Reducing emissions from diesel engines is one of the most important air quality challenges facing the country. Even with more stringent federal heavy-duty highway and nonroad engine standards set to take effect over the next decade, millions of diesel engines already in use will continue to emit large amounts of nitrogen oxides, particulate matter, and air toxics, which contribute to serious public health problems. Diesel retrofits can significantly reduce harmful air pollutants, improve the working environment of those operating diesel-powered equipment, and reduce smoke and odor from diesel engines. Diesel retrofit devices usually are not original equipment but are installed on a vehicle just prior to or after it has been serviced.
Diesel Oxidation Catalyst (DOC)
Diesel oxidation catalysts are devices that use a chemical process to breakdown pollutants in vehicle exhaust into less harmful components. More specifically, a DOC has a porous ceramic honeycomb-like structure that is coated with a material that catalyzes (causes) a chemical reaction to reduce harmful air pollutants. DOCs rarely require maintenance, come with a 100,000- to 150,000-mile warranty, can last for 7 to 15 years, and can be installed on a variety of diesel vehicles. Typically, a DOC costs between $1,000 and $1,500 and takes about 1 to 2 hours to install.
Diesel Particulate Filter (DPF)
Diesel particulate filters are ceramic devices that collect particulate matter in the vehicle exhaust stream. The heat produced by the vehicle exhaust heats the ceramic filter and allows the particles to break down (or oxidize) into less harmful components. Unlike diesel oxidation catalysts (DOCs ), DPFs must be used in conjunction with ultra-low sulfur diesel (ULSD) fuel and work best on diesel engines built after 1995. Therefore, knowing the age and type of each engine in the fleet as well as the exhaust temperature is an important part of the retrofit project. Manufacturers recommend that DPFs be cleaned about every 100,000 miles. Most DPFs come with a 100,000- to 150,000-mile warranty. Typically, a DPF costs between $5,000 and $7,000 and takes about 6-8 hours to install.
| Technology | CO | HC | PM | Per Unit Cost |
|---|---|---|---|---|
| Diesel Oxidation Catalyst (DOC) | 40% | 70% | 40% | $1,000 - $1,500 |
| Diesel Particulate Filter (DPF) 1,2 | 75% | 85% | 90% | $5,000 - $7,000 |
1) Reductions are calculated based on the use of ULSD
2) Actual emission values vary with engine design
Aftermarket Auxiliary Heaters
Aftermarket auxiliary heaters can be used to warm up engines and passenger compartments in colder climates. This equipment runs off the diesel fuel tank or the vehicle's electrical outlets and includes a timer that can be programmed to automatically start the heating function. There are three types of heaters:
- Engine Block Pre-Heaters are used to heat the engine block for a warm start. Block engine pre-heaters cost approximately $1,200 - $1,500 installed. Some of the reported benefits of these heaters are lower emissions and fuel savings, longer engine oil life, less wear-and-tear on the engine, and relatively easy installation and maintenance.
- Compartment/Engine Block Heaters warm up the engine block and passenger compartments simultaneously while using an auxiliary heater. These heaters cost approximately $2,300 - $2,500 installed. These heaters are especially useful for nighttime-activity buses and buses that transport very young and/or special-needs children. In addition, the radiant heat keeps the windows from frosting or fogging .
- Electric Plug-in Block Heaters warm up the engine block by heating the engine coolant or oil. They are powered by electricity and are available in a range of voltages and watts, drawing between 1,000 - 1,500 watts per bus per hour. They include a timer that can automatically start or turn off the heater. A heater is mounted on the engine block of each bus and is plugged in when the bus is parked at the depot. Even in the coldest climates, engines will have a "warm start." Such heaters usually cost less than $100.
Diesel Engine Repowers
Engines are replaced with a newer/cleaner engine that meets a more stringent set of engine emission standards. The older engine is then scrapped and rendered inoperable.
Diesel Engine Upgrades
Engines are upgraded to reduce their emissions by applying manufacturer recommended upgrades or kits to certified or verified configurations.
Diesel Vehicle and Equipment Replacements
Onroad and nonroad diesel vehicles and equipment are replaced with newer, cleaner vehicles and equipment that operate on diesel or alternative fuels and are certified to meet a more stringent set of engine emissions standards. The older vehicle/equipment is then scrapped and rendered inoperable.
Verified Aerodynamic Technologies
Trailer aerodynamic technologies improve fuel efficiency by minimizing aerodynamic drag and maintaining smoother air flow over the entire tractor-trailer vehicle. Trailer aerodynamic devices include gap fairings that reduce the gap between the tractor and the trailer to reduce turbulence, trailer side skirts that minimize wind under the trailer, and trailer rear fairings that reduce turbulence and pressure drop at the rear of the trailer.
Verified Low Rolling Resistance Tires
Certain tire models can provide a reduction in NOx emissions and fuel savings by reducing energy that is wasted as heat, relative to the "best-selling" new tires for line haul trucks, when used on all three axles. The options offered include both dual tires and single wide tires (single wide tires replace the double tire on each end of a drive or trailer axle, in effect turning an "18" wheeler into a "10" wheeler). Low rolling resistance tires can be used with lower-weight aluminum wheels to further improve fuel savings.
U.S. EPA-Verified Technology
The United States Environmental Protection Agency (U.S. EPA) evaluates the emission reduction performance of retrofit technologies such as DOCs and DPFs, including their durability, and identifies engine operating criteria and conditions that must exist for these technologies to achieve those reductions. Since the U.S. EPA generally receives such results of testing conducted on a limited number of diesel engine models, a conservative approach is taken to establish the verified levels of reduction. In most cases the actual reductions are higher than verified values.