EXHAUST SYSTEM FOR A POSITIVE IGNITION ENGINE

An exhaust system for a positive ignition engine, which system comprising a filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine, which filter comprising a porous substrate having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores of a first mean pore size, wherein the porous substrate is coated only on the outlet surfaces with a catalytic three-way catalyst washcoat comprising a plurality of solid particles wherein the porous structure of the washcoated porous substrate contains pores of a second mean pore size, wherein the second mean pore size is less than the first mean pore size and wherein the three-way catalyst comprises platinum and rhodium; palladium and rhodium; platinum, palladium and rhodium; or palladium only as active components supported on a high surface area oxide, and an oxygen storage component