Method of purging lean NOx trap

Details Method of purging lean NOx trap Method of purging lean NOx trap

1998-10-02

2001-06-12

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, C060S285000, C060S297000

Reexamination Certificate

active

06244047

ABSTRACT:

TECHNICAL FIELD
This invention relates to vehicle emission control and, more particularly, to a method of controlling a direct injection engine to capitalize on opportunities presented by direct injection to accomplish lean NOx trap regeneration rapidly and frequently and with fewer changes to engine operating conditions.
BACKGROUND OF THE INVENTION
Lean NO
x
traps operate cyclically. During the lean portion of the cycle (fill duration), NO
x
is adsorbed. After running lean for a period of time, the trapping efficiency becomes low and the trap must be regenerated. This is done by operating rich of stoichiometric. The hydrocarbons and CO emitted during rich operation causes the NO
x
to reduce to N
2
and O
2
. The lean part of the cycle may typically last for one minute followed by the regeneration or purge part of the cycle for one second.
It is desirable that the transition between the fill and purge portions of the cycle be imperceptible to the driver. Accordingly, the prior art strategies provide for a relatively slow transition between the fill and purge portions of the cycle. Of the transitions that must occur, including fuel flow and spark changes, the most difficult transition is the manifold pressure change. The throttle (or other valve) must be closed rapidly to accomplish the lean to rich and rich to lean transition. Even after the throttle has attained the appropriate position, it takes a few cycles for the intake manifold to fill or to purge, depending on the transition. Thus, the transition occurs over a number of engine cycles. Because of the number of parameters which must be changed, it is a challenge to do this robustly and with no torque fluctuation. Furthermore, the ramping of the conditions which are required to get to and from the purge condition negatively impacts fuel economy.
SUMMARY OF THE INVENTION
In accordance with the present invention a method of purging a NO
x
trap is proposed that provides for transitions between lean and rich fuel mixture conditions without substantial change in the manifold pressure. Thus, neither the throttle angle nor manifold filling are issues. This is accomplished by substantially increasing the fuel flow rate, i.e., the amount of fuel delivered, while retarding the spark in order to avoid any noticeable change in torque. The increased amount of fuel delivered during purge emits substantially more reducing species than in previous strategies. Thus, the required duty cycle may be substantially reduced while still obtaining the same reduction of NO
x
without paying any fuel economy penalty.
The method of the present invention is intended for operation with a direct-injection engine where the amount of fuel delivered in the combustion chamber can be changed on a cycle-by-cycle basis. The method would not be as effective in a port-injected engine because of fuel hangup on manifold surfaces. With direct injection, the purge cycle can be as short as one engine combustion event. As the trapping efficiency of the NO
x
trap diminishes with time during a trap cycle, it is desirable to purge often to keep the trapping efficiency at its highest. As will be explained more fully hereinafter, the purge time could be scheduled for a single engine cycle following 375 engine cycles of fill. An advantage of the short interval between purges is that the NO
x
trapping efficiency is improved. A further advantage is better fuel economy due to elimination of relatively slow transition phases. Also, the severe spark retard used in practicing the present invention, causes the exhaust gas temperature to increase markedly providing higher exhaust enthalpy and helping to keep the NO
x
trap active by maintaining a higher temperature.


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