Internal-combustion engines – Charge forming device – Having fuel vapor recovery and storage system
Reexamination Certificate
1999-07-06
2001-09-04
Yuen, Henry C. (Department: 3747)
Internal-combustion engines
Charge forming device
Having fuel vapor recovery and storage system
C123S1980DA
Reexamination Certificate
active
06283098
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a vehicle fuel system with on-board diagnostics for evaporative leak 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 evaporative leak testing is required to ensure that leakage from the sealed system does not exceed acceptable limits. Typical known leak 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 leak. If the bleedup is less than R, it assumed that there is no such leak. Leaks of less that a certain size 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 small leaks.
In addition, the bleedup for a particular leak size 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 leak 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 leakage into the tank during the first stage of the test.
The present invention seeks to make further improvements to evaporative fuel system leak testing to enable smaller leaks to be reliably detected under varying ambient and operating conditions.
SUMMARY OF THE INVENTION
According to the present invention a vehicle fuel system with on-board diagnostics for leak 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 two stage leak test, when the engine is running;
h) one stage of the leak test comprising:
i) evacuation of the tank with the purge valve open and the CVV closed;
ii) monitoring pressure rise in the tank with both valves closed; and
iii) recording the pressure rise dP_A over a predetermined period A following increase of pressure to a predetermined value p2;
i) the other stage of the leak test comprising:
i) venting the tank to atmospheric pressure via the CVW then sealing the tank by closing the CVW; and
ii) measuring the amount dP_B by which the pressure in the tank rises above atmospheric due to vapour generation over a period (period_B) following closure of the CVV; and
j) the ECU being arranged to calculate a value X representative of leakage from the difference between dP_A and dP_B using a scaling factor K dependent on measured values of operating conditions.
Preferably, values of K are stored in a 2-dimensional map giving a value of K for combinations of measured values of fuel level and ambient temperature. K may also be mapped against other relevant parameters. The leak test is sensitive to vapor volume in the system but it is more convenient to measure fuel level, which is simply and directly related to vapor volume since vehicles are equipped with means for sensing fuel level. The measured pressures are more directly related to fuel temperature than ambient temperature but it is more convenient to use ambient temperature, for which a sensor is usually available in most vehicles than to provide an additional temperature sensor dedicated to fuel temperature.
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.
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Drouillard James R.
Ford Global Technologies Inc.
Gimie Mahmoud
Yuen Henry C.
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