Studies on a Dual Fuel Engine with a Waste Heat Recovery Unit

Abstract

Nowadays, electrical generators are used as stand by power generators in large buildings such as educational institutions, hospitals, auditorium, etc., for providing electrical power during electrical power shutdown or failures. Owing to higher thermal efficiency and durability, compression ignition (CI) engines are mainly used in standby electrical generators. Although, diesel is mainly used as a fuel to operate such generators, utilization of different alternate fuels to reduce dependency of diesel and also reduce engine exhaust emissions is of great interest today. The main objective of dual fuel technology is to promote homogeneity of combustible charge, which leads to enhance the thermal efficiency. This research study basically follows three steps viz., production of biofuel, engine experimentation and waste heat recovery analysis. For the research work, Waste cooking oil (WCO) and spent tea waste (STW) which were disposed in the hostel premises and institute canteens at NIT Rourkela were considered as the feedstock for production of biofuel. odiesel was obtained from WCO using a two-stage esterification process. The produced waste cooking oil methyl ester (WCOME) was characterized, and the physico-chemical properties were also determined for its use as a pilot fuel in CI engine. Since the amount of STW disposed was not very high, an initial assessment was carried out to check the potential of biogas production by co-digesting STW with cow manure. The use of STW offered two advantages when co-digestion was done; (i) reduced the amount of cow manure used and (ii) reduces the anthropogenic activity. The cow manure and STW were mixed at different proportions viz 50:50, 60:40, 70:30, 80:20 and 100:0 and kept in AD1, AD2, AD3, AD4 and AD5 digesters respectively. The experiments were carried out different input parameters such as pH, carbon to nitrogen (C/N) ratio, and digestion time. From the experimental results, the highest biogas yield was obtained for AD3 which was found to be about 86.4 ml. It was also revealed that, the AD3 digester contained higher methane content of about 71% followed by AD4 (68%). Further, the experimental results obtained from the laboratory scale were predicted through a novel fuzzy regression approach.
A single cylinder four stroke, air cooled, DI diesel engine was modified to operate in dual fuel mode, where biogas serves as an inducted fuel, and diesel or biodiesel (WCOME) as a Abstract
ii pilot fuel. During the initial stages of engine experiments, biogas at a flow rate of 0.25 to 1.0 kg/h at a regular interval of 0.25 kg/h was allowed during suction stroke; whereas diesel used as a pilot fuel. The combustion, performance and emission analyses of the dual fuel engine were analyzed and compared with those of the diesel operation and recorded as baseline data. The test results indicated that, BDDFM0.75 exhibited higher cylinder peak pressure and maximum rate of pressure rise of about 58.72 bar and 3.59 bar/°CA respectively at full load. Simultaneous reduction of NO and smoke emissions of 17.2% and 4.2% respectively were observed in BDDFM0.75 operation when compared to those of diesel at full load. It was ascertained that, biogas at a flow rate of 0.75 kg/h was found to be optimum. By using the optimum biogas flow rate of 0.75 kg/h, further the experiments were carried out to increase the thermal efficiency of the test dual fuel engine by considering three parameters at three levels; advancing the pilot injection to 3°CA in the interval of 1.5°CA along with standard (23°CA), increasing the injection pressure to 240 bar with the interval of 20 bar along with standard (200 bar), and compression ratio was modified to 16.5, 17.5 and 18.5. Hence, a total of 27 sets of experiments were performed to choose the optimum condition. It was observed that, at 240 bar, 24.5°CA with a CR18.5, the dual fuel operated engine reached a maximum BTE of about 31.2% which was higher by about 4.5% than that of diesel at full load. It was also noticed that, at the optimum conditions of the engine, i.e., at 240 bar, 24.5°CA with a CR18.5, about 2.43 kW was wasted into the atmosphere through the exhaust gases, which accounted for 16.1% of the total energy supplied by the fuel (WCOME+biogas). Hence, further investigation was carried out to convert the waste heat into useful electrical energy using thermos-electric generation (TEG).
Therefore, as a third step, a mathematical model was developed to assess the performance parameters of TEG. Then, the experiments were carried out in the same test dual fuel engine at optimum engine parameters (240 bar, 24.5°CA and CR18.5 with 0.75 kg/h of biogas) using a TEG module fitted to the engine exhaust. The conversion efficiency of the TEG was measured in terms of voltage, current, temperature difference (ΔT) and power generated at different engine loading conditions. From the test results, it was found that, a maximum of 4.2 W power was generated at 75% load of the engine for 10 modules. The conversion efficiency of the TEG was found to be about 2.1% at 75% load. Theoretically, a maximum cold side temperature of the module (Tc) was found to be about 218°C, which was only 2.75% deviated from the experimental results. From the obtained results, it was understood that, ΔT played a significant role in improving the performance of a TEG system. The salient Abstract iii features of heat pipes such as no moving parts, passive energy recovery, and silent in operation could serve as a heat sink for a medium temperature range (300°C). Therefore, an attempt was made to increase the temperature difference (ΔT) across the modules by integrating or placing the heat pipe on the cold side of thermos-electric module. Experiments were conducted at two different conditions (i) with heat pipe and (ii) without heat pipe at different engine loading conditions. The results revealed that, a significant improvement in the ΔT was noticed when heat pipe was used as the heat sink. A maximum ΔT of about 65°C for n=1, which was higher by about 680% than that without the heat pipe at 75% engine load. A maximum power of about 6.1 W was generated by the TEG-heat pipe with conversion efficiency of about 2.9% at 75% engine load. The results of the research encourage implementing the idea in a diesel generator of different capacities available in the Institute campus.