NOX is created when the oxygen and nitrogen in the air react in a high pressure and temperature environment. Diesel engines operate at high compression ratios and with lean fuel mixtures. This creates an oxygen and nitrogen rich atmosphere in the combustion chamber that leads to NOX production. By drawing some of the exhaust back into the combustion chamber (exhaust gas recirculation, EGR), the amount of surplus air is reduced, creating a condition in which less oxygen and nitrogen molecules are available for reaction into NOX. Exhaust gas recirculation can be achieved in several ways.

The simplest way is to open either the exhaust valve momentarily during the intake cycle or to open the intake valve momentarily during the exhaust cycle. Both of these methods are achievable with a simple modification to the camshaft but have the disadvantage of being uncontrolled. That is to say, regardless of engine load and speed, the EGR is always operating. The Internal EGR carries a small penalty in torque capability which are depending on optimized camprofiles.

A third method of EGR is electronically controlling the opening point of the exhaust valve. This is achieved by using a solenoid-powered valve which, when opened, allows engine oil to hydraulically operate a small plunger, which slightly depresses the exhaust valve. This system has the advantage of being controlled by the engine management system but without the problems of producing the corrosive elements associated with the cooled external EGR. Further benefits are found in improved drivability and speed of engine response. This system is mostly used on high-performance engines such as those used in agricultural tractors and high-performance earth-moving equipment.

The next method of EGR is achieved by passing a small amount of exhaust gas through an external cooler and reintroducing it into the combustion system via a reed valve. This method is known as externally cooled EGR. This system allows higher power density and better torque control as the reed valve can be controlled electronically by the engine management system. By reintroducing cooled exhaust gases, the temperature of the intake air is lowered, once again contributing to a higher power density. The disadvantages are increased cooling requirements and, more importantly, the conversion of sulfur from the fuel into corrosive sulfuric acid. This method of EGR is used on-highway where lower levels of sulfur are found in the fuel.