A significant increase in the demand of internal combustion engines is leading to energy security and environmental concerns. Biogas is an attractive renewable alternative fuel with potential to substitute fossil fuels in spark-ignition engines. Its composition, however, varies widely, i.e. 45 to 80% (by volume) of methane in the mixture of methane and CO2. A typical biogas composition, however, contains about 60% of methane. Also, biogas can be upgraded to bio-methane (can substitute as a transportation fuel), which consists of 96-98% of methane in the fuel mixture. This wide variation of biogas composition affects the fuel quality (measured by methane number (MN) and Wobbe index (WI)) that influences high (limited by pre-ignition) and low (affecting cycle-to-cycle variations) operating loads of the engine. Further on, typical biogas composition degrades the engine characteristics significantly and upgrading it to bio-methane leads to a significant increase in the fuel price due to high capital investment and equipment maintenance. Moreover, the determination of the MN for a broad range of gaseous fuel compositions requires an expensive experimental set-up, considerable time and efforts. Also, available models could not predict the MN accurately for all gaseous fuel compositions. It is to be noted that a production engine generally consists of a three-way catalytic converter, thus it becomes important to operate the engine at stoichiometric air-fuel ratio (i.e., ϕ=1), and an engine speed of 1500 rpm is required for the stationary application by utilizing a four-pole generator to produce 50 Hz output frequency.
Therefore, in the present study, first a model is developed using an artificial neural network (ANN) to predict the MN for a wide range of gaseous fuels. It is followed by the development of a flexible gaseous fuel, single-cylinder, spark-ignition engine test facility for investigation purposes. Then, comparison of the characteristics of the engine operated with liquid (i.e., gasoline) and gaseous (i.e., methane) fuels is done, and also the baseline information for the biogas composition variation studies is acquired by operating the engine under methane (as the MN of pure methane is 100) for a wide range of loads with ϕ=1. Further, evaluated the effect of biogas composition variations (or variation of fuels WI) on the operational limits of the engine along with determining the engine characteristics for a wide range of operating loads. A computational model is also developed for a better understanding of the combustion process inside the engine cylinder. In addition, the amount of CO2 to be removed from the typical biogas composition is identified as a compromise between engine characteristics and biogas upgradation cost. Lastly, assessed the effect of compression ratio (CR) variation on the engine characteristics including operational limits of the engine fueled with the typical biogas composition.
For the comparison of gasoline- and methane-fueled engine, the engine was operated at the compression ratio of 8.5:1 while maintaining the same operating conditions. For the baseline study, the engine was operated under methane (or surrogate of bio-methane) at the compression ratio of 8.5:1 over a wide range of load with ϕ=1. To investigate the effect of biogas composition variation on the engine characteristics, the engine was operated under different operating conditions with ϕ=1. A wide range of biogas compositions (including bio-methane) was achieved by mixing 10%, 20%, 30%, 40%, 50% and 60% (by volume) of CO2 in the fuel mixture of methane and CO2. For the CR variation study on engine characteristics for the typical biogas composition, the CR was increased from 8.5:1 to 10:1, 11:1, 13:1 and 15:1 while maintaining ϕ=1.
The results showed that the ANN model was able to predict the MN accurately for a wide range of gaseous fuel compositions. The brake thermal efficiency increased and emissions level decreased for methane as compared to gasoline for a given operating load. The operating range of the engine is quantified for various biogas compositions, which is important to operate the engine safely and to maintain low fluctuations in the output. The low and high operating loads of the engine increased from 2.2 to 9.6 N-m and 12 to 16.5 N-m, respectively, when WI of the fuel mixture was decreased from 48.6 to 22.2 MJ/m3 (i.e., CO2 percentage is increased from 0 to 50% in the fuel mixture). The operating range decreased with the increase in CO2 percentage in the fuel mixture (or a decrease in the WI of fuel). Moreover, engine performance deteriorated for all the operating loads with the increase in CO2 percentage in the fuel mixture, and computational studies showed that it was due to a degradation in the flame propagation process. This degradation was found to be severe when CO2 was higher than 50% in the fuel mixture, even to operate the engine. The emissions (except nitric oxide) level increase with an increase in the CO2 percentage in the fuel mixture. The removal of CO2 from biogas till 20% (by volume) could provide a good compromise between biogas upgradation cost and engine characteristics. The operating range and performance of the engine significantly decreased for the typical biogas composition as compared to methane. Compression ratio variation was found to be an effective way to improve the engine performance and operating range under the typical biogas composition.