Abstract
Homogeneous combustion in reciprocating internal combustion engines (ICE) may be realized not only in free gas (e.g., homogeneous mixture compression ignition HCCI) but also using porous medium (PM) combustion. This concept has proven its advantages at steady-flame burners. It uses very porous grid of ceramic foam or metal wires heated by the flame and stabilizing so lean mixture flame at temperatures that limits significantly NOx formation. Homogeneous combustion with equalized temperature field in space avoids both excessive nitrogen oxidization and fuel partial combustion. Its use in ICEs is not without problems due to heat transfer during compression, limited maximum temperature and the possibility of delayed heat supply to expansion.
The current paper should elucidate
- general potentials of homogeneous (fast) combustion with upper temperature and pressure limits at required engine power for engines of a new generation;
- specific problems of PM combustion under unsteady conditions typical for reciprocating engines using CFD modeling.
A simple tool for cycle parameter assessment using computerized T-s diagram was developed and presented earlier. The preliminary results of it has been promising.
As the further step, a CFD model has been developed for an evaluation of a PM combustion potential and its future optimization. A simulation method based on a moveable-grid, Runge-Kutta finite volume (FV) 2.5-D model has required the introduction of new source terms concerning spatially distributed heat transfer and momentum sink due to drag in a PM domain. The geometry of a PM insert is characterized using a filament model of its matrix characterized in 3 main directions. The temperature of a PM insert surface is solved using the Fourier equation with estimated effective thermal conductivity of a solid phase.
A combustion model uses momentum and specie sources near to a fuel entry to a PM insert accompanied by very fast heat release for a part of fuel covered by oxygen available and complex chemical kinetics.
The detailed FVM model has provided preliminary results as a base for sensitivity analysis, cycle optimization and engine lay-out changes. Steep temperature gradients may occur in a PM domain due to a gradual transport of cylinder charge into a PM and a compression or combustion of pre-heated gas. NOx formation might be limited only if high temperature occurs in the zone of a rich mixture. Concerning efficiency, a premature heat supply to gas from a PM during compression is disadvantageous as well as an intensive heat transfer during combustion. The model will be used in a future optimization of the PM combustion process.