NOx  ( Oxides of Nitrogen )


(This is only "seen" by a 5-gas analyser) NOx only occures ( in meaningful quantities) with engine under load at high temperature (driving or on dynamometer). Measured in parts per million (ppm) of the total concentration of the exhaust gases.


NOx emissions rise and fall in a reverse pattern to HC emissions. As the mixture becomes leaner more of the HC's are burnt, but at high temperatures and pressures (under load) in the combustion chamber there will be excess O2 molecules which combine with the nitrogen to create NOx. NOx increases in proportion to the ignition timing advance, irrespective of variations in A/F ratio.

This gas is related to the exhaust gas detoxification systems ( in conjunction with CO and HC) , exhaust gas recirculation systems.

Those systems bring some of the inert (processed) exhaust gas back in to the engine to be burned again. This time around this gas has no O2 extra molecules and prevents high combustion temperatures and further increase in NOx formation. NOx is Very Dangerous Lethal Gas and air pollutant!


Oxides of Nitrogen (NOx), (includes nitric oxide-NO and nitrous dioxide-NO2), are formed if the combustion temperatures within the combustion chamber exceed some 2,500 degrees F. This can normally only occur when the engine is under load. When excessive temperature conditions exist, the greatest amount of NOx is typically produced at the Stoichiometric point (AFR 14.7 or Lambda of 1.0) as the engine is under a light load. If the combustion process within an engine is burning fuel at or near Stoichiometric point, NOx levels on acceleration will typically read significantly higher than those measured at cruise and during deceleration. Typically, the NOx readings at idle will be close to 0 PPM.


1) High NOx Readings. Circumstances that can lead to abnormally high NOx emissions are:

Excessive oxides of nitrogen can be caused by anything that makes combustion temperatures rise. Typical causes of high combustion temperature on fuel injected vehicles include:

            o Cooling system problems

                        - insufficient radiator airflow

                        - low coolant level

                        - poor cooling fan operation

                        - thermostat stuck closed or restricted

                        - internal radiator restriction

            o Excessively lean air/fuel mixture  ( AFR above 14.7, Lambda above 1.0 ) leading to increased engine combustion temperature

                        - leaky intake manifold gasket

                        - worn throttle shaft

            o Closed loop control system incorrectly shifted lean

            o Improper oxygen sensor operation

                        - slow rich to lean switch time

                        - rich biased 02 sensor voltage

            o Improper or inefficient operation of EGR system

                        - restricted EGR passage

                        - Malfunctioning EGR valve ( used in "secondary" burning or recycling of exhaust gases to reduce harmful emisions )

                        - EGR modulator inoperative

                        - plugged E or R port in throttle body

                        - faulty EGR VSV operation

                        - leaky/misrouted EGR hoses

            o Improper spark advance system operation - leading to increased engine combustion temperature

                        - incorrect base timing

                        - false signal input to ECM

                        - improper operation of knock retard system

            o Carbon deposits on intake valves  - again retaining unwanted temperature and preventing good combustion

            o Malfunctioning catalytic converter


 2) Low NOx readings. There is, effectively, no reading for NOx that can be characterized as too low or below optimal. NOx is naturally near 0 ppm at idle. NOx concentrations may appear normal even in a rich burning environment where the AFR is well below 14.71 (Lambda below 1.0).


Catalytic converter intervention and NOx concentrations:

High NOx readings at the tailpipe are a clear indication that there is a problem in at least one part of the system, but a NOx reading that appears within "normal" ranges or is only modestly elevated is not necessarily a reliable indicator of proper or even acceptable system performance. NOx readings at or near "normal" are possible, and not uncommon for a malfunctioning engine equipped with a properly functioning catalytic converter. In such circumstances, truly elevated pre-catalytic converter NOx levels will be masked by the catalytic converter and the potential for a NOx problem must be further evaluated in the context of other readings of abnormal gas concentrations and AFR / Lambda readings.

The engine tendency to produce NOX is reduced by re-circulating a regulated amount of exhaust gas, under controlled conditions, thus reducing the temperature and inhibiting the formation of NOX. What is produced is removed in the catalytic converter by a process called "reduction" (as opposed to "oxidation" or burning for CO and HC). The oxygen and nitrogen are forced to part company, the oxygen combining with CO to produce C02 and the Nitrogen (N ) goes back into the atmosphere. There must be some CO left to "attract" the oxygen. For this reason the Lambda value has to be held very close to 1.0. If no CO is produced the reaction will not take place. We can therefore talk about a Lambda window, which is 0.97 to 1. 03. If Lambda in not maintained in this window the catalytic converter will not be as effective!


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