Ammonia Sensor Improves Diesel Emissions Control

TECHNOLOGY FOCUSAmmonia Sensor Improves
Diesel Emissions Control TROY, MI (March 1, 2007) - Delphi Corp. has developed the first automotive ammonia sensor designed to directly measure ammonia levels in the exhaust of diesel vehicles equipped with selective catalytic reduction (SCR) aftertreatment systems. With the increasing proliferation of diesels entering the market, the company expects an increase in the use of both SCR systems and urea, the ammonia-rich compound the systems require, in diesel-powered vehicles. The increase would be in response to new emissions regulations in both the light- and heavy-duty diesel markets. Atmospheric ammonia reacts with airborne compounds such as nitric acid to create dust-sized airborne particles, which can create a smog-like haze. Ammonia emissions from vehicles are currently only a very small proportion of total ammonia emissions, which mainly originate from livestock and factories. "Vehicle exhausts are now many times cleaner than even just a handful of years ago, but that is no reason for not striving for further gains," says Guy Hachey, president of Delphi Powertrain. "With the growth of diesel vehicles using intensive SCR, ammonia emissions are a potential problem that we want to help our customers address before it becomes an environmental issue."

Delphi's new sensor uses a new ammonia-sensitive material, developed at the company's research laboratories in Troy, MI, that is deposited onto a thick-film ceramic substrate similar to the one employed in the company's oxygen sensors. The sensor is then mounted in a compact stainless steel package.
(Photo: Delphi Corp.)

Today's conventional SCR systems use an open-loop design, and using predictive algorithms, the engine's control unit estimates the urea dose. To accurately control this dose, systems will need to become closed-loop, says Hachey, which will require a post-catalyst sensor.

"This can be either a [nitrous oxide (NOx)] sensor or an ammonia sensor," says Ivan Samalot, chief engineer for exhaust sensors at Delphi's technical center in Brighton, MI. "Several vehicle manufacturers have chosen the NOx option, but the sensor technology is cross-sensitive to NOx and ammonia, so [it] can confuse one with the other. The result can be inappropriate dosing decisions that, while providing a dramatic improvement on open-loop systems, do not deliver the benefit achievable by measuring the ammonia slip directly."

The ammonia sensor is situated behind the SCR catalyst within the aftertreatment system. The SCR system helps reduce NOx emissions in diesel powertrain systems through a chemical reaction in which ammonia reacts with NOx to produce nitrogen and water. The new sensor is designed to allow direct closed-loop control of the SCR system by controlling the quantity of urea injected into the system. By directly measuring tailpipe ammonia, the sensor allows the injection of urea to be optimized to reduce not only NOx, but excess ammonia emissions as well.

The new sensor detects excess ammonia - without cross-interference from carbon monoxide and NOx - in the exhaust gas within a range of zero to 100 parts per million, allowing the urea dose to be continuously optimized. In addition to significantly reducing ammonia slip, this can create financial savings for high-mileage operators who will use significantly less urea.

Delphi officials say it also allows vehicle manufacturers to eliminate an expensive post-oxidation catalyst that would otherwise be needed to remove excess ammonia from the exhaust, and allows the size of the SCR converter to be optimized for the application. In addition to reducing the cost of the aftertreatment system, this feature can provide improved packaging. This prevents an increase in backpressure, which could harm fuel consumption if further aftertreatment systems are added.

Vehicle manufacturers in Asia, North America and Europe are working with Delphi on development programs incorporating the new ammonia sensor, which is expected to reach production during 2010.

(Source: Delphi Corp.)