Abstract
Keywords - Internal Combustion Engine, Natural Gas, Dual Fuel Engine, Pilot Injection,
Visualization
Abstract - It is very important to reduce exhaust emissions because regulations against exhaust emissions are being strengthened as well as to improve thermal efficiency against green house problem for internal combustion engine. Natural gas is considered to be one of promising alternative fuels in view of minimizing resources of petroleum, and/or utilizing as a co-generation system because of cleaner exhaust gas for internal combustion engines with high thermal efficiency. There are some types for natural gas engines, however, pilot injection type was chosen in this study because of stable ignition and durable ignition source.
A four-stroke water-cooled engine with a single cylinder was used for this study to analyze the combustion characteristics in a natural gas engine. The engine speed was constant. Natural gas was injected into an intake pipe filled with compressed air, while diesel fuel was injected into the cylinder near the TDC for igniting the natural gas and air mixture. Both injectors were controlled by a computer. The overall equivalence ratio was set from 0.5 to 0.8 and the ratio of gas oil was very small.
The combustion of the pilot injection was visualized using a high-speed video camera from the bottom of the quartz piston. The pressure history was analyzed to obtain the rate of heat release to analyze the combustion characteristics. Exhaust emissions of NOx, HC, CO and smoke were measured. A small first peak of heat release rate could be seen due to premixed combustion of diesel fuel injected in the cylinder. After that, the rate of heat release shows only a mild peak that is seen in the premixed combustion in a spark-ignition engine. Therefore, flames propagates from many ignition kernel produced by the ignition of diesel fuel toward the wall of cylinder. The effect of quantity of the gas oil injected on initial combustion process and exhaust emissions could be found.
This natural gas engine operated in intake pressure from 102 kPa (near atmospheric pressure) to 200 kPa by utilizing compressed air. The engine could operate under leaner mixture condition with increasing intake pressure without increasing instability of the combustion. In intake pressure of 200kPa, the engine operated even in equivalence ratio of 0.55 with low NOx and high thermal efficiency with maintaining the IMEP of 1.6MPa. When the mixture becomes leaner, mean effective pressure decreases suddenly and unburned hydrocarbons and carbon monoxide increase very much.