Flamelet Structure in Diesel Engines under Lean and Stoichiometric Operating Conditions

Stoichiometric operation is one possible approach for reducing in-cylinder pollutant formation in diesel engines. High levels of exhaust gas recirculation (EGR) combined with stoichiometric operation may be employed to decrease soot and NO emissions from the engine. In this work, in-cylinder conditions are estimated for a diesel engine near top dead center, prior to the start of injection, for different levels of EGR. Two modes of engine operation are considered: the first is operation with excess air such that the overall equivalence ratio is 0.5, and the second is stoichiometric operation. These conditions are employed in separate studies to understand the influence of both EGR and mode of operation on pollutant formation and ignition. N-heptane is used as a representative fuel. Its oxidation chemistry is modeled using a reduced 159-species, 1540-step mechanism. A kinetics-based soot model and NO sub-mechanism are employed to investigate pollutant formation.

For a fixed load, the studies show that peak flame temperature is lower under stoichiometric conditions than lean conditions with no EGR due to the presence of residual gases. As EGR levels increase, the difference in peak temperature between the stoichiometric and lean modes increases since oxygen concentrations are lower in the stoichiometric cases. Longer ignition delay times are observed for stoichiometric engine operation for all EGR fractions. The results show that in-cylinder NO formation is reduced for stoichiometric engine operation, compared to conventional lean operation. In-cylinder soot formation is also lower for stoichiometric operation.