Diesel engines are the workhorses of American economy, powering everything from heavy-duty construction vehicles and buses to ships.
These engines are also one of the biggest producers of harmful pollutants, churning out more smog-causing Nitrogen Oxides (NOx) and near 100 times more sooty particles than their gasoline counterparts.
As the country’s fourth largest city, Houston experiences some of the worst of these effects. Based on data collected by the Texas Commission on Environmental Quality, about one-third of NOx emissions produced in a three county region encompassing Houston are attributed to diesel engines. So operators of diesel fleets are retrofitting these vehicles with devices—classified by the U.S. Environmental Protection Agency as emerging technologies—that attempt to diminish the negative impact diesel vehicles can have on the environment.
Researchers from the Texas Diesel Testing and Research Center at the University of Houston are at the forefront of analyzing these devices ability to reduce these harmful exhaust emissions.
“The U.S. EPA recently granted us funds for technologies that are on the emerging technologies list,” said Michael Harold, lead investigator on the two stimulus grants, which total more than $2.5 million. “These are technologies that are close to commercialization, but which need more data to assess their value and performance. Using our portable emissions measurement system, we will test these two technologies in the field while they are installed on the vehicles, evaluating each ones ability to reduce NOx and particulate soot.”
Testing the Air
Created by Midwest-based companies Truck Emission Control Technologies Inc. (TECT) and Tinnerman/Shadowood, diesel researchers will test these two retrofit systems’ abilities to reduce smog-causing NOx emissions on 35 vehicles. Each will be monitored throughout the 15-month study in order to provide real-world performance data to the EPA.
Following a series of baseline tests to determine the level of pollution created by these vehicles prior to retrofitting, these researchers will supervise installation of both systems—15 of Tinnerman/Shadowood’s total emission control 2010 exhaust reform system on Houston Independent School District buses and 20 of TECT’s PM/NOx 3126 retrofit system on a combination of Texas Department of Transportation utility vehicles and HISD buses.
Once equipped with the devices, diesel center researchers then will analyze these devices’ ability to reduce NOx and soot with their portable emission measurement system, which is connected to the vehicle during operation on the road. It’s all in an effort to determine just how close each system gets to meeting their reduction goals.
Predicted to reduce emissions by as much as 65 percent or about 1.4 tons a year, Tinnerman/Shadowood uses a filter to remove excess NOx diesel engines make in excess when oxygen and nitrogen in the air react together during combustion.
As NOx and soot travel through the exhaust system, TInnerman/Shadowood's filter captures the particulate soot while lean NOx trap (LNT) stores NOx. A reformer uses mixtures of diesel fuel and air to generate a hydrogen-rich gas mixture, which reacts with the NOx stored in the trap to form ammonia. The ammonia is then stored by the selective catalytic reduction system (SCR), and used to convert the remaining NOx that slips past the trap to nitrogen gas—a naturally occurring substance in the Earth’s atmosphere.
“The filter works similar to an oven,” describes Mark Hemingway, chief scientist with Tinnerman/Shadowood. “There are cycle cleaning ovens that are locked and carry out a very hot burn out while self cleaning ovens take on this cleaning gradually over time. We have this equivalent—a self-cleaning filter.”
Much like this method, TECT’s retrofit system also uses a filter to achieve an expected reduction of near 2.4 tons a year on these vehicles by putting a twist on a NOx reduction method that’s been around since the 1970s.
The technique, called exhaust gas recirculation (EGR), actually re-circulates exhaust into the air intake of the engine. NOx is usually formed when temperatures in the combustion chamber get too high. With the addition of exhaust gases, lower oxygen and temperatures are present in the combustion chamber—cooling the process and reducing NOx formation. The drawback to this method is it increases the amount of sooty particulate matter. So the company has designed a wire mesh filter with cloth netting that uses electric current to continuously incinerate soot build-up.
“TECT is capitalizing on its particulate converter capability to accommodate and reduce high particulate emissions resulting from the activation of the EGR,” said Ray Kammel, CEO of TECT. “The primary benefit of our retrofit technology is its high reliability and durability in operation in an urban environment. An early version of the product was installed on five buses in Traverse City, Mich. more than five years ago. The units are still operating very satisfactorily.”
Emission Cleaning Verification
Twice diesel center researchers will conduct on-road-tests during the study after the retrofits are installed. Beyond this, they expect to test at least one vehicle equipped with each one of the systems on their chassis dynamometer—a kind of treadmill for vehicles—that helps researchers to measure emissions in house at their 12,000 square foot facility.
They will use their baseline tests for comparison, offering real-world data on each systems NOx reduction ability as well as each units’ overall durability to the EPA at the end of the study. Companies such as TECT and Tinnerman/Shadowood are relying on diesel center researchers results to verify their retrofit’s ability and allow them to carry their technology to the Southwest Research Institute in San Antonio for verification that can lead to their systems commercialization.
The testing these researchers are providing is not only assisting retrofit companies test their emerging technologies, but also arming diesel fleet operators with a more cost effective method for meeting air quality requirements.
“If we were to just wait until all of these diesel trucks were off the road we would not be able to comply with the EPA’s Clean Air Act,” said Rachel Muncrief, research assistant professor with the diesel center, noting diesel vehicles have a much lower turnover rate than gasoline cars—lasting 20 years or more. “So it is important to clean them up, and right now the least expensive way of doing this is with retrofits.”