Internal-combustion engines – Combustion chamber means having fuel injection only – Combustible mixture stratification means
Reexamination Certificate
2000-08-02
2002-04-02
Wolfe, Willis R. (Department: 3747)
Internal-combustion engines
Combustion chamber means having fuel injection only
Combustible mixture stratification means
C123S520000
Reexamination Certificate
active
06363908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of operating an engine. More specifically, it relates to a method for operating an engine to allow evaporative emission purge during operation of an engine in a dual combustion mode.
2. Discussion of the Related Art
Internal combustion engines can be coupled to fuel vapor recovery systems to allow purging of fuel vapors generated in the fuel tanks. Conventional systems control total purge flow based on an estimated purge fuel vapor concentration representing the amount of fuel vapor contained in the total purge flow from the fuel system. Fuel injection control is also adjusted based on this estimated purge concentration.
The present inventor developed a method for accurately determining the cylinder fuel vapor purge concentration. This invention was disclosed in U.S. application Ser. No. 09/366,124, and its disclosure is incorporated herein by reference. By knowing the cylinder fuel vapor purge concentration as well as the concentration in the fuel vapor recovery system, control of the operation of the engine can be accomplished. The inventor also has developed a method for determining the overall fuel vapor purge concentration. This invention was disclosed in U.S. application Ser. No. 09/055,500, and its disclosure is also incorporated herein by reference.
Direct injection spark ignition (DISI) engines can operate in multiple modes of operation. These modes can include a homogeneous mode where the air-fuel mixture is injected during the intake stroke; a stratified mode where additional fuel can be added during the compression stroke; and a combined or dual mode that allows injection on the intake stroke and further injection of fuel during the compression stroke. The combined mode is especially useful for high load conditions and knock resistance.
Multiple injections can also used to make transitions between the stratified and homogeneous modes and to extend the envelope of non-homogeneous operation.
In the dual mode, it is difficult to control the proper amount of fuel so that it is balanced and will not produce unburned hydrocarbons or over fuel/over torque the engine. Currently there is no way to control the total quantity of fuel burned to provide a desired torque output from the engine and to balance the amount of fuel supplied by each of the various sources (canister purge vapor, fuel injected during the intake stroke, fuel injected during the compression stroke) so that the homogeneous portion of the cylinder charge is at an air-fuel ratio that provides reliable combustion and acceptable emissions when ignited and the stratified portion of the charge provides reliable ignition as well as providing the final portion of fuel necessary to deliver the desired torque. Further, there is currently no method for maximizing the usage of fuel vapors from the purge system.
None of the prior art allows for a balanced control of the engine in the dual mode when the carbon canister is feeding purge vapor to the engine cylinders. Typically, if the purge fuel vapor is added to the engine in the prior art structures, the mixture is often too lean to burn and the fuel is wasted. In some cases, the unburned hydrocarbons are used to warm up the catalyst but this usually increases emissions and does not provide a balanced control to solve the above-mentioned problems.
SUMMARY OF THE INVENTION
An object of the present invention is to use the fuel vapor concentration to control the operation of the engine, especially when the engine is operating in dual mode.
It is a further object of the present invention to provide a method that uses purge flow from the carbon canister with a minimum impact on fuel economy and pollution.
During stratified operation of a DISI engine, the fuel is injected during the compression stroke which creates a region of combustible mixture in only a portion of the cylinder which is surrounded by a non-combustible mixture of air and exhaust residuals. In the dual mode of operation, fuel is injected on the intake stroke to provide a lean, nearly homogeneous air fuel mixture throughout the cylinder, then additional fuel is injected during the compression stroke to create a region of richer mixture that will be easier to ignite with the spark plug.
If fuel vapors from the carbon canister are introduced to the intake manifold, this fuel will be mixed throughout the cylinder. During stratified operation, the portion of the evaporative fuel that is in the rich region will burn. The fuel in the remainder of the cylinder will burn (and produce torque) only if it is rich enough.
If the overall mixture is too lean, it will not burn and this results in higher feed gas hydrocarbon emissions and increased catalyst temperatures. Also, this fuel will be consumed without producing useful work resulting in a lower fuel economy. If the overall mixture is near the lean limit of flammability, it may burn during some combustion events and not burn during others and this will result in rough engine operation. Further, if the canister is not purged during stratified operation, it can be overfilled and inadequately purged during typical operation. However, if the engine is run in a homogeneous mode more often to allow for more canister purge, the fuel economy benefit from stratified operation is reduced.
In the dual mode, it is desirable to control the total quantity of fuel burned to provide a desired torque output from the engine and further, to balance the amount supplied by each of the various sources (canister purge vapor, fuel injected during the intake stroke, fuel injected during the compression stroke) such that the homogenous portion of the cylinder charge is at and air-fuel ratio that provides reliable combustion (and acceptable emissions) once ignited and the stratified (richer) portion of the charge provides reliable ignition as well as providing the final portion of the fuel necessary to deliver the desired torque. Also, maximizing the usage of fuel vapors from the evaporative purge system is desired to maximize the purge of the carbon canister during vehicle operation. To provide the proper balance, a total fuel quantity desired is determined; this fuel is then apportioned between fuel supplied during the intake stroke and fuel supplied during the compression stroke. Further, the fuel supplied during the intake stroke is comprised of fuel vapors from the carbon canister and fuel injected during the intake stroke.
In general if Ft is the total fuel desired, then the sum of Fp (the fuel that is available from the canister purge system), Fh (the fuel injected during the intake stroke) and Fs (the fuel injected during the compression stoke) should equal Ft:
Fp+Fh+Fs=Ft.
Further, in the homogenous portion of the charge, given a mass of air in the cylinder (MA) and a desired homogenous air fuel ratio (AFh), the fuel supplied during the intake stroke should satisfy the equation:
MA/(Fp+Fh)=AFh.
The resulting air fuel ratio measured in the exhaust system (AFx) would be:
AFx=MA/(Fp+Fh+Fs)
Alternately, when it is adequate for the homogenous portion of the charge to simply be within a range of air fuel ratios (AFhMin, AFhMax) then:
AFnMin<MA/(Fp+Fh)<AFhMax
where AFnMin would be a minimum combustible air fuel ratio, and AFhMax would be the maximum air fuel ratio desired for acceptable emissions.
If the fuel quantity that will be supplied by the carbon canister is not currently known, the homogenous fuel can initially be supplied by injection. Then as flow from the carbon canister is induced the homogenous injected fuel is reduced to maintain the desired overall air fuel ratio. The amount of reduction provides the information necessary to determine the fuel content of the purge flow. Conversely, once the fuel content of the purge is known, a purge flow can be commanded that will provide as much fuel as possible without exceeding the desired homogenous air fuel ratio. If the fuel from the purge is insufficient to provide the desired, combustible, a
Ford Global Technologies Inc.
Huynh Hai
Russell John
Wolfe Willis R.
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