Reactivity controlled compression ignition (RCCI) is a potential combustion strategy to achieve high engine efficiency with ultra-low NOx and soot emissions. Fuel stratification can be used to control the heat release rate of RCCI combustion. But the in-cylinder combustion process of the RCCI under different fuel stratification degrees has not been well understood, especially at a higher engine load. In this paper, simultaneous measurement of natural flame luminosity and emission spectra was carried out on a light-duty optical RCCI engine under different fuel stratification degrees. The engine was run at 1200 revolutions per minute under a load about 7 bar indicated mean effective pressure (IMEP). In order to form fuel stratification degrees from low to high, the common-rail injection timing of n-heptane was changed from -180° CA after top dead center (ATDC) to -10° CA ATDC, while the iso-octane delivered in the intake stroke was fixed. The natural luminosity imaging results indicated that, when n-heptane was injected at -180° ATDC, the combustion process looked like HCCI combustion at first with uniform blue flames all over the combustion chamber, but bright soot spots emerged later on. With retarding n-heptane injection timing, combustion regions with higher natural luminosity moved to the edge of the combustion chamber, and the natural flame luminosity got more stratified, showing more signs of soot radiation. The flame emission spectra from RCCI combustion under different fuel stratification degrees showed band spectra of OH, CH, CH2O and C2, as well as continuous spectra of CO oxidation and soot radiation. Band spectra of OH, CH (431.4 nm) and C2 (516.5 nm)/CH2O (422-424 nm and 434.5 nm) became more distinct and appeared successively with the retarding of n-heptane injection timing from -180° ATDC to -10° ATDC.
