TDC Top Dead Center. 1. Introduction. This theoretical study is intended to study the combustion duration and the ignition delay for a mixture of hydrogen and ...
Second International Conference on advances in Engineering Sciences& Technologies, National Research Centre, Egypt, November 12-14, 2005.
Investigations in Combustion of Hydrogen Enriched Natural Gas A.El Fatih.Farrag*, M.Saber Gad* *Department of Mechanical Engineering, Engineering Research Division,National Research Centre, Dokki, Egypt
Abstract The combustion process is the most important process because the chemical energy of the fuel is converted into sensible internal energy of the charge during this process. The details of the flame propagation process influence the efficiency of energy conversion and engine performance.Combustion duration reduction leads to performance improvement and emissions reduction. The variations in compression ratio, engine speed, fuel/air ratio, equivalence ratio, spark plug location and spark advance have an effect on combustion duration and mass fraction burned. Using Hydrogen as an additive to natural gas reduces the combustion duration, increases the mass fraction burned, improves the engine performance, enhances the flame propagation and flame propagation speed, furthermore, the details of the flame propagation influences on the combustion efficiency, mass fraction burned and engine performance, as well as pollutants formation.
Keywords: Combustion Duration, Ignition Delay, Hydrogen Addition. ـ
Nomenclature F1(N) F2(Φ) F3(θs) F4(r) N r Yi Φ θs Δθ Xb
Function of Engine Speed. Function of Equivalence Ratio. Function of Ignition Delay. Function of Compression ratio. Engine speed in RPM. Engine compression ratio. The molar fraction of the fuel (i) in the fuel mixture. Equivalence ratio = actual fuel to air ratio/ stoichiometric fuel to air ratio. Ignition Delay (Degree). Combustion Duration (Degree). Mass Fraction Burned.
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Δθi Δθm
the corresponding combustion period or ignition delay for the engine when operating with the fuel component (i) on its own under the same conditions (Degree). the combustion duration or ignition delay for the mixture (Degree).
Abbreviations CA CFR CNG LPG S.I.E TDC
Crank Angle. Combustion Fuel Research. Compressed Natural Gas. Liquefied Petroleum Gas. Spark Ignition Engine. Top Dead Center.
1. Introduction This theoretical study is intended to study the combustion duration and the ignition delay for a mixture of hydrogen and natural gas to investigate the mass fraction burned which helps in ensuring the importance of using alternative fuels because of its economy and environmental effects. Considering energy crises and pollution problems today, investigations have been concentrated on decreasing fossil fuel consumption by using alternative fuels and on lowering the concentration of toxic components in combustion products combined with improvements in efficiency [1]. Among the alternative fuels such as CNG, LPG and Hydrogen have shown excellent performance over other fuels. Natural Gas (NG) as an alternative fuel has been promoted, and natural gas is being regarded as one of the most promising alternative fuels for combustion engines. Hydrogen has a number of properties that make it a highly desired alternative fuel; it has been used as a fuel in internal combustion engines, fuel cells and as an additive to conventional fuels such as gasoline or natural gas. Using hydrogen as an additive offers the possibility of enhancing the mixture by taking advantage of properties from both fuels [2]. Using hydrogen as an additive to natural gas reduces exhaust emissions, improves engine efficiency, and improves combustion efficiency and stability [3]. The combustion duration has an effect on engine performance. The combustion duration is low with the high rate of pressure rise. Pressure should be maximum before TDC to produce greater force acting through a long period of the power stroke. There will be enough time to lose some of its heat to the coolant resulting in poor performance. Fifty percent of the pressure rise is completed by the TDC, resulting in peak pressure and temperature occurring at 10-15 %after TDC which reduces the heat loss. An increase in the combustion duration causes the peak temperature and the brake mean effective pressure to decrease. This is an improvement of the thermal efficiency [3].
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2. Combustion Duration and Ignition Delay Combustion in spark ignition engine in general can be divided into three parts defined as the FIP (Flame initiation period), the FPP (Flame preparation period) and the flame termination period (FTP). The FIP is the period in which the growth of the flame kernel has proceeded to a size where the flame front for turbulent propagation is established. The transition from FIP to FPP is not marked precisely by a distinct measurable characteristic; it is arbitrarily defined in terms of the burned mass fraction which ranges from 1 % to 10 %. FIP is also called flame development angle and expressed in crank angle interval between the spark discharge and the time when a small but significant fraction of the mixture (fuel & air) mass has burned or fuel chemical energy has been released. Usually this fraction is 10 %. Though other fractions such as 1 % and 5 % can be considered. FIP is also called ignition delay. FPP is defined as the crank angle period required for 5% to 90% of the mass to be burned or the interval between the end of the flame development stage and the end of the flame propagation process. The FTP is the period where the rest of the mixture has been burned [4]. Two parameters are used to characterize the combustion process: the mass fraction burned Xb and the combustion duration Δθ. It is assumed that 1 is a known combustion duration for r1,N1,1,s1, where is a function of r,N,,and s .The following equations are defined to evaluate the relation between the combustion duration and engine speed, equivalence ratio, ignition delay and compression ratio as represented in Fig.1, Fig.2 and Fig.3 [5]. These equations are used to determine the combustion duration in S.I.E. as mentioned in Ref.[5]. F1(N) = 0.1222 + 0.9717*(N/N1) - 0.05051* (N/N1)2 F2() = 4.3111-5.6383*(/1)+20304*(/1)2 F3(s) = 1.0685-0.2902*(s/s1) +0.2545*(s/s1)2 F4(r) = 3.2989-3.3612*(r/r1) +1.08*(r/r1)2 /1 = F1*F2*F3*F4
(1) (2) (3) (4) (5)
The combustion duration at the base operating condition for a typical engine can be calculated. The base reference values are taken as: r1= 7.5, N1= 1000 RPM, Φ1= 1, θs1= -30 CA and for this case Δθ1= 24 CA. The validity of the proposed correlation for predicting the combustion duration in S.I. Engine is examined. The comparison between the measured combustion duration and those computed by empirical formula is in good agreement [5].
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Engine under consideration has the following specifications in this table:Engine Parameters Cycle No. of cylinders Cylinder Bore (mm) Cylinder Stroke (mm) Compression ratio Rated power (kW)
Specifications Four stroke Four 92 95.25 8:1 20
To improve engine performance, the combustion duration should be minimum with a high rate of pressure rise. Fifty percent of the pressure rise completed by the TDC, resulting in peak pressure and temperature occurring at 10-15 %after TDC which reduces the heat loss. An increase in the combustion duration causes the peak temperature and the brake mean effective pressure to decrease.
3. Effect of combustion duration on Mass Fraction Burned for Gasoline fuel A functional form often used to represent the mass fraction burned versus crank angle curve is the Wiebe function [6]: Xb= 1-exp [-a*((-s)/) m+1]
(5)
Where is the crank angle, s is the start of ignition, is the total combustion duration from X b 0 to X b 1 . Varying a and m changes the shape of the curve significantly. a and m are adjustable parameters that determine the shape of the curve. a= 3 and m=2 and this is shown in Fig.4 [6. From equ.5, increasing the combustion duration leads to decreasing the mass fraction burned and decreasing the mixture mass burned. From equ.5, increasing the ignition lag leads to decreasing the mass fraction burned and decreasing the mixture mass burned.
4. Combustion Duration & Ignition Delay for compressed Natural Gas Natural gas is a mixture of different gases may differ from one source to another. One component of NG is methane, which is typically up to 99% of the total volume .other constituents may include non-methane hydrocarbons such as Ethane, Propane and Butane and traces of hydrocarbons like Nitrogen, Helium, Carbon dioxide, Hydrogen Sulphide and sometimes water. Natural gas is the cleanest of all fossil fuels. It is easy to transport and store, available on demand,
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less environmental polluted and economical than gasoline. Natural gas has a higher octane number and suitable for engines of higher compression ratio. Polynomial functions were developed for and a constant (a) over a wide equivalence ratio range. The polynomial functions for a, and the adjustable parameters m were determined as follows to determine the combustion duration and mass fraction burned for natural gas as in fig.5 and fig.6 [4]: a = -11.65*+17.39 = 544.26*2 -942.71* +451.19 m=3
(6) (7) (8)
5. Combustion Duration & Ignition Delay for Hydrogen Hydrogen is superior over gasoline. With Hydrogen, the engine tends to operate at leaner mixtures; the hydrogen has a higher calorific value, lower density and lower boiling point. Operation with the hydrogen reduces the Brake Specific Fuel consumption, reduces deposits, increases oil life, and reduces engine wear. The Hydrogen has a higher calorific value and this leads to increasing the heat content of the combustion. The higher heat content leads to the higher flame speed. The combustion duration decreases as the engine speed increases. The turbulence inside the cylinder increases, leads to a better heat transfer between the burned and unburned zones. Operating at lean mixture leads to decrease the thermal energy liberated from hydrogen- air mixture, increases the ignition delay and decreasing the flame propagation speed. The combustion duration decrease as the compression ratio increases. The rise in temperature and pressure leads to increasing the flame speed. As we move the spark from the peripheral position to the center, the combustion duration decreases because of the decrease in the distance traveled by the flame. The rate of combustion should be fast that combustion duration is low with higher pressure and temperature inside the engine cylinder. The minimum combustion duration leads to decreasing the heat loss, increasing performance and improving the thermal efficiency [7].
6. Mass Fraction Burned effect on combustion duration for Hydrogen From equ.5, Mass fraction burned of the hydrogen depends on the combustion duration and the ignition delay for hydrogen. As the combustion duration decreases, the fraction residual gases decrease, the ignition delay decreases and the mass fraction burned increases as in Fig.7.
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7. Combustion Duration and ignition Delay for fuel mixture of hydrogen and natural gas Any correlations for the combustion duration and ignition delay of fuel mixtures have to be based empirically on experimental data obtained directly from measurements in the engine. The combustion duration is inversely proportional to the flame propagation speed which can be viewed in the case of fuel mixtures as a mean flame speed where the contribution of each fuel component of the mixture would be proportional to its own flame speed under similar conditions and its concentration in the mixture. A simple expression for the combustion duration of a fuel mixture based on experimental observations made in a CFR engine can be given approximately as [7] and this is explained in fig.8 and fig.9: (1/m) = (y1/1) + ( y2/2) +…………. + ( yi/i)
(9)
Where yi is the molar fraction of the fuel (i) in the fuel mixture. Δθ i is the corresponding combustion period for the engine when operating with the fuel component (i) on its own under the same conditions. Δθ m is the combustion duration for the mixture. The ignition delay of a fuel mixture can be determined by equ.9, where Δθi is the corresponding ignition delay for the engine when operating with the fuel component (i) on its own under the same conditions. Δθ m is the ignition delay for the mixture
8. Mass Fraction Burned for fuel mixture of hydrogen and natural gas Increasing the concentration of Hydrogen in a mixture of hydrogen and natural gas has some benefits which arising from the improvement in the combustion process this can be offset by the reduction of the total energy input as a result of the increased substitution of natural gas by the hydrogen. At higher rates of this substitution, combustion duration for a mixture of Hydrogen and CNG depends on the hydrogen volume percent of increase in hydrogen volume percent leads to decreasing the combustion duration and increase in mass fraction burned as in Fig.10 [7].
9. Results and Discussions Figure1 shows that operating at lean or rich mixtures tends to increase the combustion duration. This effect is more predominant at higher speeds. This is because of the lesser thermal energy liberated from the leaner mixtures which
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increases the ignition delay. The combustion duration decreases as the equivalence ratio increases as mentioned in Ref. [5]. Figure 2 shows that the combustion duration decreases as the compression ratio increases. This is because of the increase in the end of compression temperature and pressure and decrease in the fraction residual gases. This creates a favorable condition for the reduction of ignition lag and the combustion duration as mentioned in Ref.5. Figure 3 shows that the combustion duration increases as the engine speed increases, the turbulence in the combustion chamber increases which leads to combustion duration increase. Figure 4 indicates that the combustion duration increases so that The fraction residual gases decrease, the ignition delay increases which leads to the mass fraction burned increases. The combustion duration increases as the spark advance increases as mentioned in Ref.6. Figure 5 shows that the combustion duration decreases as equivalence ratio increases until at equivalence ratio ( = 0.9) after that, the combustion duration increases at lean mixture because the less thermal energy is produced. At rich mixture, the combustion duration decreases because the higher thermal energy produced as in Ref.5. Figure 6 shows that the combustion duration increases related to the combustion duration for Spark Ignition fueled with Gasoline. The fraction residual gases decrease, the ignition delay increases and the mass fraction burned increases. The combustion duration increases as the spark advance increases, this increase in combustion duration leads to decreasing the mass fraction burned as in Ref.5.. Figure7shows that the Mass fraction burned of the hydrogen decreases as the ignition delay decreases, and this leads to increasing the burning mass of the mixture.8- Figure 8 Shows that the combustion duration decreases as the hydrogen volume percentage increases. . This leads to higher thermal energy liberated from the combustion of the mixture because of the higher thermal energy liberated from Hydrogen combustion and this leads to higher flame speed as in Ref.8. Figure9 shows that the Ignition Delay decreases as the hydrogen volume percentage increases because of the higher Heat content liberated from Hydrogen combustion and this leads to the higher Heat content liberated from the combustion of the mixture and higher flame speed. Figure 10 shows that the combustion duration decreases as the hydrogen volume percentage increases and Ignition Delay decreases. This leads to higher thermal energy liberated from the combustion of the mixture because of the higher thermal energy liberated from Hydrogen combustion. And also leads to slowing the flame propagation. The pressure and temperature inside the cylinder increase as the combustion duration and ignition delay decrease. A better heat transfer between the burned and unburned zones and this leads to improvement of the thermal efficiency. 7
10. Conclusions There are some parameters such as engine speed, equivalence ratio, compression ratio and spark advance have an effect on combustion duration and ignition delay in spark ignition engine. In NG engines, the combustion duration decreases as equivalence ratio increases, the ignition delay increases and the mass fraction burned increases. In Hydrogen Engines, combustion duration decreases because of the higher flame propagation and this leads to increase in the mass fraction burned. The performance characteristics of compressed natural Gas fueled spark ignition engines can be enhanced significantly by the presence of some hydrogen with natural gas. Improvements to performance parameters such as power output, thermal efficiency and reduction in emissions can be obtained.
Combustion duration,Deg
The combustion duration has an effect on the engine operating parameters such as performance and emissions. The decreased combustion duration improve the performance and emissions of the engine. Combustion duration for a mixture of hydrogen and CNG depends on the hydrogen volume percent, which leads to decreasing the combustion duration, increase in the flame propagation speed and increase in mass fraction burned.
100 80 60 40 20 0 0.4
0.6
0.8 1 Equivalence ratio
1.2
1.4
Fig.1.combustion durations versus Equivalence ratios at N= 1500 RPM, r = 8, θs= -15.
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Combustion duration,Deg.
120 100 80 60 40 20 0 5
5.5
6
6.5
7
7.5
8
Compression ratio,r.
Combustion duration,deg.
Fig.2.combustion durations versus Compression ratios at N= 3000 RPM, Φ= 1,θs= -30.
100 80 60 40 20 0 1000
1500
2000
2500
3000
3500
Engine speed,rpm
Fig.3.Combustion durations versus Engine Speeds at r = 8, Φ= 0.9, θs= -25.
9
4000
1
Mass fraction burned
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 160
165
170
175
180
185
190
195
200
205
Crank angle,degree
Fig.4.Mass fraction burned Xb versus crank angle for Gasoline at N= 1500 RPM, r = 8, Φ= 1, θs= -15 CA.
Combustion Duration.deg.
250 200 150 100 50 0 0.3
0.5
0.7
0.9
1.1
1.3
1.5
Equivalence ratio
Fig.5.Combustion duration versus equivalence ratios for compressed natural gas.
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Mass fraction burned for NG
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 150
170
190
210
Crank angle,deg.
Mass fraction Burned for Hydrogen
Fig.6.Mass fraction burned versus Crank angles for Compressed natural gas at spark advance = 22 CA
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 173
178
183
Crank angle (Degree)
Fig.7.Mass fraction burned versus Crank angles for Hydrogen at Ignition Delay θs= 7 CA.
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Combustion Duration (Deg)
70 60 50 40 30 20 10 0 0
20
40
60
80
100
Hydrogen volume percentage %
Fig.8.Combustion duration for mixture of Hydrogen and Compressed natural gas versus Hydrogen volume percentages.
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Ignition Delay (Degree)
25 20 15 10 5 0 0
10
20
30
40
50
60
70
80
90
100
Hydrogen volume percentage %
Fig.9.Ignition delay for mixture of Hydrogen and Compressed natural gas at different Hydrogen volume percentages.
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1 0.9
Mass fraction burned for mixture
0.8 0.7 0.6 0 % H2 0.5
20% H2 40 % H2
0.4
60 % H2 0.3
80 % H2 100 % H2
0.2 0.1 0 150
160
170
180
190
200
210
220
230
Crank angle (Degree)
Fig.10.Mass Fraction Burned for mixture of Hydrogen and Compressed natural gas versus Hydrogen volume percentages.
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6- Heywood J.B."Internal Combustion Engine Fundamentals".Singapore: McGraw-Hill; 1988. 7-Yamin A.A.Jehad,Gupta H.N.,Bansal B.B.,Srivastava O.N., "Effect of Combustion duration on the performance and emission characteristics of a spark ignition engine using Hydrogen as a fuel", International Journal of Hydrogen Energy,Vol. 25, PP.581-589, 2000. 8- Karim.G.A.,Bade Shrestha S.O."A Predictive Model for Gas Fueled Spark Ignition Applications".SAE Journal,(993482),1999.
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