A typical evaporative emission control system in a modern automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the fuel tank. During conditions conducive to purging, the canister is purged to the engine intake manifold by means of a canister purge system that comprises a canister purge solenoid valve that is operated by an engine management computer. The canister purge valve is opened in an amount determined by the computer to allow the intake manifold vacuum to draw vapors from the canister through the valve into the engine.
U.S. governmental regulations require that certain future automobiles that are powered by volatile fuel such as gasoline have their evaporative emission control systems equipped with on-board diagnostic capability for determining if a leak is present in a portion of the system which includes the fuel tank and the canister. One proposed response to that requirement is to connect a normally open solenoid valve in the canister vent, and to energize the solenoid when a diagnostic test is to be conducted. A certain vacuum is drawn in a portion of the system which includes the tank headspace and the canister, and with the canister and the tank headspace not being vented due to the closing of the canister vent, a certain loss of vacuum over a certain time will be deemed due to a leak. Loss of vacuum is detected by a transducer mounted on the fuel tank. Because of the nature of the construction of typical fuel tanks, a limit is imposed on the magnitude of vacuum that can be drawn. Too large a vacuum will result in deformation and render the measurement meaningless. In order to avoid this problem, a relatively costly vacuum transducer is required. Since typical automotive vehicles are powered by internal combustion engines which draw intake manifold vacuum, such vacuum may be used for performance of the diagnostic test, but typically this requires that the engine be running in order to perform the test.
The invention disclosed in commonly assigned U.S. Pat. No. 5,191,870 provides a solution to the leak detection problem which is significantly less costly. The key to that solution is a new and unique vacuum regulator/sensor which is disposed in the conduit between the canister purge solenoid and the canister. The vacuum regulator/sensor is like a vacuum regulator but with the inclusion of a switch that is used to provide a signal indicating the presence or the absence of a leak. A diagnostic test is performed by closing the tank vent and using the engine manifold vacuum to draw, via the canister purge solenoid valve and the vacuum regulator/sensor, a specified vacuum in the tank headspace and canister. Upon the requisite vacuum having been drawn, the vacuum regulator/sensor closes to trap the drawn vacuum. If unacceptable leakage is present, a certain amount of vacuum will be lost within a certain amount of time, and that occurrence causes the switch of the vacuum regulator/sensor to give a signal indicating that condition.
U.S. Pat. No. 5,146,902 discloses a diagnostic system and method for evaluating the integrity of a portion of the canister purge system that includes the tank and canister by means of positive pressurization rather than negative pressurization (i.e., rather than by drawing vacuum). In certain canister purge systems, such a diagnostic system and method may afford certain advantages over the system and method described in U.S. Pat. No. 5,191,870. For example, certain types of leaks, for example cracked hoses and faulty gas caps, may be more susceptible to successful detection. Moreover, the evaporative emission control system may be diagnosed either with or without the automobile's engine running.
A further benefit of positive pressurization over negative pressurization is that the increased pressure suppresses the rate of fuel vapor generation in the tank, and such attenuation of fuel vapor generation during a diagnostic test reduces the likelihood that the test will give, under hot weather conditions which promote fuel vapor generation, a false signal that would erroneously confirm the integrity of the canister and tank whereas the same test during cold weather would indicate a leak.
Certain of the commonly assigned pending applications relate to introducing the pumped air into the evaporative emission system through an atmospheric vent port of the canister after that port has been closed to atmosphere by the closing of a canister vent solenoid (CVS) valve through which the canister is otherwise vented to atmosphere during non-test times. Such pumping may afford certain advantages over pumping air directly into the tank headspace.
In all of the aforementioned systems, certain variable ambient conditions are either more or less of an influence on the test accuracy. Atmospheric pressure and temperature are two such influences, and where they are significant enough, means must be provided to compensate for their variations.
The present invention relates to a new and improved system that is in certain important respects simpler, and hence more cost-effective. For example, the present invention enables a relatively expensive pressure transducer and a canister vent valve to be eliminated from the system.
Briefly, and without necessarily limiting its scope, the present invention comprises the use of a centrifugal air pump (i.e. a blower) to blow ambient air through a differential flow meter which splits the pump flow into two paths, a first flow path through a first flow sensor leading to the closed vapor headspace in the tank-canister, and a second comprising a second flow sensor in series with a calibrated orifice leading to atmosphere. The two flow sensors provide respective electric signals representative of the respective air flows through them to respective inputs of an electronic comparator circuit. The latter takes the difference and provides an electrical output signal that is indicative of that difference. The capacity of the air pump and the calibrated orifice are sized in relation to a certain range of leakage from the tank-canister headspace such that a reasonably accurate measurement of the amount of leakage can be obtained, if the leakage is in fact within that range. For gross leakage, the accuracy of the measurement may be somewhat problematic, but that will typically be unimportant since a gross leakage will in any event be indicated.
Further specific details of the construction and arrangement of the inventive system, and of the method of operation thereof, along with additional features and benefits, will be presented in the ensuing description.