Among the many discussions of the proposed California LEV III and U.S. Tier 3 emissions standards at the SAE 2012 World Congress, no topic was hotter than the challenge of particulate matter (PM, or soot) emissions regulations in the 2017-25 time frame.
The super-stringent LEV III standards, which progressively reduce the mass limit for PM emissions from the current level of 10 mg/mi to 6 mg/mi in 2017, then to 3 mg/mi in 2022—with a potential drop to an almost imperceptible 1 mg/mi by 2025—loom like dark clouds over powertrain engineers working on gasoline direct injection (GDI) and diluted/lean-burn combustion technologies, as well as advanced diesels.
“I believe we can do 3 mg without a PM filter [on gasoline engines], but I don’t know about 1 mg,” Chris Cowland, Chrysler’s Director of Advanced and SRT Powertrain, told the audience during the “2018 and Beyond Powertrains” technical session April 24.
“We’re doing a lot of work on PM filters, but there are things we don’t yet know such as, what are the OBD [onboard diagnostics] implications of particulate filters on gasoline engines? Because unlike diesel particulate filters with their active regeneration, those for gas engines are passive. It’s very different technology,” Cowland said.
Another panelist, General Motors’ Vice President of Powertrain Engine Engineering, Sam Winegarden, agreed. “It looks like 3 mg is possible, but 1 mg will be a challenge,” he said. Added a powertrain engineer in the audience who asked not to be named, “The 1-mg proposal is draconian, and so very typical of the California regulatory mind-set, because equipment to measure particulate down to that size doesn't yet exist.”
Compared with multi-port injection, DI provides greater injection accuracy, leading to improved thermal efficiency and fuel economy, and to lower greenhouse-gas emissions. But GDI also results in higher engine-out PM, due mainly to limited air/fuel mixing within the combustion chamber. This situation can lead to fuel enrichment around the spark plug, and chamber wetting through reduced droplet evaporation.
The U.S. EPA’s proposed Tier 3 regs also will include more stringent PM-mass emissions levels, said to be similar to those of LEV III. And in Europe, the Euro6 standards include a particle-mass-emission limit of 4.5 mg/km (equivalent to about 7 mg/mi). A tighter solid-particle-number emission limit is being considered, too.
Advanced exhaust gas recirculation (EGR) techniques currently in development offer a significant reduction in PM-mass emissions, as well as in solid-particle-number emissions when the engine is operating in stoichiometric mode. Under enrichment EGR also is proven to reduce soot-mass emissions; however, there may not be a similar reduction in solid-particle numbers.
Other technologies that are being investigated for LEV III and Tier 3 include secondary-air injection during rich start-up; ignition retard, cylinder head preheating; and lower-thermal-mass components aimed at fast warm-up.
The proposed LEV III standards would phase-in a new 150,000-mi (241,400-km) durability requirement.