Controlling Bus Emissions
Both, Transport Canada and the U.S. Environmental Protection Agency (EPA) have regulations that require diesel engines to reduce Oxides of Nitrogen (NOx) and Particulate Matter (PM) emissions from on-road trucks and buses. Through a combination of engine redesign, ultra-low sulfur diesel (ULSD) fuel and engine oils, new vehicle technologies will significantly cut emissions.

What are Oxides of Nitrogen (NOx) and Particulate Matter (PM)?
Oxides of Nitrogen are a regulated gaseous emission which is a collective term for emissions composed of nitrogen and oxygen. Too much NOx in the atmosphere contributes to the production of smog. Particulate matter is a regulated diesel emission composed primarily of carbon soot and other combustion by-products.

To reduce NOx emissions, newer Calgary Transit buses use a combination of Exhaust Gas Recirculation (EGR) and Variable Geometry Turbocharger (VGT) in the engine system. The EGR works by re-circulation a varying proportion of the exhaust gas back to the cylinder. This reduces the oxygen content to a lower combustion temperature resulting in a reduction of NOx formation. The Variable Geometry Turbocharger (VGT) is equipped with moveable vanes that allow the engines electronic controls to adjust the exhaust pressure in the system, ensuring there is always enough pressure for sufficient EGR flow. The diagram below illustrates the EGR valve, EGR cooler and VGT which are key components to the diesel engine designed to reduce NOx emissions.

Selective Catalytic Reduction (SCR) is one of the latest technologies used by diesel engine manufacturers capable of almost eliminating NOx emissions. SCR systems can reduce NOx emission up to 90 percent, hydrocarbons and CO emissions by 50 to 90 percent, and particulate matter (PM) emissions by 30 to 50 percent. SCR systems can also be combined with a diesel particulate filter to achieve even greater emission reduction for PM.

The SCR system injects diesel exhaust fluid (a urea solution of 32.5% high purity urea and 67.5% deionised water) through the exhaust stream of an engine. Diesel Exhaust Fluid works with the catalyst and high temperature exhaust gas to break oxides of nitrogen (NOx) to two components found in our normal atmosphere – Nitrogen (N2) and Water (H2O). The nitrogen and water exit harmlessly through the engine’s exhaust system as illustrated in the diagram below.

To reduce particulate matter, all newer Calgary Transit buses have Diesel Particulate Filters (DPFs) installed that capture soot and larger sulfate particles. These filters work by channeling engine exhaust through a porous material on the filter, trapping engine exhaust particulate matter and preventing its release into the environment. Soot accumulates in the filter and when sufficient heat is present, active regeneration occurs, burning off the soot and cleaning the filter.