Presentation SAE 2010-01-1511

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Prof. PhD Victor Gheorghiu Hamburg University of Applied Sciences, Germany

CO2-EMISSION REDUCTION BY MEANS OF ENHANCING THE THERMAL CONVERSION

EFFICIENCY OF ICE CYCLES

SAE 2010-01-1511

Return May 2010 SAE 2010-01-1511 / Victor GHEORGHIU 2

CONTENT

Introduction, Thermodynamic Ways for improving the TCE and their Restrictions

Goals of this Investigation

Analysis of classical Atk inson Cycle Implementation to aspirated and turbocharged engines

New Solution for Atk inson Cycle Implementation while Meeting usual Thermal & Mechanical Restrictions and their Analysis For Aspirated Engines

For Very High Pressure Turbocharged Engines

Ideal V,p,T-Model for easier comparison of the thermal Cycle of Turbocharged Engines at Full and Part Loads

Conclusion

Return Nov 2009 ESFA 2009 (E09A102) / Victor GHEORGHIU 3

INTRODUCTION Thermodynamic Ways for improving the TCE* of ICE cycles:

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 will be referred here as Seiliger V,p,T-cycle.

* TCE = Thermal Conversion Efficiency


Ways 1. Increasing effective Volumetric

Compression Ratio (VCR) 2. Shorten eff. compression stroke




Thermodynamic Ways for improving the TCE of ICE cycles:

1.

INTRODUCTION

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.




INTRODUCTION

Ways 1. Increasing effective volumetric

compression ratio 2. Shorten eff. compression stroke





2.

1.




INTRODUCTION







3.


2.

1.



3.

2.

1.

INTRODUCTION





increase in TCE & BMEP**

Thermodynamic Ways for improving the TCE* of ICE cycles:

4.

* TCE = Thermal Conversion Efficiency ** BMEP = Brake Mean Effective Pressure


INTRODUCTION

Ways 1. Increasing effective

volumetric compression ratio 2. Shorten effective

compression stroke 3. Completing effective

expansion stroke 4. Turbo-charging for a

concurrent increase in TCE & BMEP

Conclusion: The four well-known Thermodynamic Ways for improving the TCE* cycles lead (for both aspirated & turbocharged engines) from Seiliger to Atkinson cycle!

Schematic Pressure-Volume diagrams of classical four stroke Seiliger (left) and Atkinson V,p,T-cycles


Thermodynamic Ways for improving the TCE of ICE cycles and Consequences & Restrictions on their Implementation:

INTRODUCTION

Thermodynamic Ways 1. Increasing effective compression

ratio 2. Shorten effective compression

stroke 3. Completing effective expansion

stroke 4. Turbo-charging for a concurrent


Consequences & Restrictions Exceeding allowed pmax , Tmax , NOx (Diesel & SI), knocking (SI) etc. Decreased aspirated gas mass lower BMEP and lower TCE improvement Large displacement of the engine , heavy engine, lower TCE improvement Exceeding allowed pmax , Tmax , NOx (Diesel & SI), knocking (SI) etc.

* TCE = Thermal Conversion Efficiency ** BMEP = Brake Mean Effective Pressure


Goals of these Investigations 1. Goal of this investigation is to analyze how much Thermal

Conversion Efficiency Potential have the known Implementations of the Atkinson cycle for: a) Aspirated and b) Supercharged Engines

2. Goal is the attempt to improve these Implementations in accordance with the previous presented Restrictions without prejudice the Thermal Conversion Efficiency (TCE) of the Atkinson cycle for: a) Aspirated and b) Supercharged Engines at Full and Part Loads

New Solutions for Atkinson Cycle

Implementation are needed!


These simulations are doing here: For aspirated engine cycles with a self developed simulation tool For super-charged engine cycles with:

1. self developed Ideal V,p,T-Model for Seiliger and Atkinson cycles 2. AVL BOOST and for Validation of Ideal V,p,T-Model

Ideal V,p,T-Model is required here because: usual simulation tools have several influence parameters, analysis of their influence upon TCE is complicated and time expensive optimal combinations of these parameter which maximize TCE and

fulfill the above restrictions are very difficult to obtain.

o In all of here presented simulations entire cycle - including the gas exchange processes - are considered.

o To Enable the comparison of simulation results performed with Ideal V,p,T-Model and AVL BOOST, and for Eliminating the influence of Heat Exchange between different Simulation Variants, which is quite difficult to compensate, the Cylinder is treated here as an Adiabatic System.

Analysis Tools Presentation


Aspirated Engines - Analysis of Toyota Implementation of the Atkinson Cycle in Prius II characterized by:

Source: Toyota

Atkinson cycle

Delaying

a) Shorten of the Effective Compression Stroke by means of Delaying of the Intake Valve Closing & b) Enhancing of the Volumetric Compression Ratio (VCR)


Source: Toyota

Delaying

The questions are: 1. Is this Atkinson cycle

implementation optimal?

2. If not, why?

Atkinson cycle

Aspirated Engines - Analysis of Toyota Implementation of the Atkinson Cycle in Prius II characterized by:

a) Shorten of the Effective Compression Stroke by means of Delaying of the Intake Valve Closing & b) Enhancing of the Volumetric Compression Ratio (VCR)


log(Pressure) - Volume diagrams for 1V and 2V related to SV

SV = Standard Variant (classical Cycle for Aspirated Engines, Seiliger Cycle) 1V = SV + 100°CA Delayed Intake Valve Closing 2V = 1V + 90% Increased Volumetric Compression Ratio (VCR) ~ Toyota Implementation of the Atkinson Cycle in Prius II Note: AFR is kept in all these variants identical!

intake valve closing in SV

eo = exhaust open ec = exhaust close io = intake open ic = intake close

intake valve closing in 1V

Increased VCR in 2V

intake valve closing in 1V & 2V

Simulation of this kind of Atkinson Cycle Implementation


Indicated Fuel Conversion Efficiency – Crank Angle diagram

Pushing out of burned gases consumes more piston work in 1V as in SV because of lower pressure at exhaust valve opening & consequently of sluggish cylinder emptying

log(Pressure) - Volume diagram

Note: Toyota uses an SI aspirated engine in its Prius II which tries to achieve high efficiency by using an Atkinson cycle, where the intake valve is kept open for a large part of the compression stroke and the volumetric compression ratio (VCR) is enhanced

SV & 1V





The forth and back flow through the intake valve in 1V causes the most important losses referred to SV in the indicated fuel conversion efficiency

Fluid Mass - Volume diagram

SV & 1V

In 1V remains 46% less mass as

in SV in cylinder after scavenging





The forth and back flow through the intake valve in 1V causes the most important losses referred to SV in the indicated fuel conversion efficiency


SV & 1V & 2V


Only the increasing with 90% of volumetric compression ratio in 2V restores the value of indicated fuel conversion efficiency referred to SV

intake valve closing in 1V & 2V


In the case of aspirated engines, where the intake valve is kept open for a large part of the compression, the advantage of performing the cycle using this Atkinson implementation is of little benefit for following reasons: The Gain in Indicated Fuel Conversion Efficiency is modest and is largely dependent on the fine tuning of all parameters

The Pushing out of the burned gases consumed more piston work as in SV

The forth and back flow through intake valve causes the most important losses in the Indicated Fuel Conversion Efficiency (IFCE).

The specific power of the engine (or BMEP) is low because of the decreased retained mass of fresh charge in cylinder before combustion. As consequence a relatively large displacement and therefore heavy engine is needed to power the vehicle.

The Supercharging seem to be the necessary solution to compensate the diminishing of BMEP and for enhancing of Indicated Fuel Conversion Efficiency.

Conclusion referred to this Implementation of the Atkinson Cycle at Aspirated Engines


Turbocharged (TC) Engines Analysis of classical Atkinson Cycle Implementations

characterized by:

a) Shorten of the Effective Compression Stroke, b) Enhancing of the Volumetric Compression Ratio c) Enhancing of the Boost Pressure + intensive Intercooling

SV-TC = Standard Variant (classical Cycle for Turbocharged Engines) 1V-TC = SV-TC + Delayed Suction 2V-TC = 1V-TC + Very Delayed Suction + Increased VCR The Boost Pressure Level is chosen to meat the following Restrictions:

Without exceeding maximal Pressure on Cycle set here to ca. 210 bar Without exceeding maximal Temperature set here to ca. 2100 K Note: AFR cannot be kept more in all these variants identical!

Simulation of classical Atkinson Cycle Implementations in the following Variants:


Turbocharged Engines Simulation of classical Atkinson Cycle

Implementations in the following Variants:

Pressure - Volume diagram IFCE – Crank Angle diagram

SV-TC & 1V-TC

Delayed Suction in 1V-TC

Very Delayed Suction in 2V-TC

SV-TC = Standard Variant (classical Cycle for Turbocharged Engines) 1V-TC = SV-TC + Delayed Suction + Increased Boost Pressure 2V-TC = 1V-TC + Very Delayed Suction + Increased VCR + Very High Boost Pressure


Turbocharged Engines Simulation of classical Atkinson Cycle

Implementations in the following Variants:


SV-TC & 1V-TC

Maximal Pressure on the cycle are nearly the same in all variants



SV-TC = Standard Variant (classical Cycle for Turbocharged Engines) 1V-TC = SV-TC + Delayed Suction + Increased Boost Pressure 2V-TC = 1V-TC + Very Delayed Suction + Increased VCR + Very High Boost Pressure



Turbocharged Engines / Classical Atkinson Cycle Implementations SV-TC = Standard Variant 1V-TC = SV-TC + Delayed Suction + Increased Boost Pressure 2V-TC = 1V-TC + Very Delayed Suction + Increased VCR + Very High Boost Pressure

log(Pressure) - Volume diagram IFCE – Crank Angle diagram

SV-TC & 1V-TC

VCR

AFR

IMEP

pC , TC


IFCE

Piston Work for Scavenging is very different




Temperature - Volume diagram IFCE – Crank Angle diagram

SV-TC & 1V-TC

Maximal Temperature on the cycle are nearly the same!


VCR

AFR

IMEP

pC , TC

IFCE


Return


May 2010 SAE 2010-01-1511 / Victor GHEORGHIU 24

Fluid Mass - Volume diagram IFCE – Crank Angle diagram

SV-TC & 1V-TC

Flowing back into the intake pipe

lower retained

Fluid Mass

VCR

AFR

IMEP

pC , TC

IFCE

Although the boost pressure is very different, IMEP and IFCE are virtually the same


In 1V-TC (= SV-TC + Delayed Suction) although the boost pressure is 40% higher, TCE and IMEP are 6% less than in the SV-TC (standard version of the Seiliger cycle). For these reasons, an other approach is needed to implement the Atkinson cycle with a normal crankshaft drive. In 2V-TC (= 1V-TC + Very Delayed Suction + Increased VCR + very high boost pressure) the retained fresh charge mass into cylinder is much lower as in the SV-TC. Although the boost pressure in 2V-TC is more than five times higher at virtually the same IMEP, only a minor improvement of the TCE can be detected. This improvement can be expected to be somewhat better if AFR is kept identical in both cycles.

For this reason, the implementation of the Atkinson cycle by means of a significant delay of the suction and a strong enhancement of the charge pressure applied to an engine with classical crank mechanism does not represent a suitable solution.

Therefore, a new approach is needed to implement a real Atkinson cycle.

Conclusion referred to these Implementations of the Atkinson Cycle at Supercharged Engines


The aim here is merely to estimate what potential for increasing the Indicated Fuel Conversion Efficiency (IFCE or TCE) exists when: For aspirated engines

The losses caused by the suction and partial expulsion of the fresh charge are eliminated (Variant 3V)

For turbocharged engines

Very high boost pressure + intensive intercooling turbocharging is implemented without reducing of aspirated mass & BMEP and with respect of the upper limits for pressure and temperature on the cycle (Variant 3V-TC).

For this reason asymmetrical crankshaft drives are here introduced which have shortened compression and extended expansion strokes.

In this way the piston work for compression is strongly diminished and the full potential of the very high boost pressure turbocharging can be used.

New Solutions for Atkinson Cycle Implementation


a) Shorten of the Geometric Compression Stroke by means of an asymmetrical crank drive

b) Adapting of the Volumetric Compression Ratio (VCR) c) Very high Boost Pressure and intensive Intercooling

New Solutions for Atkinson Cycle Implementation at Aspirated and Turbocharged Engines based on:

Aspirated Engines Turbocharged Engines

Increased VCR in 3V

Relative Piston Displacement – Crank Angle diagram

Volume – Crank Angle diagram


Simulation Results for Aspirated Engines

log(Pressure) - Volume diagram IFCE – Crank Angle diagram

SV = Standard Variant (classical Cycle for Aspirated Engines, Seiliger Cycle) 1V = SV + 100°CA Delayed Intake Valve Closing 2V = 1V + 90% Increased Volumetric Compression Ratio (VCR) 3V = 2V + Modified Crank Drive Note: AFR is kept in all these variants identical!


+15% for 3V

Simulation Results for Aspirated Engines

6%

The losses caused by the suction and partial expulsion of the fresh charge in 1V & 2V are in 3V eliminated. As result a 15% higher value of the Indicated Fuel Consumption Efficiency is achieved in 3V related to SV & 2V.

In 3V the entire gas mass sucked in remains in cylinder for combustion. Although the compression stroke is much shorter, the sucked mass in 3V is 6% greater than in 2V.



3V-TC / SV-TC IFCE +17% IMEP +38%

Simulation Results for Turbocharged Engines at Full Load

The full potential of the very high boost pressure (ca. 16 bar) turbocharging can be used without exceeding of the maximal pressure (ca. 210 bar) and temperature (ca. 2100 K) on the cycle.

The VCR is reduced to 5.0 and the VER is enhanced to 27.0. The limits for maximal pressure and temperature on the cycle are respected, but the AFR are different!





The losses caused by the suction and partial expulsion of the fresh charge are in 3V-TC eliminated. The piston work for compression is minimized by fully using of the high level of boost pressure (16.70 bar).

The VCR is reduced to 5.0 and the VER is enhanced to 27.0. The limits for maximal pressure and temperature on the cycle are respected, but the AFR are different!

Temperature - Volume diagram IFCE – Crank Angle diagram




The losses caused by the suction and partial expulsion of the fresh charge are in 3V-TC eliminated. The piston work for compression is minimized by fully using of the high level of boost pressure (16.70 bar).

The limits for maximal pressure and temperature on the cycle are respected, but the AFR are different. The retained fluid mass into cylinder after suction is 12% higher as in 2V-TC.


12%


Procedure:

To facilitate the comparison the Ideal V,p,T-Model is introduced and the following parameters are kept identical in both Atkinson and Seiliger cycles:

1. expansion ratio 2. specific heat (heat per fluid mass) 3. AFR (air-fuel-ratio) 4. isentropic exponent 5. maximal pressure and temperature on the cycle 6. charge temperature of the fresh air after compressor and cooler.

The Ideal V,p,T-Model was validated by means of AVL-BOOST Simulation Tool

Analysis of Atkinson Cycle Implementation for Very High Boost Pressure Turbocharging at

Full and Part Loads


Comparison between Seiliger and Atkinson Cycle for these Engines at Full Load

Pressure - Volume (left) & Temperature - Volume Diagrams of V,p,T-Seiliger and -Atkinson Cycles with same limits for maximal pressure and temperature

VCR

VER

AFR

IMEP

pC

TC


Conclusions:

As result the Thermal Conversion Efficiency ηt in the Atkinson cycle reaches more as 25% as in the Seiliger cycle.

At the same time the Indicated Mean Pressure (IMEP) exceeds in Atkinson cycle with more as 70% that of Seiliger cycle under fulfilling of the same mechanical and thermal limits on both cycles.

Questions: 1. How is that possible?

2. Validate BOOST the simulation results of V,p,T-Model for the Atkinson cycle?

VCR

VER

AFR

IMEP

pC

TC



How is that possible?

Because in both cycles the temperature at the end of the filling are kept identical, is in the Atkinson cycle the sucked fresh charge mass much bigger as in Seiliger cycle (see figure).

That explains the much bigger indicated mean pressure of Atkinson cycle.

In addition the piston work for gas exchange processes becomes strong positive, i.e. this piston work is for Atkinson cycle only supplied and not in part consumed like in the case of Seiliger cycle.




BOOST-Validation of Ideal V,p,T-Model for Very High Boost Pressure Turbocharged Engines

VCR VER

AFR IMEP

Pressure - Volume (left) & Logarithmic Pressure - Volume Diagrams of Atkinson Cycle as V,p,T- and BOOST-Models with the same charge pressure & temperature, IMEP and maximal Temperature on the cycle.


Mass - Volume (left) & Temperature - Volume Diagrams of Atkinson Cycle as V,p,T- and BOOST-Models with the same charge pressure & temperature, IMEP and maximal Temperature on the cycle.

BOOST-Validation of Ideal V,p,T-Model for Very High Boost Pressure Turbocharged Engines


Atkinson Cycle Implementation for Very High Boost Pressure TC Engines at Part Loads

The asymmetrical crank drive has the disadvantage that at part loads - because of the very extensive expansion - the cycle stops being feasible, i.e. the pressure at the end of expansion becomes lower than the ambient pressure.

For this reason, the crank drive should also enable the variation of the VCR!

Relative Piston Displacement – Crank Angle diagram of an

Asymmetrical Crank Mechanism with Variable VCR


Atkinson Cycle Implementation for Very High Boost Pressure TC Engines at Part Loads

Correlations between: •TCE, •IMEP, •Boost Pressure, •Pressure & •Temperature before Turbine & •Energy balance at Turbocharger

The arrows show the combination of these parameters in an EOP (10 bar boost pressure and AFR = 1) & for the 1. Crank Drive Position

Enough Energy for Turbocharging

Not sufficient Energy


• The TCE of the Atkinson cycle implemented in supercharged engines with

asymmetrical crank drive is more than 25% better and the IMEP exceeds

that of the Seiliger cycle by more than 70%, while meeting the same

mechanical and thermal limits in both cycles.

•The implementation of the Atkinson cycle by means of the asymmetrical crank

drive has the disadvantage that at part loads - because of the very extensive

expansion - the cycle stops being feasible, i.e. the pressure at the end of

expansion becomes lower than the ambient pressure. For this reason, an

asymmetrical crank mechanism is required which also enables the variation of

the VCR.

•By using such a crank mechanism, it is possible to realize Atkinson cycles for

part loads even with stoichiometric AFR and without throttling.

CONCLUSION

Thank you for your attention!

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Presentation SAE 2010-01-1511

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