|Delphi’s ammonia sensor.|
Delphi has developed the world’s first automotive ammonia sensor. The new technology will allow direct closed-loop control of the SCR (Selective Catalytic Reduction) systems used by an increasing number of diesel vehicles to reduce NOx.
By directly measuring tailpipe ammonia, the sensor allows the injection of urea (an ammonia-rich compound required by the SCR system) to be optimized and ammonia emissions reduced.
Control of urea injection is expected to become a rapidly increasing priority as SCR levels increase to meet new emissions regulations in both light and heavy duty diesel markets.
A vehicle’s SCR system injects urea into the exhaust stream ahead of the NOx reduction catalyst. The ammonia in the solution reacts with the exhaust gas, with conversion into nitrogen and water. Unreacted ammonia is expelled with the exhaust gasses—“ammonia slip”.
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.
In addition to its use in heavy-duty applications, a number of automakers have indicated they will use urea SCR to meet current Tier 2 Bin 5 requirements in the US and expected Euro 6 requirements from 2014. (Others, such as Honda, are endeavoring to not have to use a urea-based SCR system. Earlier post.)
Today’s SCR systems are open loop, so the urea dose is estimated by the engine control unit using predictive algorithms. To accurately control the dose, systems will need to become closed loop, which will require a post-catalyst sensor.
This can be either a NOx sensor or an ammonia sensor. Several vehicle manufacturers have chosen the NOx option, but the sensor technology is cross-sensitive to NOx and ammonia, so 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.—Ivan Samalot, chief engineer for exhaust sensors, Delphi Technical Center
Delphi combined expertise in oxygen sensors with its materials expertise to develop a new type of ammonia sensor for automotive applications. A new ammonia sensitive material, developed at the Delphi Research Laboratories in Troy, Mich., is deposited onto a thick film ceramic substrate similar to the one proven in Delphi’s oxygen sensors. The sensor is then mounted in a compact, highly durable stainless steel package also based on Delphi’s proven oxygen sensor technology.
The new sensor detects excess ammonia in the exhaust gas within a range of zero to 100ppm, allowing the urea dose to be continuously optimized. 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.
As well as taking cost out of the aftertreatment system, this feature allows substantially improved packaging. This will be important for the anticipated growth in light-duty vehicle applications and also prevents an increase in back-pressure, 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.