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Technique of Biogas-HHO Gas Supply for SI Engine

Bui Van Ga, Bui Thi Minh Tu, Truong Le Bich Tram, Bui Van Hung
Danang University of Science and Technology-The University of Danang
54, Nguyen Luong Bang Street, Danang, Vietnam
Email: buivanga@ac.udn.vn
 
International Journal of Engineering Research & Technology (IJERT), Volume 08, Issue 05 (May 2019), pp. 669-674
https://www.ijert.org/research/technique-of-biogas-hho-gas-supply-for-si-engine-IJERTV8IS050448.pdf

 

Abstract:The paper presents a new technique to control the air-fuel ratio of a SI engine fueled with biogas-HHO gas based on the simulation of pressure variation in the intake manifold. With a fi-correction valve integrated into the conventional gaseous fuel supplying valve, the equivalence ratio of the mixture can be controlled in a narrow gap around the stoichiometric value. The fi-correction valve is in action under the effect of vacuum which decreases as decreasing of engine speed or/and increasing of flow obturation. Without fi-correction valve, the equivalence ratio of biogas-HHO gas fueling engine decreases significantly as increasing of engine speed. In this case, with the same gas supply condition, the suitable mixture for low engine speed becomes too poor at high engine speed; conversely, optimal mixture for high engine speed becomes too rich at low engine speed. The compact gaseous fuel supplying kit included a mixer, a main fuel injection valve and a fi-correction valve can provide a stable equivalence ratio at any engine speed regimes. This new concept of air-fuel ratio control can overcome the inconveniences of the conventional system and it has demonstrated as an appropriate technique for biogas-HHO gas supplying to SI engine in practice.
Keywords: Renewable Fuels; Biogas; HHO Gas; Biogas Engines; Air to Fuel Ratio

Main results:


Fig. 7a and Fig. 7b present the effects of engine speed on variation of the equivalence ratio f according to the crankshaft rotation angle of engine fueled with biogas-HHO gas in two cases of  throttle valve position: a=0° and a=45°. Under a fixed fuel supply condition, when the engine speed increases, the equivalence ratio strongly decreases. As mentioned above, when engine speed increases, the vacuum level increases, this is in favor to introducing more fuel to the cylinder. But in this case, the time (in s) for fuel supply period is reduced. Fig. 3a and Fig. 3b show that with given vacuum pressure, the crankshaft angle of the valve opening (in terms of the crankshaft rotation angle, °CA) increases with increasing engine speed. However, the amount of gas introduced into the intake manifold depends on the time of the valve opening (in seconds). When the engine operates at high speed, at the same time interval in seconds, the crankshaft rotates a larger angle than that at low speed. It results in a reduction of fuel introduced to the cylinder. The magnitude of reducing of f is almost independent with the butterfly valve position. Thus, the conventional gas supplying system should be modified to meet the requirements of the engine fuel with biogas-HHO gas.


Fig. 8a and Fig. 8b present the variation of equivalence ratio as the engine is fueled by conventional gaseous fuel supplying valve. Fig. 8a shows that when the fuel injection condition is fixed for f=1 at a=0° independently engine speed, the equivalence ratio changes very slightly as changing of butterfly valve position. In average, f changes from 1 to 1.05 as a varied from 0° to 45° (Fig. 8a). But when the fuel injection condition is fixed for f=1 at n=1500 rpm independently throttle valve position, the equivalence ratio drops down sharply as increasing of engine speed (Fig. 8b). The equivalence ratio decreases from 1 to 0.65 as engine speed increases from 1500rpm to 3000rpm in this condition.
 

 

 

Fig. 9: Schema of biogas-HHO supplying system for SI engine


The above results show that it is not necessary to adjust the equivalence ratio according to the opening level of throttle valve, but it is important to control air-fuel ratio as varying engine speed. Due to the equivalence ratio varies strongly with engine speed in the same fuel supply condition and the same throttle opening position, if the mixture is adjusted suitably for low-speed engine operation, then at high engine speed the mixture is too poor, thus, the engine cannot operate. Conversely, is adjust the optimal mixture for high speed operation, then when the engine is operating at low speed, the mixture will be too rich, beyond the limit of combustion. So, the air-fuel ratio must be adjusted by supplying a supplement quantity of fuel as increasing of engine speed. The basic principle of the technique of biogas-HHO supply is based on the effect of engine speed on vacuum in intake manifold analyzed above.
The schema of biogas-HHO supply system is shown in Fig. 9. It is a compact gaseous fuel supplying valve with the input is connected to biogas source and the outputs are connected to the biogas injectors. The normal close van inside the compact valve in controlled by the balance between the spring tension and vacuum pressure applied on the membrane. In this schma, venturi 2 is the original one of the engine and venturi 1 is new one. The tension of the spring 1 and the spring 2 can be adjusted by the screws. As the diaphragm diameter of the compact valve has been fixed, the timing of closing, opening of the valve, i.e. the interval time of the gas supply can be adjusted through adjusting the spring tension so that the valve operates within the desired vacuum range.



Fig. 10 shows the variation of equivalence ratio f when the engine operates at full load regime with different engine speeds fuel with biogas enriched by HHO through the compact valve. The injection time of biogas is variable according to the vacuum at the cross section S2 and S4. The injection time of HHO is fixed. Valve 2 is closed at low speed and it is opened gradually as increasing of engine speed due to increasing of vacuum pressure. It can be observed that when the f-correction valve is added, the equivalence ratio can be adjusted around f=1 under different engine speeds. Besides the adjustment of equivalence ratio by the f-correction valve, the presence of HHO gas in fuel mixture improves combustion properties, allows an extension of combustion limit. This helps to recover the stability of the engine as speed changes suddenly.

 

 
Conclusion:
 
Based on the above analysis, the following conclusions can be drawn:

  • The maximum vacuum at the downstream venturi is about 10kPa higher than that at the upstream venturi. Vacuum at the venturi throat increases significantly with engine speed and the peak of the vacuum curve tends to shift toward the end of the intake process as increasing of engine speed.
  • The vacuum at the throttle venturis decreases as decreasing of engine speed or/and increasing of flow obturation. The tendency of variation of vacuum pressure at the upstream venturi and at the downstream venturi not quite different but the absolute value of vacuum in the second case is about 40% higher than that of the first case.
  • With the conventional gaseous fuel supplying system, the equivalence ratio f of biogas-HHO fueling engine decreases significantly as increasing of engine speed. With the same gas supply condition, the suitable mixture for low engine speed becomes too poor at high engine speed; conversely, optimal mixture for high engine speed becomes too rich at low engine speed.
  • A supplement fuel supplying valve is needed to correct the equivalence ratio of the engine fueled with biogas-HHO. The valve operates under effect of vacuum at the downstream venturi to supply a compensation quantity of fuel as increasing of engine speed.
  • The compact kit composed by a mixer, a compact biogas valve (included main injection valve and fi-correction valve), providing a stable equivalence ratio around stoichiometric value, demonstrated appropriate technique for biogas-HHO supplying to SI engine in practice.
 
   

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