The increased limitations to both NOx and soot
emissions have pushed engine researchers to rediscover gasoline
engines. Among the many technologies and strategies, gasoline
direct injection plays a key-role for improving fuel economy and
engine performance. The paper aims to investigate an extremely
complex task such as the idle operating engine condition when the
engine runs at very low engine speeds and low engine loads and
during the warm-up. Due to the low injection pressure and to the
null contribution of the turbocharger, the engine condition is far
from the standard points of investigation. Taking into account the
warm-up engine condition, the analyses are performed with a
temperature of the coolant of 50°C.
The paper reports part of a combined numerical and experimental
synergic activity aiming at the understanding of the physics of
spray/wall interaction within the combustion chamber and particular
care is used for air/fuel mixing and the combustion process
analyses. In order to properly describe the engine condition,
different injection strategies are investigated. Late and early
injection strategies are deeply analyzed and compared in terms of
combustion stability and pollutant emissions.
UV-visible imaging and spectral measurements are carried out in
real engine with wide optical accesses... Measurements are
performed in the optically accessible combustion chamber realized
by modifying a real engine. The cylinder head was modified in order
to allow in the fourth cylinder the visualization of the fuel
injection and the combustion process with high spatial and temporal
resolution.
The 3D-CFD engine simulations are reproduced by means of the
commercial code Star-CD. Due to the warm-up condition and the many
physical sub-models a numerical methodology is implemented and
particular care is used to boundaries conditions analyses. CFD
analysis is used to find a possible explanation of the high
cycle-to-cycle variability. The experimental and numerical
comparisons, in terms fuel mixing and front flame propagation, give
an explanation of the idle condition.