Table 1: The most important properties of porous materials for engine applications.

ParametersRange of values (required/expected)

Specific surface areaLarge: must be adapted to particular application for supporting individual processes

Heat transport propertiesExcellent: especially important for PM engine concept with combustion in porous reactor in order to permit homogeneity of combustion zone

Heat capacityLarge: 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 propagationHigh 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 distributionTypical 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 shapePrincipally all available shapes are suitable: cells must be open and have radial connections

Thermal shock resistanceHigh: Especially for in-cylinder application and for PM engine concept. Depending on reactor location and kind of assembly within engine components

Corrosion resistanceHigh: especially in atmosphere of burned gases inside the engine cylinder

Electrical propertiesFor 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 temperatureDepends on application: for example for combustion in PM reactor 𝑇 m a x < 2 0 0 0  K; for vaporization and mixture formation 𝑇 m a x < 1 5 0 0  K; for two-stage combustion concept 𝑇 m a x < 1 8 0 0  K

Porous medium mechanical stabilityImportant 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 componentsVery 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 geometryImportant: for all engine applications (adapting to available space and shape)

Long-time stabilityShould be very high: as yet almost unknown area for engine applications