A compression ignition internal combustion engine for a heavy-duty diesel vehicle comprising an exhaust system, which exhaust system comprising a composite oxidation catalyst (12, 42) and a soot filter substrate (44, 50) disposed downstream from the composite oxidation catalyst, which composite oxidation catalyst (12, 42) comprising: a substrate, preferably a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; two catalyst washcoat zones (1, 2) arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1 and comprising a first catalyst washcoat layer, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end of a second catalyst washcoat zone having a length L 2 and comprising a second catalyst washcoat layer, wherein L 2 < L, wherein the second catalyst washcoat zone is defined at a second end thereof by the second substrate end, and wherein the first substrate end of the composite oxidation catalyst (12, 42) is oriented to an upstream side and wherein the first catalyst washcoat zone comprises a first refractory metal oxide support material and two or more platinum group metal components supported thereon comprising both platinum and palladium at a weight ratio of platinum to palladium of <1; and the second catalyst washcoat zone comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon; and a washcoat overlayer extending axially from the first substrate end for protecting at least part of the underlying first catalyst washcoat zone from phosphorus and/or zinc poisoning when in use, thereby to retain activity of the first catalyst washcoat zone to generate an exotherm from an increased concentration of hydrocarbon fuel present in exhaust gas relative to normal operating conditions for heating the downstream filter, which washcoat overlayer comprising a particulate metal oxide having a loading of >48.8 g/l (>0.8g/in 3) and wherein the particulate metal oxide has a mean pore diameter of ≥ 10 nm and/or the washcoat overlayer has a mean interparticle pore diameter of ≥ 10 nm, wherein the particulate metal oxide in the washcoat overlayer supports a platinum group metal, which is platinum or a combination of platinum and palladium at a Pt:Pd weight ratio of ≥ 1:1, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is greater than a total platinum group metal loading in the second catalyst washcoat zone and wherein the first catalyst washcoat zone comprises one or more first alkaline earth metal components, preferably barium, supported on the first refractory metal oxide support material. a substrate, preferably a honeycomb flow-through substrate monolith, having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end; two catalyst washcoat zones (1, 2) arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L 1 and comprising a first catalyst washcoat layer, wherein L 1 < L, is defined at one end by the first substrate end and at a second end by a first end of a second catalyst washcoat zone having a length L 2 and comprising a second catalyst washcoat layer, wherein L 2 < L, wherein the second catalyst washcoat zone is defined at a second end thereof by the second substrate end, and wherein the first substrate end of the composite oxidation catalyst (12, 42) is oriented to an upstream side and wherein the first catalyst washcoat zone comprises a first refractory metal oxide support material and two or more platinum group metal components supported thereon comprising both platinum and palladium at a weight ratio of platinum to palladium of <1; and the second catalyst washcoat zone comprises a second refractory metal oxide support material and one or more platinum group metal components supported thereon; and a washcoat overlayer extending axially from the first substrate end for protecting at least part of the underlying first catalyst washcoat zone from phosphorus and/or zinc poisoning when in use, thereby to retain activity of the first catalyst washcoat zone to generate an exotherm from an increased concentration of hydrocarbon fuel present in exhaust gas relative to normal operating conditions for heating the downstream filter, which washcoat overlayer comprising a particulate metal oxide having a loading of >48.8 g/l (>0.8g/in 3) and wherein the particulate metal oxide has a mean pore diameter of ≥ 10 nm and/or the washcoat overlayer has a mean interparticle pore diameter of ≥ 10 nm, wherein the particulate metal oxide in the washcoat overlayer supports a platinum group metal, which is platinum or a combination of platinum and palladium at a Pt:Pd weight ratio of ≥ 1:1, wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per litre of substrate volume (g/L) is greater than a total platinum group metal loading in the second catalyst washcoat zone and wherein the first catalyst washcoat zone comprises one or more first alkaline earth metal components, preferably barium, supported on the first refractory metal oxide support material.