Mixer Design for A High Performance Biogas SI Engine Converted From A Diesel Engine

Mixer Design for A High Performance Biogas SI Engine Converted From A Diesel Engine

Bui Van Ga, Tran Van Nam

The University of Danang 41 Le Duan, Danang, Vietnam

Abstract

The performance and pollution emission of biogas engines strongly depend on the fuel- air equivalence ratio of the mixture. Thus an appropriate design of the mixer is the main issue in converting an existing diesel engine into a biogas spark ignition engine.

This paper presents some results of simulation and experiment research on a venturi type mixer for a biogas SI engine converted from a ZH1115 diesel engine. The results show that the fuel-air equivalence ratio of the mixture is less dependent on the opening of the butterfly valve which controls the mixture but it sharply depends on CH4 concentration in the biogas and/or on section of the biogas supplying pipe. At full load, the fuel-air equivalence ratio is slightly changed in relation to the engine speed but at partial load, it strongly depends on the engine speed, particularly at low regime. The dimensionless diameter of the biogas supplying pipe can be expressed by a power relationship with CH4 concentration in biogas with an exponent of – 0.515.

Keywords: Biogas, Renewable energy, SI engine, Simulation, Mixer

1. Introduction

   Biogas is an attractive renewable source of energy for rural areas. It can be produced from organic wastes, such as dung of animals, plant matter and other wastes of agriculture production. Approximately two-thirds of biogas (in volume) is methane and the rest is mostly carbon dioxide. As a fuel, biogas has a low energy density on the volume because of its high CO2 content. The burning velocity of biogas is low, just at 25 cm/s as against 38 cm/s for LPG, due to the reason that carbon dioxide may change the combustion behavior of the airfuel mixture. A large quantity of CO2 present in biogas lowers its calorific value, burning velocity and flammability range compared with those of natural gas. The self-ignition temperature of biogas is high and hence it resists knocking which is desirable for engines with a relatively high compression ratio to maximize thermal efficiency.

   Power and thermal efficiency of biogas engines reached their highest values with the RAFR between 1.05 and 0.95 [1]. Under these conditions, HC and CO emissions were relatively low but the NOx values were relatively high. Power and thermal efficiency were reduced for leaner mixtures, particularly, though engine speed was increased, emissions were all reduced [1]. Mixtures richer or leaner than this optimal point will cause incomplete combustion or slow down the burning rate and hence lead to a drop in thermal efficiency. Chulyoung Jeong et al. observed that the maximum values of generating efficiency, cylinder pressure, and NOx emissions were obtained at an EAR of around 1.2 [2], which is slightly higher than the values reported by [1].

   A high compression ratio spark ignition engine for biogas can be built by replacing the injectors of a diesel engine by a spark plug and modifying the pistons. It is necessary to maintain a proper ratio between the fuel gas and air in order to attain good combustion [3]. However, since NOx production in this condition is relatively high, two alternative approaches, slight

   retardation of spark timing or a little leaner operation which can be applied to control NOx emission without sacrificing considerable thermal efficiency, were suggested as an optimum and practical operating point for use in an actual biogas site in the future [4].

   The supply of the right mixture of air and fuel is therefore of utmost importance for the performance of a biogas spark ignition engine [5]. Further, the engines operating close to the stoichiometric air/fuel ratio display lower levels of emissions of toxic gases. Enhanced methane concentration in biogas (as in methane enriched biogas) significantly improves the engine performance and reduces emissions of hydrocarbons [5].

   Thus it is necessary to design appropriate mixers in order to ensure the right mixture with various biogas composition and pressure. In Vietnam, the research team GATEC of the University of Danang [6] has carried out a lot of studies on biogas engine. The results of researches allowed application of biogas on engines in rural areas which is very helpful for climate change mitigation [7]. As the original engines are diverse in structure and dimensions, it is difficult and costly to carry out experiments for determination of basic parameters of appropriate mixers [8-9]. The simulation of mixers will be useful to predict characteristics of the flow under different operation conditions so that we can identify basic dimensions of the mixer corresponding to the size of each engine.

   In this research, we study the characteristics of a mixer designed for a biogas spark ignition engine converted from a Jandong ZH1115 diesel engine. The objective of the research is to identify the section of biogas supplying pipe to ensure normal operation of the engine fueled with different components biogas. The experiences taken from this research can be applied on other kinds of diesel engine.

2. Method of Study

   1. Mixer Design

      The ZH1115 diesel engine with bore of 115mm, stroke of 115mm, and compression ratio of 17 reaches power of 24HP at rated speed of 2,200 rpm. The engine is converted into a biogas spark ignition engine by replacing injection systems with an electronic spark ignition system and a mixer mounted on the intake

      manifold.

      Figure 1 shows the longitudinal section of the mixer system for this ZH1115 engine. A venturi injector of 33mm interior diameter is mounted on a 2.5mm diameter pipe at the narrowest section of biogas supply. The mixer is disposed of for 2 valves: a ball valve for biogas

      flow control with flow diameter of 18mm and a

      Ball valve for biogas flow conrol

      22

      18

      52,5

      96

      Butterfly valve for mixture flow control

      49 4

      30°

      33

      85,5

      2,5

      7°

      50

      R24,5

      2

      46

      78

      butterfly valve for mixture flow control. The open angle of the biogas ball valve (compared to

      Figure 1: Mixer designed for biogas SI engne converted from ZH1115 biogas engine

      the vertical axis) changes from = 0 (completely closed) to = 90 (fully open). The open angle

      of the butterfly valve (compared to the center line of the mixer) changes from = 0 (fully open) to = 70 (fully closed). Relationships between open angle and the flow section of the intake manifold are shown in Table 1.

      Table 1: Relationship between opening of butterfly valve and flow section of intake manifold

      ()	0	10	20	30	40	50	55	70
      Flow section (%)	100%	97%	87%	72%	53%	34%	21%	0%

   2. Experimental setup

   The experimental setup is

   10 11

   introduced in Figure 2. This

   8

   4

   experimental testing is conducted

   9

   3

   with the Froude dynamometer on site

   2

   of biogas production. The ZH1115 5

   6

   engine is converted into the spark

   12

   7

   ignition engine with the compression 1

   ratio of 12. The biogas supplying pipe

   with variable diameter in accordance with the CH4 concentrations of biogas. Biogas from the digester is conducted by two different filtration

   Figure 2: Experimental setup

   1. ZH1115 biogas engine; 2. Mixer; 3. Air flowmeter; 4. Air inlet; 5. Biogas supplying pipe; 6. Biogas flowmeter; 7.

   Froude dynamometer; 8. GFM 435 Biogas analyzer; 9. Biogas mixture bag; 10. Rich biogas source; 11. Poor biogas source; 12. Computer

   systems. The first system removes only H2S by

   bentonite. The second system removes simultaneously H2S and CO2 by means of NaOH solution. Biogas mixture coming from these two sources with different concentrations of CH4 is supplied to the engine. Compositions of biogas are measured by a biogas analyzer GFM 435. Air

   mass flow is measured by ABB a flow meter. Biogas mass flow is measured by a Sigma flow meter.

   Experimental data are transferred to computer via A/D card and Labview software.

   3. Simulation

   The simulation study has been conducted by using the FLUENT CFD Software in ANSYS

   14. The 3D meshing of the mixer (Figure 3) has been established by means of the SOLIDWORKS Software. The boundary conditions include air inlet pressure p_in_air = 0, biogas inlet pressure p_biogas_in = 50Pa. The mixture outlet pressure is calculated based on the crankshaft speed and the structure parameters of the engine.

   Density of mixture is supposed to be =1,293kg/m3. Average mass flow rate of mixture

   during the intake process is

   m 1.293.Vh .2n

   60

   (kg/s). The vacuum pressure on the intake

   manifold is p 1 V2 . Mass flow rate of

   2

   mixture is also given as m KSV . In other words, mass flow rate m is proportional to speed

   V whereas p is proportional to V2, so it is

   proportional to m2.

   In order to exclude the coefficient of proportion in the FLUENT calculation, we firstly suppose boundary conditions p_mix_out as p_mix_out_propos. The calculation results will

   Figure 3: Meshing 3D mixer SOLIDWORKS Software

   give us the supposed mass flow rate: mmix_out_propos (kg/s). Hence the pressure at mixer

   outlet with known mass flow rate of the mixture during intake process mn will be identified by the following expression:

   m 2

   p _ mix _ out p _ mix _ out _ propos n

   m

   mix _ out_ propos

   Table 2: Boundary conditions at mixer outlet

   n (rpm)	1000	1200	1400	1600	1800	2000	2200
   p_mix_out (Pa)	-1608	-2316	-3153	-4118	-5212	-6435	-7786

   Pressure at the mixer outlet p_mix_out when the open angle of the butterfly valve is 30, the open angle of the biogas ball valve is 75, and the biogas fuel contains 70 CH4 is illustrated in Table 2.

   With mass flows of air and biogas given by simulation of each case, we can then calculate the equivalence ratio of mixture supplied to the engine.

   Figures 4a, 4b, 4c and 4d introduce calculation results of velocity field, contour of dynamic pressure, contour of O2 and CH4 concentrations on the symmetrical surface of the mixer with open angle of the butterfly valve at 30. We can predict homogeneity of mixture through the mixer with help based on these results.

   Velocity (m/s) 23

   120

   O2 Concentration (kg/kg)

   18

   100

   80

   12

   60

   40 6

   20

   0

   0,2

   4,60 CH4 Concentration (kg/kg)

   7,5.103

   Dynamic pressure (Pa)

   3,35

   2,30

   1,15

   0,00

   5,0.103

   2,5.103

   0,0

   Figure 4: Flow velocity vector (a), contour of dynamic pressure (b),

   concentrations of CH4 (c) and O2 (d) on the symmetrical plane of the mixer.

3. Results and Discussion

   1. Simulation Prediction

      Figure 5a introduces variations of fuel-air equivalence ratio of the mixture versus speed of an/the engine fueled with biogas containing 60% of CH4. The biogas ball valve is fully opened (90). The butterfly valve is opened at positions of 34%, 72%, and 100%, respectively. The engine speed in each case ranges between 1,000 rpm and 2,200 rpm. The calculation results show that when an engine operates on full load curves (with butterfly valve being 100% open),

      of mixture is almost stable ( changes from 1.3 to 1.4). When the engine operates on partial

      load curves, the curve of the fuel-air equivalence ratio varies in function of engine speed: the

      steeper the engine speed is, the smaller the butterfly valve open level becomes. The change in concentrations at high-speed positions is less than at low-speed positions. At any opening levels of the butterfly valve, when the engine runs at rated speeds between 1,800 rpm and 2,200 rpm, fuel-air equivalence ratio of the mixture changes narrowly from 1.02 to 1.10.

      1.25

      1.28

      1.20

      1.20

      1.15

      1.10

      1.05

      1.00

      1000 1200 1400 1600 1800 2000 2200

      n (vòng/phút)

      n (rpm)

      a.

      1.23

      1.18

      1.13

      1.08

      1.03

      1000 1200 1400 1600 1800 2000 2200

      n (vòng/phút)

      n (rpm)

      b.

      1.15

      1.10

      1.05

      1.00

      0.95

      1000 1200 1400 1600 1800 2000 2200

      n (vòng/phút)

      n (rpm)

      c.

      Figure 5: Fuel-air equivalence ratio vs engine speed

      M6C4, ball valve 90 (a); M7C3, ball valve 75 (b); M9C1, ball valve 60 (d); Butterfly valve opening 34% (),72% () and 100% ()

      Figure 5b introduces the same results with biogas fuel containing 70% of CH4 and an opening level of the biogas ball valve at 75. Figure 5c introduces the same results with biogas fuel containing 90% of CH4, with the opening level of the biogas ball valve at the position of 60. The results manifest the changing principle of in terms of n similar to the case in which biogas contains 60% of CH4. With biogas containing 90% of CH4, if the opening level of the biogas ball valve is at 60, the mixture is poor. In order to increase fuel-air equivalence ratio of the mixture in these cases, we can increase the opening levels of the biogas ball valve. On the contrary, in the case that the fuel contains 60% – 70% of CH4, we can reduce the opening level of the biogas ball valve so as to reduce fuel-air equivalence ratio of the mixture.

      The above results show that, when opening levels of the biogas valve and the butterfly valve are given, the fuel-air equivalence ratio of the ixture is slightly reduced as decreasing of the engine speed slows. The bigger the opening level of the butterfly valve, the lower the changing degree of becomes. When CH4 concentration in biogas is varied, we can adjust the biogas ball valve to achieve the best fuel-air equivalence ratio . This adjustment can be made once for each kind of fuel. Table 3 summarizes the results of calculations on the opening levels of the biogas ball valve corresponding to the biogas containing different percentages of CH4 concentration. The results show that with given a CH4 concentration, we can choose an appropriate opening level of biogas ball valve so that the fuel-air equivalence ratio is in optimal range according to [1] at any opening level of butterfly valve.

      Table 3: Effect of butterfly valve opening from 34% to 100% on fuel-air equivalence ratio at given CH4 concentration and opening level of biogas ball valve (n=2,200 rpm)

      %CH4 in biogas	60	70	80	90
      Opening levels of biogas ball valve ()	90	75	65	60
      	1.02-1.08	1.03-1.09	0.97-1.01	0.98-1.02

      1.25

      1.20

      1.15

      1.10

      1.05

      1.00

      1.30

      1.25

      1.20

      1.15

      1.10

      1.05

      1.00

      1.20

      1.15

      1.10

      1.05

      1.00

      0.95

      Vol. 3 Issue 1, January – 2014

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      a.

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      b.

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      c.

      Figure 6: Fuel-air equivalence ratio vs opening levels of butterfly valve; M6C4, ball valve 90 (a); M7C3, ball valve 75 (b); M9C1, ball valve 60 (c); n=1,000 rpm (), n=1,400 rpm (); n=1,800 rpm () and n=2,200 rpm ()

      Figures 6a, 6b and 6c introduce the effect of engine speed on in relation to the opening level of the butterfly valve with biogas containing 60%, 70% and 90% of CH4 with opening levels of the biogas ball valve shown in Table 3. The results show that when engine speed and

      position of the biogas ball valve are fixed, fuel-air equivalence ratio of mixture is reduced as the opening levels of the butterfly valve are increased. When the butterfly valve is fully open (the engine operates on full load curves), the fuel- air equivalence ratio of the mixture is almost unaffected by the engine speed. Therefore the mixer must surely supply the best mixed

      %Vol CH4

      Figure 7: Mixture concentrations proportional to CH4 concentrations in biogas (ball valve opening 75 ; butterfly valve opening 72%; n=2,200rpm)

      components when the engine operates on full load curves. Under partial load operation,

      slightly increases when the opening level of the butterfly valve is reduced. The results show that with 34% opening of the butterfly valve at 1,000 rpm engine speed, is about 1.25 compared with its value of approximately 1 a fully opened butterfly valve with biogas containing 60-70% of CH4. Even when the mixture is richer as the engine runs on partial load curves, is within combustible limit range.

      1.30

      1.25

      1.20

      1.15

      1.10

      1.05

      1.00

      0.95

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      a.

      1.20

      1.15

      1.10

      1.05

      1.00

      0.95

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      b.

      1.15

      1.10

      1.05

      1.00

      0.95

      30 40 50 60 70 80 90 100

      % m bm ga

      % opening butterfly valve

      c.

      Figure 8: Fuel-air equivalence ratio vs opening levels of butterfly valve n = 1,000 rpm (a); n = 1,800 rpm (b); n = 2,200 rpm (c);

      M6C4, ball valve 90 (); M7C3, ball valve 75 (); M8C2, ball valve 65 (); M9C1, ball valve 60 ().

      However, as the biogas ball valve position, butterfly valve position and engine speed are fixed, changes considerably in accordance with the concentration of CH4 in biogas fuel. Figure 8 shows that at engine speed of 2,200 rpm, the biogas ball valve is open up to 75 and the butterfly valve is open 72%, reaches 0.85 and 1.65 with biogas containing 60% and 90% CH4, respectively. Therefore, to obtain an appropriate fuel-air equivalence ratio of the mixture as

      CH4 concentration in biogas changes, we must change the opening levels of the biogas ball valve.

      Figures 8a, 8b and 8c introduce the effect of biogas fuel and opening levels of the biogas ball valve on change of in function of opening levels of the butterfly valve. With a given engine speed, the tangent of curves are almost similar, independent of CH4 concentration in biogas fuel. So if we adjust the position of the biogas ball valve so that for a given opening level of the butterfly

      % opening butterfly valve

      Figure 9: Variation of fuel-air equivalence ratio in accordance with butterfly valve opening levels M6C4, ball valve 90 ();

      M7C3, ball valve 74 (); M8C2, ball valve 67 (); M9C1, ball valve 61 ().

      valve we obtain the same of mixture, then we can represent a linear relationship between and the opening level of the butterfly valve.

      Figure 9 illustrates the variation of in function of butterfly valve opening levels with biogas containing 60%, 70%, 80%, and 90% of CH4 and opening levels of the biogas ball valve of 90,

      74, 67, and 61, respectively. At 34% opening

      0.37

      y=deq /dad

      0.35

      0.33

      0.31

      0.29

      0.27

      y=2.9x- 0,515

      x=%Vol CH4

      level of the butterfly valve, is in range between

      1.06 and 1.08. When the butterfly valve is fully opened oscillates from 1.02 to 1.05. These results

      60 70 80 90

      Figure 10: Relationship between dimensionless diameter of biogas supplying pipe and CH4 concentration in biogas fuel (n=2,200 rpm, =10.02)

      show that the tangent of the curves is -0.0006 (if the opening level of the butterfly valve is in percentage).

      As the tangent of the curves is very slight, we can consider is unchanged in relation to opening levels of the butterfly valve. Contrarily, of the mixture changes sharply in relation to the opening levels of the biogas ball valve and CH4 concentrations in biogas fuel. This means that for a given biogas, we need to determine the size of the pipe that supplies biogas to the mixer in relation to the size of the air admission pipe so that is in optimal range observed by Huanga et al. [1] or by Jeong et al. [2] at any level of butterfly valve opening. In this case, we need not equip the biogas ball valve with the supplying pipe.

      Figure 10 introduces the variation of dimensionless diameter y=deq/dad of the biogas supplying pipe in accordance with the concentration of CH4 in the fuel in case of ZH1115 biogas engine. When CH4 concentration in biogas increases, the amount of biogas supplied to the

      mixture must decrease to ensure that of the mixture is unchanged. Because mass flow rate m is proportional to the flow section S, in other words, it is proportional to the square of the dimensionless diameter of the biogas-supplying pipe y, or the diameter y is in proportion to m0,5. Otherwise, to keep constant when the CH4 concentration in fuel increases, the mass flow rate of fuel decreases. This means that dimensionless diameter of biogas supplying pipe is proportional to x-0,5. Figure 10 shows that the exponent of the curve is -0.515. The absolute value of the exponent is slightly higher than 0.5. This is reasonable because when the biogas mass flow rate changes, the air mass flow rate is also changed to ensure the constantvalue of .

   2. Experiment Valuation

   Figures 11a, 11b, and 11c introduce the variation of equivalence ratio versus engine speed at full load regime. Biogas contains 60%, 73%, and 87% of CH4 concentrations. Biogas supplying pipes are selected with diameters of 18mm, 16mm, and 14.5mm, respectively, corresponding to the relationships shown in Figure 10. During experimentation, the butterfly valve is fully open. The results show that equivalence ratios given by simulation are fitted well to their values given by experiment with different CH4 concentration in biogas. This confirms that the relationship in Figure 10 is reasonable.

   1.11

   n

   atio

   mul

   Si

   1.09

   ent

   rim

   xpe

   E

   1.07

   1.09

   1.07

   n

   lati

   mu

   Si

   1. o

      1.03

      n

      latio

      mu

      Si

      ime

      Ex

      1.01

      per

      nt

      1.05

      m)

      (rp

      n

      1.03

      1000 1200 1400 1600 1800 2000 2200

      a.

      1.03

      )

      pm

      n (r

      1.01

      1000 1200 1400 1600 1800 2000 2200

      b.

      0.99

      m)

      (rp

      n

      nt

      ime

      per

      Ex

      0.97

      1000 1200 1400 1600 1800 2000 2200

      c.

      Figure 11: Comparison fuel-air equivalence ratio given by simulation and by experiment at full load regime with biogas containing 60% CH4 (a), 73% CH4 (b) and 87% CH4 (c)

      Figure 12 introduces full load characteristic curves of ZH biogas engine fueled with biogas containing 60%, 73%, and 87% CH4 concentrations. The results show that at the speed of 2,500rpm, the maximum power of engine run by the biogas containing 87% of CH4 is 21 HP. Calculated power via proportions of CH4 passing into a/the cylinder of the engine fueled with biogas containing 73% and 60% of CH4 will be 20.58HP and 20.03HP, respectively. The experimental data are suitable for the first case, but as for the final case (biogas containing 60% of CH4), the real power is much lower than that of the calculation. This is because of incomplete combustion due to high concentration of CO2 in biogas. The suitability of the power proportions when the ZH1115 engine is run by biogas containing different concentrations of CH4 affirms that the relationships between the biogas supplying pipes with the CH4 concentrations in biogas

      shown in Figure 7 are accurate.

      The results of this research are very useful to convert an existing engine running on diesel that is largely used in rural areas into biogas engine. Previously for converting a diesel engine into biogas engine we have to conduct a lot of experimental tests to determine appropriate parameters of the mixer. This takes a lot of time and money. Now thanks to this new method of

      22

      %C

      73

      H4

      %C

      87

      Pe (HP)

      18

      CH4

      0%

      6

      H4

      14

      )

      rpm

      n (

      10

      6

      600 1000 1400 1800 2200 2600 3000

      Figure 12: Performance curves of the ZH1115 when operating with biogas containing different CH4 concentrations

      simulation we can orient the technology of conversion. This way can be applied generally to any kind of diesel engine. This is very helpful in reducing the cost of conversion that will encourage numerous farmers to use biogas in their machines. It is an effective contribution to climate change mitigation.

4. Conclusion

For this paper, we have studied the optimal parameters of a mixer in order to transform a typical diesel engine to a biogas spark ignition engine which can produce high effectiveness. We have drawn the following conclusions from the results.

1. With venturi type mixer designed for biogas SI engine, the equivalence ratio is less dependent on the opening of the butterfly valve which controls the mixture flow but it sharply depends on CH4 concentration in biogas and/or on sections of the biogas supplying pipe.

2. At full load, the equivalence ratio given by the mixer is slightly changed in relation to engine speed but at partial load, it strongly depends on engine speed, particularly at low regime.

3. The dimensionless diameter of the biogas supplying pipe can be generally expressed by a power relationship with CH4 concentration in biogas with exponent of -0.515.

References

1. Jingdang Huanga and R. J. Crookesb, Assessment of simulated biogas as a fuel for the spark ignition engine, Fuel, Vol. 77, No. 15, Dec.1998, pp. 17931801.

2. Chulyoung Jeong, Taesoo Kim, Kyungtaek Lee, Soonho Song, Kwang Min Chun, Generating efficiency and emissions of a spark-ignition gas engine generator fuelled with biogashydrogen blends, International Journal of hydrogen energy, Vol. 34, No. 23, Dec. 2009, pp. 9620-9627.

3. R. Chandra, V.K. Vijay, P.M.V. Subbarao, T.K. Khura, Performance evaluation of a constant speed IC engine on CNG, methane enriched biogas and biogas, Applied Energy, Vol. 88, No. 11, Nov. 2011, pp. 3969-3977.

4. E. Porpatham, A. Ramesh, B. Nagalingam, Investigation on the effect of concentration of methane in biogas when used as a fuel for a spark ignition engine, Fuel, Vol. 87, No. 8-9, Jul. 2008, pp. 16511659.

5. Cheolwoong Park, Seunghyun Park, Yonggyu Lee, Changgi Kim, Sunyoup Lee, Yasuo Moriyoshi, Performance and emission characteristics of a SI engine fueled by low calorific biogas blended with hydrogen, International Journal of hydrogen energy, Vol. 36, No. 16, Aug. 2011, pp. 10080-10088.

6. www.dongcobiogas.com

7. Bui Van Ga, Tran Van Nam, Truong Le Bich Tram, Engines fueled by biogas: A contribution to energy saving and climate change mitigation. The 6th Seminar on Environment Science and Technology Issues Related to Climate Change Mitigation, Japan-Vietnam Core University Program, Osaka, Japan, Nov. 2008, pp. 26-28.

8. Bui Van Ga, Tran Van Nam, Tran Thanh Hai Tung, Le Minh Tien, Le Xuan Thach, Study of Performance of Biogas Spark Ignition Engine Converted from Diesel Engine, The International Conference on Green Technology and Sustainable Development, Hochiminh City, Vietnam, Sep. 2012, pp. 29-30.

9. Bui Van Ga, Tran Van Nam, Le Minh Tien, Le Xuan Thach, Effect of CH4 concentration, spark timing and compression ratio to biogas engine performance, National Review of Transport, Iss. 5, 2013, pp. 7-9 and 13 (in Vietnamese).