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Energy 36 (2011) 760e769

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Energy

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Theoretical performance limits of a syngasediesel fueled compression ignitionengine from second law analysis

Bibhuti B. Sahoo a, Ujjwal K. Saha b,*, Niranjan Sahoo b

aCentre for Energy, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, IndiabDepartment of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India

a r t i c l e i n f o

Article history:Received 16 July 2010Received in revised form17 December 2010Accepted 17 December 2010Available online 26 January 2011

Keywords:AvailabilityDual fuelExergySyngasLoad

* Corresponding author. Tel.: þ91 361 2582663; faxE-mail address: saha@iitg.ernet.in (U.K. Saha).

0360-5442/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.energy.2010.12.045

a b s t r a c t

The present study is an attempt to investigate a syngasediesel dual fueled diesel engine operation undervarying load conditions from the second law point of view. The fuel type in dual fuel operation is ach-ieved by varying the volumetric fractions of hydrogen (H2) and carbon monoxide (CO) content in syngas.It is revealed that increasing the hydrogen quantity of syngas increases the cumulative work availabilityand reduces the destroyed availability. This enhancement is due to a better combustion process andincreased work output when a high amount of H2 quantity is employed. At lower loads, the in-cylindercombustion temperatures are reduced in case of the dual fuel combustion. Hence, the destructionavailability is increased due to poor combustion and reduced heat transfer availability losses. When theengine is operated beyond 40% load, the destroyed availability reduced due to higher combustiontemperature and pressure. The increase in the both exhaust gas and cooling water availabilities arereflected in an increase in second law efficiency with increasing load. The dual fuel cumulative workavailability is increased at higher loads and thus, the exergy efficiency is increased.

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1. Introduction

The use of new, alternative, and clean-burning fuels as primaryenergy resources in internal combustion (IC) engines is a globalinterest now-a-days to achieve lower pollutant emissions andhigher fuel economy. Towards this, the compression ignition (CI)engine of the dual fuel type has been employed to utilize variousalternative gaseous fuel resources in place of conventional dieselengines [1]. In a CI dual fuel diesel engine operation, the gaseous fuelismixedwith air in the inletmanifold. Thehighoctane index gas andair mixture is sucked and compressed like in a conventional dieselengine. However, the compressed airegas mixture does not auto-ignite due to lack of good enough ignition quality of gaseous fuel.Hence, it is fired by combustion of a pilot diesel fuel spray and themixture undergoes a multi-point ignition inside the cylinder [2].

The most common gaseous fuels used in a CI dual fuel dieselengine are: natural gas (NG), liquefied petroleum gas (LPG),hydrogen (H2), biogas, landfill gas, sewage gas, digester gas andsyngas etc. Among these fuels, syngas is ideally a mixture of twodiatomic molecules H2 and carbon monoxide (CO) produced bygasifying a solid fuel feedstock (such as coal or biomass). Over the

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years, depending on how it is formed, the gaseous mixture of COand H2 has had many names such as producer gas, town gas, watergas, synthesis gas, and syngas etc. to name a few. In principle,syngas can be produced from any hydrocarbon feedstock such asNG, naphtha, residual oil, petroleum coke, coal and biomass [3].Mixtures of H2 and CO could serve as an alternative spark ignition(SI) fuel due to their high anti-knock behavior [4,5]. However, theaddition of H2 to CO tends to increase combustion temperaturewhich increases nitric oxide (NO) emissions under stoichiometric SIcombustion [6]. Hence, the use of H2 and CO mixtures is moreappropriate in lean burn conditions where combustion tempera-tures are moderated by excess air like in a CI diesel engine. Also,these mixtures could serve in ‘dual fuel’ mode that operates underCI using a pilot injection of diesel fuel [7]. Again, in their publishedwork, Garnier et al. [8] have suggested the use of syngas in dieselengines with dual fuel mode for mechanical and electrical appli-cations. To apply syngas as a regular fuel, it is crucial to assess itseffect on a dual fuel engine, and its performance with that ofa traditional diesel engine from the second law viewpoint.

Whether it is an IC engine or not, the application of traditionalfirst law theory to a thermodynamic system fails to give the bestinsight into the engine’s operation [9e11]. Therefore, the secondlaw analysis should be coupled to the first law one. The second lawanalysis provides the knowledge of when and where the availableenergy is lost or destroyed in the engine system. Evaluation of

Nomenclature

AbbreviationsA availability, kWA/F mass airefuel ratioBHP brake horse power, hpBTDC bottom dead centreCA crank anglecc cubic centimeter, cm3

LHV lower heating value, kJ/kg

NotationsCp specific heat, kJ/kg-K_m mass flow rate, kg/hP0 ambient pressure, barPego exhaust gas pressure out from engine, barReg exhaust gas constant, kJ/kg-K

T0 ambient temperature, KT1 cooling water inlet temperature, KT2 cooling water outlet temperature, KT3 calorimeter water inlet temperature, KT4 calorimeter water outlet temperature, KT5 calorimeter exhaust gas inlet temperature, KT6 calorimeter exhaust gas outlet temperature, K

Symbolsd dieselpd pilot dieselg gaseg exhaust gasw watercw cooling waterin inputi ino out

B.B. Sahoo et al. / Energy 36 (2011) 760e769 761

available energy determines themaximumpossible performance ofa thermodynamic system [12]. In addition, impact of processchange in the system in terms of system losses is also assessed.These findings help in reducing the availability loss to improve theperformance of the engine in terms of efficiency and power output[13]. In an IC engine operation, availability analysis reveals theengine processes and subsystems where the capability of workingmedium to produce useful mechanical work is: (a) destroyed due tothermodynamic irreversibilities, such as combustion, heat transfer,mixing, throttling, friction, etc., and (b) lost due to undesirableavailability (exergy) transfers, such as heat transfer to the cylinderwalls and loss of thermal energy to the exhaust [14]. These systemlosses data are used to develop an explanation of why engineperformance is changed by the variation of design and operatingparameters like speed, load and type of fuel etc.

Availability analysis from the second lawapplication is not a newtechnique [15]. This type of analysis has used for many years forevaluating stationary systems [16,17] and automotive engines[10,18e23]. Alkidas [24] presented availability balance equations fora diesel engine on an overall basis. Various Cummins EngineCompany researchers [10,19,25,26] presented a quantification ofengine irreversibility. They reported the throttling and thermalmixing losses along with the well established combustion irre-versibility. In contrast, Van Gerpen and Shapiro [11] performeda detailed theoretical analysis for the closed part of the cyclebringing in focus the controversial term of chemical availability. Thechemical availability is shown to be significant and necessary toobtain an accurate estimate of the irreversibility. A different type ofstudy, the combined energy and exergy analysis can be used todetermine an optimumengine condition. In this context, the energyefficiency found maximum at a speed of 2040 rpm whereasmaximum exergy efficiency found at a speed of 2580 rpm for a SIengine operation [22]. In another study, Sahoo et al. [23] selected theoptimumengine speedof adiesel engine for a given throttleopeningposition under a single loading condition using second law analysis.

A thermodynamic cycle simulation was used to obtain theavailability characteristics as functions of speed and load for a SIengine [27]. The availability destroyed (as a percentage of the fuelavailability) by the combustion process resulted in the rangebetween 5 and 25%. This fraction found lowest for the highest speedand load. The author also investigated the effects of compressionratio (CR) and expansion ratio (ER) on second lawparameters for a SIengine [28,29]. This study examined the cases of part load andwideopen throttle condition at different CRs and ERs. In another paper,

Chavannavar and Caton [30] studied parametrically the destructionof availability at constant pressure, constant volume, and constanttemperature combustion processes for isooctane vapor and airmixtures. The results of this work showed that the availabilitydestruction decreased with increasing reactant temperatures. Theeffect of the reactants mixture pressure on availability destructionshowed less pronounced.

Recently, Rakopoulos and co-workers are carrying out theirsignificant effort on availability analysis research in IC engines usingof various alternative fuels including syngas. Rakopoulos and Kyr-itsis [31] developed a method for computing both combustionirreversibility and working medium availability in a diesel enginecylinder. They calculated analytically combustion irreversibility asa function of fuel reaction rate using the second law analysis anda chemical equilibriumhypothesis. The fuelsmethane andmethanolshowed better competence as compared with dodecane fuel. Thesame authors [32] studied the availability balance computationallyusing a zero-dimensional model during combustion of hydrogen-enriched natural and landfill gases in engine cylinders. From thesecond law point of view, hydrogen combustion showed qualita-tively different from the combustion of hydrocarbon fuels. Rako-poulos and Michos [33] developed a multi-zone combustion modelfor prediction of SI engine performance andNO emissions. Theyalsopresented an availability analysis of the same engine with varyingload by using syngas as fuel [34]. The computed NO emissions fromthe multi-zone model for various engine loads found to be in goodagreement with the respective experimental ones.

1.1. Availability concept

The concept of ‘availability’ (also called ‘exergy’) is introduced bythe second law of thermodynamics. The availability of a thermody-namic system is defined as the maximum useful mechanical workthat can be produced when the system is brought to thermal,mechanical and chemical equilibriumwith its environment throughreversible processes. It is an extensive property of the system anddepends on both the state of the system and on the properties of theenvironment. The state of the environment is referred to as the deadstate, defined by the environmental temperature, pressure andcomposition. In availability analyses of thermal systems, it is cust-omary to divide the availability content of a system into twoparts [9]:

i. The thermo-mechanical availability: It refers to the maximumuseful mechanical work extractable as the system comes into

B.B. Sahoo et al. / Energy 36 (2011) 760e769762

thermal and mechanical equilibrium with the surroundingatmosphere. The mass of the system is not permitted to passor chemically react with the environment. The thermal andmechanical equilibrium are achieved when both thetemperature and pressure of the system are equal to that ofthe environment. This specific state of the system is called therestricted dead state.

ii. The chemical availability: One part of the chemical availabilityof a system concerns only the system’s species that are alsopresent in the environment, known as diffusion availability.Whereas, the other part, called reactive availability, concernsthe amount of work developed by allowing species of thesystem to chemically react with substances of the environ-ment in order to form also environmental species [30]. Thesystem achieves the chemical equilibrium when any of itscomponents unable to interact in any way with those of theenvironment in order to produce work.

1.2. Research objectives

The first and second law analysis should be coupled together inorder to complete the theoretical treatment of an IC engineoperation. In this way, the analysis provides both general perfor-mance calculations with the details of the overall thermodynamicsof engine operation. The present work will apply the thermo-mechanical availability analysis to a four-stroke, direct injection(DI) and constant speed CI diesel engine operation using syngas asfuel under a dual fuel mode. In this work, the effects of variouscombinations of H2/CO volumetric ratio rich syngas on the dualfuel engine performance are examined from the second lawperspective. Finally, the outcomes of dual fuel mode are comparedto that of diesel mode. Specifically, the effects of fuel type on allexisting availability terms: brake power output, coolant heattransfer, exhaust losses, exergy efficiency, airefuel ratio, andirreversibility, are explored by both first and second law ofthermodynamics.

2. Experimental investigation

2.1. Test engine and measuring devices

The first law and second law analyses are applied to a KirloskarTV1 Make diesel engine fueled with diesel and variable H2/COvolumetric composition syngas. It is a single-cylinder, constantspeed, water-cooled, four-stroke, and DI diesel engine. The mainengine specifications are: bore 87.5 mm, stroke 110 mm, compres-sion ratio 17.5, displacement volume 661 cc and maximum power(with diesel fuel) 5.2 kW (7 BHP) at 1500 rpm. The engine load isvariedwith an eddy current type dynamometer. The engine featureswith a conventional fuel injection systemwith the static diesel fuelinjection timing of 23� BTDC. The injection nozzle features threeholes of 0.3 mm diameter with a 120� spray angle. The engine isprovidedwith ahemispherical combustion chamber. Engine coolingis accomplished by circulating water through the jackets of theengine block and the cylinder head. A piezoelectric pressure trans-ducer ismounted in flushwith the cylinder head surface tomeasurethe cylinder pressure. The liquid fuel is supplied to the engineinjection pump from fuel tank under gravity feed. The air flow intothe engine is monitored by passing the intake air through an air boxwith orifice meter and manometer. Two individual flow meters arealso inbuilt with the engine data acquisition system to control thewater flow through the engine and calorimeter to cool them. Forcombustion diagnostics, the in-cylinder pressure is measured usinga piezo sensor (PCB Make) mounted on the engine cylinder. The

crank angle (CA)measurement is sensedbyanoptical sensorand thedata are acquired on a Personal Computer (PC). The varioustemperatures are measured using K-type thermocouples fitted onrespective positions. For performance analysis purpose the engine isfacilitated with ‘Enginesoft’ software.

A few additional components such as gas mixer, flow meter,non-return valve, pressure regulator and gas carburetor etc. areincorporated into the original diesel engine setup for executing thedual fuel operation. The schematic of dual fuel engine is shown inFig. 1. A two-stage pressure relief valve reduces the high pressure ofindividual gases (H2 and CO) supply from the storage cylinder to gasmixer through non-return valves and flow meters. The gas mixerblends a mixture of individual gas components H2 (99.99% purity)and CO (99.95% purity) to prepare simulated syngas. In order toachieve a better mixing of H2 and CO, the turbulence is generatedby feeding individual gas at an angle into the gas mixer. The pres-sure of syngas flow inside the gas mixer is reduced by providingwire mesh in the path of gas flow. A gas carburetor is fastened inbetween engine intake manifold and air suction side. The inletpressure of syngas into the carburetor from the gas mixer is kept atatmospheric condition as that of air through a single stage pressureregulator. The simulated syngas is supplied through an obliqueprotruding designed gas nozzle with an angle of 30� to the hori-zontal axis of carburetor. This ensures a homogeneous airesyngasmixture because the required venturi of the carburetor is created bythis design approach.

2.2. Fuel type

In this study, both liquid and gaseous fuels were served as dualfuel in a diesel engine, namely, standard diesel as liquid andsimulated syngas as fuel-gas. The diesel oil was used as the pilotfuel in dual fuel operation. While the primary fuel, syngas, wassimulated by mixing two pure gases, namely, H2 and CO in the gasmixer. The fuel-gas quality is determined by the H2 and CO content,expressed in percentage (%) by volume. Table 1 summarizesimportant properties of fuels utilized in the experiments. The lowerheating value (LHV) of syngas was calculated theoretically from theeach elementary gas LHV. In this experimental work, a total of fourdifferent types of syngas fuels were tested in dual fuel mode. Thevolumetric fraction of H2 content in fuel-gas was varied to 100, 75,50 and 0% of the total H2 and CO contained syngas. Hence, thebalance was the CO content in the syngas fuel.

2.3. Engine test procedure

In order to establish the basis for comparison of dual fuel esti-mated results, a baseline test with 100% diesel fuel was also con-ducted. To ensure the consistency of the observations, engine testswere conducted as per experimental design as given in Table 2. Theengine was operated at different load levels ranges fromaminimum of 20% load to amaximum of 100% loadwith an intervalof 20% for both diesel and dual fuel mode. The complete experi-mental matrix was repeated for at least three times to recordaverage experimental data for analysis purpose. The engine wastested for baseline results followed by dual fuel experiments. Thebaseline experiment was carried out with the engine operating ondiesel fuel only. Engine operation was started with diesel fuel andrun for few minutes at 1500 rpm under no-load condition to warmup and reach stable operating conditions. The water flow wasadjusted to 250 and 70 l/h for the engine cooling and calorimeterrespectively according to the engine supplier instructions. Then, asper experimental design a load level was set for engine operation.Once the engine reached the steady-state condition, air and fuelflow rates along with the various temperature readings were

Fig. 1. Schematic diagram of the dual fuel engine setup.

B.B. Sahoo et al. / Energy 36 (2011) 760e769 763

recorded and insertedmanually to the computer software program.The data were converted into engineering units and, are updatedand displayed on a monitor at every second.

Now, for dual fuel operation, the H2 and CO gases were suppliedfrom respective high pressure cylinder to an outlet pressure of1e2 bar using two-stage pressure relief valves. The simulatedsyngas was specified as per its H2 and CO volumetric composition,for an example H2:CO ratio of is 1:1 for H2:COT50:50 syngas. Theproportion of H2 and CO in syngas was controlled throughoutthe dual fuel operation by adjusting the individual gas flow rate.The required flow rates of H2, CO and syngas, were achieved bymanual adjustment of the control valves, and were measuredseparately using calibrated flow meters. For stable operatingconditions, at a set engine load, the syngas fuel valve was openedslowly and allowed the fuel-gas to enter from mixer to gas carbu-retor. The homogeneous airegas mixture from carburetor was thensucked into the cylinder to take part in the dual fuel combustion.The syngas flow was increased till engine shows signs of knock.This decided the maximum gas flow for the dual fuel operation.During the process, engine speed increased due to added extrachemical energy from gaseous fuel. To maintain the constantengine operating speed of 1500� 50 rpm and also, same poweroutput as of diesel mode, supply of diesel to the enginewas reducedby adjusting diesel cut-off valve. Finally, the cut-off valve waslocked manually at the rated engine speed. Now, for a steady-stateoperation, again the same input manual parameters, as describedfor baseline tests, were inserted into the computer softwareprogram for the dual fuel results. Once all the necessary testreadings were sorted out, the normal diesel oil operation of theengine was restored by shutting syngas flow and adjusting diesel

Table 1Physical and chemical fuel properties.

Properties Diesel Syngas

Chemicalcomposition

C12H26 100% H2þ 0% CO

Density (kg/m3) 850 0.085Calorific value (MJ/kg) 42 119.81Cetane number 45e55 e

Stoichiometric A/F ratio 14.92 34.3Energy density (MJ/Nm3) 2.82 2.87

cut-off valve to original position. The engine is now in a position fora change in engine loading or use of different type of syngas fuel foranother set of experimental results starting from baseline readings.This above experimental measurement procedure was repeated asper the experimental design. At the end of whole experimentaldesign, syngas flow rate was ceased completely and the engine wasmade to run at a steady-state condition using only diesel at no-loadcondition before shut down.

3. Thermodynamic analysis

Initially, the first law analysis is presented for both the dieseland dual fuel modes. This analysis is shown in order to assist thecomprehension of the second law analysis to follow.

3.1. First law analysis

The energy input (Qin) in any IC engine is contained in its fuel.This amount of input energy is then converted into other forms[20,35]. In an engine, the input chemical energy of fuel is usuallyconverted to the following forms [23]:

(a) Useful work output or shaft energy (Pshaft);(b) Energy transferred to cooling water (Qcw);(c) Energy transferred to the exhaust gases (Qeg); and,(d) Uncounted losses (Quncounted) due to friction, radiation, heat

transfer to surroundings, operating auxiliary equipment, etc.

The amount of each of these energies stated above evaluated onthe basis of the first law of thermodynamics is now described. The

75% H2þ 25% CO 50% H2þ 50% CO 0% H2þ 100% CO

0.38 0.67 1.923.09 14.81 10.112e e e

8.075 4.58 2.452.38 2.70 3.79

Fig. 3. Availability distributionwith varying fuel input as a function of load (dual fuel 1).

Table 2Experimental design for data collection.

Mode ofoperation

Fuel Designationof operation

Engineoperation

Diesel fuel 100% Standarddiesel

Baseline test Speed: 1500� 50 rpmLoad: 20, 40, 60, 80, 100%Injection timing: 23� BTDC

Dual fuel Primary: syngas(H2:COT100:0)

Dual fuel 1 - do -

Pilot: dieselDual fuel Primary: syngas

(H2:COT75:25)Dual fuel 2 - do -

Pilot: dieselDual fuel Primary: syngas

(H2:COT50:50)Dual fuel 3 - do -

Pilot: dieselDual fuel Primary: syngas

(H2:COT0:100)Dual fuel 4 - do -

Pilot: diesel

B.B. Sahoo et al. / Energy 36 (2011) 760e769764

input energy (Qin) to the diesel engine is the amount of fuel energycontent in the supplied fuel and it is given by,

For a diesel mode,

Qin ¼ ��_md=3600

�� LHVd�; kW (1)

For a dual fuel mode,

Qin ¼h�

_mpd=3600��LHVpd

iþ ��

_mg=3600��LHVg

�; kW (2)

The energy converted to shaft output,

Pshaft ¼ Brake power output; kW (3)

The heat loss from the engine block to the cooling water is givenby,

Qcw ¼ ��_mw=3600

�� Cpw � ðT2 � T1Þ�; kW (4)

The energy wasted in form of exhaust gas losses is evaluated by,

Qeg ¼ ��_meg=3600

�� Cpeg � ðT5 � T6Þ�; kW (5)

The physical property of the exhaust gas (Cpeg) is determinedfrom the energy balance of exhaust gas calorimeter. The variation ofCpeg with exhaust gas temperature is considered here for the moreaccurate analysis.

The amount of the uncounted losses is determined by per-forming an energy balance and is given by,

Quncounted ¼hQin �

�Pshaft þ Qcw þ Qeg

�i; kW (6)

Fig. 2. Availability distribution with varying fuel input as a function of load (dieselfuel).

3.2. Second law analysis

The second law analysis indicates various forms of energy thathave different levels of ability to do useful mechanical work. Thisability to perform useful mechanical work is defined as availability[35]. In an IC engine, the availability input (Ain) which contained inits chemical availability of fuel is converted into other exergy forms[20]. In an engine, the input availability in fuel energy is convertedto the following forms [23]:

i. Useful work output or shaft availability (Ashaft);ii. Availability transferred to cooling water (Acw);iii. Availability transferred to the exhaust gases (Aeg);iv. Uncounted availability destructions (Adestroyed) due to friction,

radiation, heat transfer to surroundings, operating auxiliaryequipment, etc.

The amount of each of these availability transfers evaluated onthe basis of the second law of thermodynamics is now explained.Chemical availability of fuel or input availability,

For diesel mode [10],

Ain ¼ �1:0338� �

_md=3600�� LHVd

�; kW (7)

For dual fuel mode,

Ain ¼hApd þ Ag

i; kW (8)

where, Apd ¼ ½1:0338� ð _mpd=3600Þ � LHVd�; kW, Ag ¼ ½0:985�ð _mg=3600Þ � LHVg�; kW (for the only H2 content gaseous fuels[36]); and, Ag ¼ ½0:95� ð _mg=3600Þ �HHVg�; kW (for the comp-licated structure gaseous fuels [37])

Fig. 4. Availability distributionwith varying fuel input as a function of load (dual fuel 2).

Fig. 5. Availability distributionwith varying fuel input as a function of load (dual fuel 3). Fig. 7. Availability input distribution at different engine load.

B.B. Sahoo et al. / Energy 36 (2011) 760e769 765

The availability associated with the shaft work,

Ashaft ¼ Brake power output; kW (9)

The availability associated with the cooling water,

Acw ¼ Qcw � ��_mw=3600

�� Cpw � T0 � lnðT2=T1Þ�; kW (10)

The availability associated with the exhaust gas is given by,

Aeg ¼ Qeg þ��

_meg=3600�� T0 �

�Cpeg lnfT0=T5g

� Reg ln�P0=Pego

��; kW (11)

Considering the complete combustion of fuel, first, the molec-ular weight (MW) (kg/kmol) of products of combustion is calcu-lated. Then, the specific gas constant of the exhaust gas (Reg) isdetermined from the thermodynamic relation, Reg¼ Ru/MW, (kJ/kg-K), where, Ru is the Universal gas constant (8.314 kJ/kmol-K).

The uncounted availability destruction is determined from theavailability balance as,

Adestroyed ¼hAin �

�Ashaft þ Acw þ Aeg

�i; kW (12)

The exergy efficiency (hII) is the ratio of total availabilityrecovered from the system to the total availability input into thesystem. The recovered availability includes Ashaft, Aeg and Acw.

hII ¼ ðAvailability recovered=Availability inputÞ¼ 1�

�Adestroyed=Ain

�(13)

Fig. 6. Availability distributionwith varying fuel input as a function of load (dual fuel 4).

4. Results and discussion

A detailed presentation of the syngasediesel dual fuelperformance compared with diesel mode has been provided inRef. [38,39]. The experimental observation data are retrieved herefor the second law analysis purpose. The availability balance foreach one of the five fuel types used during both the diesel anddual fuel operations as a function of engine load are presented inFigs. 2e6. This is a balance between the various terms of thesecond law analysis described by Eqs. (7)e(13), namely, fuelavailability input, work availability, availability exchange throughcooling water and exhaust gas, availability destroyed, and exergyefficiency. During the engine operation, as load increases, thericher fueleair mixture increases combustion temperature.Therefore, increased work availability and reduced heat transferavailability losses are obtained, as percentages of the fuel chem-ical availability. For this, an increase in the exergy efficiency isresulted at higher loads for all the tested fuels. Specifically, thedual fuel operations are favored thermodynamically at higherloads since their exergy efficiencies improve significantly ascompared to low load conditions. Because of the improvedcombustion syngas at higher loads, (mainly) the exhaust gasavailability and cooling availability (although of small quantity)are increased. In addition, the shaft availability of the fuels isincreased for an increased load. Therefore, when load is increased,the added cumulative availabilities increased the exergy effi-ciency. However, this efficiency is decreased slightly after the 80%load due to the poor combustion of fuels where the oxygenavailability diminished. On the contrary to exergy efficiency trend,

Fig. 8. Shaft availability distribution at different engine load.

Fig. 9. Cooling water availability distribution at different engine load. Fig. 11. Destroyed availability distribution at different engine load.

B.B. Sahoo et al. / Energy 36 (2011) 760e769766

after the 80% load, the destroyed availability is increased due tothe raised heat transfer from the engine.

Fig. 7 depicts the availability distribution on the basis of kW ofavailable energy input to the diesel and dual fuel operations. Whenload is raised tomaintain a higher power output at higher loads, thesupply of fuel chemical energy in to the engine cylinder is increased.In the process, at higher engine loads, the shaft availability iscalculated against the amount fuel exergy input. The quantity of fuelexergy input for the engine operation at a given load mostlydepends on the energy content of the fuel type and effectivecombustion of the fueleair mixture. Although some tested gaseousfuels seem to have higher energy content than diesel fuel, however,at lower loads of 20e40%, all the testeddual fuel operations requiredhigher fuel exergy input as compared to dieselmode. This is becauseof their poor combustion characteristics in the low temperatureenvironment. As the load was increased, the differences in fuelexergy input reduced under dual fuel modes as opposed to dieselmode for their improved combustion. Again, at high temperaturezone of 100% load, the chemical energy requirement increased forthe dual fuel operations due to the diminishing of oxygen avail-ability needed for the complete combustion.

Figs. 8e11 present the diesel and dual fuel mode comparison ofshaft, cooling water, exhaust gas and destroyed availability withload with respect to their respective fuel chemical availability. Asdiscussed earlier, the shaft work produced at different loads of bothdiesel and dual fuel modes are same. But the shaft availability as a %fuel input for the fuels is different due to the difference in fuel’s

Fig. 10. Exhaust gas availability distribution at different engine load.

input chemical availability as shown in Fig. 7. At low loads (20% and40%), the shaft availability of dual fuel mode is very poor ascompared to diesel mode. However, this value improved as the loadis increased. Generally, increase in the load results the enhance-ment of the combustion process, increasing the combustiontemperatures and the peak cylinder pressure and reducing thecombustion duration. The maximum shaft availability recorded atmaximum thermal efficiency point (Fig. 12) of 80% load andthereafter, it reduced slightly up to 100% load. The shaft availabilityis found highest in case of diesel mode than all the tested dual fuelmodes at all ranges of load. For diesel mode, at 80% load, the shaftavailability is found as 20.2%.Whereas, at same loading, this value isfigured as 19.8, 17.5, 15.8 and 15.6% for the 100, 75, 50 and 0% H2syngas dual fuel modes respectively (Fig. 8). The cooling availabilityof dual fuel operations is very little due to the intensive cylinderwall loss (Fig. 9). Only a maximum of about 1% fuel input coolingavailability is accessible from dual fuel operations as compared toabout 2e3% to that of diesel mode. This is because, in comparison tothe much higher chemical energy input during dual fuel operationsto that of diesel mode, the level of increase in engine cooling watertemperature of dual fuel modes are less than that of diesel mode.

The dual fuel operations of syngas fuels produced about 100 �Chigher exhaust gas temperatures as compared to diesel mode at allranges of load (Fig. 13). This is due to the late and inadequatecombustion time of gaseous fuels under dual fuel mode [38,39].This leads to higher exhaust gas availability for the dual fuel modes(Fig. 10). The 100% H2 syngas dual fuel mode produced maximumexhaust gas availability as this operation recorded maximumtemperature in its exhaust. The maximum exhaust gas availability

Fig. 12. Variation of brake thermal efficiency with engine load.

Fig. 13. Variation of exhaust gas temperature with engine load.Fig. 15. Effect of engine load on total raised work availability.

B.B. Sahoo et al. / Energy 36 (2011) 760e769 767

is found as 17.2% for 100% H2 syngas at 80% load as compared to thatof 14.2% for diesel mode. Whereas, for 75 and 50% H2 syngas fuelsthe maximum of this value is found as 14.3 and 10.6% for respec-tively at same load condition. However, the maximum exhaust gasavailability for 100% CO syngas mode is observed as 9.8% but at100% load. This is due to the better combustion of CO gas at highertemperature zone. Due to these huge availability losses throughexhaust gas, the efficiencies of dual fuel operations are lower thanthat of diesel mode. Therefore, it can be concluded that the exhaustgas available energy loss must be reduced to improve dual fuelengine performance. The amount of destroyed availability (asa percentage of fuel input) is decreased with increasing load. This isdue to the fact that, as load increases, greater values of fueleairequivalence ratio cause greater temperatures inside the cylinderand it results better combustion of gaseous fuels. At low loads of20% and 40%, poor combustion of syngas fuels causes less coolingwater and exhaust gas availabilities i.e., higher destroyed avail-ability (Fig. 11). The destroyed availability was found minimum atthe maximum efficiency condition of 80% engine load. Diesel modeshowed the minimum destroyed availability loss (61.8%) among allthe tested fuel modes. While at the same loading condition, theminimum of this value was found to be 62, 67.4, 72.3 and 74.8% forthe 100, 75, 50 and 0% H2 content syngas operations respectively.

The availability balance is affected by the content of the syngasfuel as shown in Fig. 14. The percentage of fuel availability thatdestroyed is reduced with the increase in H2 content in syngas.These results are in agreement with the results of H2/NG blendsreached in Ref. [31]. This decreasewith increasing H2 content is dueto the entropy generation [40] and more specifically, for betterairefuel mixture combustion [31,32]. For the 100% H2 syngas mode,the destroyed availability is noticed least as compared to other

Fig. 14. Exergy efficiency and destroyed availability as a function of hydrogen content.

syngas fuels. Contrary to the trend of destroyed availability, thesecond law efficiency increased with increasing H2 content. This isa combined effect of reduction of combustion irreversibility andincrease in the maximum temperature of the cycle, which causedefficiency gains during the operation.

Fig. 15 shows the shaft availability (kW) as a function of load,along with the increase in work availability from both the exhaustgas and cooling water for all the tested fuel types. The availabilityresults showed that, as the load increased, the dual fuel operationsgenerated more increase in the cumulative exhaust gas and coolingwater availabilities. This allowed the more of the availability acces-sible for conversion to work availability. The drawback of dual fueloperations due to its poor efficiency can be resolved by accessingabout 1.5e4 kW of work availability losses through an effectiveexhaust gas heat recovery system. The increase in the gross workoutput availability increased the corresponding exergy efficiency(Fig.16). At 80% load, the second lawefficiency is observedhighest incase of diesel mode and recorded a maximum of 38.2%. While atsame loading condition, when the volumetric fraction of H2 insyngas increased from 0% to 50, 75 and 100%, the maximum secondlaw efficiency enhanced from 25.2% to 27.2, 33.6 and 38%, respec-tively accompanied by a simultaneous reduction in the destroyedavailability. For higher H2 content syngas, the energy input into thecylinder increased, and as a consequence, the correspondingcumulative work availability also increased. Moreover, increase inthe H2 content in syngas resulted an improvement of the combus-tion process from the second law viewpoint. Therefore, at higherloads, it can be seen that 75% and 100% syngas combustion givecomparative exergy efficiency as compared to that of diesel mode.This demonstrates that dual fuel engine operations cannot beignoredon thebasis of their lowerefficiency in adiesel enginewhichwas actually designed for the standard diesel fuel. Therefore, a dual

Fig. 16. Comparison of exergy efficiency as a function of engine load.

B.B. Sahoo et al. / Energy 36 (2011) 760e769768

fuel engine must be provided with its exhaust gas heat recoverysystem for a better efficiency figure along with other benefits.

5. Error analysis

The uncertainty associated with the dual fuel engine perfor-mance calculation is estimated using sequential perturbationtechniques [41,42]. It includes contributions from individualuncertainties in measurement mass of diesel, syngas and air flow(1%); water flow (2%); lower heating value calculation (1%); enginespeed (0.5%); engine load (0.5%); and temperature (0.5%). Based onthe above values, the calculated engine performance is believed tobe accurate within �3%.

6. Conclusions

A second law analysis was performed on a single-cylinder,constant speed and direct injection diesel engine. The engine wastested for the diesel mode with diesel fuel and dual fuel operationswith different types of syngas fuel. Availability equations wereapplied on varying load experimental results of both diesel anddual fuel modes. The various kinds of availability terms (i.e. shaft,cooling water, exhaust gas and destroyed availability) werecompared and discussed.

B At higher loads, the syngas dual fuel operations are advan-tageous from the second law perspective. With increasingload, the destroyed availability decreases due to highercombustion temperature and pressure, and therefore, theexergy efficiency increases.

B As load increases, fuel supply into the engine cylinderincreases. This added chemical energy from the fuelsincreases the amount of kW fuel input availability of engine.Thus, increase in load increases the shaft availability (asa percentage fuel input) of all the tested fuels.

B The exergy efficiency of 100% H2 syngas differs by an amountbelow 0.5% only to that of diesel mode (38.2%) at maximumefficiency point. It demonstrates that hydrogen is an effectivegaseous fuel in a diesel engine under dual fuel operation.

B The maximum thermal efficiency of 100, 75, 50 and 0% H2content syngas dual fuel modes were found as 19.8, 18.3, 16.1and 15.7% respectively at 80% load. However, at this bestefficiency loading point, the maximum exergy efficiencies arefound as 38, 33.6, 27.2 and 25.2% for the same syngas fuelsrespectively. This indicates that dual fuel operations canincrease their work availability by accessing, mainly, theirexhaust gas availability loss (about 8e17% of fuel input).

B The coolingwater availability from diesel mode is determinedas about 2e3% of fuel input at all ranges of load. However, thisvalue is of maximum 1% of fuel input only for dual fueloperations due to the lower cylinder wall loss and higherexhaust gas temperature.

B At maximum exergy condition of 80% load, increase in thevolumetric fraction of H2 in syngas from 0% to 100%, results inan increase of second law efficiency about 34%. This increaseis accompanied by an increase in the work availability fromexhaust gas and cooling water.

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