DETROIT – Bone-chilling weather gripping the upper Midwest creates the ideal environment for Faurecia to market its latest technology intended to limit oxides of nitrogen emissions from new diesel engines.
In extreme cold, it may take up to 30 minutes before operating temperatures allow a selective catalytic reduction (SCR) system to effectively scrub NOx emissions through the dosing of urea into the exhaust stream.
A chemical reaction within the catalyst chamber neutralizes the NOx. Conventional systems in passenger cars use liquid urea as the reductant because it is abundant and safe to store, but it requires conversion to ammonia through thermal decomposition in order to be effective.
Urea will freeze in extreme temperatures, and it can take about a half-hour before it thaws to the point it is adequately filtering out NOx, says Robin Willats, chief engineer-innovation for Faurecia Emissions Control Technologies.
Faurecia has developed its Ammonia Storage and Delivery System (ASDS) to replace urea with pure gaseous ammonia, which can begin doing its job within two minutes on even the coldest days, Willats says.
Another way to treat NOx in extreme cold is to pair SCR with a NOx trap, but he says that solution is expensive, requiring two different systems.
“The main issue limiting NOx performance is operating temperatures, when you’re getting out in really cold weather,” he says during a demonstration of the technology at the North American International Auto Show, which ends Sunday.
In stop-and-go urban driving cycles, diesel engines need more time to heat up. Willats says a urea-based system will not begin scrubbing NOx until the exhaust gas reaches at least 180º C (356º F). However, gaseous ammonia will get to work earlier, by 150º C (302º F).
Even below 150º C, Willats says the ammonia-based SCR system has been proven to convert more than 50% of NOx, compared with 28% for a conventional urea system.
“In that case, we’re emitting 35% to 40% less NOx,” he says. “What that means is more NOx that’s in the environment in heavily populated areas” with a urea-based system.
Faurecia expects ASDS to become the new world standard for NOx reduction in diesel engines. NOx is considered a greenhouse gas that is a key component of smog.
In Europe, the next round of Euro 6 emissions legislation will focus more heavily on real-world conditions, which will require more effective NOx treatment at low temperatures.
In the U.S., Willats says NOx regulations will become more stringent beginning in 2017, and a key focus for many automakers will be in the area of cold-start.
“We think this is one technology to address, how do you get faster startup on the FTP (Federal Test Procedure) cycle,” he says.
But first there are several hurdles for Faurecia’s technology to overcome, such as packaging.
Most diesels today store urea in a tank, and the fluid must be refilled, about every 10,000 miles (16,093 km). In addition to the tank, conventional systems use a pump, urea lines and a doser or injector to spray the fluid into the exhaust stream.
Faurecia’s ASDS module replaces the tank with a self-contained ammonia module under the floorpan. An electronic control unit governs its operation, and no doser or injector is required. Instead, a simple pipe connects the ammonia cartridge to the exhaust stream ahead of the SCR catalyst.
The unit actually contains two ammonia cartridges – the large main one and a smaller “starter” cartridge that heats up more quickly so it can treat NOx more rapidly. The cartridges contain strontium chloride salt, which forms a crystal lattice to store the ammonia.
When the cartridge is depleted, it must be replaced, which means the vehicle owner or a technician needs access to it. The supplier suggests a removable cap in the floor of the vehicle, perhaps behind the front seats, so the cartridge can be pulled out and replaced.
Overall, Willats says the module would be about half the size of a urea tank, resulting in a weight savings of up to 22 lbs. (10 kg). Another option is to use larger ammonia cartridges so they would not have to be replaced as often, but they would be heavier.
Faurecia says several automakers and commercial-truck manufacturers are studying the ASDS technology, and that the goal is to price it competitively with urea-based SCR. No contracts have been signed yet, Willats says.
“With a urea system, you have the complexity of needing heated lines to avoid freezing in the urea lines,” he says, adding that ammonia does not require additional heating once it leaves the cartridge. “And you have a more complicated injector with a urea system.”
Even on a scorching summer day, “high temperatures are not an issue for the gaseous ammonia” system, Faurecia says, noting the ability to place the module close to the engine.