Page 34 - Methods For Monitoring And Diagnosing The Efficiency Of Catalytic Converters A Patent - oriented Survey

16 Methods for Monitoring and Diagnosing the Eficiency of Catalytic Converters

the absorbent in the form of nitric acid ions NO3 (fig. Sa). In this way, NO, is absorbed in the
NO, absorbent.

When the oxygen concentration in the in-flowing exhaust gas is lowered, the production of
NO2 is lowered and the reaction proceeds in an inverse direction (NO, -+N02), and thus nitric
acid ions NO; in the absorbent are released in the form of NO2 from the NO, absorbent. In this
case, components such as HC and CO, which exist in the exhaust gas, react with the oxygen
0; or (I2. on the platinum Pt and are oxidized. After oxygen 0; or 0'- on the platinum Pt are
consumed by HC and CO in the exhaust gas, NO, released from the NO, absorbent as well as
NO, emitted from the engine are reduced by the HC and CO remaining on the platinum Pt. This
oxidation of the HC and CO consumes the oxygen component existing near the NO, absorbent,
and the concentration of oxygen in the atmosphere around the NO, absorbent is lowered. Also,
the NO2 released from the NO, absorbent reacts with the HC and CO in the exhaust gas as
shown in Fig. 8b and is reduced to N2. In this way, when the NO2 on the surface of the
platinum Pt reacts with HC and CO in the reducing agent, and when the NO2 no longer exists
on the surface of the platinum R, the NO2 is successively released from the absorbent.
Accordingly, when HC and CO components exist in the in-flowing exhaust gas, the NO, is
released from NO, absorbent and quickly reduced to N2. The HC and CO component in the
exhaust gas immediately react with the 0, or 02- on the platinum Pt and are oxidized, and
subsequently if the HC and CO still remain after the 0;or 02-on the platinum Pt are
consumed, the NO, released from the absorbent and the NO, emitted from the engine are
reduced.

In diesel engines, oxidation catalytic converters are used to convert a large part of the
hydrocarbon constituents of the soluble organic fraction (SOF), as well as gaseous HC, CO,
odor creating compounds and mutagenic emissions.

The NO, reduction in diesel engines and lean-bum petrol engines is achieved by the so called
selective catalytic reduction (SCR) catalytic converters. A reducing agent is introduced in the
exhaust gases upstream of the catalytic converter to promote reduction of the NO,. Such
reducing agents are: hydrocarbons, ammonia, urea, hydrogen etc. More information on
supplying reducing agents to the exhaust gases can be found in EP0709129 (1996),
EP0737802 (1996), EP0723805 (1996), EP0537968 (1993), EP0498598 (1992).

In some other cases like the one described in EP0510498 (1992), an ammonia synthesizing
catalytic converter is introduced in the exhaust system that transforms part of the NO, of the
exhaust gases to ammonia. The ammonia produced plays the role of a reducing agent and
reacts with the remaining of nitrogen oxides of the exhaust gases to produce nitrogen so no
external addition of ammonia is necessary.

Other ways to reduce NO, in the exhaust gases is to recirculate part of the exhaust gases (e.g.
10%) back to the engine or to inject water in the combustion chamber. The recirculation
systems are called EGR (Exhaust Gas Recirculation) systems. The recirculated exhaust gas
decreases the flame temperature in the engine cylinder and provides a shortage of oxygen in