NOx reduction technology
NOx is produced when the oxygen and the nitrogen in the air react with each other in a high-temperature environment. Diesel engines operate with high compression ratios and lean fuel mixtures. This results in rich oxygen and nitrogen atmosphere in the combustion chamber which leads to formation of NOx.
This feedback of part of the exhaust gases into the combustion chamber (exhaust gas recirculation, EGR) reduces the quantity of excess air so that a condition with fewer available oxygen and nitrogen molecules is created which can bond as NOX in a chemical reaction.
An exhaust gas feedback can be produced in different ways.
Internal EGR
The simplest way is either to open the outlet valve during the aspiration stroke or the inlet valve during the outlet stroke. Both methods can be implemented by simple modification of the camshaft but have the disadvantage that they cannot be deactivated. This means that the EGR always operates regardless of the engine load and speed. The internal, uncontrolled EGR has a slight disadvantage at some engine operating points with regard to the torque acceptance which depends on optimised camshaft profiles.
Controlled internal EGR
Another EGR method is the electronic control of the opening point of the outlet valve. The outlet valve stroke for the EGR is implemented by hydraulic connection with the inlet cam. The connection can be broken by means of a solenoid valve which deactivates the EGR. The advantage is that this system is controlled by the engine management system without having the disadvantage of cooled external EGR, corrosive materials. Other advantages are an improved driving behaviour and a faster response of the engine. The system is mainly used in high-performance equipment such as tractors, factory floor vehicles and building machinery.
Cooled EGR
A third EGR method is to feed a defined amount of exhaust gas through an external cooler and then feed it back into the combustion system through a clack valve. This method is known as external, cooled EGR. The system enables a higher power density and better torque control because the switching valve can be controlled electronically by the engine management system (definition of the EGR amount). The engine in-cylinder temperature is reduced by feeding back cooled exhaust gases which leads in turn to a higher power density. The disadvantages are higher cooling requirements and, more important, the transformation of sulphur from the fuel to corrosive sulphuric acid. This is one of the reasons why all manufacturers of high performance diesel engines are demanding the distribution of sulphur-free diesel fuel for this solution. DEUTZ uses especially suitable materials to avoid corrosion.
