SIMULATION OF COMBINED OTTO AND RANKINE CYCLE IN AN I.C.ENGINE

8/20/2019 SIMULATION OF COMBINED OTTO AND RANKINE CYCLE IN AN I.C.ENGINE

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IPASJ International Journal of Mechanical Engineering (IIJME)Web Site: http://www.ipasj.org/IIJME/IIJME.htm

A Publisher for Research Motivation........ Email: [email protected]

Volume 3, Issue 7, July 2015 ISSN 2321-6441

Volume 3, Issue 7, July 2015 Page 28

ABSTRACTTypical internal combustion engine lose about 65% of the fuel energy through the engine coolant, exhaust and

surface radiation. Most of the heat generated comes from converting the chemical energy in the fuel to mechanical

energy and in turn thermal energy is produced. In general, the thermal energy is unutilized and thus wasted. This

paper describes the analysis of a waste heat recovery (WHR) system that operates on a Rankine cycle. This WHR

system involves a modified engine assembly to reduce losses associated with compression and exhaust strokes in a

four-cycle engine. The wasted thermal energy recovered from the coolant and exhaust systems generate a high

temperature and high pressure working fluid which is used to increase efficiency of the engine.

Keyword- Internal Combustion engine, Waste heat recovery (WHR), Coolant.etc

1.

INTRODUCTION1.1. Literature Review

Waste heat recovery involving a Rankine cycle utilizes sensible enthalpy from the hot exhaust gases coming out of the

engine to heat the working fluid to superheated vapor and then the sensible enthalpy from the vapor is used to obtain

useful work. Rankine cycles have been explored by automotive and power generation industry for many years. Brands,

et al. [1] achieved WHR in a six cylinder, 14.5 L, Cummins NTC-400 diesel engine rated at 298 kW at 2100 RPM by

turbo-compounding. This involved the use of a power turbine to recover energy from the exhaust gas. The authors

demonstrated a 12.5% improvement in power and 14.8% net improvement in fuel economy due to WHR by Rankinecycle turbo-compounding.

Chen, et al. [2] reviewed many methods incorporated by various investigators to improve engine efficiency. They came

to the conclusion of a possible multi-stage Rankine cycle with the 1st stage operating on water followed by a 2

nd stage

operating on R-11 (organic solvent) to recover high temperature exhaust heat and to enable low temperature exhaust

WHR respectively. They also predicted a 15% improvement in efficiency through WHR.

An ORC system operating on tri fluoroethanol designed for use with a Class 8, long-haul vehicle diesel engine was

tested for improvements in engine efficiency. A 12.5% increase in highway fuel economy was achieved with this

system.

Teng, et al. [3] analyzed a supercritical ORC system of WHR from heavy duty diesel engines. The exhaust WHR was

analyzed from the perspectives of the first and second law of thermodynamics. They predicted up to a 20%

improvement in engine power using a supercritical ORC.

The Rankine cycle efficiency of various wet, dry and isentropic fluids was examined by Chammas, et al. [4]. They presented a concept to recover waste heat from high and low temperature of the exhaust and engine coolant

respectively. They concluded that to eliminate the need of a superheating apparatus, the Rankine cycles should operate

on dry or isentropic fluids. Simulations predicted a 32% improvement in fuel economy and also the energy from both

exhaust and engine coolant can be recovered to a certain limit.

Stationary Internal Combustion engines were investigated by Vaja, et al. [5] for WHR using a thermodynamic analysis.

They predicted a 12% improvement in thermal efficiency. Various working fluids were tested and benzene showed the

highest improvement. A critical heat exchanger was needed to be designed in their analysis to achieve the predicted

results.

The improvement in engine efficiency using Rankine cycles for WHR has been analyzed, simulated and tested by

various individuals for many types of engines. A common conclusion obtained from the literature review is that using

Rankine cycles for WHR has the potential to improve the overall engine efficiency. Based on the results of the above

discussed papers, the current research project of a novel waste heat recovery mechanism for an I.C. engine using an

ORC was continued.

Thermal energy is predominantly released through the coolant and exhaust systems of a typical internal combustion

engine i.e. the fossil fuel energy can be converted into 35%Break Power only and the heat energy is wasted through the

exhaust 35%, cooling system 30% and radiation loss of 5%. The proposed Waste Heat Recovery system makes use of

SIMULATION OF COMBINED OTTO AND

RANKINE CYCLE IN AN I.C.ENGINEM.SuryaNarayana

1, Dr.N.HariBabu

2.

1. M.Tech Student, Department of Mechanical Engineering, Aditya Institute of Technology and Management, Tekkali

2. Professor, Department of Mechanical Engineering, Aditya Institute of Technology and Management, Tekkali,

Srikakulam Dist.,-532201.


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this wasted heat and thus improves fuel conversion efficiency of the internal combustion engine. It makes use of a

Rankine cycle to convert the thermal energy into useful work.

Fig.1.1. Distribution of fuel energy

The Rankine cycle is a thermodynamic cycle that converts heat into work [6]. The Rankine cycle system consists of a

turbine, pump, condenser and boiler.

Fig.1.2. Rankine cycle and its characteristics

1.2.

Combined Cycle: [Otto & Rankine cycle]

It is the combination of Otto and rankine cycle. The Otto cycle heat energy is released to the exhaust & cooling system

and these wasted heats is used to super heat the working fluid in rankine cycle.

Fig.1.3.T-S diagram for combined cycle

The recovery of heat energy from the exhaust and cooling systems is done by transferring it to the vapor in the heat

exchanger that serves as the boiler.The combined cycle increases the efficiency without increasing the initial cost

generally. Consequently many new power plants operate on combined cycle, to increase the efficiency.

Working of Combined Cycle

It mainly consists of

Engine

2-Heat Exchangers

a. Engine exhaust heat exchange

b.Engine cooling heat exchanger

Condenser

Pump


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Fig.1.4. Novel waste heat recovery system

The 2-heat exchangers are placed in between the engine cooling system and engine exhaust system. These 2-heat

exchangers are used to super heat the working fluid in Rankine cycle, therefore the high pressure and temperature of

working fluid vapor is expanded in Engine vapor expansion chamber. The expansion of vapor takes place when the

piston moves BDC to TDC in compression and exhaust strokes.

The expansion of vapor helps to push the piston upwards. After expansion the low pressure and temperature of working

fluid enters in the condenser. In condenser, vapor is condensed to saturated liquid and this saturated liquid is pumped

to the heat exchangers by increasing the pressure in the pump. And thus the cycle is again repeated.

The working fluid of the first heat engine having low in its entropy than a second one though it completes cycle.

Subsequent heat engine may extract energy from the waste heat of working fluid in first engine. And by combiningthese cycles, the overall net efficiency of the system may be increased by 30 – 40%. Combing two or more

thermodynamic cycle’s results in improved overall efficiency and reducing fuel costs.

1.3. Heat exchanger between the coolant system and the working fluid

This heat exchanger is used to extract the thermal energy from the coolant system and raise the temperature of the

working fluid. 100% propylene glycol replaces the commonly used ethylene glycol/water mixture within the coolant

system. This was chosen to achieve an increased coolant temperature without the concern of boiling since the boiling

point of propylene glycol; (370℉) is higher than that of an ethylene glycol/water mixture (225℉). Also, the working

fluid had an increased temperature of 250℉ before entering the exhaust heat exchanger, which reduced the size, back

pressure and cost of the heat exchanger. Figure 1.5 shows a schematic of the single pass coolant heat exchanger used in

the simulation.

Fig.1.5. Coolant heat exchanger

This heat exchanger is used to vaporize and superheat the working fluid. For a gas-to-gas heat exchange process, a

compact heat exchanger is the optimum choice for the exhaust heat exchanger.


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2. ANALYSIS OF HEAT RECOVERY SYSTEM2.1. Engine Operation

Fig.2.1 shows the schematic diagram of the modified 4-stroke engine cylinder assembly with two steam power strokes

per two crankshaft revolution.

Fig.2.1.Modified Engine Cylinder Assembly

The residual steam from the previous stroke is expelled into the condenser at 1 atm, 212 ℉ where the vapor condenses

to liquid. This liquid is pumped to a higher pressure and enters the coolant heat exchanger which increases the

temperature of the working fluid. Finally, the working fluid undergoes a phase change in the exhaust heat exchanger

and it is superheated to 800psia, 747℉. This superheated vapor is then used in the next stroke.

The superheated vapor helps in removing the hot exhaust gases from the engine during the exhaust stroke and the

working fluid follows the closed loop again.

2.2. Working Fluids Used in Rankine Cycle

The efficiency of the Rankine cycle is limited by the use of the working fluid. In a Rankine cycle system, the working

fluid is reused continuously and follows a closed loop. Water is a commonly used working fluid but becomes inefficient

for WHR at temperatures below 370℃. For temperatures below 370℃, the use of organic fluids increases the Rankinecycle efficiency. An Organic Rankine Cycle (ORC) is a Rankine cycle that uses organic fluid. Fig.2.2. shows the

efficiencies of different working fluids versus turbine inlet temperatures. There may be an issue of formation of liquid

droplets on the turbine blades during the expansion process which can be eliminated by an Organic Rankine Cycle

(ORC).

Fig.2.2.Efficiencies of different working fluids for various turbine inlet temperatures

Fig.2.3.T-s diagrams for different fluids


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Based on the desired operating conditions (low temperature around 40 and high temperature around200 , low

pressure around 100kPa and high pressure around 10MPa), refrigerant fluid have better efficiency than isobutene

fluid.

Typ

e

AHA

RE Num

ber

Molecul

arFormula

Molec

ularMass

(kg/m

ol)

Norm

alBoilin

g

point(

)

Criti

calTem

perat

ure

( )

Critical

pressure(absolute

kPa)

HF

C

R-

245f

a

C3H3F

5

134.04

77

15 154.

05

3,640

HC

FC

R-

123

C2HF3

Cl2

152.92

74

27.6 183.

68

3,662

HC

FC

R-

113

C2Cl3F

3

160.92

1

47.7 214 3417

Fig.2.4. Comparison of several suitable working fluid properties.

2.3. Physical and Chemical Properties of R-123

Appearance - Colourless liquid

Physical state - Liquid

Molecular weight - 152.9

Chemical formula - CHCL2CF3

Odour -Faint ethereal and sweetish

Specific gravity (water 1.0) -1.47@70℉ (21.1 )

Solubility in water (weight %) - 0.21%70℉ (21.1 )Boiling point - 82.2℉ (21.1 )

Melting point - -107 (-160℉)

Vapour pressure - 11.4 psia@70℉ (21.1 )

Vapour density (air 1.0) - 5.3

2.4. Assumptions in the present analysis of Rankine cycle

Present analysis considers engine is operating with Otto cycle and the wasted heat is recovered through exhaust heat

exchanger and this wasted heat is recovered to increase the efficiency of the engine by introducing the rankine cycle as

the secondary cycle. The heat rejection at 4-1 process in Otto cycle and this rejected heat is supplied to the Rankine

bottoming cycle.

After expansion the vapour reaches to 10 degrees above the atmospheric temperature and pressure .Effectiveness of a

heat recovery device is considered as 1. That means heat rejection in Otto cycle is equal to the heat supplied to the

Rankine cycle.3. C-ProgrammeAs the manual calculations are difficult due to complexity, calculations are made using c -programme and the graph

has been plotted as per results.

3.1. Objective Function

The objective function is the function which is maximized by the optimization of variables. The efficiency of the system

is hence written in terms of compression ratio in the system. The optimization of the geometry variables is analyzed

and yet to be implemented on I.C Engines.

3.2. Temperatures and pressures in terms of geometric variables:

The temperature and pressures at some points can be written in terms of geometric variables. Note that at a point

T4=533K in Otto cycle, T4 = T1 (In Rankine cycle), condenser pressure P2 = saturation pressure (Psat) at 60˚C = 2.90bar.

The remaining temperatures and pressures and efficiencies are calculated and the values are compared using MAT Lab

programme. The program for combined cycle or waste heat recovery system is as shown in below:


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4. RESULTSThe various parameters such has network of Otto, compression ratio, vapor pressure and superheated temperature are

given input to the program and by considering above parameter values, the resulting output values are shown in below

1. Initially the input parameters values are, Wotto= 15kw, cr = 7, P1=P4 =14bar and T4 (Otto) = T1 (Rankine) = 533k


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2. varying the compression ratio value how it affects the combined efficiency. Input parameter values given to the

program is Wotto = 15kw, cr = 8, P1=P4 =14bar and T4 (otto) = T1 (Rankine) = 533k and then the output results are

shown below.

3. Varying the vapor pressure how its effects the combined efficiency. Therefore the input values are Wotto = 15kw, cr

= 7, P1=P4 =19.6 bar and T4 (Otto) = T1 (Rankine) = 533k then output values are shown below.

4. Increasing the network of Rankine how its effects the combined efficiency and other values shown below.


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Fig.4.1. Compression Ratio Vs Combined Cycle Efficiency

The compression ratio increases the efficiency of combined cycle also increases.

Fig.4.2. Network of Otto Vs Combined Cycle EfficiencyWhen the network of Otto cycle varies from 0 to 5 KW the efficiency of combined cycle continuously increases and

from 5 to 10KW the efficiency first increases and then decreases. From 10KW to 20KW the efficiency of combined

cycle is constant.


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Fig.4.3. Pressure Vs Combined Cycle EfficiencyWhen the pressure in the Rankine cycle increases the efficiency of combined cycle decreases slightly.

5. MAT LABThe Values obtained in c-programming is imported into MAT Lab simulation and compared the results by plotting the

graph. As the compression ratio increases the efficiency of combined cycle also increases.

5.1. Programme for Combined efficiency Vs Compression ratio at different enthalphies

.

GRAPH. 5.1: HI=574.44 KJ/KG


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GRAPH .5.2: H1=574.14 KJ/KG

GRAPH.5. 3: H1=573.97 KJ/KG

GRAPH.5. 4: H1=569.01 KJ/KG


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GRAPH.5. 5:H1=566.33 KJ/KG

5.2. Programme for Net work and combined efficiency

When the network of Otto cycle varies from 0 to 5 KW the efficiency of combined cycle continuously increases and

from 5 to 10KW the efficiency first increases and then decreases. From 10KW to 20KW the efficiency of combined

cycle is constant.


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GRAPH .5.6 :H1=574.14KJ/KG

GRAPH .5.7 :H1=573.97KJ/KG

GRAPH.5.8 :H1=569.01KJ/KG

GRAPH 5.9 :H1=566.33KJ/KG


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5.3. Programme for pressure and combined efficiency

When the pressure in the Rankine cycle increases the efficiency of combined cycle decreases slightly.

GRAPH 5.10 Pressure Vs combined efficiency

6. CONCLUSIONFrom the analysis, it has been identified that there are large potentials of energy saving through the use of waste heat

recovery technologies in I.C engines. Waste heat recovery entails capturing and reusing of waste heat from internal

combustion engines &through heat generated by exhaust gases and using it for heating the working fluid andgenerating mechanical or electrical work. It would also help to recognize the improvement in performance and

emissions of engine.


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With the implementation of modified engine cylinder design, calculations and analysis there is a measurable

improvement in the overall efficiency of the engine nearly 20% increase of overall efficiency of the engine.

REFERENCES[1]

Brands MC, Werner JR, Hoehne JL, Kramers S. Vehicle testing of Cummins turbocompound diesel engine, SAE

Paper No. 810073, 1981.

[2]

Chen SK, Lin R. A review of engine advanced cycle and Rankine bottoming cycle and their loss evaluations, SAE

Paper No. 830124, 1983.

[3]

Teng H., Achieving high engine efficiency for heavy-duty diesel engines by waste heat recovery using supercritical

organic-fluid Rankine Cycle, SAE Paper No. 2006-013522, 2006.

[4]

Chammas, R.E. and D. Clodic, Combined Cycle for Hybrid Vehicles. SAE International, 2005 (2005-01-1171).

[5]

I. Vaja, A. Gambarotta, Internal combustion engine (ICE) bottoming with organic rankine cycles (ORCs), Energy,

35 (2) (2010), pp. 1084–1093.

[6] Cengel, Y.A. and M.A. Boles, Thermodynamics An Engineering Approach. 6th ed. 2008.

[7] JasdeepS.Condle, Final Report-Novel Waste Heat Recovery Mechanism For An I.C Engine, MICHIGAN

UNIVERSITY-2012.

[8] Miers, S.A., Armstead, J.R., Final Report - Simulation Analysis of Automotive Waste Heat Recovery with

DYNAMAXTM Technology, June 16th, 2011.[9] Miers, S.A., Armstead, J.R., Supplemental Report - Simulation Analysis of Automotive Waste Heat Recovery with

DYNAMAXTM Technology, August 5th, 2011.

[10] Organic Rankine Cycle (ORC) http://www.orcycle.be/index.php/en/orctheorie.

[11] Liu, B.-T., K.-H. Chien, and C.-C. Wang, Effect of Working Fluids on Organic Rankine Cycle for Waste Heat

Recovery. Energy, 2004. 29: p. 1207-1217.

[12]

Srinivasan, K.K., Mago, P.J., Krishnan, S.R., Analysis of exhaust waste heat recovery from a dual fuel low

temperature combustion engine using an Organic Rankine Cycle, Energy, Volume 35, Issue 6, June 2010, Pages

2387-2399.

Documents

SIMULATION OF COMBINED OTTO AND RANKINE CYCLE IN AN I.C.ENGINE