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SI Engine Fuelled by Biogas Seminar Report
For understanding performance and combustion parameters at various compression ratio a single cylinder diesel engine was modified to operate as a biogas operated spark ignition engine. The engine was operated at 1500 rpm at throttle opening of 25% and 100% at various equivalence ratios. The performance, emissions and combustion characteristics with different compression ratios are compared. It has been found from the results that the higher the compression ratio, the higher the brake thermal efficiency. When the compression ratio was above a critical value of 13:1, brake power and thermal efficiency increased little. At higher compression ratios above 13:1, increased NOx, HC, and CO emissions were measured .In the study of hydrogen blending in biogas engine a 8-L spark ignition engine fueled by biogas with various methane concentrations which we called the N2 dilution test was performed in terms of its thermal efficiency, combustion characteristics and emissions. The engine was operated at a constant engine rotational speed of 1800 rpm under a 60 kW power output condition and simulated biogas was employed to realize a wide range of changes in heating value and gas composition. The N2 dilution test results show that an increase of inert gas in biogas was beneficial to thermal efficiency enhancement and NOx emission reduction. H2 fractions ranging from 5 to 30% were blended to the biogas and the effects of hydrogen addition on engine behavior were evaluated. The engine test results indicated that the addition of hydrogen improved in-cylinder combustion characteristics, extending lean operating limit as well as reducing THC emissions while elevating NOx generation.
Biogas is typically composed of 40-60% of methane and rest of inert gases such as nitrogen(N2) and (CO2).however its main composition varies by origin.There for it is difficult to establish optimum and consistent air- fuel mixture conditions and combustion phasing in order to ensure a stable operation of I C engines. In addition a significant amount of inert gases contained in biogas affects in-cylinder combustion charecteristics,reducing flame propagation speed which increase average length of combustion duration and ignition delay.One of the efforts to overcome these drawbacks is a rise of compression ratio in a biogas engine.it enables improvements in engine performance, especially in thermal efficiency and power output. However, an increased compression ratio can also exacerbate knock tendency and produce more nitrogen oxides (NOx) and hydrocarbon (HC) emissions.Although the presence of inert gases in biogas can suppress knock vulnerability and NOx emission, modifying piston (in an SI based engine) or replacing the diesel injectors by spark plugs (in a CI based engine) to achieve a high compression ratio will require extra cost. Another approach for better engine performance is a biogas diesel dual fuel engine.In this case, mixture of gaseous fuels and fresh air is supplied to a cylinder and then, a small amount of diesel-like fuel is injected to ignite the combustible mixture. Since it is usually converted from a diesel CI engine, the dual fuel engine has a high compression ratio. This means that it can achieve higher efficiency than a biogas dedicated SI engine. In addition, the injected diesel fuel behaves as multi-point ignition sources so that higher burning speed and relatively complete combustion can occur inside a combustion chamber. The other way to improve biogas engine performance is to add hydrogen (H2) in gaseous fuels. Hydrogen is well known for its excellent combustion characteristics and has been considered as a combustion enhancer for a gas engine fueled by natural gas or methane.Several researchers reported that use of H2-natural gas or H2-methane blend fuels cannot Furthermore, this multi-point ignition can make in-cylinder combustion process less sensitive to composition fluctuation of biogas, leading to stable onsite engine operations. Despite these advantages, the dual fuel engine is expensive together with high maintenance cost compared with a SI engine. It also suffers the possibility of frequent injector failures due to reduced cooling effect by a decrease in injected fuel and has issues on engine durability due to high in-cylinder pressure in power generation applications The other way to improve biogas engine performance is to add hydrogen (H2) in gaseous fuels. Hydrogen is well known for its excellent combustion characteristics and has been considered as a combustion enhancer for a gas engine fuelled by natural gas or methane.Several researchers reported that use of H2-natural gas or H2-methane blend fuels can not only extend lean operation limit of the engine but also decrease HC and carbon monoxide (CO) emissions while increasing NOx.However, most of these studies have focused on transportation applications.In addition, since all these previous works were carried out with biogas of relatively high methane concentration (more than 60%) and target power less than 20 kW, further research on a power generation engine with lower quality biogas less than 40% methane fraction needs to be done which is actually the case requiring an assistance of combustion promoter such as hydrogen.
In this study, an experimental investigation on a naturally aspirated (NA), 8-L spark ignition engine fueled by biogas with various methane concentrations e which we called the N2 dilution test e was performed in terms of its thermal efficiency, combustion characteristics and emissions. The engine was operated at a constant engine rotational speed of 1800 rpm under a 60 kW power output condition and simulated biogas was employed to realize a wide range of changes in heating value and gas composition. Then, as a way to achieve stable combustion for the lowest quality biogas, H2 addition tests were carried out in various excess air ratios. H2 fractions ranging from 5 to 30% were blended to the biogas and the effects of hydrogen addition on engine behavior were evaluated. Moreover, as one of the main results, a set of optimum operating conditions for maximum efficiency and minimum emission was suggested in terms of excess air ratio,spark ignition timing, hydrogen addition rate and so on.
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