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Biogas spark ignition engine


Lê Xuân Thạch

          Biogas spark ignition engine converted from diesel engine presents a lot of advantages: robust structure, high compression ratio, no need liquid fuel for operation... It is really an appropriate solution for energy saving and pollution control in our country.
           In this work, ZH1115 diesel engine has been converted into biogas spark ignition engine. Diesel fuel injection system was removed and replaced by electronic ignition system. Combustion chamber of the engine was tested with two types: omega combustion chamber and plane combustion chamber. Compression ratio of the engine can be changed by cutting off the piston head with different thickness. By this way, we can test the performance of the engine with compression ratio varried from 9 to 14. Advance ignition timing angle of the engine is adjusted by changing the position of the ignition timing sensor mounted on block of engine. Biogas-air mixture is supplied to the engine via a three-way mixing device. Simulation of flow through mixing device allows us to determine its optimal basical size with biogas containing different concentrations of CH4.
          The experiment was carried out in the field with Froude dynamometer. Concentration of CH4 in biogas is varried by mixing two different sources: (1) without CO2 filtration and (2) with CO2 filtration.
          Combustion process in combustion chamber of biogas engines is simulated via the fluid dynamic software FLUENT. Comparison between results given by simulation and experiments is carried out in some cases. The comparison allows us to adjust the parameters of the model and then we can predict the performance of biogas engine running in different conditions by simulation without experimental data.
          The results of the research allows us to draw the following conclusions:
         1. The presence of CO2 in biogas reduces burnning velocity of the fuel-air mixture but it makes an increasing of capacity of antidetonation. So biogas fuel should fit to low speed and high compression ratio engine. Therefore, converting diesel engine into biogas spark ignition engine is appropriate solution both in technology and economy.



         2. Research methodology combined cobustion simulation by software FLUENT and experimental testing on Froude dynamometer is very efficiency, it allows us to limit costs of experiements but ensure the reliability of research results.


         3. Inthe calculation ofcombustion in biogas spark ignition engine converted from diesel ZH1115 engine, we can use the standard k-eturbulence model, Partial Primixed combustion model with laminar burning speed given by empirical formula and turbulent combustion coefficient selected by ff=1.2. Default parameters pre-installed in FLUENT can be selected for other parameters.


         4. Burning velocity of biogas-air mixture decreases with decreasing of CH4 concentration in biogas fuel. Thus when increasing of engine speed or/and reducing of CH4 concentration in biogas, an increasing of spark timming angle is needed to ensure optimal operation of engine. Biogasengine with compression ratio e=12, powered by biogas containing 60% CH4 has optimum spark timming angle in the range from 34°to 42°as engine speed changes.




            5. When converting diesel engine to biogas spark ignition engine, its compression ration should be reduced  to the optimum value. In the case of biogas engines ZH1115 run at speeds of 2200 rpm, optimum compression ratio ranges is from 11.5 to 12.5..


         6. Due to burning velocity of biogas-air mixture is lower than diesel case, we should maintain turbulent movement of flow in combustion chamber for improving the efficiency of the engine. Simulation results show that indicating work of ZH1115 biogas engine with piston flat is lower than that of omega combustion chamber about 22% when running at speed of 2200 rpm.


           7. The three-way mixing device can supply stable mixture for biogas engines. When cross section of biogas suppling pipe is fixed, variation of mixture composition is not considerable with change in the throttle opening and engine speed. When CH4 composition in biogas fuel changes, cross section of  biogas supplying pipe should be changed. For ZH1115 biogas engine with original air supply pipe, the equivalent diameter of biogas supplying pipe can be represented by the expressionD(mm)=166.X-0,5443,where X is the percentage (%) of CH4 in biogas by volume



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