| abstract |
A positive ignition engine comprising an exhaust system, which comprises a catalysed filter for filtering particulate matter from exhaust gas emitted from a positive ignition internal combustion engine, which filter comprising a porous substrate having a total substrate length and having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a first porous structure containing pores of a first mean pore size, wherein the porous substrate is coated with a washcoat composition which is a NOx absorber catalyst washcoat composition comprising at least one precious metal; or a selective catalytic reduction (SCR) catalyst washcoat composition, wherein a second 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, which NOx absorber catalyst washcoat or SCR catalyst washcoat being axially arranged on the porous substrate as a first zone comprising the inlet surfaces of a first substrate length less than the total substrate length and a second zone comprising the outlet surfaces of a second substrate length less than the total substrate length, wherein the sum of the substrate length in the first zone and the substrate length in the second zone is ≧100%, wherein:n (i) a washcoat loading in the first zone >second zone; or (ii) where the washcoat composition is a NOx absorber catalyst washcoat composition, both a washcoat loading and a total precious metal loading in the first zone >second zone,n nand wherein the first zone is disposed upstream of the second zone. |