One promising approach to reduce pollutants from compression ignition engines is the Partially-Premixed- Combustion in which engine out emissions can be reduced by promoting mixing of fuel and air prior to auto-ignition. A great interest for a premixed combustion regime is the investigation on fuels with different reactivity by blending diesel with lower cetane number and higher volatility fuels. In fact, fuels more resistant to auto-ignition give longer ignition delay that may enhance the fuel/air mixing prior to combustion.
During the ignition delay period, the fuel spray atomizes into small droplets, vaporizes and mixes with air. As the piston moves towards TDC, as soon as the mixture temperature reaches the ignition point, instantaneously some pre-mixed amount of fuel and air ignites. 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 test engine was optical 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% gasoline by volume (G20). The cetane number lower than pure diesel fuel of this blend produced longer ignition delay for enhanced in-cylinder fuel/air mixing. Two injection pressures (70 and 140 MPa), several injection timings at two EGR rates were tested.
UV-visible emission spectroscopy was used for detecting the chemical markers of combustion process. Chemiluminescence signals, due to OH, HCO, CO and CO2 emission bands, were identified. OH emission was correlated to NO measured at the engine exhaust. The soot spectral feature in the visible wavelength range was correlated to soot engine out emissions.