Insights into Cold-Start DISI Combustion in an Optical Engine Operating at -7 degrees C

Particulate Matter (PM) emissions reduction is an imminent challenge for Direct Injection Spark Ignition (DISI) engine designers due to the introduction of Particulate Number (PN) standards in the proposed Euro 6 emissions legislation aimed at delivering the next phase of air quality improvements. An understanding of how the formation of combustion derived nanoparticulates in engines is affected by the engine operating temperature is important for air quality improvement and will influence future engine design and control strategies. This investigation has examined the effect on combustion and PM formation when reducing the engine operating temperature to-7 degrees C. A DISI single-cylinder optical research engine was modified to simulate a range of operating temperatures down to the proposed-7 degrees C. A high-speed 9 kHz optical investigation of the in-cylinder combustion and fuel spray along with incylinder pressure measurements was completed with the engine motored and fired at 1500 rpm during homogeneous and stoichiometric combustion conditions. Results show striking differences between the flame growth structures at various operating conditions with the notable presence of significant fuel-rich regions which are understood to be prominent areas of PM formation. Measured engine performance parameters such as Indicated Mean Effective Pressure (IMEP) and Mass Fraction Burned (MFB) times correlated with the observed differences in combustion characteristics and flame growth speed. Flash boiling of the fuel spray was present in the fully heated engine case and significantly reduced the penetration of the spray plume and the likelihood of piston crown and cylinder liner impingement. A clear link was shown between operating temperature, engine performance and in-cylinder combustion parameters which contribute to the formation of PM.