Effect of Gaseous Fuels and Coolant Temperature on Performance and Combustion Characteristics of a Low-Speed Single Cylinder Diesel Engine

Today, diesel engines provide the main power source for the world equipment e.g., common propulsion generators in industry and agriculture. These engines are widely used due to their high combustion efficiency, reliability, compatibility, and cost-effectiveness. However, diesel engines are one of the most critical consumers of fuel which in turn causes some environmental pollution. One of the convenient and low-cost ways to reduce the pollution of these engines is dual-fuel mode and the use of gaseous fuels as an alternative fuel. This study investigated the effect of blending CNG and LPG with neat diesel in dual-fuel mode. Besides, the variation in engine coolant temperature on engine performance characteristics was experimentally studied.

The experimental apparatus consisted of a stationary, four-stroke, naturally aspirated, water-cooled, single-cylinder compression ignition engine. To control the engine load, an electrical dynamometer was made using a 7.5 kW three-phase generator and coupled to the engine as a cradle. A load cell was used to determine the force applied to the generator. The engine speed was monitored continuously by a tachometer. Fuel consumption was measured by using a weight method. A thermostat with variable temperature was used to control the temperature of the engine coolant. To measure the mass flow of air entering the cylinder, an airbox with a sharp edge orifice was used. For this study, factorial experiments in the form of a randomized complete block design with three replications were utilized to analyze the data statistically. The studied parameters were three levels of fuel ratio (100% diesel, 20% diesel and 80%± 2% CNG, 20% diesel and 80%±2% LPG), 11 engine speeds (1500 to 1600 rpm with 10 rpm intervals), and three engine coolant temperatures (50, 60, and 70 °C). All experiments were conducted in the governor control mode.

The results showed that the torque, brake power and brake mean effective pressure (BMEP) in the diesel-CNG mode at all engine speeds and in the diesel-LPG mode at low engine speeds significantly increased compared to pure diesel. The increases in these parameters in the diesel-CNG mode were 18.67%, 19.56% and 19.85%, and in the diesel-LPG mode were 14.02%, 13.86% and 14.2%, compared to those related to the pure diesel, respectively. This increase could be due to the high calorific value of gas fuels and improvement of combustion inside the cylinder due to the formation of homogeneous charge. At low engine speeds, the reductions in the brake specific fuel consumption (BSFC) and brake specific energy consumption (BSEC) for coolant temperature 60 °C were 11.21% and 10.77%, compared to coolant temperature 50 °C, respectively. Also, the BSFC and BSEC for diesel-CNG dual-fuel mode decreased by 8.12% and 10.81%, respectively. These values for the diesel-LPG dual-fuel mode were 5.4% and 2.4%, respectively. The brake thermal efficiency (BTE) also showed a significant increase at high speeds and when using the dual-fuel operational mode. However, raising the coolant temperature due to reducing the heat losses of the engine increased the BTE. The increases in BTE for coolant temperatures 60 and 70 °C were 7.19% and 4.37%, compared to the coolant temperature of 50 °C, respectively. When using the engine in dual-fuel mode, the volumetric efficiency due to reducing the air ratio showed a significant reduction. These diesel-CNG and diesel-LPG dual-fuel mode values were 20.31% and 24%, respectively. Furthermore, raising the coolant temperature diminished the volumetric efficiency. The reduction in volumetric efficiency for the coolant temperatures of 60 °C and 70 °C were 6.84% and 19.91% compared to the coolant temperature of 50 °C, respectively.

The following conclusions can be deduced based on this study:The use of gaseous fuels as the main fuel and with a small amount of diesel in compression ignition engines is possible and improves the engine's performance characteristics.In the diesel-CNG mode, torque, brake power and BMEP at all engine speeds and in the diesel-LPG mode at low engine speeds significantly increased compared to pure diesel because of improved combustion inside the cylinder.At low engine speeds, increasing the coolant temperature reduced the BSFC and BSEC. Also, in the dual-fuel mode compared to the engine with baseline diesel fuel, the BSFC and BSEC were significantly lower due to the higher calorific value of gaseous fuels and higher power generation.The BTE at high engine speeds and when the engine was in dual-fuel mode showed a significant increase. Also, increasing the coolant temperature due to reducing the heat losses of the engine increased the BTE.When using the engine in the dual-fuel mode, due to the volume of air replaced by the gas, the volumetric efficiency showed a significant reduction. Also, raising the coolant temperature diminished the volumetric efficiency.Overall, it can be stated that the use of a diesel-CNG dual-fuel mode with a coolant temperature of 60 °C at entire engine speeds has the best outputs on the performance and combustion characteristics of the engine.