On some versions of NOx catalytic converters, post-fuel injection can lead to NOx afterburning. This causes the main fuel injection to continue for 200°after TDC during the power stroke and the exhaust stroke. Post-fuel injection delivers a certain amount of fuel to the exhaust gases.
Unlike the control and main injections, the injected fuel does not ignite, but takes the residual heat from the exhaust gases in the form of steam. During the exhaust stroke, the mixture of exhaust gases and fuel enters the exhaust system through the exhaust valves. Some of the fuel is returned to the afterburning cylinders via the EGR system, and has the same effect as pilot fuel injection. NOx catalytic converters use the fuel in the exhaust gases as an actuator to reduce the NOx content in the exhaust gases.
Since post-fuel injection dilutes the engine oil, this injection must be approved by the engine manufacturer.