Technologies to Reduce Greenhouse Gas Emissions from Mobile Sources

Anthropogenic activities, particularly the burning of fossil fuels, have changed the composition of the atmosphere in ways that threaten dramatic changes to the global climate. Signs of climate change are evident worldwide and additional changes will have serious impacts on our nation's future. Some of the important greenhouse gas emissions from fossil fuel combustion include: carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), and black carbon. In addition climate change is also impacted negatively by higher ground level ozone emissions. Ozone levels are in turn linked to hydrocarbon and NOx emissions from mobile and stationary sources.

Emissions from the transportation sector are a large and important source of greenhouse gases (GHGs), contributing about 33% of the CO₂ emissions in the U.S. As such, controlling GHG emissions from the transportation sector is essential to the overall efforts to alleviate long-term impacts on the climate. There are a large set of technologies that can significantly reduce mobile source emissions of CO₂, N₂O (as well as other NOx emissions), CH₄, and black carbon. Detailed below are some of the impacts emission control technologies can have on reducing greenhouse gas emissions from mobile sources:

• CO₂: As the largest source of U.S. greenhouse gas emission, CO₂ from fossil fuel combustion has accounted for approximately 79% of global warming potential (GWP)-weighted emissions since 1990, growing slowly from 77% of total GWP-weighted emissions in 1990 to 80% in 2006. Of the total, transportation activities accounted for 33% of CO₂ emissions from fossil fuel combustion in 2006. Over 60% of the emissions resulted from gasoline consumption for personal vehicle use and the remaining emissions came from other transportation activities, including combustion of diesel fuel in heavy-duty vehicles. There are a large set of technology combinations that are available to reduce greenhouse gas emissions from passenger vehicles and light-duty trucks, including fuel efficient, state-of-the-art and future advanced gasoline and diesel powertrains.

  Implicit in federal and state greenhouse emission analyses is the ability of these advanced powertrain options to meet the applicable criteria pollutant emission standards, such as CO, NOx, and non-methane organic gases (NMOG). All of these advanced, light-duty powertrain options combined with the appropriately designed and optimized emission control technologies can meet all current and future federal and state criteria emission requirements. In this manner, advanced emission controls for criteria pollutants enable advanced powertrains to also be viable options for reducing greenhouse gas emissions. A range of powertrain technologies, including engine turbochargers, advanced fuel systems, variable valve actuation technology, advanced transmissions, hybrid powertrain components, and powertrain control modules that can be applied to both light-duty gasoline and diesel powertrains to help improve overall vehicle efficiencies that in turn can result in lower CO₂ emissions.

  Light-duty diesel powertrains will see increased interest in North America because of their high fuel efficiency and relatively lower greenhouse gas emissions compared to gasoline engines (on the order of 20-40% higher fuel efficiency and 10-20% lower CO₂ emissions for diesel engines compared to comparable gasoline engines). Advanced emission controls for controlling diesel particulate emissions and NOx emissions from diesel engines allow light-duty diesel engines to achieve comparable criteria pollutant emission levels to gasoline engines. Significant criteria emission reductions from diesel vehicles can be achieved through the use of several technologies, including:

  • Diesel Particulate Filters (DPF) can achieve up to, and in some cases, greater than, 90% reduction in diesel particulate matter (PM). To date, more than five million DPFs have been installed on light-duty diesel vehicles operating in Europe. New "clean diesel" models available in the U.S. will be equipped with DPFs to meet EPA or ARB emission standards for diesel PM.
  • Selective catalytic reduction (SCR), lean NOx trap catalysts and combinations of these two technologies can be used to significantly reduce NOx emissions from diesel vehicles. These NOx-based catalysts for diesel combustion strategies are capable of reducing NOx emissions from 70 to 90%.

  For gasoline vehicles, direct injection technology enables gasoline engines to achieve greater fuel efficiency. Gasoline direct injection offers CO₂ emissions reductions ranging from 5% to 20% depending on how it is implemented and the base engine to which it is compared. Again emissions controls ensure that these more fuel efficient gasoline engines meet tough EPA or ARB criteria emission regulations:

  • Under stoichiometric conditions, a three-way catalyst can significantly reduce emissions of NOx, HC and CO.
  • Under lean combustion conditions, similar emission control technologies used on diesel vehicles can be used to reduce emissions from lean, gasoline direct injection powertrains. These include particulate filters to reduce PM emissions, and SCR and lean NOx trap catalysts to reduce NOx emissions.

  Diesel-electric and gasoline-electric hybrid vehicles, that combine either a diesel or gasoline engine with elements of an electric-drive powertrain, offer a range of CO₂ emission reduction possibilities, and again advanced emission controls allow these powertrains to meet even the toughest criteria emission regulations. Emission controls for gasoline and diesel engines are also generally compatible with low carbon, alternative fuels (e.g., gasoline blends with renewable ethanol or biodiesel blends) that can provide additional reductions in mobile source greenhouse gas emissions.

• Black Carbon: Black carbon is a major component of particulate matter emissions from mobile sources and is believed to have a significant net atmospheric warming effect by enhancing the adsorption of sunlight. Black carbon is a dominant absorber of visible solar radiation in the atmosphere. Anthropogenic sources of black carbon are transported over long distances and are most concentrated in the tropics where solar irradiance is highest. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are thought to be the second strongest contribution to current climate change, after CO₂ emissions. For example, in the Himalayan region, solar heating from black carbon may be just as important as CO₂ in the melting of snowpacks and glaciers. Estimates of the global warming potential of black carbon are in the range of 60% of the warming potential of CO₂. It is estimated that 70% of the black carbon emissions from mobile sources are from diesel-fueled vehicles, with the assumption that 40% of gasoline PM is black carbon and 60% of diesel PM is black carbon.

  Black carbon from diesel vehicles can be significantly reduced through emission control technology. High efficiency diesel particulate filters (DPFs) on new and existing diesel engines provide nearly 99.9% reductions of carbon emissions. To meet EPA's 2007 heavy-duty engine PM standards, all on-road heavy-duty diesel engines are now equipped with high efficiency DPFs. It is estimated that the installation of DPFs will reduce PM emissions from U.S. heavy-duty diesel vehicles by 110,000 tons per year. Because older diesel engines emit significant amounts of PM, there are also significant opportunities to reduce black carbon emissions through diesel retrofit programs that make use of retrofit DPF technology. The number of vehicles retrofitted, the number of programs, and the interest in new program for DPFs has grown significantly over the past few years with more than 250,000 DPFs installed as retrofits to date in a variety of world markets. Retrofit filters can provide large benefits in human health through reductions in diesel PM and climate change benefits black through reductions in black carbon emissions on both existing, on-road and off-road diesel engines.

• N₂O: While total N₂O emissions are much lower than CO₂ emissions, N₂O is approximately 310 times more powerful than CO₂ at trapping heat in the atmosphere. One of the main anthropogenic activities producing N₂O in the U.S. is fuel combustion in motor vehicles. In 2006, N₂O emissions from mobile combustion were approximately 9% of total U.S. N₂O emissions. It is estimated that the N₂O emissions account for about 2% of the total GHG emissions from a typical light-duty vehicle. On late model light-duty gasoline vehicles, modern three-way catalyst-based emission control technology is effective at controlling nitrous oxide emissions. Tightening of NOx emission standards over time with the parallel introduction of more effective emission control systems have resulted in lower emissions of N₂O from today's vehicles compared to older vehicles certified to less stringent NOx standards. The performance of NOx emission control technologies for diesel vehicles such as SCR catalysts and lean NOx trap catalysts can also be optimized to minimize N₂O emissions from diesel engines.
• CH₄: According to the United Nation's International Panel on Climate Change (IPCC), methane is more than 20 times as effective as CO₂ at trapping heat in the atmosphere. Over the last 250 years, the concentration of CH₄ in the atmosphere has increased by 148%. Methane emissions from mobile sources are emitted from exhaust from vehicles using hydrocarbon fuels but the anthropogenic contribution of road transport to the global methane inventory is less than 0.5%. Emissions of CH₄ are a function of the type of fuel used, the design and tuning of the engine, the type of emission control system, the age of the vehicle, as well as other factors. Although CH₄ emissions from gasoline vehicles are small in terms of global warming potential when compared to N₂O emissions, they can be high in natural gas-fueled vehicles, as methane is the primary component of natural gas. On light-duty gasoline vehicles, modern three-way catalyst-based emission control technology is effective at reducing all hydrocarbon exhaust emissions including methane. Tightening of hydrocarbon emission standards over time with the parallel introduction of more effective emission control systems have resulted in lower emissions of methane from today's vehicles compared to older vehicles certified to less stringent standards. Catalyst designs can also be optimized in concert with engine control strategies to oxidize methane exhaust emissions from motor vehicles including vehicles that operate exclusively on natural gas or bi-fuel vehicles that can operate on either natural gas or gasoline.