What is NOx?

The topic of diesel emissions – and in particular nitrogen oxides (NOx) – has become a major talking point in recent years.

Understanding the ins and outs of the systems that help reduce these emissions has never been more important for a technician.

But what is NOx? And how do you diagnose NOx problems? There are several factors you need to be aware of if you are trying to get to the bottom of such an issue.

Nitrogen and oxygen are present in the atmosphere so both diesel and gasoline engines are affected.

During combustion, these elements combine in an endothermic reaction to form nitrogen oxides, or NOx.

The amount of NOx formed varies with the peak combustion temperature – as that rises, so does the rate of NOx formation.

Diesel engines produce more oxides of nitrogen due to the engine operating on excess air; this is due to the absence of a conventional throttle valve.

Oxygen and nitrogen react in the presence of high temperatures and combustion pressures to form NOx gases.

The temperature in a diesel engine’s combustion chamber can reach as high as anywhere between 1,500°C and 1,800°C, which is optimal for the formation of NOx gases.

How is NOx treated?

NOx gasses can be treated in two ways:

  • The formation of the chemical bond being disrupted during the combustion process (exhaust gas recirculation, internal and external)
  • The reduction of NOx gases using exhaust gas after-treatment devices (catalytic reactions)

An exhaust gas recirculation (EGR) valve can be fitted to a vehicle. It regulates the quantity of (spent) exhaust gases recycled into the intake manifold.

This method works effectively because the recycled exhaust gases are inert and essentially “dilutes” the fresh mixture charge.

The result is the overall peak combustion temperature is reduced.

The EGR system is operated during conditions where there is a surplus of free oxygen such as part load and cruising conditions.

Alongside NOx, other emissions from diesel vehicles include carbon monoxide, hydrocarbons, sulphur and particle matter.

The current Snap-on diagnostic software upgrade allows you to boost your knowledge of NOx matters with on-screen guidance and technology classes.

These include:

  • NOx Accumulator
  • Fuel Additive Versus Urea Solution
  • NOx Emissions
  • Diesel Particulate Filter

Find out more about the new Snap-on diagnostic software upgrade.

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