Experimental investigations of the combustion process of n-butanol/diesel blend in an optical high swirl CI engine

In diesel engines, fuel is injected into the engine cylinder close to the end of the compression stroke. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air. As the piston continues moving towards TDC, the mixture temperature reaches the fuel ignition point, causing instantaneous ignition of some pre-mixed amount of fuel and air. The balance of fuel that does not burn in premixed combustion is consumed in the rate-controlled combustion phase, also known as diffusion combustion. Fuel composition, charge dilution, injection pressure as well as injection timing are the main factors that influence combustion and emission formation in the compression ignition engine. In order to evaluate the effects of these factors on in-cylinder spray combustion and soot formation, UV-visible digital imaging and natural emission spectroscopy were applied in a single cylinder high swirl compression ignition engine. The engine was optically accessible and equipped with a common rail multi-jets injection system. Combustion tests were carried out using commercial diesel and a blend of 80% diesel with 20% n-butanol (BU20). Two injection pressures (70 and 140 MPa), two injection timings (11 CAD BTDC and 3 CAD BTDC) and a low and high EGR rate were tested. UV-visible emission spectroscopy was used for the detection of the chemical markers of combustion process. Chemiluminescence signals, due to OH, HCO and CO2 emission bands were detected. OH emission was correlated to NO measured at the exhaust. The soot spectral feature in the visible wavelength range was correlated to soot engine out emissions.