Diesel engines move goods around the globe; UVA engineers are working on reducing emissions
As a technical remedy to climate change, it is easy to reduce CO2 emissions from passenger transport; We can switch from gasoline engines to electric motors that run on renewable energy sources.
In contrast, “decarbonising” heavy trucks is difficult. No technology today has the power and efficiency to replace diesel engines in moving goods over long distances by land and water, which is unfortunate as society’s increased shipping demands are driving diesel consumption soaring.
Researchers at the University of Virginia’s School of Engineering and Applied Science are taking on the challenge of finding which diesel fuels burn the cleanest, under which conditions and with the lowest compensation costs for the environment. Your particular focus is on minimizing the production of laughing gas or N2O.
William S. Epling, Professor and Chair of Chemical Engineering, is leading the effort along with Co-Principal Investigators Lisa Colosi-Peterson, an associate professor in the Department of Engineering Systems and Environment, and Chloe Dedic, an assistant professor in the Department of Mechanical and Aerospace and space technology.
Also contributing as lead researchers are Robert Davis, William Mynn Thornton Professor of Chemical Engineering, and Chris Paolucci, Assistant Professor of Chemical Engineering.
The project is supported by a $ 1.7 million grant through the Environmental Convergence Opportunities program of the Chemical, Bioengineering, Environmental, and Transportation Systems Division of the National Science Foundation. The program requires that members of the research team have different expertise and perspectives in order to find imaginative solutions to difficult and pressing societal challenges.
For this support, the team members must find a common basis in the research questions before they can deal with the solution to the technical problem – in a group of people whose expertise and perspectives differ from one another due to the design.
“It was harder than you could imagine, but it was intentionally tough,” said Colosi-Peterson, an environmental engineer who joined the team because she realized that diesel engines are not going to go away anytime soon. “I think NSF is very clever to have such a big incentive because it took a lot of thought to come up with something that really uses all of our individual skills and interests.”
Colosi-Peterson is an expert in life cycle assessments, a computer model-based field of study that examines the environmental price of products, processes, or services used by people. Dedic studies combustion and reaction flow systems and is a rising star in the development of new laser-based techniques for non-intrusive measurements of the complex chemical reactions and fluid dynamics that occur during combustion. Epling is known in his field of catalysis, particularly for his work on reducing pollutant emissions from diesel engine exhaust systems.
Explains the technical problem (s)
When fuel is burned – whether based on biomass or petroleum – not everything is burned. The exact chemical composition of the leftover hydrocarbons will depend on a number of factors, including the type of fuel used. To meet Environmental Protection Agency emissions regulations, the catalytic converter in your car and the aftertreatment systems in diesel tractor units purify these exhaust gases through chemical reactions after they leave the engine.
What comes out of the tailpipe at any given time also depends on variables such as whether the engine is hot or cold, how well the air and fuel have mixed in the engine, and the age of the catalyst, which becomes less effective over time. The team is studying these emissions by and large, including carbon dioxide, methane, and soot particles, but the main target is N2O.
Here’s the thing: N2O isn’t made in your engine.
“It’s made over your catalytic converter, which is supposed to clean the exhaust gases. We want to understand how a diesel catalyst produces N2O depending on the type of fuel, ”said Epling. “When we know which hydrocarbons cause the most or the least N2O to be produced, we can think about what type of fuel is the right one to minimize N2O production.”
Why the focus on N2O? The EPA, which has historically been more concerned with air pollution than climate change, didn’t start regulating N2O for diesel exhaust until 2011 after numerous studies showed that the global warming potential of the gas, which scientists call GWP, is 298 times greater than that of carbon dioxide. This is because N2O traps more heat than other greenhouse gases.
Carbon dioxide is still the largest greenhouse gas emitter from diesel because it emits so much, said Carlos Weiler, a Ph.D. Student in Eping’s laboratory who conducts the catalysis experiments for the project. The industry also has no good options for curbing N2O production.
“So we have to look at the other products that are being molded,” said Weiler. “They’re all harmful to the environment in some ways, but when it comes to the global warming potential of diesel exhaust, N2O is pretty strong.”
The team’s approach
Weiler will conduct experiments with various fuel inputs and catalyst combinations under the guidance of Epling and Davis, another widely recognized catalysis expert. Essentially, the unburned hydrocarbons from burned fuel are directed into a catalytic reactor that simulates a diesel aftertreatment system and Weiler measures which gases are leaking. Weiler will share his findings with Sugandha Verma, a PhD student in Paolucci’s computer catalysis group who will use computer simulations of catalytic reactions to predict outcomes and suggest further laboratory experiments.
But to understand how or how much N2O is produced by the catalytic converter, one needs to know which hydrocarbons are leaving the engine. This is where Dedic comes in. Measuring the remainder of a fuel after engine combustion is difficult because the compounds are structurally very similar. Traditional spectroscopy, which relies on how light interacts with different molecules, struggles to distinguish one hydrocarbon from the next. Techniques exist to count the number of carbon and hydrogen molecules, but this requires taking gas samples from the engine during combustion, which may change the chemistry.
“The best scenario is to take a measurement without interrupting the sample where the reaction is taking place,” said Dedic. “Then you get a real measurement of what is going on in your reactor.”
Dedic approaches these challenges from two angles. First, her team is developing a simplified reactor – a burner that can safely burn liquid diesel formulations in the laboratory – to isolate combustion chemistry from other engine effects such as fluid dynamics. The second uses ultra-fast laser sources to develop new measurement techniques for the project.
“We want to study these molecules on the same time scales in which they react and collide with one another. We can observe molecular vibrations and rotations in time to provide more information than you can get from conventional frequency-resolved spectroscopy, ”said Dedic.
The bigger picture
While Dedic’s lab identifies diesel products for Eping’s team to experiment with, Colosi-Peterson’s LCA group takes a systems approach and designs models that use the lab data to predict the cost of implementing the fuels in the real world. For example, if biomass-based diesel fuels produce less N2O, how much agricultural or carbon-sequestering forest area would then be lost for fuel production? How much energy will we use to grow biofuels and where will this energy come from? Another important question that Colosi-Peterson will investigate is whether the reduction of N2O from diesel emissions is significant or insignificant when compared to the effects of other pollutants.
The project creates a back and forth dynamic between the experimentalists and the modelers in order to better integrate the three research groups, which would normally work independently of one another. It’s a more proactive approach to improving the environmental performance of new technologies in development, Colosi-Peterson said, rather than waiting for the technology to roll out and then assessing its societal costs.
“Bill and Chloe might say, ‘Here are a few things we are considering,’ and I plug all of that data into my model and say, ‘Well, this is what would happen if we did this on a large scale. Is there any way to make this part of the process a little less emissive? ‘
“And so I think this desire to work more closely together on the laboratory scale is meant to avoid some of that trial and error and to be more aware of the kind of basic research that Bill and Chloe do.”
The team’s research will be sophisticated in its science but holistic in its approach, Epling said, referring to the project’s title, “A Holistic Effort to Decarbonise Diesel for Heavy Duty: Targeted Combustion and Emissions Catalysis Research to Improve Life Cycle Performance.” He hopes that the collaboration will lead to a more nuanced understanding at the political level of the extent to which proposed strategies – such as a regulation to reduce N2O emissions from diesel engines – can lead to meaningful improvements in global warming.
One way to ultimately improve the policy is to provide cross-training for the students working on the project who are just starting their PhD. Studies, said Colosi-Peterson.
“It’s really powerful when the student who makes the catalyst and reacts in the lab is faced with questions about the difference in their technology or its consequences,” she said.
“Conversely, Bill or Chloe may question some of the values my students use in their system-level analysis, basically the assumptions we make when we model policy making and legislation. I think it is very beneficial when the students have to think on a laboratory scale and on a system level. “