Full Length ArticleExperimental studies on cyclic variations in a single cylinder diesel engine fuelled with raw biogas by dual mode of operation
Introduction
Alternative fuels are attracting attention due to strict emission norms, depleting fossil fuels and their ever increasing prices. Biogas has emerged as a promising alternative fuel from the past several years which can be used in different applications including transportation, stationary power generation etc [1], [2]. Biogas is considered to be a clean and efficient fuel for operating IC engines due to its broad ignition boundary, ability to generate a homogeneous mixture, higher hydrocarbon ratio as well as larger auto ignition temperature [3], [4], [5], [6]. In gasoline engines, knocking can be controlled by using biogas because of higher auto ignition temperature of the biogas. On the other hand, biogas cannot be used as a direct fuel in diesel engine, hence with few modifications are necessary for effective use of biogas such as dual fuel combustion [7]. Vibrations from the diesel engine are the vital factor in design, performance of the engine and convenience for the passengers. The engine vibrations are undesirable as it may damage the engine components, creates instability of the engine, motion sickness and discomfort to the users. The engine vibrations are mainly originated from cycle-by-cycle variations occurring in the combustion chamber. The cyclic variations in the diesel engine are mostly due to the instabilities of a fuel injection system or long ignition delay [35], [36], [37].
These problems are inter-associated with the concept of cycle-by-cycle variation causing vibration and instability of the engine. So this work describes the complete outline of the cycle-by-cycle variations in a diesel engine when biogas is used along with it. Thus, it is important to study combustion analysis of the diesel engine fuelled with biogas.
Normally, the problem of cyclic variations is observed in petrol engines caused due to the change in the burning rate for every cycle. Due to the cyclic variation in the amount of fuel, motion of gases in cylinder, presence of air and exhaust gases in the cylinder, composition of mixture variations across the spark plug which causes a difference in combustion are also the reasons [8], [9], [10]. On the other hand, cycle variations are quite less in case of diesel engines due to its nature of dominance in non-premixed combustion. However, the instabilities of a fuel injection system or long-lasting ignition delay are also considered for the cause of cyclic variations. [11].
Many researchers have observed cycle-by-cycle variations or cyclic variations as a challenging task because of its destructive effects on the engine's performance which inhibits power loss, decrease in brake thermal efficiency as well as raise in emissions. Wing [12] discovered that due to the variation in injection timing for a rotary fuel-pump injected diesel engine cyclic variations occurred for every cycle. Koizumi et al. [13] observed that as the mass of the fuel injected is varied the combustion variations in every cycle developed is analyzed in the indicated mean effective pressure in the indirect injection diesel engine. Schmillen et al. [14] concluded that combustion variation cannot be analyzed by observing the variation of fuel injection during the combustion process. Ozdor et al. [15] examined the effects of cyclic variations in case of the petrol engine, from the results he concluded that cyclic variations are caused due to the random fluctuations at equivalence ratio as well as the turbulent behaviour of the flow field. These variations promote the development of laminar speed variations, partial stratification. Shoji [16] studied the cycle variations of a diesel engine by observing the direction of the fuel spray and performance at low load and low speed. Zhong et al. [17] explored the variations in a common rail diesel engine (CRDi) with direct injection by varying the rate of injection correlated with maximum temperature (Tmax), maximum cylinder pressure (Pmax) and Indicated mean effective pressure (IMEP). Zhang et al. [18] articulated that the in-cylinder flow field is also one of the reasons for variations occurring in the performance of a diesel engine. Kouremenos et al. [19] interpreted the combustion fluctuations happening in the diesel engine by using the technique called as stochastic analysis and concluded that ignition delay and injection timing had no role on causing cyclic variations in peak cylinder pressure. Rakopoulos et al. [20] analyzed the nature of variations in the combustion of a diesel engine by the means of probability density functions of the different parameters associated with the combustion. Quangang et al. [34] examined the cyclic variations of diesel engine operated with methanol by dual fuel mode. They performed the investigation to study the effects of engine load for various injection timing as well intake temperature, methanol substitution percent by considering In-cylinder pressure of 100 combustion cycles. From the results researchers concluded that the stability of the fumigated engine is comparable with neat diesel combustion but on the other hand magnitude of cyclic combustion of dual mode varies at lower loads. Moreover, work related to cyclic variations for NOx emission reduction as well as using various fuels like natural gas, LPG or methane, neat biodiesel, fumigated methanol etc, for different operating conditions was investigated by the researchers [20], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47].
The combustion characteristics of the diesel engine operated with the biogas by dual fuel combination are not easy to analyze and in the dual fuel mode gaseous fuel is added with compressed air which is more vulnerable to combustion noise due to the cause of cyclic variations. Novelty of the present work lies in utilizing the biogas obtained from the food waste as an alternative fuel for conventional diesel fuel and to operate the biogas with the diesel engine by dual mode. No major studies were found on the effect of higher proportions of biogas operated in the diesel engine. Moreover, the combustion studies related to cyclic variations of biogas in the diesel engine by dual fuel mode are less [21]. Hence an objective is framed in the present work to understand the concept of the cycle-by-cycle variations of diesel-raw biogas by dual fuel mode operation in a computerized single cylinder diesel engine for different operating conditions.
The biogas which is used for conducting the research work is obtained from the biogas plant which is locally available as seen in Fig. 1 is situated 14.1 km away from NITK campus. All the biodegradable wastes are processed by the plant which is having two tonnes capacity.
Biogas is produced by the anaerobic digestion process with food waste collected from various hotels across the city. The biogas plant produces 140–180 m3 of gas and 120–150 kg of compost every day. The process of extracting the biogas was a challenging task because the pressure coming out from the digester was at very low pressure and it is difficult to fill the gas in a cylinder or tyre tubes. In order to overcome this issue, we came up with a plan of extracting biogas by using a compressor provided with necessary fittings.
The extraction of biogas process using a compressor is shown in Fig. 2(a and b). Fig. 2(a) shows the gas exit from the digester connected with a hosepipe and it is interconnected to the inlet of the compressor. Then the outlet of the compressor is connected to the inlet of a heavy vehicle tyre tube. As the compressor is switched on, the biogas is sucked and compressed to an operating pressure of 7 bar. The compressed biogas exits out from the compressor and gets filled into the tyre tube as observed in Fig. 2(b). After this stage, the tyre tube containing compressed biogas is transported back to NITK campus.
The tube filled with biogas is kept in an internal combustion engine research laboratory. It is convenient to operate the biogas when it is filled in a cylinder rather than directly with the tyre tube. In the next stage, the biogas is transferred from the tube to cylinder as shown in Fig. 3(a and b). The Fig. 3(a) shows the biogas before filled into the cylinder and Fig. 3(b) shows the biogas filled into the cylinder. The process of extracting biogas from the tube and filling into the cylinder is the same as the process of extracting biogas from MCC. After filling the compressed biogas into the cylinder it is ready to operate with the diesel engine. Table 1 shows some of the properties of selected fuels.
Section snippets
Experimental setup and methodology
The experimental studies were conducted in IC engine research laboratory, NITK campus. The engine test rig used to conduct the research work is a computerized diesel engine, single cylinder, four stroke, naturally aspirated, water cooled and direct injection. Different loading conditions are attained by the means of eddy current dynamometer which is coupled to the engine by regulating the voltage as well as current supply. Engine operation is controlled and different inputs are provided by the
Cylinder pressure (P-Θ)
Fig. 7 shows the variations of the average cylinder pressure for 100 combustion cycles with diesel and various biogases at maximum brake power of 4.84 kW. All the biogas from BG20 to BG60 showed higher peak cylinder pressure than diesel at max brake power. Due to higher auto-ignition temperature and presence of carbon dioxide, the start of ignition gets delayed for biogas mixtures compared to diesel. Diesel fuel showed lower peak cylinder pressure of 56.5 bar occurring at 10° CA aTDC for max BP
Conclusions
In this article, cycle-by-cycle variations of diesel-raw biogas operated in dual fuel mode at full load and constant speed conditions is investigated. From the food waste, biogas is generated with higher methane concentration and is operated in a diesel engine by dual fuel mode for different proportions from BG20 to BG60 with a step of 10% and gets mixed with air by mass respectively. Using a data acquisition system the combustion parameters are measured for 100 consecutive combustion cycle.
CRediT authorship contribution statement
C. Jagadish: Conceptualization, Methodology, Writing - original draft, Investigation, Visualization, Writing - review & editing. Veershetty Gumtapure: Supervision, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to express gratitude to Mangalore city corporation (MCC), Karnataka, India for providing biogas and department of mechanical engineering NITK, Karnataka, India to conduct experiments.
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