As exhaust gas passes through the converter, DOC oxidizes and purifies gaseous HC and CO, lube oil of total particulate matter(TPM), and unburned fuel SOF by taking advantage of the theory that catalyst and exhaust react.
For gasoline engine, operating air-fuel ratio is close to theoretical one, while diesel engine is operated in rarefied air, thus TWC is not available. Therefore diesel engine is designed to most remove HC, CO and SOF with diesel oxidation catalyst.