Because of the simple chemical reaction reason, to reduce NOx on a compression ignition engine, the chemical composition of the exhaust gas must first be changed. There are two ways to accomplish NOx reduction goal — selective catalytic reduction (SCR) and NOx traps (or NOx Absorbers).

The leading solution for 2010 is Selective Catalytic Reduction (SCR) — an emissions-reduction technology with the ability to deliver near-zero emissions of Nitrogen Oxides (NOx), a smog-causing pollutant and greenhouse gas. This technology requires treating the exhaust stream with a spray of diesel exhaust fluid (DEF) — an ammonia based solution. DEF, along with the exhaust heat and a catalyst, converts NOx into nitrogen and water vapor, which are clean, harmless and present in the air we breathe every day. Three forms of DEF can be used in SCR systems: pure anhydrous ammonia, aqueous ammonia and urea.

Urea SCR System

The system will still use the existing engine architecture, diesel oxidation catalyst (DOC) and diesel particulate filter (DPF), with additional SCR hardware. This technology allows the engine to function at optimal combustion temperatures, which increases fuel efficiency and reliability. The main components of the SCR system are the SCR catalyst, the DEF injection unit, the DEF storing tank and the DEF dosing control unit — a commonly closed-loop control. The system is benefited from closed-loop control when appropriate exhaust gas sensors are used.

An ammonia sensor, which is based on a non-equilibrium electrochemical principle and outputs emf signals, has been developed by Delphi for use as a feedback element in closed-loop control of urea injection in a diesel SCR after-treatment system. Closed-loop control of SCR dosing enables the SCR system to be robust against disturbances and to meet conformity of production (COP) and in-use compliance norms.

DEF is a solution of purified water and urea, an organic nitrogen compound that turns to ammonia when heated. When DEF is injected into the exhaust pipe downstream of the engine, the heat of the engine exhaust gases decomposes DEF into ammonia and CO2. Vaporized DEF and hot exhaust gases enter a catalytic converter located in the exhaust system after the DPF (diesel particulate filter). The catalytic converter contains a catalyst, which is a substance that causes or accelerates a chemical reaction, without being affected itself. When the NOx reacts inside the catalyst with the ammonia, the harmful NOx molecules in the exhaust are converted to harmless nitrogen and water.

Selective Catalytic Reduction with urea injection is already a proven technology that uses basic chemistry to reduce NOx emissions through a process that is simple, extremely efficient, very reliable and safe. Above figure show the 2007 emissions and 2010 emissions system configuration with focus of particulate matter and NOx emission reduction.

Ammonia SCR System

Selective Catalytic Reduction (SCR) of NOx using an ammonia compound as reductant has been used for many years in stationary diesel engine applications, as well as for mobile applications. In the SCR process, NOx reacts with the ammonia, which is injected into the exhaust gas stream before a special SCR Catalyst.

The main chemical reaction which occurs in ammonia SCR system is shown by this Equation:
4NO+ 4NH3 + O2 = 4N2 + 6H2O

The ammonia SCR process requires precise control of the ammonia injection rate. An insufficient injection may result in unacceptably low NOx conversions. An injection rate that is too high results in release of undesirable ammonia to the atmosphere. These ammonia emissions are known as ammonia slip. The ammonia slip increases at higher NH3/NOx ratios. The stoichiometric NH3/NOx ratio is about 1.0.

An ammonia SCR system for the automotive diesel engines works in the same way of urea SCR system with the exception of the injection control system. The ammonia SCR control system is an open loop configuration, where a pre-programmed map of engine NOx emissions is used to control the ammonia injection rate. This open loop configuration is capable of some 95% NOx reductions. The automotive industry recognizes that it offers the most effective SCR solution for mobile applications.

The System is comprised of sensors for exhaust gas temperature, intake air temperature, engine load information, throttle position, engine rpm, exhaust back-pressure, the electronic control unit (ECU) and a SCR converter. The ECU is responsible for controlling the amount of ammonia being injected for maximum NOx reduction and includes circuitry for the redundant fail-save and OBD (On Board Diagnostic) system.

The ECU calculates the correct amount of Ammonia needed, by «reading» the information supplied by the various sensors and compares these values with the appropriate point of the factory-programmed ECU Map. The fail-save system assures that possible ammonia leaks do not go undetected and the OBD system alarms the vehicle operator of any problems. High quality electromagnetic valves assure auto-shutoff of the ammonia supply in case of accidents or system malfunctions.
The prevention of ammonia slippage and a high NOx conversion rates is guaranteed by the advanced Electronic Control Unit (ECU) and high-sensitivity engine and ambient parameter sensors. Without an advanced ECU, low to no ammonia slippage and high NOx reduction cannot be guaranteed.

Ammonia SCR systems have several benefits over urea SCR systems: