Engine control with a fuel vapor purge system

Details Engine control with a fuel vapor purge system Engine control with a fuel vapor purge system

1999-08-02

2001-05-29

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, C060S286000, C123S520000

Reexamination Certificate

active

06237328

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention relates to fuel vapor purge in direct injection spark ignition engines.
BACKGROUND OF THE INVENTION
In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air/fuel operation in which the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to conventional three-way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx.
Direct injection engines are also coupled to fuel vapor recovery systems to allow purging of fuel vapors. Conventional systems allow fuel vapor purging in the stratified mode only when the catalyst temperature is high enough to convert the unburned hydrocarbons. In other words, since the fuel vapor is inducted with the fresh charge, it forms a homogenous air/fuel mixture. Then, when the fuel is directly injected during the compression stroke to form a stratified mixture, only the stratified fuel burns since the homogenous fuel vapor mixture is too lean. Therefore, unburned fuel vapor may exit the engine cylinder. In some circumstances ,to remove these vapors, the catalytic converter may have to be above a certain temperature, known to those skilled in the art as the light-off temperature. Therefore, fuel vapor purging during the stratified mode is restricted until the catalyst has reached this light-off temperature. Such a method is described in U.S. Pat. No. 5,245,975.
The inventor herein has recognized a disadvantage with the above approach. When operating in the stratified mode, less heat is generated to increase or maintain catalyst or NOx trap temperature. Thus, it takes a long time after a cold start before purging is allowed. Also, with some catalyst formulations and catalyst configurations, the catalyst may not maintain this light-off temperature during certain conditions and purging is only possible in the homogeneous mode, thereby reducing fuel economy and allowable purge time. In other words, when operating in the stratified mode, catalyst or NOx trap temperature may fall below the light-off temperature, preventing purge and thereby limiting stratified operation.
SUMMARY OF THE INVENTION
An object of the invention herein is to control an emission control device temperature without degrading fuel economy performance.
The above object is achieved, and problems of prior approaches overcome, by a control method for an engine having multiple combustion chambers coupled to an emission control device, the method comprising the steps of: operating in a stratified mode where fuel is injected during a compression stroke of the engine; providing an indication of temperature of the emission control device; purging fuel vapors into the combustion chambers when said temperature is below a predetermined threshold, thereby providing unburned hydrocarbons and excess oxygen to react exothermically and increase said temperature to allow continued stratified operation; and discontinuing said stratified operation when a sum of said injected fuel and said purged fuel vapors is greater than a predetermined value.
Purged fuel vapors during stratified operation are inducted into the cylinder and form a homogeneous mixture that is too lean to combust, even when the engine is operating in a stratified mode. These unburned fuel vapors, along with oxygen from the unburned portion of the charge inducted, will react exothermically in the emission control device. This exothermic reaction will generate heat that will increase the emission control device to a desired level. By monitoring and controlling the amount of fuel vapor ingested into the cylinders, this exothermic reaction can be used to maintain the catalyst temperature under conditions where the temperature would otherwise be lower than desired for maximum system efficiency. However, since at least some of the fuel vapor is not producing driving torque, there is a fuel economy penalty for maintain temperature of the emission control device. When the sum of purge fuel and injected fuel to operate lean and maintain catalyst temperature is greater than the sum of purge fuel and injected fuel to operate at stoichiometry, where purge fuel will produce driving torque, stratified operation is discontinued.
An advantage of the present invention is increased fuel economy.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.


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