THE COMPLIANCE QUESTION
It is the “have to use it” issue that is the EPA's chief concern when it comes to SCR as a 2010 compliance solution (www.epa.gov
). “Engine makers and OEMs using EGR and SCR will both demonstrate compliance with the 2010 standard in the same way — they must both run certain required emissions tests and submit the data gathered for certification,” says Byron Bunker, center director for the EPA Heavy-Duty On-Highway Center. “What is special about SCR is the required urea. We want to make sure that the certifiers are doing all they can to assure that end users of the SCR system will do what they should do to keep it operating properly.
“There are lots of things manufacturers can do to make a big difference,” he continues. “They can do more than anyone else to make sure that urea is readily available, for instance, just like any other replacement fluids required for a vehicle, such as oil or coolant. They can also use sensors and other onboard systems to make sure the driver knows when the urea tank needs to be refilled and that the system is durable and not easy to disable.”
In March 2007, the EPA issued guidance on the emissions certification procedures the agency wants to see for on-road diesel vehicles. Specifically, the agency outlined five elements it expects manufacturers to provide in order to have an SCR system certified as meeting 2010 requirements: an escalating driver warning system to alert the driver to the fact that the urea tank is approaching empty; a driver “inducement” (some means to ensure that the driver will not operate the vehicle without the reducing agent); a NOx or urea sensor or some other mechanism to prevent the use of an incorrect or diluted reducing agent; plus a durable and tamper-resistant design.
Since EGR systems require no action on the part of the vehicle operator and are much less prone to tampering or misuse, EPA concerns about compliance are correspondingly smaller, too. “Engine makers relying on EGR to meet 2010 NOx levels may be able to calculate NOx output rather than directly measuring NOx in the exhaust stream,” says Gary Parsons, global OEM and industry liaison manager for Chevron Oronite Co. LLC (www.chevron.com
). “For instance, based on engine mapping, they may be able to use the angle of the EGR flow valve and known EGR flow rates as an indirect indicator of the NOx levels being produced and simply map the NOx levels created in the engine under various conditions.”
In other words, SCR systems will need some way of directly measuring NOx levels at the tailpipe, while EGR can assume tailpipe NOx emissions based on engine operating parameters.
When it comes to SCR and EGR, it is not strictly accurate to talk in terms of either/or, adds Parsons. “Practically speaking, SCR is not going to entirely replace EGR come 2010,” he says. “Even engine makers using SCR will probably have to use some level of EGR to reach the extremely low NOx levels required.”
Neither EGR nor SCR may be the end of the emissions reduction road. Although the EPA says it has no plans for regulations past 2010, there are other emissions reduction technologies and other approaches being explored today.
“Right now we have no plans for regulations past 2010,” says Bunker. “After all, we don't have far to go past 2010 to get to zero NOx emissions. That last little bit tends to be the most expensive to get so we have to weigh the cost to the benefit. Technologies will also continue to get better. We spend a lot of time ourselves investigating new technologies, especially combustion technologies.”
“Everybody is working on cleaner diesel combustion solutions,” agrees Parsons, “especially on what is called Homogeneous Charge Compression Ignition [HCCI] where air and fuel are premixed in the cylinder, more like a gasoline engine, but still ignited by compression as in a diesel engine.
“As a fuel, gasoline is very difficult to spontaneously ignite, which is why gasoline engines require a spark plug for ignition,” Parsons explains. “Octane is the term that is used to describe gasoline's ability to resist ignition. Diesel fuel, on the other hand, is less flammable than gasoline but much easier to ignite under the proper conditions, which is why diesel engines use compression rather than a spark plug for igniting the fuel.
“Cetane is the term used to describe diesel's ability to spontaneously ignite under pressure,” Parsons continues. “The goal with HCCI and partial HCCI is to create a better combustion technology plus a fuel with the right balance of octane and cetane, something that will ignite uniformly and at exactly the right moment for optimum, lower-temperature combustion, with enough heat to burn cleanly but at a low enough temperature to avoid producing NOx.”
HCCI is not the only alternative approach to emissions reduction that is being explored. So-called “NOx Absorbers” (or “Lean NOx Traps” or “Hydrocarbon-SCR Systems”) use hydrocarbons rather than ammonia to reduce NOx. They are already being used in some light-duty engines and have the advantage of not requiring an additional reductant like urea.
Today's Dodge Ram pick-up trucks equipped with Cummins engines, for instance, use Lean NOx Traps and are already able to meet 2010 emissions standards. However, NOx Traps currently rely on catalysts made with expensive precious metals to get the job done and have not yet been proven effective in heavy-duty applications.
Honda Motor Co. Ltd. has announced that it will introduce a diesel that meets 2010 standards via aftertreatment — and without the need for urea. Instead, Honda's system creates its own ammonia from diesel fuel and uses it to convert NOx into nitrogen and water during normal engine operation. Since diesel fuel is used in the NOx after-treatment system, however, there is a slight fuel economy penalty in these systems.
Canadian company NxtGen (www.nxtgen.com
) says it is commercializing a system that can omit the need for SCR technology and also clean the diesel particulate filters introduced in 2007. The system makes use of a non-catalytic “synthesis gas” generator that converts diesel fuel, mixed with about 2% of the airflow from the exhaust manifold, into hydrogen and carbon monoxide. The resulting gaseous mixture can be pumped into the particulate filter and NOx absorber to regenerate them.