An innovative approach to the study of combustion and emission
formation in modern diesel engines has been applied to a EURO V
diesel engine equipped with an indirect-acting piezo injection
system.
The model is based on the joint use of a predictive
non-stationary 1D spray model, which has recently been presented by
Musculus and Kattke, and a diagnostic multizone thermodynamic model
developed by the authors.
The combustion chamber content has been split into homogeneous
zones, to which mass and energy conservation laws have been
applied: an unburned gas zone, made up of air, EGR and residual
gas, several fuel/unburned gas mixture zones, premixed combustion
burned gas zones and diffusive combustion burned gas zones. The 1D
spray model enables the mixing process dynamics of the different
fuel parcels with the unburned gas to be estimated for each
injection pulse; therefore, the equivalent ratio time-history of
each mixture zone can be estimated. A separate set of zones has
consequently been generated for each pulse, according to a similar
conceptual approach to that introduced by Dec.
A premixed burned gas zone is generated as combustion takes
place. This zone progressively oxidizes the mixture zones of the
pulse, until they are completely consumed. If the average
equivalence ratio of the premixed burned gas zone is higher than
unity, diffusive burned gas zones are generated to complete
combustion.
The global heat release rate is calculated on the basis of the
experimental pressure signal, as the approach is of the diagnostic
type. The main model results are the mass and temperature
evolutions of the zones, along with the equivalence ratio values of
the different mixture zones at the start of combustion. In the
literature, this value has been shown to be significantly related
to the soot formation rate.
The diagnostic tool includes predictive submodels for the
calculation of the pollutant emissions. In other words, NO
formation is modeled by means of thermal Zeldovich and prompt
mechanisms; CO is calculated via the Bowman equations; soot
formation is modeled by means of an expression that is derived from
Kitamura et al.'s, results, in which an explicit dependence on
the local equivalence ratio at the start of combustion is
considered; soot oxidation is modeled via the
Nagle-Strickland-Constable formulation; the THCs are calculated by
accounting for the effects of spray overmixing, injector sac and
hole volumes, and spray impingement.
The model outcomes can be reported in the well-known φ-T
diagrams, which offer a synthetic representation of the local
conditions during the fuel/unburned gas mixing processes and during
combustion for each single injection pulse.
The diagnostic approach has been applied to a EURO V diesel
engine equipped with indirect-acting piezo injectors, at both
medium-low and medium-high load/speed conditions. The effects of
EGR rate variations have been also investigated in order to assess
the capability of the model to take the changes in the charge
chemical composition into account. The main results have shown that
the combustion of the pilot injection mainly occurs at
stoichiometric/lean premixed conditions, as it is responsible for
NOx but not for soot formation. The main injection
combustion initially occurs in rich premixed conditions, a result
that confirms the Dec conceptual model. No spray impingement
occurred in the analyzed data as far as THC formation is concerned,
and the main contribution to THC emissions at the engine exhaust
was due to the injector sac and hole volumes. However, the
contribution of spray overmixing increased at medium-low loads.
Finally, it has been confirmed that EGR is not an effective
means of decreasing the average φ value at the start of combustion
to reduce soot formation.