Potential of Porous-Media Combustion Technology as Applied to Internal Combustion Engines
Table 1
The most important properties of porous materials for engine applications.
Parameters
Range of values (required/expected)
Specific surface area
Large: must be adapted to particular application for supporting individual processes
Heat transport properties
Excellent: especially important for PM engine concept with combustion in porous reactor in order to permit homogeneity of combustion zone
Heat capacity
Large: must be adapted to particular application. In case of PM engine defines engine dynamic properties and influences cycle thermodynamics. Also influences cold-start conditions
Permeability to flow and flame propagation
High porosity: For gas or liquid flow more than 80% porosity, preferably more than 90% porosity with open pores (cells). Pore size influences different processes in PM volume. For example, an optimum pore size for fuel distribution in space, for heat recuperation and for combustion process is different
Pore size and size distribution
Typical foam pore density from 8 ppi to 30 ppi: for flame propagation under pressure see Pecklet number criterion. Directly influences pressure losses, multijet splitting by injection, homogenization in the volume. For nonfoam structures, for example, 3D-frame structure, the pore size and distribution in space may be optimized for particular application
Pore shape
Principally all available shapes are suitable: cells must be open and have radial connections
Thermal shock resistance
High: Especially for in-cylinder application and for PM engine concept. Depending on reactor location and kind of assembly within engine components
Corrosion resistance
High: especially in atmosphere of burned gases inside the engine cylinder
Electrical properties
For direct heating: high electrical resistance and homogeneous energy distribution (preferably voltage 12 V and current 10 to 80 A). Attainable temperatures up to 1500 K. Could be important for cold-start conditions
Available maximum temperature
Depends on application: for example for combustion in PM reactor K; for vaporization and mixture formation K; for two-stage combustion concept K
Porous medium mechanical stability
Important under high temperature and pressure conditions: very critical factor in case of ceramic material mounted in piston top. Accelerations up to 500 times earth acceleration typical and large temperature gradients usual
Porous medium fitting within engine components
Very critical: especially in case of ceramic materials. Possible gluing using high temperature ceramic glues. Important also that porous reactor at one time cold and at another may be very hot. Metal foams could be more useful if application temperature high enough
Variable geometry
Important: for all engine applications (adapting to available space and shape)
Long-time stability
Should be very high: as yet almost unknown area for engine applications
