SIMULTANEOUS INCREASING OF THERMAL CONVERSION
EFFICIENCY AND BMEP WHILE REDUCING EMISSIONS
Victor Gheorghiu, Prof PhD ME Hamburg University of Applied Sciences, Germany
AVL AST 2012
23 – 24 Oct 2012 Heidelberg
Content Introduction Concept regarding strict Implementation of Atkinson cycles =
Ultra-Downsizing Goals of this Investigation BOOST Simulation Tool and Model & Setting of the Simulations 1st Goal: Investigation Case of Optimal Ratio between Internal
and External Expansions & Compressions 2nd Goal: Atkinson cycles for part & full load even with
stoichiometric AFR (λ = 1), without throttling, intensive EGR, mixture stratifying, HCCI etc.
3rd Goal: Evaluation of the maximum improving potential of Ultra-Downsizing performances by means of an ideal V,p,T-Model for avoiding high optimizing effort of all BOOST model parameters
Conclusion 2 23-24.10.2012
23-24.10.2012 3
Introduction Thermodynamic Ways for Improving TCE* of Engine Cycles and therefore for CO2 Emission Reduction:
Ways 1. Increasing effective compression
ratio 2. Shorten eff. compression stroke
(e.g. delaying intake valve closing) 3. Completing eff. expansion stroke
(e.g. delaying exh. valve opening) 4. Turbo-charging for a concurrent
increase in TCE & BMEP
Schematic Pressure-Volume diagram of a four stroke Seiliger cycle, where the Heat Release is modeled by constant Volume (2-3v), Pressure (3v-3p) and Temperature (3p-3). For this reason this Ideal Cycle can be referred as Seiliger V,p,T-cycle.
Effective compression and expansion strokes of classical ICE cycles (usual named as Seiliger cycle) are almost identical.
* TCE = Thermal Conversion Efficiency
23-24.10.2012 4
Introduction Thermodynamic Ways for Improving TCE* of Engine Cycles and therefore for CO2 Emission Reduction:
Ways 1. Increasing effective compression
ratio 2. Shorten eff. compression stroke
(e.g. delaying intake valve closing) 3. Completing eff. expansion stroke
(e.g. delaying exh. valve opening) 4. Turbo-charging for a concurrent
increase in TCE & BMEP
Schematic Pressure-Volume diagram of a four stroke Seiliger cycle, where the Heat Release is modeled by constant Volume (2-3v), Pressure (3v-3p) and Temperature (3p-3). For this reason this Ideal Cycle can be referred as Seiliger V,p,T-cycle.
Effective compression and expansion strokes of classical ICE cycles (usual named as Seiliger cycle) are almost identical.
* TCE = Thermal Conversion Efficiency
1.
23-24.10.2012 5
Introduction Thermodynamic Ways for Improving TCE* of Engine Cycles and therefore for CO2 Emission Reduction:
Ways 1. Increasing effective compression
ratio 2. Shorten eff. compression stroke
(e.g. delaying intake valve closing) 3. Completing eff. expansion stroke
(e.g. delaying exh. valve opening) 4. Turbo-charging for a concurrent
increase in TCE & BMEP
Schematic Pressure-Volume diagram of a four stroke Seiliger cycle, where the Heat Release is modeled by constant Volume (2-3v), Pressure (3v-3p) and Temperature (3p-3). For this reason this Ideal Cycle can be referred as Seiliger V,p,T-cycle.
Effective compression and expansion strokes of classical ICE cycles (usual named as Seiliger cycle) are almost identical.
* TCE = Thermal Conversion Efficiency
1.
2.
23-24.10.2012 6
Introduction Thermodynamic Ways for Improving TCE* of Engine Cycles and therefore for CO2 Emission Reduction:
Ways 1. Increasing effective compression
ratio 2. Shorten eff. compression stroke
(e.g. delaying intake valve closing) 3. Completing eff. expansion stroke
(e.g. delaying exh. valve opening) 4. Turbo-charging for a concurrent
increase in TCE & BMEP
Schematic Pressure-Volume diagram of a four stroke Seiliger cycle, where the Heat Release is modeled by constant Volume (2-3v), Pressure (3v-3p) and Temperature (3p-3). For this reason this Ideal Cycle can be referred as Seiliger V,p,T-cycle.
Effective compression and expansion strokes of classical ICE cycles (usual named as Seiliger cycle) are almost identical.
* TCE = Thermal Conversion Efficiency
1.
2.
3.
23-24.10.2012 7
Introduction Thermodynamic Ways for Improving TCE* of Engine Cycles and therefore for CO2 Emission Reduction:
Ways 1. Increasing effective compression
ratio 2. Shorten eff. compression stroke
(e.g. delaying intake valve closing) 3. Completing eff. expansion stroke
(e.g. delaying exh. valve opening) 4. Turbo-charging for a concurrent
increase in TCE & BMEP
Schematic Pressure-Volume diagram of a four stroke Seiliger cycle, where the Heat Release is modeled by constant Volume (2-3v), Pressure (3v-3p) and Temperature (3p-3).
* TCE = Thermal Conversion Efficiency * * BMEP = Brake Mean Effective Pressure
1.
2.
3. 4.
8
Introduction
Ways 1. Increasing effective
compression ratio 2. Shorten eff. compression
stroke (e.g. delaying intake valve closing)
3. Completing eff. expansion stroke (e.g. delaying exh. valve opening)
4. Turbo-charging for a concurrent increase in TCE & BMEP
Conclusion: These four Thermodynamic Ways for improving TCE lead from Seiliger to Atkinson cycle, i.e. to a cycle with shorted effective compression stroke!
Schematic Pressure-Volume diagrams of classical four stroke Seiliger (left) and Atkinson V,p,T-cycles
9
Introduction
Consequences & Restrictions of their Implementation:
Thermodynamic Ways
1. Increasing effective volumetric compression ratio (VCR)
2. Shorten effective compression stroke
3. Completing effective expansion stroke
4. Turbocharging for a concurrent increase in TCE & BMEP
Consequences & Restrictions
Exceeding pmax , Tmax , NOx (Diesel & SI) limits, knocking occurrence (SI)
Decreased aspirated fluid mass lower BMEP & lower TCE improvement
Large displacement of the engine heavy engine, lower TCE improvement
Exceeding pmax , Tmax , NOx (Diesel & SI) limits, knocking occurrence (SI)
23-24.10.2012
Introduction
23-24.10.2012
1. Atkinson cycles have been implemented so far mostly with symmetrical crank mechanisms, where intake valves are closed very late on cycle. Thus, a part of charge sucked into cylinder is pushed back to intake pipes, and effective compression stroke is in this way decreased. This quasi implementation shows no noticeable improvements of IFCE and, hence, it will not be discussed here in detail.
2. Real Atkinson cycles can be implemented only with help of asymmetrical crank mechanisms. This strict Atkinson cycle implementations allow to use concurrently very high boost pressures (to increase IMEP) and higher VCR (to enhance IFCE) and to set them much more independently of each other compared to Seiliger cycles.
Possible Ways for Atkinson Cycle Implementation:
10
Introduction
1. Quasi-Atkinson cycles with symmetrical crank mechanisms by means of very late intake valves closing
Possible Ways for Atkinson Cycle Implementation:
11 Schutting, E, Neureiter, A, Fuchs, Ch., Schwarzenberger, T, Klell, M, Eichlseder, H, Kammerdiener, T: Miller- und Atkinson-Zyklus am aufgeladenen Dieselmotor, MTZ 06 / 2007
Introduction
23-24.10.2012
1. Atkinson cycles have been implemented so far mostly with symmetrical crank mechanisms, where intake valves are closed very late on cycle. Thus, a part of charge sucked into cylinder is pushed back to intake pipes, and effective compression stroke is in this way decreased. This quasi implementation shows no noticeable improvements of IFCE and, hence, it will not be discussed here.
2. Real Atkinson cycles can be implemented only with help of asymmetrical crank mechanisms. This strict Atkinson cycle implementations allow to use concurrently very high boost pressures (to increase IMEP) and higher VCR (to enhance IFCE) and to set them much more independently of each other compared to Seiliger cycles.
Possible Ways for Atkinson Cycle Implementation:
12
15 23-24.10.2012
Requirements: To raise IFCE and IMEP
simultaneously: The engine must be
highly turbocharged Compression stroke must
be much shorter as expansion stroke and VCR accordingly adapted.
Most of compression of working gas should occur outside of cylinder and Most of expansion within cylinder.
Advantages: As an important part of
compression takes place beyond cylinder, this high compressed fresh charge can be cooled intensively before it is sucked in.
Following moderate compression within cylinder leads to lower temperature peaks during combustion process and, consequently, to less NOx emissions.
Concept regarding strict Implementation of Atkinson cycles = Ultra-Downsizing
16 23-24.10.2012
Requirements: To raise IFCE and IMEP, to
limit pressure & temp. peaks during combustion even for stoichiometric mixture, simultaneously: Very high turbocharged
engine Optimized Ratios
between internal and external compression and expansion parts.
VCR must be varied accordingly.
Advantages: Lower temperature peaks
during combustion and thus less NOx emissions.
Realize real Atkinson cycles for part and full loads even with stoichiometric AFR and without throttling, intensive external EGR, mixture stratifying, HCCI…
Only 3-way catalysts are sufficient for after-treatment in this case.
Concept regarding strict Implementation of Atkinson cycles = Ultra-Downsizing
17 23-24.10.2012
1. To look for optimum ratio between internal (i.e. within cylinder) and external (within turbines) expansions of working gas, which leads simultaneously to maximizing IFCE and enabling sufficiently high values of IMEP.
2. To make possible the implementation of Atkinson cycles for part and full loads even with stoichiometric AFR and without throttling and/or intensive EGR.
3. To evaluate the maximum improving potential of Ultra-Downsizing performances, however avoid the high optimizing effort of all BOOST model parameter.
Goals of this Investigation Simulation
Tool:
BOOST*
BOOST*
Self made analytical
model
* AVL BOOST www.avl.com/boost1
18 23-24.10.2012
BOOST Simulation Tool and Model BOOST*-Model considers true
geometrical dimensions of engine components and losses caused by friction and heat transfer.
Power balances of all three turbochargers (TC) determine actual boost pressure level.
Expansion processes in turbines (Tx) are described by means of their discharge coefficients (µTx). When boost pressure required for preserving pressure limit on cycle is low, superfluous TC are kept for simplicity and comparability in use (i.e. are not bypassed here).
Inta
ke s
ide
Exh
aust
sid
e
19 23-24.10.2012
Setting of Simulations
Most parameters of BOOST model are selected for a hypothetical engine and are kept unchanged for all simulations, e.g.: All geometrical dimensions (with exception their of crank mechanism) Valve timing Wall temperatures, heat transfer coefficients, efficiencies and pressure
losses of intercoolers (target efficiency = 0.75, target pressure drop = 5 kPa), friction coefficient in pipes (0.019), blow by gap size of cylinder, frictional characteristic curve of engine etc.
Efficiency of turbochargers (compressor efficiency = 0.75, turbocharger overall efficiency = 0.5)
AFR, engine speed Combustion parameters:
Simple Vibe function (for modeling of heat release) Different positions of TDC on Atkinson cycles are compensated by
choosing a suitable start of combustion (SOC), so that combustion begins in all cycles uniformly at 15°CA before TDC.
20 23-24.10.2012
1st Goal: Investigation Case of Optimal Ratio between Internal and External Expansions
In Investigation Case A (IC A) VCR is varied and VER is kept steady Expansion stroke is kept
unchanged and compression stroke is varied significantly to allow modification of ratio between internal and external expansions.
Atkinson (Atk) cycles are implemented by varying eccentric radius exx of crank mechanism used.
Seiliger cycle is realized with zero eccentric radius.
TDC
Seiliger
Rel
ativ
e Pi
ston
Pos
ition
[-]
Crank Angle [°]
Exp
ansi
on S
troke
Com
pres
sion
Stro
ke
Inta
ke S
troke
Exh
aust
Stro
ke
Table shows: VER VCR, µTx, n (engine speed), AFR, SOC, CD (combustion duration), mVibe (exponent of Vibe heat release function), IFCE, IMEP, max(p) and max(T) (maximum cylinder pressure and temperature), pMP8 and TMP8 (mean boost pressure and temperature; i.e. at measuring point MP8) and pMP12 and TMP12 (mean exhaust back pressure and temperature; i.e. at MP12) for cylinder 1 (C1). 21
Parameter and Simulation Results for IC A
MP8
MP12
C1
Inta
ke s
ide
Exh
aust
sid
e
MP8 MP12 C1
Turbine discharge coefficients µTx are tuned in all cycles for reaching max(p) ≈ 230 bar. For reaching approximately same expansion rate in all three turbines, their µTx are set at the same level and compensated with cross sections ratios of turbine output pipes. Hence, only µT3 is adapted for each cycle to meet max(p) ≈ 230 bar. 22
Parameter and Simulation Results for IC A
MP8
MP12
C1
Inta
ke s
ide
Exh
aust
sid
e
23
Parameter and Simulation Results for IC A
Cylinder pressure (logarithmic) - Displacement volume diagram with valves timing for all cycles
IFCE - Crank angle (right axis) and Displacement volume – Crank angle diagram for all cycles
eo denotes exhaust open, ec exhaust closed, io intake open, ic intake closed
Atk e62
TDC
IFC
E [-
]
-33%
Forced exhaust
24
Parameter and Simulation Results for IC A
In IC A for all Atkinson cycles the aspirated gas mass changes only slightly
Seiliger (VCR=7)
Seiliger (VCR=15)
eo denotes exhaust open, ec exhaust closed, io intake open, ic intake closed
Atk e62
Gas Mass - Displacement Volume diagram with valve timing for all cycles
Cylinder Pressure (logarithmic) - Displacement Volume diagram with valves timing of all cycles
25 23-24.10.2012
Parameter and Simulation Results for IC A Trends arise from analysis of table values: All Atkinson show better IFCE values than Seiliger cycles. Seiliger cycles reach higher IMEP values because of longer
intake stroke and thus of more aspirated gas mass. IMEP follows IFCE variation and is mostly independent of boost
pressure variation in all Atkinson cycles. Highest IFCE value of Atkinson cycles is not reached in variant
with highest VCR, but in variant where VCR is ≈50% of VER.
26 23-24.10.2012
In Investigation Case B (IC B) VER and VCR are simultaneously varied
Exp
ansi
on S
troke
Com
pres
sion
S
troke
All four strokes are simultaneously varied by setting of parameter g while eccentric radius is kept steady to e32. Dashed curve shows
null position (g = 0) where a) expansion & exhaust and b) intake & compression strokes are identical (like IC A).
Inta
ke
Stro
ke
Exh
aust
S
troke
Dis
plac
emen
t Vol
ume
[lite
r]
Crank Angle [°] VIR = Volumetric Intake Ratio VXR = Volumetric Exhaust Ratio
2nd Goal: Atkinson cycles for part & full load even with λ = 1, without throttling and/or intensive EGR
27 23-24.10.2012
IC B = simultaneously variation of VER & VCR by steady eccentric radius (e32)
VIR = Volumetric Intake Ratio VXR = Volumetric Exhaust Ratio
Turbine discharge coefficients µTx should be set appropriately (while parameter g is varied) for fulfilling restriction for maximal cylinder pressure max(p) ≈ 230 bar.
All four volumetric ratios are varied simultaneously by setting parameter g.
null position
≈ 230 bar
max(p) µT1
µT3 µT2
Parameter g Parameter g
28 23-24.10.2012
IFCE varies in all OPs only within a 6% wide band while: Residual gas resides < 7% IMEP varies between 8.5
and 42 bar, max(T) varies between
1800 and 2300 K by unchanged heat release! max(p) within cylinder is
kept steady at ≈230 bar
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
6%
IFCE Residual Gas
IMEP max(T)
29 23-24.10.2012
IFCE varies in all OPs only within a 6% wide band while: … Boost pressure varies
between 2.5 and 12 bar Boost temperature does
not exceed 360 K Maximal cylinder back
pressure reaches ≈11 bar Max. temperature before
T3 achieves only ≈1000 K
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
6%
IFCE Residual Gas
IMEP max(T)
Boost pressure
Boost temp.
Back pressure
Temp. before T3
Parameter g Parameter g
pMP8 TMP8
pMP12 TMP12
30
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
6%
IFCE Residual Gas
IMEP max(T)
Boost pressure
Boost temp.
Back pressure
Temp. before T3
Parameter g Parameter g
pMP8 TMP8
pMP12 TMP12
Comments: TMP12 achieves only 1000 K
because of extended expansion within cylinder. As benefit turbine wheel must
not be protected against higher exhaust gas temperature. As disadvantage a higher
cylinder back pressure pMP12 is required for achieving desired boost pressure pMP8.
31
Comments: Required cylinder back
pressure pMP12 diminishes the level of IFCE by ca. 25% because of consumed work for forced exhaust. The load independence of
these IFCE losses is quite unexpected, but …
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
-25%
Forced exhaust
IFC
E
Dis
plac
emen
t Vol
ume
[lite
r]
Crank Angle [°]
32
Comments: The load independence
of these IFCE losses is quite unexpected, but if the difference between cylinder pressure at eo and cylinder back pressure is noted, the positive effect of free exhaust becomes evident.
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
Forced exhaust Cyl
inde
r Pre
ssur
e (lo
garit
hmic
) [ba
r]
Volume Displacement [liter]
Free
exh
aust
33
IC B: Parameter & Performance at Full & Part Loads (stoichiometric AFR, without Throttling & EGR)
6%
IFCE Residual Gas
IMEP max(T)
Boost pressure
Boost temp.
Back pressure
Temp. before T3
Parameter g Parameter g
pMP8 TMP8
pMP12 TMP12
Big Question: How much can IFCE be improved, if all the BOOST model parameter (like heat release, valve times, turbocharging etc.) would be optimized?
23-24.10.2012
34 23-24.10.2012
3rd Goal: Evaluation of the maximum improving potential of Ultra-Downsizing performances by
means of V,p,T-Model for avoiding the high optimizing effort of all BOOST model parameters
In the case of supercharged engines, the number of parameters which influence the IFCE and BMEP is very high.
As a consequence, the effort to achieve combinations of parameters which maximize the performances of the real (by BOOST) cycle becomes difficult and very time expensive.
For these reasons, the V,p,T analytical model of ideal open cycles have been developed for this purpose (see Appendix).
In ideal V,p,T-cycle the heat is partially released isochorically (2 – 3v), isobarically (3v – 3p) and isothermally (3p – 3).
35 23-24.10.2012
… In ideal V,p,T-cycle the heat
is partially released • isochorically (2 – 3v), • isobarically (3v – 3p), • isothermally (3p – 3).
The amounts of heat released isochorically and isobarically depend on the targets for maximum pressure and temperature on the cycle (i.e. isochorically up to pmax, isobarically up to Tmax and the rest of the heat is released isothermally).
Cyl
inde
r Pr
essu
re [b
ar] (
loga
rithm
ic)
Displacement Volume [liter]
Some Details of V,p,T-Model for Open Cycles V,p,T (dashed curves)
with optimized valve timing heat release turbocharging …
BOOST (solid curves)
eo, ec = exhaust open / closed io , ic = intake open / closed
36 23-24.10.2012
In the analytical V,p,T model the thermal properties (i.e. isentropic exponents κc for unburned and κe for burned parts) of the working fluid are kept constant throughout the cycle. The entire fuel mass is added to the cylinder gas mass in the
state 3v of the cycle. The mass contribution of the exhaust rest gas part is also taken
into consideration. The available heat (from fuel combustion) is decreased by the
amount of heat transferred to cylinder wall. In this case, compression, combustion and expansion can be treated adiabatically. The backpressure pT behind the cylinder (equivalent of MP12
from the BOOST model) is computed by means of energy balance at the turbocharger for the desired boost pressure.
Some Details of V,p,T-Model for Open Cycles
38
Maximum improving potential of Ultra-Downsizing performances
Comparison between simulations doing for IC B by BOOST and V,p,T-Model
Optimized heat release of the V,p,T-cycle : • isochorically (2 – 3v) • isobarically (3v – 3p) • isothermally (3p – 3)
eo exhaust open, ec exhaust closed, io intake open, ic intake closed
V,p,T (dashed curves) with optimized valve timing heat release turbocharging …
Cyl
inde
r Pr
essu
re [b
ar] (
loga
rithm
ic)
Cyl
inde
r Pr
essu
re [b
ar]
Displacement Volume [liter] Displacement Volume [liter]
39 23-24.10.2012
Maximum improving potential of Ultra-Downsizing performances
V,p,T (dashed curves)
Comparison between simulations doing for IC B by BOOST and V,p,T-Model (max(T) limited by 2100 K)
Cyl
inde
r G
as M
ass
[gra
m]
Cyl
inde
r Te
mpe
ratu
re [K
]
Displacement Volume [liter] Displacement Volume [liter]
1 2
3v 4
5
6
7
41 23-24.10.2012
Maximum improving potential of Ultra-Downsizing performances
Parameter g Parameter g Parameter g Parameter g
IFCE
IMEP
pMP8 TMP8
pMP12 TMP12
Residual Gas
κc κe
ψ
λ
1 − ψ − θ
θ
ma
Hea
t Rel
ease
Rat
es
Ther
mal
Pr
oper
ties
42 23-24.10.2012
Maximum improving potential of Ultra-Downsizing performances
Parameter g Parameter g Parameter g Parameter g
IFCE
IMEP
pMP12 TMP12
Residual Gas
κc κe
ψ
λ
1 − ψ − θ
θ
ma
VCR VER
Hea
t Rel
ease
Rat
es
Ther
mal
Pr
oper
ties
43 23-24.10.2012
CONCLUSION
The optimum ratio between internal (i.e. within the cylinder) and external (i.e. within turbines) expansions of the work gases which maximize IFCE is reached when the VCR is close to ca. 50% of VER.
An asymmetrical crank mechanism which permits in addition to vary the VCR makes possible to realize Atkinson cycles for part and full load even with stoichiometric AFR and without throttling and/or intensive EGR.
The presented comparisons between V,p,T and BOOST simulations show that this analytical V,p,T model of ideal open cycles can simulate a real cycle relative accurate and predict correctly the upper limit of cycle performances under given engine operating conditions.
Thank you for your attention!
44 23-24.10.2012
Contact Information
Victor GHEORGHIU Prof. PhD ME HAW Hamburg University of Applied Sciences Faculty TI, Engineering and Informatics Dpt. MP, Mechanical Engineering Berliner Tor 21 20099 Hamburg, Germany Tel.: + 49 40 42875-8636 Fax: + 49 40 42875-8799 [email protected] [email protected] www.haw-hamburg.de/pers/Gheorghiu www.victor-gheorghiu.de
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