Lire la première partie de la thèse

Dulbecco, Alessio

Description

Niveau: Supérieur, Doctorat, Bac+8
Lire la première partie de la thèse

• both conventional


• ifp-c3d solver


• hcci combustion regimes


• angle ?


• conventional diesel


• fuel evaporation


• narrow angle


• engine operating


• diesel bowl


• injection sprays

Sujets

• Renault

Informations

• Publié par: profil-zyak-2012
• Nombre de lectures: 17
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Lire la première partie de la thèse Chapter 3
Dual-CM engine validation
In this chapter, the dual-CM, developed in chapter 2, is validated against two types of
results :
• experimental data from engine test-bench are used to validate the overall behavior
of dual-CM,
• 3D CFD simulation results obtained with the IFP-C3D solver, section 3.2, are
used to validate more specific aspects of the dual-CM, and in particular aspects
associated to fuel evaporation and mixing. Although IFP-C3D calculations are
not exact, they give results close to experiments and above all they allow, thanks
to the Diesel combustion model ECFM3Z developed by Colin et al. [34], to give
access to the basic physics of combustion in Diesel engines, which is unfeasible with
experimental measurements.
Concerning this chapter, after a brief description of the tested engine, section 3.1, and
of the IFP-C3D solver, section 3.2, the methodology used for determining the simulation
initial conditions starting from the experimental data is detailed, section 3.3. Then, the
dual-CM results are investigated and compared with the IFP-C3D results, section 3.4.
Furthermore, the sensitivity of the dual-CM to isoparametric variations and its
adaptation capability to a wide range of engine operating conditions are presented in section 3.5
and section 3.6, respectively. To conclude, statistics relative to the dual-CM behavior
on the entire engine operating domain are presented, section 3.7.
3.1 Tested engine : Renault G9T NADI™ concept engine
The tested engine is a turbo-charged Renault G9T NADI™ concept engine [93]. The
specifications of the engine are given in table 3.1. This engine has the particularity
Engine type in-line 4 cylinder
Bore [mm] 87:0
Stroke [mm] 92:0
Connecting rod [mm] 149:9
Compression ratio [-] 14:0
3Unitary piston displacement [cm ] 546:91
Injection system Bosch GmbH CRI 2.2
Table 3.1: Renault G9T NADI™ concept engine specifications.
144145
of being able to operate in both conventional Diesel and HCCI combustion regimes.
Figure 3.1 shows the original scheme published in the Narrow Angle Direct Injection
(NADI™ ) concept patent. A better illustration of the NADI™ concept principle is
Figure 3.1: Scheme of the NADI™ concept.
given in figure 3.2. Figure 3.2 shows the difference between the conventional Diesel bowl
Figure 3.2: Difference between the conventional Diesel bowl and the Narrow Angle Direct
Injection (NADI™ ) bowl. The angle indicates the opening of the solid angle containing the
axes of the fuel injection sprays.
and the NADI™ concept bowl. In particular, in a conventional Diesel configuration the
injection sprays impact the outer part of the bowl and are then wall-guided towards the
inner part of the combustion chamber. Differently, in the NADI™ concept, because of
the narrow angle between the sprays ( < 100 ), the injections directly impact the
inner part of the bowl and are then wall-guided towards the outer part of the combustion
chamber. When compared to conventional Diesel engines, the NADI™ concept geometry
has shown to have two main advantages [93] :
• reducing the liquid fuel cylinder wall impingement, which is the cause of unburned
hydrocarbon emissions,
• improving the mixing of fuel and ambient gases in the combustion chamber,
especially for engine operating points having high EGR rates, as common for HCCI146
engine operating points. As a consequence, the emissions of NO and soot arex
reduced.
In order to put in evidence the impact of the piston shape on the spray dynamics, two
different bowls were used, figure 3.3. As will be shown in sections 3.4 and 4, the geometry
0
−0.1
−0.2
−0.3
−0.4
−0.5
−0.6
−0.7
−0.8
−0.9
−1
0 0.2 0.4 0.6 0.8 1
x [−]
Figure 3.3: Two different profiles of NADI™ concept piston bowls (out of scale).
of the piston bowl can have a strong influence on the mixing process of injected fuel with
the in-cylinder ambient air.
3.1.1 The engine operating domain
The Renault G9T NADI™ engine was installed on an engine test-bench and tested on the
entire engine operating domain [95]. Figure 3.4 represents the engine operating domain
expressed in terms of Indicated Mean Effective Pressure (IMEP) and engine speed.
The region of the engine operating domain associated to HCCI operating conditions,
thatisthelowloadandlowenginespeedregion, isratherunstableanddifficulttocontrol.
For this reason a thorough investigation of this region was performed, figure 3.4. This
experimental database represents the basis on which the overall behavior of the dual-CM
has been validated. In section 3.7, the statistics of the results of the dual-CM on the
entire engine operating domain are presented.
3.2 The 3D Computational Fluid Dynamics Code IFP-C3D
IFP-C3D is a 3D Fluid Dynamics (CFD) code using unstructured meshes
and Arbitrary Lagrangian Eulerian (ALE) finite volume method dedicated to ICE
simu1lations. IFP-C3DintegratesacombustionmodelentirelydevelopedatIFP:ECFM3Z[34] .
1The ECFM3Z equations are reported in appendix B.
y [−]147

Conventional Diesel
25
HCCI
20
15
10
5
0
500 1000 1500 2000 2500 3000 3500
Crank speed [rpm]
Figure3.4: Renault G9T NADI™ concept engine operating domain. Each circle corresponds to
a different engine operating point : empty and filled circles correspond to conventional Diesel and
Diesel HCCI conditions, respectively. The solid line represents the upper load limit as function
of the engine speed.
Nowadays, this model is widely used by car manufacturers for several and diversified
applications. In particular, ECFM3Z is able to compute conventional Diesel and Diesel
HCCI combustion regimes. This model has been retained as a reference model especially
for two reasons :
• it is able to compute all the combustion regimes : auto-ignition, premixed flames
and diffusion flames,
• the details given by a 3D calculation permit us to have a deep understanding of the
flow in terms of turbulence, fuel mixture fraction distribution and fuel evaporation
rate otherwise inaccessible with experimental devices.
Principle of ECFM3Z
Figure 3.5 shows the different zones computed by the model :
• a pure ambient air zone, which in the most general case contains a perfectly stirred
? ?
u bmixture of pure air and EGR A =A +A ,
? ?
u b• a pure gaseous fuel zone F =F +F ,
? ?
u b• a zone in which ambient air and fuel are mixed with each other M =M +M .
IMEP [bar]148
As seen, each of these zones is further subdivided into two parts representing the
unburned and burned gases. On the left of figure 3.5 is represented the unburned gas
u u u b b bregion (A +M +F ), while on the right the burned gas region (A +M +F ).
Figure 3.5: Schematic representation of the ECFM3Z combustion model.
Diesel combustion process dynamics in ECFM3Z
Figure 3.6 shows the initial state of the combustion chamber : the gaseous mixture
inside the reactor is a perfectly stirred mixture of pure air and eventually EGR at a
given thermodynamical state. At this stage, all the gases belong to the pure ambient
uair zone of the unburned gases (A ), figure 3.5. When the liquid fuel is injected in the
Figure 3.6: Diesel combustion process dynamics in ECFM3Z : initial state.
ucylinder, a second zone containing pure gaseous fuel appears in the cylinder (F ). Once
the pure ambient air and pure fuel zones coexist, they exchange mass with each other.
uConsequently, a third zone, containing pure air, EGR and gaseous fuel, is formed (M ),
figure 3.7. At this stage, the whole gaseous mixture belongs to the unburned gas region,
figure 3.5. Once the mixing region is created, the computation of the auto-ignition delay149
Figure 3.7: Diesel combustion process dynamics in ECFM3Z : mixing phase.
of the gaseous mixture starts. When the auto-ignition delay is attained, the combustion
process begins. At this point, three other zones relative to the burned gas region appear
figure 3.8 :
b u• a zone containing pure ambient air, A , identical to A ,
b u• a zone containing pure fuel, F , identical to F ; this zone is fed by the liquid fuel
mass evaporation rate, which is partitioned between the burned and burned pure
fuel zones,
b• a zone containing a gaseous mixture of fuel, air and products of combustion, M .
Figure 3.8: Diesel combustion process dynamics in ECFM3Z : auto-ignition of the gaseous
mixture.
The mixing process between pure ambient air and pure fuel takes place in the burned gas
region, too, figure 3.9. Thus, the mass fluxes from pure ambient air and pure fuel feed
the mixing zone and burn in diffusion combustion. This kind of modeling is integrated
in each computational cell used for representing the physical volume of the combustion
chamber. One of the main contributions of the ECFM3Z approach is the representation
of the mixing dynamics, which is capital for a correct computation of the combustion
process. A fundamental variable of the ECFM3Z approach is the progress variable, c.