Canister purge strategy for a hybrid electric vehicle

Details Canister purge strategy for a hybrid electric vehicle Canister purge strategy for a hybrid electric vehicle

2001-01-03

2003-05-06

Moulis, Thomas N. (Department: 3747)

Internal-combustion engines

Charge forming device

Having fuel vapor recovery and storage system

C123S179160

Reexamination Certificate

active

06557534

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a Hybrid Electric Vehicle (“HEV”) where a vehicle system controller or engine controller determines if a canister collecting fuel vapor needs to be purged during vehicle idle.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and pollutants of automobiles and other vehicles powered by Internal Combustion Engines (“ICEs”) is well known. Vehicles powered by electric motors have attempted to address these needs. However, electric vehicles have limited range and limited power coupled with the substantial time needed to recharge their batteries. An alternative solution is to combine both an ICE and electric traction motor into one vehicle. Such vehicles are typically called Hybrid Electric Vehicles (“HEVs”). See generally, U.S. Pat. No. 5,343,970 (Severinsky).
The HEV has been described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In others, the electric motor drives one set of wheels and the ICE drives a different set.
Other configurations have developed. A Series Hybrid Electric Vehicle (“SHEV”) is a vehicle with an engine (most typically an ICE) which powers a generator. The generator, in turn, provides electricity for a battery and motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. A Parallel Hybrid Electrical Vehicle (“PHEV”) is a vehicle with an engine (most typically an ICE), battery, and electric motor combined to provide torque to power the wheels of the vehicle.
A Parallel/Series Hybrid Electric Vehicle (“PSHEV”) has characteristics of both a PHEV and a SHEV. The PSHEV is also known as a torque (or power) splitting powertrain configuration. Here, the torque output of the engine is given in part to the drive wheels and in part to an electrical generator. The electric generator powers a battery and motor that also provide torque output. In this configuration, torque output can come from either source or both simultaneously. In this configuration the vehicle braking system can even deliver torque to drive the generator to produce charge to the battery.
The desirability of combining an ICE with an electric motor is clear. The combination provides the opportunity to reduce the ICE's fuel consumption and pollutants with no appreciable loss of performance or range of the vehicle. Nevertheless, there remains substantial room for development of ways to optimize these HEV's operational parameters.
One such area of improvement is the HEV's tailpipe and evaporative emission control systems. Tailpipe emissions require very tight control of the Air to Fuel ratio (A/F). Controlling the A/F ratio requires an oxygen sensor to measure the amount of oxygen leaving the engine after combustion. A controller then monitors the oxygen levels and controls the amount of fuel provided by the injectors in an attempt to create an optimal A/F ratio, thereby reducing unwanted emissions.
Controlling the A/F ratio becomes more complex when fuel vapor is considered. Fuel vapor is generated in the fuel system (tank and lines) because of the heat of the fuel when the engine is running at its stabilized operating temperature. If not managed properly, the vapor can build, causing the fuel vapor pressure to increase to the point where the vapor can leak out of the fuel system into the atmosphere as unwanted evaporative emissions. Thus, a charcoal canister is typically installed between the fuel tank and the engine to collect the fuel vapor. Over time, the canister becomes full and must be emptied or purged. In order to purge, a vapor management valve (VMV) is opened in a controlled manner by a VMV controller, thereby allowing the fuel vapor into the intake manifold, as long as there is sufficient vacuum present inside the manifold. During the purging process, the A/F controller maintains the optimum A/F ratio (and thus compensates for the additional fuel vapor entering the cylinders) by adjusting the fuel amount delivered by the injectors. Then, the VMV controller determines when the canister is empty and closes the VMV. Specifically, the VMV controller determines the canister's condition by estimating how much fuel vapor is being drawn into the intake manifold and cylinders. The amount that the A/F controller must correct the fuel delivery through the fuel injectors when the purging process is occurring reflects how much fuel vapor is coming from the vapor canister and causing the A/F disturbance.
Although it is desirable to purge the canister as quickly as possible, the rate of purging must be controlled. If the purge valve opens too quickly, especially if the intake manifold is in a high vacuum condition, the A/F controller may not be able to compensate fast enough for the incoming fuel vapor. This, in turn, can cause the A/F ratio to become too lean and causes poor engine combustion. In a conventional vehicle, if the A/F is too lean, the engine could stall. Thus, in conventional vehicles (and perhaps some HEV configurations), even though the vapor canister can be purged faster if the VMV is opened quickly and if higher vacuum conditions are present in the intake manifold, the risk exists that the engine may stall.
HEVs present additional purge problems. First, the engine is not always running, particularly during idle conditions (when the vehicle is not in motion). The canister can still store vapor, but it is not possible to purge the canister if the engine is not running.
Second, some HEVs run the engine at near wide-open throttle conditions (when the engine is running) because it is more fuel-efficient. However, little or no vacuum is available to draw the vapor into the intake manifold when the VMV is opened. This, in turn, makes it very difficult to purge the vapor canister.
Finally, most engine control systems implement an adaptive fuel strategy that “learns” or “adapts” the long term fuel shifts in the fuel delivery system caused by variation in fuel system components (injectors and mass air flow sensor). A typical engine control system does not allow the purging process to occur while the adaptive fuel shifts are learned because the purging process introduces A/F ratio shifts that should not be attributed to the fuel delivery system but rather to purge vapor. Thus, for the reduced amount of time that the engine is running during an HEV drive cycle, the adaptive fuel and purge strategies are competing for time to accomplish their objective.
The aforementioned issues make it difficult to purge the vapor canister often enough during a given HEV drive cycle. This increases the risk that fuel vapors will be released into the environment, which is not consistent with current emission goals and standards. Therefore, it is desirable to develop a method of purging the canister of an HEV to minimize the release of fuel vapor to the environment.
SUMMARY OF THE INVENTION
The present invention provides a method and system for purging a vapor canister in an HEV. Even though this invention is for an HEV, it uses a conventional-type purge control strategy that runs normally when the engine is “on” and conventional purging conditions are met (such as the adaptive fuel strategy is not running). This strategy includes the vehicle idle modes encountered in an HEV drive cycle where the engine is required to be “on” for reasons other than purging the vapor canister. The reasons include but are not limited to battery charging and running the air conditioner if mechanically driven by the engine front end accessory drive belt, etc.
When the engine is running, it is not always at an optimal point for purging (low vacuum or adaptive fuel strategy is running). Further, since most vehicle idle modes have the engine “off”, the vapor canister status and purge must be monitored at appropriate times to insure efficiency and emissions goals are met. The best opportunity for doing this is when the vehicle is at idle.
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