A diesel oxidation catalyst (DOC) is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. This layer contains a small, well dispersed amount of precious metals such as platinum or palladium.  As the exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles (unburned fuel and oil) are oxidized, thereby reducing harmful emissions.  

Diesel Oxidation Catalysts enable a chemical reaction that removes approximately 20% of PM, and reduces HC by nearly 50%3.  These catalysts range in price from $1,000-$2,000, depending upon catalyst composition and installation charges.  DOCs can last upwards of 10 years, and require little to no maintenance once installed.  Installation of a DOC can take between 1-3 hours.

Diesel Particulate Filters (DPF)

Capable of removing 60-90% of PM, HC, and CO when used correctly4.  DPFs are best suited for buses newer than 1995 in order to ensure that ideal temperatures are reached in the exhaust stream to decompose particles that accumulate in the filter.  A DPF has a working life span between 7 and 15 years, and costs between $8,000-$10,0005. Applicants for 20 or more buses may apply for funding for particulate filter regenerators, which must be used for all DPFs, including the ones that will be used in new 2007 diesels.

Diesel Multi-Stage Filter (DMF)

This muffler removes 71-75% of PM, HC, and CO.  The DMF is priced between $6,000- $8,000, and requires minimal maintenance.  This unit is considered to be an effective compromise between the less expensive DOC and the more efficient DPF5.

Crankcase Ventilation System (CCV)

The crankcase is the source of the vast majority of pollution that impacts students on the bus.  Emissions from the engine compartment seep through cracks and openings in windows and doors to create conditions in which the particulate matter concentrations inside the bus may be many times that of ambient conditions.

Crankcase filters ultimately reduce emissions of particulate matter to the cabin by nearly 100%.  Priced near $400-$700 each, CCVSs achieve the highest emission reductions to the cabin per dollar.  The CCVS must be maintained on a regular basis by replacing the internal filter at each oil change, at a cost of approximately $50.

The CCVS can be used in conjunction with DMFs, DOCs, or DPFs.  Both tailpipe and crankcase retrofits are beneficial to students and the public, reducing pollutants that reach the ambient air, as well as reducing what seeps into the cabin of buses.