The evolution of Diesel engines in the last decades has been driven by the stringent pollution legislation and increasing environmental concern. In this context the Diesel engine appears like a very good candidate due to its naturally high efficiency. However, in order to further reduce engine out pollutant and green house gases to the required limits, a detailed study of the fuel-air mixing and combustion processes is necessary. An investigation of those processes by Laser Induced Fluorescence (LIF) techniques is presented. The mixing process is studied by tracer LIF, enabling a statistical analysis of the mixture structure and of the effect of injection parameters. The combustion process is studied by simultaneous formaldehyde, Poly-Aromatic-Hydrocarbons (PAH) and OH LIF, giving access to detailed information on the combustion process. A strong coupling is observed between the two processes.
mass concentration field is illustrated in Figure 1 for a set of injection conditions.
High Pressure Direct Injection (HPDI) has established itself as a proven technology which allows the efficient reduction of engine-out emissions in DI Diesel engines. But although HPDI is becoming a standard, the consequences of this new technology on mixing and combustion processes inside the combustion chamber have yet to be further understood because the physical phenomena involved are extremely complex, in particular since the two processes occur on overlapping time scales and therefore a strong coupling is happening.
For a detailed investigation of the physical
processes occurring in the combustion chamber,
optical diagnostics are the best candidates since
they are the only way to obtain two dimensional
quantitative measurements inside the combustion
chamber. Various techniques are available for the
study of mixing[
Tracer LIF is appropriate for the study of the
mixing process since it can provide
twodimensional quantitative information appropriate
for statistical analysis. The first step of the
application of tracer LIF to the measurement of fuel
distribution in the jet is to develop a method to obtain
normalized fuel mass concentration fields. In ref.[
Figure 1 : Schematics of tracer LIF images configuration and palette connecting image colors and fuel mass concentrations.
2.5 ] 2 ,[ m rag 1.5 o it s eh 1 m u l o V0.5
A B F A B 0 0 1 2 3 4 5
Vapor fuel mass concentration, [kg/m3]
A statistical analysis of the mixture fields obtained by tracer LIF was carried out and is illustrated in Figure 2. It showed that the Diesel jet mixture can be separated in two distinctive zones: the mixing zone on the upstream sides, where air entrainment due to shear turbulence dominates the jet dynamic, and the stagnation zone at the tip where the jet pushes away the dense surrounding gases. The latter is characterized by a lower mixing rate since small scales turbulence are missing.
Such analysis was then carried out to investigate the effect on the mixing process of different injection parameters such as injection pressure (illustrated in Figure 3), nozzle hole diameter, injection duration.
The combustion of Diesel jets is a complex process involving different steps from auto-ignition to the stabilization of a diffusion flame, and different regimes of combustion (premixed, diffusion, soot formation...).
In order to carry out a detailed analysis of the
different combustion regions, a simultaneous
formaldehyde, PAH and OH LIF technique was
developed and applied to the Diesel jet configuration
[
Figure 4 shows a typical combined image obtained with this technique during the diffusion limited combustion. A detailed analysis of the structure of those images was carried out in order to identify the different regions of combustion and understand the evolution of the process, in particular the transition from premixed auto-ignition to diffusion controlled combustion.
The results were then synthesized by a conceptual model of Diesel jet combustion illustrated in Figure 5.
One important result that came out of those investigations is the clear connection between the mixture and combustion structures, as well as for the transition from auto-ignition to diffusion limited combustion than for the structure of the diffusion flame itself.
The study of mixture and combustion processes
of Diesel jets by LIF clearly brought up the
advantage of using such techniques for this kind of
complex investigation. Furthermore, this kind of
research can clearly benefit in the future from the
improvement of LIF technique. Indeed in recent
years, a lot of efforts has been made to obtain
more quantitative information with LIF[
Also, it will be interesting in the future to use the LIF techniques presented here to study more complex configurations. For instance multiple injection strategies are a promising technology to further reduce engine out emission, and a more detailed analysis of the physical phenomena involved during those strategies will certainly help the optimization process of Diesel combustion systems towards cleaner engines.