Abstract
It is very important to reduce exhaust emissions and to increase thermal efficiency of internal combustion engines because regulations against exhaust emissions, such as NOx, particulate matter, hydrocarbons, and carbon monoxide, are becoming stricter. The greenhouse effect, caused by CO2 emissions, is also a problem. Natural gas is considered to be one of several alternative fuels suitable for minimizing the side effects of petroleum, or for use in a co-generation system because they produce a cleaner exhaust gas for internal combustion engines with higher thermal efficiency. There are several types of natural gas engines, such as the open-chamber or pre-chamber type or engines using electric spark or micro-pilot ignition with gas oil. In this study, the pilot injection type with open-chamber ignition was chosen because of its 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 natural gas engines. Natural gas was injected into an intake pipe filled with compressed air, while gas oil was injected into the cylinder near the top dead centre for igniting the natural gas and air mixture. The injector for pilot gas oil was controlled by a common rail system. The overall equivalence ratio was set from 0.45 to 0.6 and the ratio of the gas oil was about 3% on a heat value basis.
The combustion was visualized from the bottom of the quartz piston using a high-speed video color camera. The initial combustion caused by the diesel spray could be observed. The pressure history was analyzed to obtain the rate of heat release and determine the combustion characteristics. Exhaust emissions of NOx, HC, CO, and smoke were measured. Many ignition kernels were produced by the ignition of gas oil. They initiated flame propagation of natural gas and air mixture, and the flames then moved toward the wall of cylinder. The effect of the number and diameter of the holes of the injector on initial combustion process and exhaust emissions could easily be found.
This natural gas engine operated at an intake pressure of 100-200 kPa by utilizing compressed air. The engine could operate under leaner mixture conditions with increasing intake pressure with stability of the combustion. At an intake pressure of 200 kPa, the engine operated with an equivalence ratio of 0.60 with low NOx and high thermal efficiency while maintaining the same indicated mean effective pressure of 1.6 MPa. As the mixture became leaner, the indicated mean effective pressure and the thermal efficiency decreased, while the levels of unburned hydrocarbons and carbon monoxide increased. This experiment determined that the quantity of gas oil affects NOx emissions.
Keywords:Dual Fuel Engine, Internal Combustion Engine, Natural Gas, Pilot Injection, Visualization