Exhaust gas purification system for a lean burn engine

Details Exhaust gas purification system for a lean burn engine Exhaust gas purification system for a lean burn engine

1999-12-21

2001-01-02

Denion, Thomas (Department: 3748)

Power plants

Internal combustion engine with treatment or handling of...

By means producing a chemical reaction of a component of the...

C060S274000, C060S281000, C060S289000, C060S280000, C060S307000

Utility Patent

active

06167698

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to exhaust systems for engines and, more particularly, to an exhaust after-treatment system for a low emission, lean-burn internal combustion engine.
BACKGROUND OF THE INVENTION
Catalysts are generally used as a means for removing pollutants such a HC, CO and NO
x
from the exhaust gas of internal combustion engines. The ability of a catalyst to remove NO
x
in the exhaust gas falls rapidly, however, when the air-fuel ratio of the exhaust gas becomes leaner. Lean burn engines such as diesel engines, operate at overall air-fuel ratios leaner than stoichiometry. As a result, they have improved fuel economy. On the other hand, conventional three-way catalysts do not function properly at such lean air-fuel ratios because of the relatively high concentration of oxygen in the exhaust gas. For this reason, such engines are typically equipped with a lean NO
x
catalyst (LNC) and/or a selective catalytic reduction (SCR) catalyst.
LNC and SCR catalyst can chemically reduce NO
x
into the components of CO
2
, H
2
O, and N
2
by utilizing hydrocarbons in the exhaust gas stream. To increase the NO
x
conversion efficiency of such exhaust gas after-treatment systems, injectors are used to inject reductants such as gasoline, diesel fuel or urea into the exhaust gas upstream of the catalyst.
The quantity of reductant injected is very small, and must be finely atomized prior to introduction in the exhaust gas flow upstream of the LNC or SRC. Conventional reductant injection systems utilize an air assisted injector in connection with a series of electrically or mechanically driven air and reductant pumps to introduce the air/reductant into the exhaust stream. Such systems have the obvious drawback of additional cost, noise and weight associated with the separate dedicated electrical or mechanical air and reductant pump system.
SUMMARY OF THE INVENTION
One object of the present invention is to provide an improved exhaust gas purification system. The foregoing and other objects and advantages are achieved through the provision of an exhaust gas purification system for an engine equipped with a turbocharger and an exhaust pipe having a NO
x
conversion catalyst therein. The system comprises a reservoir for storing pressurized air received from the turbocharger, a reductant injector in fluid communication with a reductant supply, and a mixing chamber in fluid communication with the reservoir and connected to the reductant injector. The reductant injector is responsive to an injection signal for atomizing and injecting a quantity of reductant into the mixing chamber. The mixing chamber is in fluid communication with the exhaust pipe for introducing a quantity of air and reductant mixture into the exhaust gas flow upstream of the NO
x
conversion catalyst. In one aspect of the invention, the system includes an engine control unit, a valve connected between the reservoir and the mixing chamber, and a differential pressure transducer positioned between the mixing chamber and the exhaust pipe for measuring the pressure differential across the mixing chamber and exhaust pipe upstream of the NO
x
conversion catalyst. The engine control unit controls the valve to introduce a quantity of pressurized air from the reservoir into the mixing chamber to maintain the pressure differential between the mixing chamber and exhaust pipe substantially constant.
One advantage of the present invention is that it eliminates the cost, weight, and durability concerns associated with a separate dedicated electrically or mechanically driven air pump. Other features and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.


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