J. Mahdavi , M.R. Modarres Razavi , H. Niazmand
Department of Mechanical Engineering
University of Ferdowsi
Mashhad, Iran, 9177948944
Email: ja_mahd@yahoo.com
ABSTRACT
Hydrogen is considered as an ideal alternative fuel in and Hydrogen (H only water (H
2
2
). Burning hydrogen theoretically produces
O) with no CO, CO
2
, HC or fine particles, and since it can be produced without causing any ecological internal combustion engines with some shortcomings such as high NOx emission, tendency of preignition and knock, and low power output. In this study, a mathematical model is developed to predict the operating characteristics of a spark ignition hydrogen fuelled engine coupled with an exhaust gas recirculation (EGR) system. The proposed system of recycling disorder, hydrogen as a future fuel has been drawing greater attention. Besides being the cleanest burning chemical fuel, hydrogen can be produced from water (using nonfossil energy
[1]) and conversely, it forms water again in a combustion of engine exhaust gases improves the engine performance and eliminates pollutants emissions by producing only water. The effects of various parameters such as compression ratio, quasi equivalence ratio, supply flow rates and percentage of exhaust gas recirculation (EGR) to the engine performance are studied process.
Many researchers have studied the effects of using hydrogen as a fuel (pure or blended with another fuel) on engine performance and pollutant emissions [2–9] and have considered hydrogen as an ideal alternative fuel.
In this study, hydrogen having the following advantages and compared with the previous analytical data obtained for a
CFR engine [1].
INTRODUCTION
Energy crisis and pollution problems have raised the number of investigations on the different methods of increasing efficiency and lowering the concentration of undesired components in combustion products such as using alternative fuels. Air pollution, in fact, has become a serious problem in large urban areas, and a major portion of it has been linked to the internal combustion engines. The incomplete combustion of hydrocarbons with air as an oxidant in conventional engines results in the production and emission of a wide range of pollutants comprising carbon monoxide, unburned hydrocarbons, oxides of nitrogen, smoke and such toxic substances like fine particles. In addition to the air pollution problem, further pressure is built up on all concerned in research and development to find out the alternative sources of energy to replace the rapidly depleting petroleum resources.
This alarming fact has led researchers to try hard in finding the alternatives to the presently used fuels, which could be pollution-free, environmentally friendly, and also economical and compatible to use in automobiles. Among the suggested alternatives, gaseous fuels have shown very good performance over others like alcohol. Among the gaseous fuels suggested are: CNG (mainly Methane, CH
4
), LPG (mainly Propane, C
3
H
8
) over gasoline is considered as an alternative fuel.
1.
Reduced deposits and increased combustion efficiency due to more homogeneous mixture formation arising from the higher molecular diffusivity of hydrogen in air.
2.
Reduced engine oil dilution and increased oil life.
3.
Reduced engine wear, hence increased engine life.
4.
Higher compression ratios could be utilized and thus the related increased efficiency might weaken the problem of the reduced power output due to the reduction in volumetric efficiency.
5.
Elimination of CO, CO
2
and HC emissions.
6.
Increased fuel economy due to possible operation at ultra lean mixtures.
With hydrogen as a fuel, it is obvious that hydrocarbons and carbon monoxide do not exist and leaving only oxides of nitrogen and steam to be in the exhaust gases. A further refinement can eliminate the oxides of nitrogen from exhaust gases by burning hydrogen with oxygen instead of air [1].
Considering these introducing remarks, the prime purpose of this work is to study the feasibility of the development of a clean S.I. engine with no undesirable exhaust emissions.
Therefore, in this work, hydrogen burns only with oxygen instead of air in a S.I. engine. With regard to high flame temperature and burning velocity of stoichiometric mixture of hydrogen and oxygen, by using diluent or recycling the exhaust gases to the hydrogen-oxygen mixture or using the rich mixture of hydrogen-oxygen, the peak cylinder temperature and
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pressure can be controlled. The proposed system of recycling of engine exhaust gases improves the engine performance and eliminates pollutant emissions by producing only water. Three different schemes are studied in this work. Two schemes of exhaust gases recycling systems are considered; in the first scheme, a portion of the exhaust gases is recycled directly to the engine intake and a stoichiometric mixture in the engine intake is produced. In the second one, first water vapor is partially removed from the exhaust gases through condensation and then the remaining gases are recycled to produce a rich mixture in the engine intake. The third scheme is related to the engine performance using different diluents without the recycling system.
HYDROGEN AS AN ALTERNATIVE FUEL
For any fuel to be considered as an alternative fuel, it has to fulfil certain criteria. The basic criteria for selecting any alternative fuel are:
1.
Availability: the fuel has to be in abundant supply or, preferably, derives from renewable sources.
2.
High specific energy content.
3.
Easy transportation and storage.
4.
Minimum environmental pollution and resource depletion.
5.
Good safety and handling properties.
Hydrogen has proved its superiority over gasoline in most of these criteria [4-7]. It can be noticed from Table 1 that the hydrogen fuelled engine tends to operate at leaner mixtures, making engine operation more economical. With a higher calorific value, lower density and lower boiling point, hydrogen fuelled engine operation and life are significantly improved with respect to gasoline fuelled engines. From the viewpoint of engine performance parameters, operation with hydrogen reduces the specific fuel consumption. However, because of the loss of volumetric efficiency, mainly due to lower molecular weight of hydrogen, these engines tend to produce about 20% less power compare to gasoline fuelled engines.
The burning velocity of hydrogen-air mixture is about six times higher than that of the gasoline-air mixture. As the burning velocity rises, the actual indicator diagram approaches closer to the ideal diagram and a higher thermodynamic efficiency is achieved [5]. In addition to the efficiency contribution, high flame speeds reduce the end gas residence times in the cylinder, and thus reduce the chance of knock to occur. This follows from the belief that the existence or nonexistence of knock depends essentially on a race between the arrival of the flame front at the end gas region and the autoignition of the end gas.
Hydrogen fuelled reciprocating engines operates in rich and lean conditions effectively, considering wide flammability limits of hydrogen. The hydrogen fuel when mixes with air produces a combustible mixture, which can be burned in a conventional spark ignition engine at an equivalence ratio much below the lean flammability limit of a gasoline-air mixture. The resulting ultra lean combustion produces low flame temperatures and leads directly to lower heat transfer through the engine walls, higher engine efficiency and lower NOx emission [5].
The self-ignition temperature of the hydrogen-air mixture is greater than that of the other fuels especially gasoline and, therefore hydrogen has higher octane number and tendency to resist knock.
Using a gaseous fuel (hydrogen) rather than a liquid fuel
(gasoline) for the short periods during cold start and warm up, avoids problems of cold fuel evaporation, uneven distribution of the fuel to the different cylinders due to the presence of a liquid film on the walls of the intake manifold and the unwanted large variations in supplied air-fuel ratio during transient conditions such as acceleration and deceleration [3].
Therefore, the results of different studies clearly establish that hydrogen fuel can increases the effective efficiency of the engine and thus reduces the specific fuel consumption.
ANALYSIS OF THE PROPOSSED SYSTEM
A carbureted (or injected) hydrogen fuelled engine generally develops lower maximum power and higher NOx emissions compared to an equivalent gasoline engine due to the restricted airflow and the increase of maximum temperature inside the cylinder, respectively [3]. To reduce the amount of
NOx emissions, the hydrogen fuelled engine can be operated with lean equivalence ratios. The lean operation of the hydrogen engine gives lower levels of NOx emissions compared with that of a pure gasoline operation, but with more deterioration in engine power. In order to increase the power output, the hydrogen CFR engine can employ higher compression ratio and intake pressure (with supercharging). With further increase in compression ratio, the engine power output decreases due to unstable combustion. However , the maximum power output of a hydrogen engine is limited by the loss of combustion control, usually described as preignition. Therefore increasing intake pressure is a more effective method to increase the power output of the hydrogen fuelled CFR engine rather than increasing its compression ratio [3].
In present work, an exhaust recycling system is proposed where intake pressure increases through recycling exhaust gases with constant supply flow rate; therefore the thermal efficiency and power output improve. Also, in this system hydrogen burns only with oxygen.
Figure 1 is a schematic diagram for operating conditions when the condenser is used in recycling loop. According to this figure, a supply mixture flows into the system at "1" and mixes with the recycle stream "2" at "3". The total exhaust flow rate passes through the condenser. In this case, the only mass leaving the system is in the form of liquid water at point "6".
Therefore, to prevent mass accumulation and to achieve the steady conditions, the supply mixture is selected as stoichiometric mixture of hydrogen and oxygen. To have the best performance and to control effectively the peak cylinder temperature and pressure, as mentioned before, the quasi equivalence ratio at the engine intake is selected larger than 1.
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Therefore, the recycling system allows unreacted hydrogen to be recycled into the intake, after a portion of steam is removed by the condenser. Hence, it is expected that the overall efficiency does not necessarily fall in rich mixtures as it does in conventional engines.
In the condenser, it is assumed that a portion of water vapor is condensed and extracted from the exhaust gases. Therefore, the remaining recycled stream is saturated with water vapor at the condenser pressure and outlet temperature. Also, the mass flow of water liquid leaving the condenser is the same as the mass flow rate of the supply stream for steady flow condition.
In order to prevent the condensation of water vapor in the engine intake, it is necessary to reheat the recycled stream after it leaves the condenser and before it mixes with the fresh supply mixture. Since gases leaving the condenser are saturated with water vapor, their impact with the fresh supply at a lower temperature will cause condensation, especially if the recycled stream is not preheated. Therefore, a heat exchanger should be included in the system as shown in Fig. 1 which utilizes some energy from the engine exhaust gases to reheat the recycled stream. Also a control device is used to adjust the intake mixture temperature. In reality this is simply a temperature transducer in the intake stream which is linked to a flow-divider in the recycled stream to divide the flow between the heat exchanger and its bypass.
When the condenser is bypassed, a portion of the engine exhaust leaves the system at "6" to prevent mass accumulation, while the remaining part of the exhaust is recycled to the engine intake. Therefore, in this case, the supply mixture is not necessarily to be the stoichiometric ratio. But, since in rich or lean mixtures, unreacted fuel or oxygen in the exhaust of the system can be considered as a direct waste, the supply mixture is chosen stoichiometric. This type of operation is presented schematically by Fig. 2.
It is also necessary to cool the recycled gases in order to prevent an unstable type of operation where both intake and exhaust temperatures creep upwards. As a result, a cooler (a heat exchanger) which can control and maintain the recycled gas temperature at the designed value should be added to the system. For the operations presented here, the intake temperature was maintained at 400 K to ensure that condensation of water vapor would not take place in the intake system.
In each two cases (with or without condenser), fresh charge
"3" enters to the cylinder and mixes with residual gases from previous cycle. For second and subsequent cycles, the composition of residuals is the same as that of the exhaust from the previous cycle. For the first cycle, however, no values are available for the composition of the residuals. If a good estimation for residuals is provided for the first cycle, the computation time required to achieve steady operation of the engine and recycle system is reduced considerably. An accurate estimation of residuals is provided for the first cycle from the complete combustion of entering fresh charge to the cylinder.
As with the residuals, good estimations of the composition and flow rate of the recycled stream are provided for the first cycle from the conservation of mass and chemical species [9].
The simulation program which is used in the present work is based on the previous works by different researchers [2–13] and is an extension of the work of Taylor [1]. His model is largely modified to cover a wide range of engines with recycling system.
A computer quasi one-dimensional model simulating the compression, combustion and expansion processes of spark ignition engine cycles with all species of exhaust emissions is developed for hydrogen fuel. The combustion chamber is generally divided into the burned zone and the unburned zone separated by a flame front. The first law of thermodynamics, equation of state and conservation of mass and volume are applied to the burned and unburned zones. The pressure is assumed to be uniform throughout the cylinder charge. Flow into and out of crevices, and leakages of gases from the cylinder are not accounted. No pressure losses are considered in the system and the pressure drop across the condenser and heat exchanger are assumed to be negligible. A system of first order ordinary differential equations is obtained for the pressure, mass, volume, temperature of the burned and unburned zones and the heat transfer from the burned and unburned zones through the engine walls.
The mass fraction burning rate is modeled by the following equation (triangular burning model): d dx d

 dx







 d dx d

 dx





 max max

 max
  e .
i .
  e .
i .

 e .
c .

 
 e .
c .
max

 e .
i max







 max e .
c .
(1)
Where θ is the crank angle, θ e.i.
, θ e.c.
and θ max
are the crank angels at the end of ignition lag, the end of combustion period and at a location associated with the maximum mass burning rate; dx/dθ is the mass burning rate and (dx/dθ) max
is the maximum mass burning rate.
The instantaneous heat interaction between the cylinder content (burned and unburned zones) and its walls is calculated by using the Annand's model based on the convective and radiation heat transfer. The cylinder pressure and the temperatures of the burnt and unburned zones are predicted using energy, mass and volume balance equations and the equation of state.
A chemical reaction scheme involving six species (H
2
, H,
O
2
, O, OH, and H
2
O) and four reaction steps is considered in the calculation of combustion product concentrations. The following four reaction equations are used:
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1 / 2 H
2

H
1
1 / 2
H
/
2
2 O
2

O
H
2

1 /

1 /
2 O
2
2 O
2


OH
H
2
O
(2)
More details of the developed model, the equations and the solutions are presented in reference [11].
The modeled engine used for validation of the program, is the CFR (variable compression ratio) single cylinder four stroke engine with cylinder bore of 82.6 mm, stroke of 114 mm and engine speed of 3000 rpm.
The results of the mathematical model are then verified with the results obtained by a zero-dimensional Taylor's model
[1]. The various simplifying assumptions made by Taylor are substituted by real considerations. Some of the differences between the model used by Taylor and the present work are: using combustion model to consider the effect of burning time, considering different heat transfer correlation, using the better estimation of thermodynamic properties of working fluid throughout the engine cycle, using better chemical reaction scheme involving 6 species and four reaction steps, …
Figures 3 and 4 show the results of this comparison. Any difference in the results can be interpreted by the simplification done in model of reference [1]. It is obvious that, despite of the difference in the results, the variation trends for all of the curves are similar.
Figure 3 shows the predicted trend in indicated thermal efficiency as a function of compression ratio. The trend of increasing efficiency with increasing compression ratio is what will be expected because of increasing expansion stroke.
With the operating conditions represented in Fig. 4, the indicated thermal efficiency increases with increasing quasi equivalence ratio at the engine intake (quasi equivalence ratio is defined in this work as the ratio of the hydrogen-oxygen ratio to the stoichiometric hydrogen-oxygen ratio). The rising efficiency with respect to increasing quasi equivalence ratio can be attributed to a number of factors. Perhaps the main contribution to the increased efficiency at higher quasi equivalence ratios is that of the accompanying lower peak cylinder temperatures, which result in less thermal dissociation. With less thermal dissociation, more fuel is reacted, releasing more energy in the position of peak cylinder temperature and pressure.
Furthermore, the lowering of peak cylinder temperatures will result in less specific heats and heat transfer to the walls, which will enhance efficiency. A factor which may have played a lesser role in the trend of efficiency is the fact that the specific heat ratio of hydrogen is slightly higher than that of the mixture.
Therefore, higher quasi equivalence ratios, with higher concentrations of hydrogen, tend to increase the specific heat ratio of the mixture.
The comparison of the results with reference [1] indicates that the model developed can be used with greater degree of accuracy.
RESULTS AND DISCUSSIONS
Results and discussions of this study are presented in the following three sections:
1.
Engine performance with condenser.
2.
Engine performance with direct exhaust recirculation
(EGR).
3.
Engine performance using diluent without recycling system.
ENGINE PERFORMANCE WITH CONDENSER
Figures 5 and 6 show the effect of supply flow rate of the stoichiometric hydrogen-oxygen mixture to the system (point
"1" in Fig. 1) on engine performance with considering condenser. To control peak cylinder temperature and pressure, as described before, the quasi equivalence ratio at the cylinder intake is chosen 5 (a rich mixture). The system which is proposed here includes exhaust gas recirculation system so that any unreacted hydrogen can be recycled and used in subsequent cycle. Hence, the thermal efficiency and engine power output will improve.
As the supply flow rate increases and the quasi equivalence ratio at the engine intake fixes, the recycled flow rate increases, leading to a higher intake pressure in the cylinder.
According to Fig. 5, the peak cylinder temperatures and pressures can be controlled effectively and reduce to a reasonable range if it is needed. As it can be seen from this figure, the peak cylinder pressure is increased with higher supply flow rates to the system due to the increase of the intake pressure. Also, the rise in peak cylinder temperature with increased supply flow rate is linked to the higher sensible energy released.
Also, according to Fig. 6, increasing the supply flow rate causes to increase the thermal efficiency and power output mainly due to the increase of cylinder pressure.
ENGINE PERFORMANCE WITH DIRECT EXHAUST
RECIRCULATION
The effects of supply flow rate to the system and the percentage of engine exhaust recycled on the performance engine are shown in Figs. 7-11. In this case, the intake mixture into the cylinder is chosen as stoichiometric mixture of hydrogen and oxygen as well as the supply mixture to the system.
By increasing the EGR, the recycled flow rate is increased.
Since supply flow rate to the system is held constant, the entering charge to the cylinder, and hence absolute intake pressure will be increased (see Fig. 7). Therefore, as shown in
Fig. 8, the peak cylinder pressure will be increased.
Figure 9 shows the variation of indicated power as a function of supply flow rate. The predicted trend is what we expected, namely, an increase in engine power output by increasing the supply flow rate.
Referring to Fig. 10, the peak cylinder temperatures can be controlled effectively by the percentage of the exhaust which is recycled.
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The effects of specific heats and thermal dissociation on efficiency are lowerd by decreasing the peak cylinder temperature and, therefore, the efficiency improves (see Fig.
11).
The thermal efficiency of such a system, however, is lower than the previous system which included the condenser.
Since the stoichiometric mixture of hydrogen and oxygen is entered into the engine, the exhaust gases consist only water vapor and a portion of them can be easily diverted to the intake system which causes the concentration of water vapor in the mixture to raise. But in the previous system (having the condenser), a portion of water is removed from the condenser, thus reducing the concentration of water vapor in the mixture.
The specific heat ratio of water vapor is lower than oxygen and hydrogen. As a result, high concentrations of water vapor with higher specific heat ratio leading to a lower thermal efficiency of the system with direct exhaust recycling.
ENGINE PERFORMANCE USING DILUENT WITHOUT
RECYCLING SYSTEM
The operating procedure is to provide constant supply flow rate of stoichiometric hydrogen and oxygen mixture to the engine (40 gr/min), while the quantity of inert diluent added to the intake mixture is varied. Stoichiometric mixtures are chosen for supply stream because they represented the worst operating condition case with respect to the control of peak cylinder temperatures and pressures.
The effects of the type (helium, argon and krypton) and concentration of diluent in the mixture on the engine performance are shown in Figs. 12-14. Oxygen index, OI, is defined as:
OI

[ O
2
]

[ O
2
[
]
Diluent ]
(3)
Where [ ] refers to the molar concentration of the species in the bracket.
The range of oxygen indices is chosen, to give effective control on peak cylinder temperatures.
Figure 12 shows only slight variation of engine efficiency due to the variation of diluent concentrations (or oxygen indices). Also, the thermal efficiency in this case is higher than the two previous system considered because of the high specific heat ratio of these diluents.
By increasing the amount of diluent, the peak cylinder pressure is increased due to the fact that, while engine speed is held constant, an increase of diluent flow rate provides a supercharging effect and cause the intake cylinder pressure to rise. Finally, the higher peak cylinder pressure is achieved as shown in Fig. 13.
As shown in Fig. 14 the peak cylinder temperatures can be controlled effectively by the addition of diluents.
Figures 13 and 14 indicate that the variations of the peak cylinder temperature and pressure have similar trend for all the diluents chosen in this study. Also, as shown in Fig. 12, the engine power output and the thermal efficiency when helium is considered as a diluent compared to the other diluents used in this work, is considerably lower. Moreover, krypton is predicted to have a slight edge on argon from an efficiency standpoint.
Based on the predicted efficiency characteristics of the three inert diluents considered, and the fact that krypton typically costs almost five times as much as argon, it is considered that argon will be a good diluent choice in a spark ignition hydrogen fuelled engine.
CONCLUSIONS
This study is focused into a number of systems thought to be feasible for the employment of hydrogen-oxygen mixtures in a S.I. engine. The following conclusions are obtained from this work.
1.
The results indicate that it will be feasible to operate an engine on hydrogen-oxygen mixture within the proposed systems. Also, a hydrogen-oxygen engine would have some distinct advantages, especially in view of the depleting petroleum resources, and the air pollution problems.
2.
By feeding a stoichiometric mixture of hydrogen and oxygen to the system while excess hydrogen is circulated within the recycling system with condenser, the rich mixture at the engine intake can be provided.
Therefore, the peak cylinder temperature and pressure can be controlled effectively and the thermal efficiency and the power output improve.
3.
Results show that a practical system can be operated successfully using hydrogen-oxygen mixtures in a S.I. engine. It is possible to run an engine on stoichiometric mixture, by cooling and recycling a portion of the exhaust gases for dilution purposes. The performance of such a system from a thermal efficiency standpoint, however, is not good.
4.
When the exhaust recycling system is not employed, the peak cylinder temperatures can be controlled by adding the diluent to the mixture. When certain inert diluents are employed, the thermal efficiencies predicted for the engine are considerably higher than those reported for the recycling system cases.
5.
Based on the predicted efficiency characteristics of the three inert diluents considered, argon is considered as a suitable diluent in a spark ignition hydrogen fuelled engine.
REFERENCES
[1] Taylor, M. E., 1972, "A feasibility study of a hydrogenoxygen engine", M.Sc. Thesis, Department of Mechanical
Engineering, The University of Calgary, Canada.
[2] Yamin, J. A. A., Gupta, H. N., Bansal, B. B., and
Srivastava, O. N., 2000, "Effect of combustion duration on the performance and emission characteristics of a spark
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ignition engine using hydrogen as a fuel", Int. J. Hydrogen
Energy, 25(6): pp. 581-589.
[3] Al-Baghdadi, M. A. S., and Al-Janabi, H. A. S., 2003, "A prediction study of a spark ignition supercharged hydrogen engine", Energy Conversion and Management,
44(20): pp. 3143-3150.
[4] Karim, G. A., 2003, "Hydrogen as a spark ignition engine fuel", Int. J. Hydrogen Energy, 28(5): pp. 569-577.
[5] Al-Baghdadi, M. A. S., 2004, "Effect of compression ratio, quasi equivalence ratio and engine speed on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel", Renewable
Energy, 29(15): 2245-2260.
[6] Al-Baghdadi, M. A. S., and Al-Janabi, H. A. S., 1999, "A prediction study of the effect of hydrogen blending on the performance and pollutants emission of a four stroke spark ignition engine", Int. J. Hydrogen Energy, 24(4): pp.
363-75.
[7] Petkov, T., Veziroglu, T. N., and Sheffield, J. W., 1989,
"An outlook of hydrogen as an automotive fuel", Int. J.
Hydrogen Energy, 14(7): 449–74, 1989.
[8] Yusaf, T. F., Yusoff, M. Z., Hussein, I. and Fong, S. H.,
2005, "A quasi one-dimensional simulation of a 4 stroke spark ignition hydrogen fuelled engine", American J.
Applied Sci., 2(8): pp. 1206-1212.
[9] Mahdavi, J, 2006, "Investigation of spark ignition hydrogen fuelled engines operating with diluents and exhaust gases recycling system", M.Sc. Thesis,
Department of Mechanical Engineering, Ferdowsi
University of Mashhad, Iran.
[10] Ramos, J. I., 1989, "Internal Combustion Engine
Modeling", Hemisphere Publishing Corporation, New
York.
[11] Ferguson, C. R., 1986, "Internal Combustion Engines
Applied Thermosciences", John Wiley & Sons, Inc.
[12] Heywood, J. B., 1988, "Internal combustion engine fundamentals", McGraw-Hill; New York.
[13] Benson, R. S., 1977, "Advanced Engineering
Thermodynamics (Second Edition)", Pergamon Press Ltd.
Table 1. Fuel properties at 289 K and 1 atm
Property
Density at 298 K (kg/m 3 )
Flammability limits in air
Autoignition temperature in air
(K)
Flame speed (m/s)
Adiabatic flame temperature
Hydrogen Gasoline CNG
0.0824
0.1-7.1
858
1.85
2480
5.11
~0.7-4
550
0.37-0.43
2580
0.717
0.4-1.6
723
0.38
2214
Lower heating value (MJ/kg) 119.7
44.79
45.8
Figure 1. Schematic flow diagram showing operation when the recycle condenser was in use
Figure 2. Schematic flow diagram showing operation when the recycle condenser was bypassed
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Figure 3. Thermal efficiency vs. compression ratio with fixed quasi equivalence ratio at intake (

= 5)
Figure 5. Peak cylinder temperature and pressure vs. supply flow rate with fixed quasi equivalence ratio at intake (

= 5)
Figure 4. Thermal efficiency vs. quasi equivalence ratio of the intake mixture to the cylinder
Figure 6. Effect of supply flow rate to the system on the engine performance with fixed quasi equivalence ratio (

= 5)
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Figure 7. Effect of EGR and supply flow rate to the system on cylinder intake pressure
Figure 9. Effect of EGR and supply flow rate to the system on power output
Figure 8. Effect of EGR and supply flow rate to the system on peak cylinder pressure
Figure 10. Effect of EGR and supply flow rate to the system on peak cylinder temperature
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Figure 11. Effect of EGR and supply flow rate to the system on thermal efficiency
Figure 13. Effect of oxygen index on peak cylinder pressure with the use of various diluents.
Figure 12. Effect of oxygen index on thermal efficiency with the use of various diluents.
Figure 14. Effect of oxygen index on peak cylinder temperature with the use of various diluents.
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