A phenomenological model for the description of unburned hydrocarbons emission in ultra-lean engines

Abstract

The adoption of lean-burn concepts for internal combustion engines working with a homogenous air/fuel charge is under development as a path to simultaneously improve thermal efficiency, fuel consumption, nitric oxides, and carbon monoxide emissions. This technology may lead to a relevant emission of unburned hydrocarbons (uHC) compared to a stoichiometric engine. The uHC sources are various and the relative importance varies according to fuel characteristics, engine operating point, and some geometrical details of the combustion chamber. This concern becomes even more relevant in the case of engines supplied with natural gas since the methane has a global warming potential much greater than the other major pollutant emissions. In this work, a simulation model describing the main mechanisms for uHC formation is proposed. The model describes uHC production from crevices and flame wall quenching, also considering the post-oxidation. The uHC model is implemented in commercial software (GT-Power) under the form of “user routine”. It is validated with reference to two large bore engines, whose bores are 31 and 46 cm (engines named accordingly W31 and W46). Both engines are fueled with natural gas and operated with lean mixtures (λ > 2), but with different ignition modalities (pre-chamber device or dual fuel mode). The engines under study are preliminarily schematized in the 1D simulation tool. The consistency of 1D engine schematizations is verified against the experimental data of BMEP, air flow rate, and turbocharger rotational speed over a load sweep. Then, the uHC model is validated against the engine-out measurements. The averaged uHC predictions highlight an average error of 7% and 10 % for W31 and W46 engines, respectively. The uHC model reliability is evidenced by the lack of need for a case-dependent adjustment of its tuning constants, also in presence of relevant variations of both engine load and ring pack design.

1D/3D simulation procedure to investigate the potential of a lean burn hydrogen fuelled engine

“…Finally, a refined model for the estimation of unburned fuel is adopted, considering the filling/emptying of the crevices, the flame wall quenching, and the fuel post-oxidation [36]. The cylinder model is coupled with additional volumes, one that schematizes the piston crevice region, and a second additional crevice volume, positioned above the dead volume.…”

Potential of hydrogen addition to natural gas or ammonia as a solution towards low- or zero-carbon fuel for the supply of a small turbocharged SI engine

“…A multi-zone approach is used to estimate the amount of NO x and CO. On one hand, the extended Zeldovich mechanism is used to evaluate the NO kinetics [31], on the other hand, a two-step reaction scheme [32] is used to describe the CO kinetics. A refined model for the evaluation of the unburned fuel is implemented, considering both the filling/emptying of the crevices volume, the flame wall quenching, and the fuel postoxidation, as explained in a previous work of the authors [30].…”

A Tabulated Chemistry Multi-Zone Combustion Model of HCCI Engines Supplied with Pure Fuel and Fuel Blends

“…A refined model for the evaluation of unburned fuel is implemented, considering the filling/emptying of the crevice volume, and the fuel post-oxidation [36]. The top land of the piston ring pack is assumed as the unique crevice volume, and the gas trapped in it is expected to be in thermal equilibrium with piston walls.…”

“…The top land of the piston ring pack is assumed as the unique crevice volume, and the gas trapped in it is expected to be in thermal equilibrium with piston walls. The piston wall temperatures are not available in literature works from which the data of the engines under study were extracted, and hence they are assigned in the simulations according to the authors' experience on other engines operated with ultra-lean mixtures [36]. When released from the crevices during the expansion phase, the uHCs are partially consumed according to a simplified chemical kinetic mechanism [37].…”