Exhaust emissions regulations primarily focus on the reduction of Particulate Matter (PM) and oxides of nitrogen (NOx).  As regulated emissions strive to achieve near-zero emissions levels, understanding the impact across the world becomes more challenging. 


Emissions Overview

What is in diesel exhaust?  Diesel exhaust is made up of chemical compounds, each regulated differently throughout the world.


Particulate Matter (PM)

Particulate matter is a collective name for substances deposited on a filter as diluted exhaust is passed through it.  Particulate matter is a mixture of solid particles and liquid droplets found in the air.  The particles are by-products of carbon particles, heavy hydrocarbons from fuel and lubricating oil, sulfates, nitrates and ash. 

Particulate matter is the “black smoke” or soot that comes out of the exhaust pipe or a variety of sources, including trucks and cars.  This soot is formed when insufficient air of low combustion temperature prohibits complete combustion of the free carbon.

Particulate matter can also be a product of the environment and includes natural dusts and pollens as well.


Oxides of Nitrogen (NOx)

NOx is a collective term that encompasses NO, a colorless gas, and NO2, a yellowish-brown gas.  NOx is formed from a reaction between nitrogen (N2) and oxygen (O2) at high temperatures.  This process, referred to as combustion, forms a toxic, reddish-brown gas – the main component in “smog.”

Oxides of Nitrogen are referred to as “NOx,” where x represents a changing proportion of oxygen to nitrogen.


Hydrocarbons (HC)

Hydrocarbon is a collective name that refers to over 100 different chemical compounds made up of hydrogen (H) and carbon (C).  Hydrocarbons are a product of incomplete combustion and fuel evaporation.  Lube oil that gets past the piston rings and valve guides in the engine become hydrocarbon emissions.  These compounds are found in the gas and particulate phases, and are another key component in “smog.”  They can react with NOx in the presence of sunlight and form ozone.

Hydrocarbon emissions are potential issues for diesels at low loads, when in cold conditions and while motoring.


Carbon Monoxide (CO)

Carbon Monoxide is an invisible, odorless gas that forms when carbon fuel does not burn completely (incomplete combustion).  Incomplete combustion can be caused by low gas temperatures, short residence times or a high fuel/air ratio at the combustion site.

Vehicle emissions are the main source of CO in our air.  Diesels generally have low CO emissions due to their lean combustion process.

At a low level of exposure, CO is typically not a problem, except to people with cardiovascular disease.


Sulfur Oxides (SOx)

Sulfur oxides are gases formed when fuel that contains sulfur (mainly coal and oil) is burned.  In diesel emissions, SO2 originates from sulfur in fuel as well as lubricating oil.  The SO3 present in the system reacts with H2O (in either the dilution tunnel or in the atmosphere) to produce sulfuric acid.  To control these compounds, sulfur content in the fuel must be limited and controlled. 

SOx are comprised of sulfur dioxide (SO2) and sulfur trioxide (SO3), which is an oxidation product of SO2.  Coal-fired power plants account for the majority of SOx pollutants in the air.