A natural gas engine equipped with a mechanism that introduces exhaust gas into a cylinder during an intake stroke, and in which an amount of diesel fuel injected into a cylinder is set to a diesel fuel amount for idling condition across an entire operating region of an engine, engine output is increased or decreased by increasing or decreasing an amount of a natural gas fuel, and fuel injection of the diesel fuel into a cylinder is performed using multi-injection in a high load region in which the accelerator opening degree is greater than a preset first opening degree.

A system for reducing internal combustion engine emissions includes a main exhaust line, a multi-particulate filter, and a valve connected between the main exhaust line and the multi-particulate filter. The multi-particulate filter contains a first particulate filter, a second particulate filter, a first ozone injector, a second ozone injector, and an oxidation catalyst. A process for reducing internal combustion engine emissions includes flowing exhaust emissions through a main exhaust line and alternating filtering the exhaust emissions with a first and second particulate filter. Alternating includes adjusting one or more valves to bypass the second particulate filter and filter the exhaust emissions with the first particulate filter while injecting the second particulate filter with ozone. The process also includes adjusting the position of the one or more valves to flow exhaust emissions to the second particulate filter while the first particulate filter receives injected ozone.