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°C.
A DISI single-cylinder optical research engine was modified to
simulate a range of operating temperatures down to the proposed
-7°C. A high-speed 9 kHz optical investigation of the in-cylinder
combustion and fuel spray along with in-cylinder 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.
