Fuel chemistry and cetane effects on diesel homogeneous charge compression ignition performance, combustion, and emissions

Abstract

The effects of cetane number (CN) on homogeneous charge compression ignition (HCCI) performance and emissions were investigated in a single-cylinder engine with port fuel injection, using intake air temperature for control. Commercial fuels and blends of the diesel secondary reference fuels were evaluated, covering a CN range from 19 to 76. Sweeps of intake air temperature showed that low-CN fuels needed higher intake temperatures than high-CN fuels to achieve ignition. As a function of intake air temperature, each fuel passed through a point of maximum indicated mean effective pressure (i.m.e.p.). High-CN fuels required a combustion phasing approximately 10 crank angle degrees (CAD) earlier than the lowest CN fuels in order to prevent misfire. The high-CN fuels exhibited a strong low-temperature heat release (LTHR) event, while no LTHR was detected for fuels with CN ≤ 34. All of the fuels yielded comparable NOx emissions (< 6 ppm at 3.5 bar i.m.e.p.) at their respective maximum i.m.e.p. timeing. Low-CN fuels were prone to excessive pressure rise rates and NOx emissions at advanced phasing, while high-CN fuels were prone to excessive CO emissions at retarded phasing. These results suggest that the products of LTHR, which are high in CO, are more sensitive to the quenching effects of cylinder expansion. Engine speed was found to suppress LTHR since higher engine speed reduces the time allowed for the LTHR reactions. In addition to measurements of standard gaseous emissions, additional sampling and analysis techniques were used to identify and measure the individual exhaust HC species including an array of oxygenated compounds. In addition to high concentrations of formaldehyde and other low molecular weight carbonyls, results showed an abundance of organic acids, ranging from formic to nonanoic acid. Concentrations of high molecular weight partially oxidized species were highest for the high-CN fuels at retarded phasing, and are believed to be a direct product of LTHR.