Internal-combustion engines – Charge forming device – Having fuel vapor recovery and storage system
Reexamination Certificate
2000-02-22
2001-04-17
Moulis, Thomas N. (Department: 3747)
Internal-combustion engines
Charge forming device
Having fuel vapor recovery and storage system
Reexamination Certificate
active
06216674
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a vehicle fuel system with on-board diagnostics for vapor integrity testing.
BACKGROUND OF THE INVENTION
Vehicle fuel systems are required to control emission of fuel vapor. This is done by collecting vapor emitted from the fuel tank in a purge canister containing carbon to absorb the vapor. The canister is purged of collected vapor when the engine is running by drawing air through the canister into the engine, relying on manifold vacuum. The system is sealed except for venting to the atmosphere via the purge canister. On-board vapor integrity testing is required to a warning is given if vapor loss from the sealed system exceeds predetermined levels. Typical known vapor integrity testing systems are described U.S. Pat. Nos. 5,333,590 and 5,765,121.
The latter patent describes a basic test in which the manifold vacuum is used to pump out the fuel tank and the return of tank pressure to atmospheric (“bleedup”) is monitored. If bleedup exceeds a certain threshold value R the system is determined to have an unacceptable vapor integrity. If the bleedup is less than R, it assumed that vapor integrity is acceptable. Low level loss of vapor integrity cannot be reliably detected with this basic system because vapor generation from fuel in the tank can cause pressure in the evacuated system to recover more rapidly than air ingress due to a low level loss of vapor integrity.
In addition, the bleedup for a particular level of vapor integrity depends on vapor volume, that is the volume of free space above the fuel tank and in the purge canister and connecting passages. Vapor volume is itself directly related to fuel level.
Thus, in order to improve the sensitivity of the basic bleedup test, measures must be taken to correct for different operating conditions, particularly the fuel level and the rate of vapor generation in the tank.
For example, U.S. Pat. No. 5,333,590 uses a threshold value R which is not fixed but is related to vapor volume and fuel temperature.
It is also known to improve the sensitivity of vapor integrity testing by using a two stage test. The first stage is a bleedup test in which pressure increase over a certain period (period_A) is measured. A second stage is carried out in which pressure rise of the closed system from atmospheric over a second period (period_B) is monitored. The second stage gives an indication of vapor generation in the tank under prevailing conditions. A constant scaling factor is used to deduct a proportion of pressure rise found during the second stage to provide a value which more closely represents the level of bleedup due to air ingress into the tank during the first stage of the test.
A source of error that is not dealt with in the existing systems described above arises from variations in temperature of the gaseous contents of the tank at the start of bleedup, due in the main to variations in the evacuation. Evacuation results in the temperature of the vapor contents being reduced below ambient temperature by an amount which depends on the nature of the evacuation (fast, slow, early or late). Without any compensation for such temperature variation, a worst case error in may be equivalent to a hole diameter of around 0.5 mm. Errors of this magnitude are not acceptable when small leaks equivalent to 0.5 mm diameter hole are required to be detected.
SUMMARY OF THE INVENTION
According to the present invention a vehicle fuel system with on-board diagnostics for vapor integrity testing comprises:
a) a fuel tank for containing fuel for delivery to an internal combustion engine;
b) a purge canister connected to the space in the tank above the fuel;
c) a canister vent valve (CVV) for connecting the purge canister to the atmosphere;
d) a purge valve for connecting the purge canister to the engine; and
e) an electronic control unit (ECU) arranged for monitoring pressure and fuel level in the tank and other engine, vehicle and ambient conditions and for controlling opening and closing of the valves;
f) the CVV and the purge valve being controlled by the ECU for venting the tank to atmosphere via the purge canister (purge valve closed, CVV open), and for purging vapor from the canister by allowing air to be drawn through the canister by manifold vacuum (both valves open);
g) the ECU being arranged to carry out a periodic vapor integrity test, when the engine is running;
h) the vapor integrity test including:
i) evacuation of the tank with the purge valve open and the CVW closed (evacuation phase);
ii) monitoring pressure rise in the tank with both valves closed (bleedup phase); and
iii) developing an indication of vapor integrity from time and pressure values measured during the bleedup;
in which:
pressure in the tank is measured at intervals during the evacuation phase and a correction is made to the vapor integrity indication based on the values and timing of the pressure measurements made during the evacuation phase, the correction being effective to reduce errors in the vapor integrity indication due to temperature variations in the air/vapor in the tank at the commencement of bleedup due to variations in the evacuation phase.
The improved fuel system test contemplated by the invention is preferably implemented using the vehicle's existing electronic engine control unit and the fuel system pressure sensor which is used for other purposes. As a consequence, the benefits of the invention may be obtained at very little additional cost.
These and other features and advantages of the present invention may be better understood by considering the following detailed description of a preferred embodiment of the invention.
During the course of this description, frequent reference will be made to the attached drawings.
REFERENCES:
patent: 5243944 (1993-09-01), Blumenstock
patent: 5251592 (1993-10-01), Seki et al.
patent: 5765539 (1998-06-01), Isobe et al.
Drouillard Jerome R.
Jaguar Cars Limited
Moulis Thomas N.
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