Diagnostics of Mixing Process Dynamics, Combustion and Emissions in a Euro V Diesel Engine 2011-24-0018

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 NO_x 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.