Analysis of combustion in diesel engines fueled by directly injected natural gas

A single-cylinder two-stroke (DDC 1-71) diesel engine has been fueled with natural gas directly injected at high pressure into the engine cylinder. Prior to injection of the natural gas, a quantity of diesel fuel is injected into the cylinder (from the same injector) to provide for gas ignition. Tests have been conducted at medium load and speed over a wide range of injection timing, and with both conventional diesel and gas/diesel operation. With natural gas fueling, significant reduction in nitrogen oxide emissions have been measured without significant loss in efficiency, relative to conventional diesel operation. Using measurements of cylinder pressure development, a new method of combustion analysis has been used to estimate mass burning rate, burned gas temperature, and rate of nitrogen oxide (NO) generation. The method uses a nonlinear regression technique to determine the distribution with crank angle of the cylinder heat loss rare. The method assumes that NO formation takes place within one turbulent mixing lime following combustion of each fuel-air increment. Comparison of measured and calculated NO concentration in the exhaust over the whole range of injection timing shows that for both conventional diesel and gas/diesel operation the effective turbulent mixing period is equivalent to 4 degrees of crank angle at 1250 RPM. The results demonstrate that a mass burned method can be used to infer cylinder temperature distributions and NO formation rate as well as the progress of combustion.