Future emissions standards for passenger cars require a
reduction of NOx (nitrogen oxide) and CO₂ (carbon
dioxide) emissions of diesel engines. One of the ways to reach this
challenge while keeping other emissions under control (CO: carbon
monoxide, HC: unburned hydrocarbons and particulates) is to reduce
the volumetric compression ratio (CR). Nevertheless complications
appear with this CR reduction, notably during very cold operation:
start and idle. These complications justify intensifying the work
in this area.
Investigations were led on a real 4-cylinder diesel 13.7:1 CR
engine, using complementary tools: experimental tests, in-cylinder
visualizations and CFD (Computational Fluid Dynamics) calculations.
In previous papers, the way the Main combustion takes place
according to Pilot combustion behavior was highlighted.
This paper, presents an in-depth study of mixture preparation
and the subsequent combustion process.
Using laser illumination techniques, in-cylinder visualizations
highlight - even at low engine speed during cranking operation -
the existence of swirl motion moving the fuel being vaporized
inside the chamber. In these conditions, visualizations and
computations show that the vicinity of the glow plug and one
injector spray leads to a combination of adequate temperature and
air/fuel ratio and so to the combustion initiation. CFD
calculations also point out that the "stair shape"
evolution of heat release can come from a progressive propagation
of combustion from one spray to another.
Using the available tools, it appears also that some troubles
encountered in cold idle can be attributed to inadequate engine
control.
The tools developed (experimental tests, in-cylinder
visualizations, and calculations) have proven quite effective and
complementary capabilities in order to investigate and to
understand the physical phenomena encountered during cold operation
(start and idle). Even if correlations are not always easy to
establish between these phenomena and engine performance: speed
stability or smoke opacity, these tools will allow to progress in
this area and to further improve engine behavior and emissions.
