CAC
Charge air cooler
CAR
Passenger car
ηQHE
Heat exchanger quality
ηRankine
Rankine efficiency
EGER
Exhaust gas energy recovery
G
Generator
HE
Heat exchanger
80
Δexp
Specific enthalpy change in
the expander
60
Δh fp
Specific enthalpy change in
the feed pump
40
ΔhHE
Specific enthalpy change in
the heat exchanger
20
ICE
Internal combustion engine
kW
Kilowatts
M
Electric motor
NEDC
New European Driving Cycle
P WM
Thermal power output of
w orking medium
PEG
Thermal power output of
exhaust gas
P CW
Thermal power output of
c ooling water
P rec_mech
Recuperated mechanical power
q
Specific heat
qin
Specific heat input
T
Turbine
T EGupstreamHE
Exhaust gas temperature
upstream of heat exchanger
VD
Displacement
w
Specific work
w out
Specific work output
w useful
Specific useful work
200
180
160
Torque [Nm]
140
120
100
0
Full load torque, base engine VD =1.4 l
Full load torque, VD =1.22 l
Additional torque from EGER
0
1000
2000
4000
5000
6000
7000
➐Torque curves for base engine, downsized engine and expander
consumption of internal combustion
engines. However, its effect is severely
limited in driving cycles typically applied
in automobile homologation, such as the
NEDC. An overall system made up of
combustion engine and exhaust gas
energy recovery must therefore be configured in a way that also reduces consumption shortly after cold starting and
at low engine loads.
The results presented show that this is
made possible on the basis of a close-cou-
❽Fuel saving in the ECE, EUDC and NEDC with (yellow) and without downsizing (blue)
2
pled heat exchanger and a heat exchanger
downstream of the catalytic converter in
conjunction with downsizing. Here,
downsizing is realised by using waste
heat up to full load. The heat exchanger
upstream of the turbine performs two
functions. It utilises the advantage of hotter engine exhaust gases to improve efficiency and helps to reduce engine enrichment in the way necessary for component
protection. It was also possible to show
that downsizing and using exhaust gas
energy are in no way contradictory but
complement each other in a positive manner. On this basis, the model presented
was capable of doubling the potential to
reduce consumption in the NEDC.
1.4-l base
1.22-l downsized
0.8
0
Reduction in fuel consumption [%]
3000
Engine speed [rpm]
-2
-4
-3.8
-4.7
-6
-8
-7.6
-7.4
-10
-9.8
-12
ECE
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EUDC
NEDC
REFERENCES
[1] Dingel, O.; Semper, T.; Ambrosius, V.; Seebode, J.:
Abwärmerekuperation: Welche Alternativen gibt es
zum thermoelektrischen Generator? 3 rd IAV conference “Thermoelectrics”, Berlin, 2012
[2] Jakobi, M.: Chemische Wärmespeicher zum Heizen und Kühlen von Fahrzeugen. 3 rd IAV conference
“Thermoelectrics”, Berlin, 2012
[3] Zegenhagen, T.: Dampfstrahlkälteanlage zur
Kälteerzeugung aus Abgaswärme. 3 rd IAV conference
“Thermoelectrics”, Berlin, 2012
[4] Kitte, J.: Bedarfsorientierte Modell- und Simulationsarchitektur am Beispiel der ganzheitlichen
S ystemdimensionierung eines mehrflutigen Thermogenerators. 3 rd IAV conference “Thermoelectrics”,
Berlin, 2012
[5] Fließbach, E.: Studie zur Energiebilanz eines
Verbrennungsmotors mit E-Booster und Abgasenergierückgewinnung. Diploma, HTW Dresden, 2012
[6] Neukirchner, H.; Arnold, T.: Untersuchungen zu
Abdichtungen von Dampfexpansionsmaschinen. 17 th
International Sealing Conference, Stuttgart, 2012
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