What causes 'after-burning' in a diesel engine, and what are its consequences?

After-burning in a diesel engine refers to the continued combustion of fuel in the exhaust system, after the main combustion event in the cylinder is supposedly complete. Several factors can cause this. One primary cause is incomplete combustion within the cylinder itself. This can be due to poor fuel atomization, inadequate air-fuel mixing, insufficient oxygen, or incorrect fuel injection timing. If the fuel is not completely burned in the cylinder, unburned or partially burned hydrocarbons are carried over into the exhaust system. Another cause is dribbling injectors. If the injector nozzle is damaged or worn, it might leak fuel into the cylinder late in the combustion cycle, or even during the exhaust stroke. This fuel will not have sufficient time or oxygen to burn completely and will continue to burn in the exhaust. Additionally, a delayed or prolonged fuel injection event can also contribute to after-burning. The consequences of after-burning are several. Firstly, it leads to increased exhaust gas temperatures, which can damage exhaust valves, turbochargers, and exhaust piping. This can cause premature failure of these components. Secondly, it significantly increases emissions of unburned hydrocarbons (HC) and carbon monoxide (CO), both of which are harmful pollutants. Thirdly, after-burning reduces engine efficiency, as the energy released by the combustion in the exhaust system is not used to produce useful work. In severe cases, after-burning can even lead to exhaust system fires or explosions.