Experimental Investigation of Combustion Stability and Particle Emission from CNG/Diesel RCCI Engine

This paper presents the experimental investigation of combustion stability and nano-particle emissions from the CNG-diesel RCCI engine. A modified automotive diesel engine is used to operate in RCCI combustion mode. An open ECU is used to control the low and high reactivity fuel injection events. The engine is tested for fixed engine speed and two different engine load conditions. The tests performed for various port-injected CNG masses and diesel injection timings, including single and double diesel injection strategy. Several consecutive engine cycles are recorded using in-cylinder combustion pressure measurement system. Statistical and return map techniques are used to investigate the combustion stability in the CNG-diesel RCCI engine. Differential mobility spectrometer is used for the measurement of particle number concentration and particle-size and number distribution. It is found that advanced diesel injection timing leading to higher cyclic combustion variations. Too advanced diesel injection results in a partial burn/misfire operating condition. The results indicate that the double diesel injection strategy has a relatively higher concentration of nucleation mode particles and it increases with advancing the diesel injection timings. Typical bimodal lognormal shape of particle-size and number distribution curve is shifted to uni-modal shape by increasing the mass of port-injected fuel.