What Emissions are Measured in Diesel Exhaust?

The development of today’s clean Ultra Low Sulfur Diesel (ULSD) fuel notwithstanding, the way diesel fuel burns makes it one of the dirtiest of all motor transportation fuels. The numbers and characteristics of diesel exhaust pollutants require more sophisticated engine management and exhaust scrubbing technologies than any other fuel. For these reasons, diesel engines require more involved and complicated engineering and manufacturing standards.

There are several types of emissions that are regulated in the United States. Tailpipe emissions standards classify and regulate these recognized pollutants:

• Diesel Particulate Matter (PM) – These are the ultra-fine particles that are suspended in the diesel exhaust. PM causes the black, sooty smoke that’s given diesels a bad name.
  
  
• Nitrogen Oxides (NO_x) – This is the broad family of oxides of nitrogen. Though there are several components, the regulated portions of NO_x are nitric oxide (NO) and nitrogen dioxide (NO₂). These are the acidic gases that contribute to smog formation and acid rain.
  
  
• Hydrocarbons (HC) – These are generally referred to as unburned hydrocarbons—it’s basically raw, unburned fuel that escapes through the exhaust.
  
  
• Carbon Monoxide (CO) – Carbon monoxide is a lethal, odorless gas that forms in the exhaust stream from the incomplete combustion of fuel.