LUXEMBOURG — The world’s first automotive ammonia sensor has been developed by Delphi Corp. 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.
SCR is the emissions-control technology used in Europe to meet environmental regulations. Several heavy truck OEMs in North America have declared that SCR is the preferred system that will be explored for meeting upcoming 2010 emission rules mandated by the EPA. Car and small truck manufacturers also are committed to SCR solutions in diesel applications to comply with Tier II Bin 5 regulations for light-duty vehicles.
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.
“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,” said Guy Hachey, president of Delphi Powertrain.
Atmospheric ammonia reacts with airborne compounds such as nitric acid to create dust-sized airborne particles, which can create a smog-like haze.
A vehicle’s SCR system injects ammonia, in the form of liquid urea, into the exhaust stream ahead of the NOx reduction catalyst. The ammonia reacts with the gas, converting it into nitrogen and water. Unreacted ammonia, known as ‘slip,’ is expelled with the exhaust gasses, the company explains.
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,” said Ivan Samalot, chief engineer for exhaust sensors at Delphi’s technical center in Brighton, Mich. “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.”
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 popular oxygen sensors.
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, the company says.
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