WO1993025882A1

WO1993025882A1 - Evaporative emission system leak test method and apparatus therefor

Info

Publication number: WO1993025882A1
Application number: PCT/US1993/004962
Authority: WO
Inventors: Kenneth W. Padgitt
Original assignee: Motorola Inc.
Priority date: 1992-06-15
Filing date: 1993-05-26
Publication date: 1993-12-23

Abstract

A method for testing for leaks in an automotive evaporative emission system is provided. This includes providing a fuel tank (101) containing a gaseous fuel material (104). Then, measuring a pressure of the gaseous fuel material in the fuel tank and providing a reference pressure (505). Then, measuring a pressure in the fuel tank (101) after providing the reference pressure and providing a vacuum change pressure (509). Then, providing an indication (511) in response to a comparison of the vacuum change pressure and a predetermined fraction of the reference pressure indicative of pressure leakage in the fuel tank (101). Additionally, an apparatus (100) for embodying the method is disclosed.

Description

EVAPORATIVE EMISSION SYSTEM LEAK TEST METHOD AND

APPARATUS THEREFOR

Field of the Invention

This invention is generally directed to the field of evaporative emissions systems and more specifically to testing an evaporative emissions system.

Background of the Invention

Evaporative emissions systems are typically found on vehicles including; passenger cars, light duty trucks, and medium duty vehicles. These systems are designed to prevent unnecessary emission of hydrocarbon vapors into the atmosphere. These emissions are primarily composed of gasoline vapors leaking from a vehicle's fuel tank to the air. In a typical system the fuel tank is vented into a canister filled with charcoal that performs a filtering function. The charcoal traps the hydrocarbon molecules from the polluting vapors, preventing them from leaking to the atmosphere. Periodically the canister needs cleaning. This is done by purging the trapped hydrocarbon molecules by drawing fresh air through the canister into an air intake manifold of the vehicle's engine. The engine then burns these hydrocarbon molecules in the combustion process.

Near term regulation requires the monitoring of the vehicle's evaporative emission system to ensure integrity of operation. This requirement specifies the checking of the function of electromechanical components associated with the vehicle's evaporative emissions system and the absence of leaks in the system. More specifically, the California Air Resources Board, CARB, specifies in their proposed On Board Diagnostic π, or OBD π, requirement to verify the presence of air flow from the evaporative emissions system to the air intake manifold of the engine and to perform a vacuum check of the evaporative emissions system. This requires detecting system leaks equivalent to an orifice larger than 0.040 inches in diameter.

Current solutions use a differential pressure sensor integral to the evaporative emissions system. Differential pressure sensors are used to compensate for changes in atmospheric pressure. This is necessary because a vehicle can be located at a wide variety of altitudes. This subjects the evaporative emissions system to a wide variety of ambient pressures. Ignored, this change of ambient pressure change would swamp out the detection of a significant leak. For instance an atmospheric pressure difference due to an altitude span of 14,000 feet is about 180 inches of water. This compares to about 10 to 15 inches of water change to be detected when performing a check of the evaporative emissions system. Periodically, any intentional path from the evaporative emissions system to the atmosphere is closed between the fuel tank and an engine intake manifold. This causes a pressure drop in the evaporative emissions system because of a vacuum provided by the engine. Some systems measure the rate of drop of pressure in the evaporative emissions system. Others check the pressure at a predetermined time after the evaporative emissions system is closed, while others may check the rate of change of pressure once the vent valve has been reopened. These pressures are measured using a pressure sensor. If a significant leak is present the change in pressure will be too low and a leak is recognized. The application of differential pressure sensors in these evaporative emissions systems is problematic. The environment vehicles are expected to operate in is very destructive to electrical devices such as differential pressure sensors. This is particularly true in the case of an evaporative emissions system because it is mounted on the lower extremity of the vehicle. This exposes the differential pressure sensor to elements such as; water, gasoline, oil, salt, battery acid, road dirt, and antifreeze to name a few. These chemicals will destroy the differential pressure sensor. Differential pressure sensors inherently have two ports for sensing. One is directed toward the fuel tank and the other toward the atmosphere. The port directed toward the atmosphere will be subjected to the destructive elements described earlier. The port directed toward the fuel tank will be subjected to the destructive fuel in the tank. Eventually the differential pressure sensor will fail, yielding the evaporative emissions system inoperative. This is clearly unacceptable because undetected pollutants may be dumped into the atmosphere. Additionally, because the pressures to be detected are very small, sensors of this type have problems with linearity and exhibit sensitivity to mounting stresses, both of which tend to degrade the accuracy of the measurement.

What is needed is an improved evaporative emissions system test system including a more reliable pressure sensor.

Summary of the Invention

A method for testing for leaks in an automotive evaporative emission system is provided. This includes providing a fuel tank containing a gaseous fuel material. Then, measuring pressure of the gaseous fuel material in the fuel tank and providing a reference pressure in accordance with the measured pressure. Then, applying a vacuum pressure to the fuel tank after providing the reference pressure. Then, measuring pressure in the fuel tank after applying the vacuum pressure and providing a vacuum change pressure in accordance with the measured pressure. Then, providing an indication signal in response to a comparison of the vacuum change pressure and a predetermined fraction of the reference pressure, wherein the indication signal is indicative of a change of pressure in the fuel tank; and therefore a pressure leakage in the fuel tank.

Additionally, an apparatus for embodying the method is disclosed.

Brief Description of the Drawings

FIG. 1 is a schematic block diagram of an apparatus for testing for leaks in an evaporative emissions system, in accordance with the invention.

FIG. 2 is a schematic diagram showing details of a pressure sensor used in FIG. 1.

FIG. 3 is a diagram of signals extracted from various locations of the circuit shown in FIG. 1 illustrative of the operation of an evaporative emissions test system.

FIG. 4 is a flow chart illustrating a method of testing for leaks in an evaporative emissions system, in accordance with the invention.

FIG. 5 is a flow chart illustrating an amplitude based method of testing for leaks in an evaporative emissions system, in accordance with an alternative embodiment of the invention.

FIG. 6 is a flow chart illustrating a time based method of testing for leaks in an evaporative emissions system, in accordance with an alternative embodiment of the invention.

Detailed Description of a Preferred Embodiment

In a preferred embodiment a method and apparatus are disclosed teaching the detection of leaks in an evaporative emissions system. This is accomplished by measuring ambient pressure in a fuel tank and providing a reference pressure. Then drawing a vacuum in the fuel tank. If the system is sealed, the pressure in the fuel tank will drop. A predetermined time later, the pressure in the fuel tank is measured and a vacuum change pressure is provided. This vacuum change pressure is compared with a predetermined fraction of the reference pressure. A fault, or leak is indicated if the vacuum change pressure is greater than the predetermined fraction of the reference pressure. The preferred embodiment teaches the measurement of pressure in the fuel tank. Of course, pressure could be measured at any location in the evaporative emissions system of equal pressure. In FIG. 1 an evaporative emissions system induding a test apparatus is detailed. A control module 131 is the central controlling element of the test apparatus. It is constructed using a Motorola M68HC05B6 microcontroller. Of course, another equivalent microcontroller or analogous circuit could be used. In a preferred embodiment this microcontroller is programmed in accordance with firmware, or a leak test routine, represented in flow chart form, presented in FIG. 4 and described later.

The control module 131 has a vent valve control output 135 that generates a vent valve control signal 134 for opening and dosing a vent valve 113. This vent valve control signal 134 drives a control input 117 to the vent valve 113. The vent valve 113 has an input port 119 and an output port 115. The input port 119 is coupled or decoupled to the output port 115 responsive to the vent valve control signal 134. The control module 131 also has a purge valve command output 136 that generates a purge valve control signal 137 for opening and dosing a purge valve 121. This purge valve control signal 137 drives a control input 125 to the purge valve 121. The purge valve 121 has an input port 123 and an output port 127. The input port 123 is coupled or decoupled to the output port 127 responsive to the purge valve control signal 137.

The control module 131 also has a test signal output 143 that generates a test signal 147 for driving a test signal input 141 to a smart, in this case absolute, pressure sensor 139. The pressure sensor 139 also has an absolute pressure output 140 for providing a pressure signal 145 indicating absolute pressure in a measurement port 138 of a fuel tank 101. This pressure signal 145 is connected to an input 132 of the control module 131. The M68HC05B6 microcontroller reads this pressure signal 145 into an analog input port. Additionally, the pressure sensor 139 provides a fault output 133 that generates an indication, or fault, signal 144 that is provided to an input 130 the control module 131. The M68HC05B6 microcontroller reads this fault signal 144 into one of its digital inputs, and if active provides a fault indication by driving an evaporative emissions system leak indicator lamp 157. Alternatively this leak indication may be provided to a diagnostic data bus in a vehide.

In FIG.'s 4 and 5 firmware is presented in flowchart form representing parts of alternative embodiments of the invention. This firmware will alternatively be programmed into the M68HC05B6 microcontroller and control the apparatus illustrated in FIG. 1. This will be detailed later.

The remainder of FIG. 1 shows elements common to an evaporative emissions system. These indude a fill port 102 to the fuel tank 101, and a gaseous output port 103 from the fuel tank 101. Reference number 104 points to a hydrocarbon based gaseous fuel material present in the fuel tank 101. The fuel tank is connected, via a conduit 152, to an input port 107 of a filter, or charcoal canister 105. The charcoal in the charcoal canister 105 is used to trap the hydrocarbon molecules of the gaseous fuel material in the fuel tank. The charcoal canister 105 also has another input port 111 that is connected to the output port 115 of the vent valve 113. The charcoal canister 105, also has an output port 109 that is connected, via a conduit 153, to the input port 123 of the purge valve 121. The purge valve output 127 is connected to an engine intake manifold 129. The engine intake manifold 129 is later used as a vacuum pressure source.

FIG. 2 shows a schematic diagram detailing the pressure sensor 139 used in FIG. 1. This sensor 139 is comprised of an absolute pressure sensor 201 with integral signal processing circuits. These drcuits indude a conventional sample and hold drcuit 202 constructed of a transmission gate 205, a capadtor 209 and a unity gain buffer amplifier 213. The transmission gate 205 receives an absolute pressure signal 256 at a signal input 203. This signal 256 is indicative of the absolute pressure in the fuel tank 101, provided by the absolute pressure sensor 201. The transmission gate 205 also receives a test signal 147 at a control input 207. This test signal 147 has an on-state and an off-state. This test signal 147 is provided by the control module 131, and is used by the sample and hold circuit 202 for gating the absolute pressure signal 256 to the capadtor 209. The absolute pressure signal 256 is also provided to the absolute pressure output 140 described earlier in FIG. 1. When the test signal 147 is in the off-state, the transmission gate 205 is enabled and gates the absolute pressure signal 256 of the pressure sensor 201 to the capadtor 209. In this condition the sample and hold circuit 202 is sampling the absolute pressure signal 256. When the test signal 147 transits to the on-state, the transmission gate 205 is disabled and the absolute pressure signal 256 of the pressure sensor 201 is not provided to the capadtor 209. In this condition the sample and hold circuit 202 is holding the absolute pressure signal 256. When in the hold state, the capadtor 209 holds a reference pressure signal 211. This reference pressure signal 211 is provided at the output 215 of the sample and hold circuit 202.

The output 215 of the sample and hold drcuit 202 is conneded to an input 216 of a sealer. The sealer is constructed of resistor 217 and resistor 219. The selection of the values of these resistors 217 and 219 determines a magnitude of a signal 218 a predetermined fraction of the reference pressure signal 211, representative of a absolute pressure signal 256 provided by the absolute pressure sensor 201, and is provided to an input 221 of an analog signal comparator 223. The magnitude of the signal 218 will be detailed later when the circuit operation is detailed. The comparator 223 also has an input 227 for receiving the absolute pressure signal 256 provided by the output of the absolute pressure sensor 201. The comparator 223 has an output for providing a signal 229, indicative of the input signals 218 and 256 for driving a data input 206 of a D- type flip-flop 231. This D-type flip-flop 231 also has a falling transition sensitive dodc input 233 driven from the test signal 147, a set input 235 driven from a power up reset circuit comprised of a capadtor 237 and a resistor 239, and an output 225 for providing a fault signal 144. The output 225 is reflective of a complement of the data input 206 when the dodc input 233 transits to the off-state. Note that in the preferred embodiment this sensor 139 is employed. In alternative embodiments described later, a conventional absolute pressure sensor is applied and the fault detection burden is shifted from the sensor's circuitry to the alternative firmware operating in the M68HC05B6 microcontroller.

FIG. 3 is a diagram of signals extracted from various locations of the circuit shown in FIG. 1 illustrative of the operation of the evaporative emissions test system. Illustrated are the vent valve control signal 134, the purge valve control signal 137, and the test signal 147, all derived from the control module 131. Also shown is the fault signal 144, and the absolute pressure signal 256, both provided by the sensor 139.

In FIG. 4 is a flow chart illustrating a method of testing for leaks. FIG.'s 1, 2, 3, and 4 will all be referred to in the following description of operation. As mentioned earlier, in the preferred embodiment, the microcontroller in the control module 131 is programmed with the firmware represented in the flow chart in FIG. 4. The test sequence essentially measures a reference pressure in the evaporative emissions system, draws a vacuum on the evaporative emissions system and expects the pressure in the evaporative emissions system to drop. If the pressure does not drop significantly, as determined by a predetermined fraction of the reference pressure then a leak is deterted in the evaporative emissions system. If a leak is detected then an indication is provided.

At the appropriate time, such as when a vehicle is started, the evaporative emissions system is checked for leaks. Until now the test signal 147, generated by the microcontroller in the control module 131, provided at test signal output 143 and driving the test signal input 141 to the pressure sensor 139, has been in the off- state. This enabled the sample and hold circuit 202 to sample the absolute pressure signal 256 during a time between reference times ti and t2, as shown in FIG. 3.

In step 401, the firmware, or leak test routine, is initiated - starting a test mode of operation in the evaporative emissions system. Next, in step 403, the on-state of the test signal 147 is generated. This test signal 147 can be seen transiting to an on-state at reference time t2 in FIG. 3. This causes the sample and hold circuit 202 to hold the absolute pressure signal 256 establishing the reference pressure 211. This reference pressure 211 will be essentially the ambient pressure of the evaporative emissions system. In FIG. 3 the absolute pressure signal 256 is shown, between reference times ti and t2 to be about 405 inches of water. Therefore, taking this measurement ensures that any errors due to altitude, temperature, linearity, or mounting stress sensitivity effects will effectively be eliminated. As mentioned earlier, the sample and hold drcuit 202 outputs this reference pressure signal 211 to a sealer constructed of resistors 217 and 219. These resistors 217 and 219 are used to scale the reference pressure 211 to provide the signal 218 described earlier as a predetermined fraction of the reference pressure signal 211. This predetermined fraction of the reference pressure signal 211 is shown as 403 inches of water in FIG. 3 at reference number 301.

Next, in step 405, the routine commands the vent valve 113 dosed. This is done by generating the vent valve control signal 134, that drives the control input 117 of the vent valve 113. This vent valve control signal 134 can be seen transiting to an on-state at reference time t2 in FIG. 3. This causes the vent valve 113 to dose, causing a decoupling between the input port 119 and the output port 115. This prevents the flow of any fresh air from an external atmosphere. Next, in step 407, a delay of one second is imposed. This is in order to allow the vent valve 113 to finish dosing. Of course, other times could be chosen.

Next, in step 409, the purge valve 121 is opened. This is done by generating the purge valve control signal 137 that drives the control input 125 of the purge valve 121.

This purge valve control signal 137 can be seen transiting to an on-state in FIG. 3 at reference time t3- This causes the purge valve 121 to open, causing a coupling between the input port 123 and the output port 127. Of course, one of ordinary skill in the art will recognize that alternatively a single signal could be used in place of the three independent signals 147, 134, and 137 controlling the sensor 139 the vent valve 113 and the purge valve 121 respectively.

This coupling between the input port 123 and the output port 127 of the purge valve 121 allows a gaseous path between the fuel tank 101 and the engine intake manifold 129. Because the engine intake manifold is connected to a running engine the pressure at the manifold is lower than the pressure in the fuel tank and a vacuum pressure is sourced into the evaporative emissions system. This causes a drop in pressure in the evaporative emissions system. This drop in pressure, after the engine intake manifold 129 is connected to the fuel tank 101, is called the vacuum change pressure. The result of this can be seen in the falling response of the absolute pressure signal 256 in FIG. 3. Because this absolute pressure signal 256 drives the input 227 of the comparator 223, the comparator output 229 will reflect a comparison of the predetermined fraction of the reference pressure signal 211, and the absolute pressure signal 256, or vacuum change pressure signal. As long as the vacuum change pressure signal is greater than the predetermined fraction of the reference pressure signal 211 the output of the comparator 229 will remain in a logical low state. If the vacuum change pressure signal falls below this predetermined fraction of the reference pressure signal 211 then there is no significant leak and the output of the comparator transits to a logical high state.

Next, in step 411 a predetermined time, in this case 5 seconds dapses. This allows the pressure in the evaporative emissions system to drop further. Note that this predetermined time is independent of any actual leakage in the fuel tank.

Next, in step 413, the test signal 147 is commanded to transit to the off-state. In FIG. 3 this is shown at reference time t4. If the pressure in the evaporative emissions system, as measured by the absolute pressure sensor 201, has not fallen below the predetermined fraction of the reference pressure, in this case 403 inches of water, then a fault indication is provided at the output 133 the pressure sensor 139.

This is accomplished by transferring the logic state of the comparator's output 229 to the sensor's output 133. When the test signal 147 transits to the off-state at reference time t4 the dodc input 233 of the D-type flip-flop 231 responds by transferring the complement of the data input 206, which represents the logical state at the output of the comparator 229, to the output 225. In the case of a leak in the evaporative emissions system, the absolute pressure signal 256, or vacuum change pressure, will not fall as much as expected. This condition is shown in FIG. 3 by reference number 303 where the vacuum change pressure only fell to 404.5 inches of water by reference time t4. In this case, the input 227 to the comparator 223 will be greater than the input 218 of the comparator. This will cause the comparator output signal 229 to remain in a logical-low state. Because this logical low-state is provided to the data input 206 of the D-type flip-flop 231 when the test signal 147 transits to an off-state, the output 225 of the D-type flip-flop 231 will provide a fault signal 144. This is indicated by the logical-high state as shown by reference number 309 in FIG. 3.

If there is no leak in the evaporative emissions system the absolute pressure signal 256, or vacuum change pressure will fall at least as much as expected. This is shown by reference number 307 in FIG. 3. Because of this, the fault signal 144 will remain in a logical-low state, as shown by reference number 305 in FIG. 3, indicating no leak.

Next, in step 415 the microcontroller in the control module 131 reads the fault signal 144 and if a fault is indicated, by a logical-high state, the evaporative emissions system leak indicator lamp 157 is driven on.

Next the evaporative emissions system is returned to a normal mode of operation. This is done by opening the vent valve in step 417, and dosing the purge valve in step 419, then exiting the leak test routine in step 421. The vent valve control signal 134 and the purge valve control signal 137 are shown to transit to a logical-low state at reference time .

FIG. 5 is a flow chart illustrating the alternative firmware for testing for leaks in an evaporative emissions system. In this alternative embodiment a conventional absolute pressure sensor is used as the pressure sensor 139, and the fault detection burden is shifted from the sensor's circuitry to the alternative firmware operating in the M68HC05B6 microcontroller. In step 501 the test routine is initiated. Next, in step 503, the absolute pressure is measured by inputting the absolute pressure signal 145 provided from the output 140 of the pressure sensor 139.

Next, in step 505, the vent valve is dosed as described earlier. Then, in step 507, the purge valve is opened, as described earlier. This provides a vacuum pressure from the engine intake manifold 129, via the evaporative e-nissions system components, to the fuel tank 101. This causes a reduction in pressure in the evaporative emissions system-hence the fuel tank 101.

Next, in step 509, the pressure in the fuel tank is remeasured, after a predetermined time interval, providing a vacuum change pressure. Then, in step 511, an indication is provided when the vacuum change pressure is greater than a predetermined fraction of the reference pressure, then exiting the leak test routine in step 513.

Of course, a time based strategy could also be applied as follows. FIG. 6 shows this time based strategy. In step 601 the test routine is initiated. Next, in step 603, the absolute pressure is measured by inputting the absolute pressure signal 145 provided from the output 140 of the pressure sensor 139.

Next, in step 605, the vent valve is dosed as described earlier. Then, in step 607, the purge valve is opened, as described earlier. This provides a vacuum pressure from the engine intake manifold 129, via the evaporative emissions system components, to the fud tank 101. This causes a reduction in pressure in the evaporative emissions system-hence the fuel tank. In step 609, the pressure in the fuel tank is continually measured, providing a vacuum change pressure, until the vacuum change pressure is less than a predetermined fraction of the reference pressure.

In step 611, an indication is provided when the time duration of the test in step 609 exceeds a predetermined time limit. This predetermined time limit is chosen to correspond to a leak rate indicative of a significant leak mechanism in the system. Then exiting the leak test routine in step 613.

In condusion, an improved evaporative emissions system leak test method has been described. This indudes new and useful apparatus to that end. This improved invention ensures that any errors due to temperature, linearity, or mounting stress sensitivity effects will effectively be eliminated. Additionally, an absolute pressure sensor is used to compensate for altitude effects, eliminating the need for a differential pressure sensor with its attendant inadequades.

What is daimed is:

Claims

1. An evaporative emission system leak test method comprising the steps of: providing a fuel tank containing a gaseous fuel material; measuring pressure of the gaseous fuel material in said fud tank and providing a reference pressure signal in accordance with said measured pressure; applying a vacuum pressure to said fud tank after providing said reference pressure; measuring pressure of the gaseous fud material in said fud tank after applying said vacuum pressure and providing a vacuum change pressure signal in accordance with said measured pressure; and providing an indication signal in response to a comparison of said vacuum change pressure signal and a predetermined fraction of said reference pressure signal, wherein said indication signal is indicative of a change of pressure in said fud tank and therefore a pressure leakage in said fuel tank.

2. A method in accordance with daim 1 wherein said comparison provided in said step of providing said indication signal occurs at a predetermined time after applying said vacuum pressure to said fud tank.

3. A method in accordance with daim 2 wherein said predetermined time is independent of pressure leakage in said fuel tank.

4. A method in accordance with daim 3 wherein said predetermined time is a fixed time.

5. A method in accordance with daim 1 wherein said step of providing said indication signal comprises determining a time it takes for said vacuum pressure to reach a level determined by and a predetermined fraction of said reference pressure signal.

6. A method in accordance with daim 5 wherein said comparison of said vacuum change pressure signal and a predetermined fraction of said reference pressure signal comprises comparing amplitude magnitudes of these signals.

7. A method in accordance with daim 6 wherein said signals, the amplitude magnitudes of which are compared in said indication signal providing step, comprise analog signals and wherein said comparison is implemented by an analog signal comparator.

8. A method in accordance with daim 5 wherein said step of providing said indication signal comprises comparing said time to a predetermined time limit to determine fuel tank leakage, wherein said predetermined time limit is representative of a predetermined leakage rate.

9. An evaporative emission system leak test method comprising the steps of: providing a fuel tank containing a gaseous fuel material; providing a filter connected to said fud tank for receiving the gaseous fuel material; providing a vacuum pressure source; providing a valve having a control port, said valve connected between said filter and said vacuum pressure source, wherdn the control port selectivdy enables a gaseous path between said fuel tank, induding said filter, and said vacuum pressure source; measuring pressure of the gaseous fuel material in said fuel tank and providing a reference pressure; providing a control signal to the control port of said valve enabling a gaseous path between said fud tank, induding said filter, and said vacuum pressure source; measuring pressure in said fuel tank a predetermined time, independent of leakage in said fud tank, after enabling the gaseous path and providing a vacuum change pressure; and providing an indication when the vacuum change pressure is greater than a predetermined fraction of the reference pressure.

10. An evaporative emission system leak test apparatus comprising: a fud tank containing a gaseous fuel material; vacuum means for applying a predetermined pressure; connection means, responsive to a control input, for selectivdy providing a coupling or a decoupling between said fuel tank and said vacuum means; and testing means for comparing a reference pressure measured while said connection means provides, responsive to a control signal provided by said testing means to the control input of said connection means, a decoupling between said fud tank and said vacuum means, to a vacuum pressure measured a predetermined time after said connection means provides, responsive to a control signal provided by said testing means to the control input of said connection means, a coupling between said fud tank and said vacuum means, and for providing a leak indication if the vacuum pressure measured is greater than a predetermined fraction of the reference pressure.

11. An apparatus in accordance with daim 10 wherein said vacuum means comprises an engine intake manifold.

12. An apparatus in accordance with daim 10 wherein said testing means for measuring pressure comprises an absolute pressure sensor.

13. An evaporative emission system leak test apparatus comprising: a fud tank having a supply port and a measurement port, said fud tank containing a gaseous fud material; a filter having an intake port, an output port and a fresh air port, wherein the intake port is coupled to the supply port of said fud tank for receiving the gaseous fuel material; a vent valve having a control port, an input port and an output port, wherein the output port is coupled to the fresh air port of said filter and wherein the control port enables coupling and decoupling between the input port and the output port; a purge valve having a control port, an intake port, and an output port wherein the input port is coupled to the output port of said filter and wherein the control port enables coupling and decoupling between the input port and the output port; an engine intake manifold having an input port connected to the output port of said purge valve; a pressure sensor coupled to the measurement port of said fud tank for providing an output responsive to the pressure in said fuel tank; and a control module having a first output coupled to and controlling the control port of said vent valve, a second output coupled to and controlling the control port of said purge valve an input coupled to the output of said pressure sensor, wherdn said control module measures a reference pressure provided at the output of said pressure sensor while the second output provides a control signal to the control input of said purge valve causing a decoupling between said fuel tank and the input port of said engine intake manifold, then measures a vacuum pressure provided at the output of said pressure sensor a predetermined time after the second output provides a control signal to the control input of said purge valve causing a coupling between said fud tank and the input port of said engine intake manifold, then provides a leak indication if the vacuum pressure measured is greater than a predetermined fraction of the reference pressure.

14. An apparatus in accordance with daim 13 wherein said pressure sensor is an absolute pressure sensor.

15. A pressure sensor for use in an evaporative emission system leak test system comprising: a pressure sensor having a sensing area and a pressure output indicative of a pressure applied to the sensing area; a sample and hold circuit having a signal input, a control input and an output, wherein the signal input is conneded to said pressure sensor for recdving the pressure output, and wherein the signal input, responsive to the control input, provides a signal representative of the pressure output provided by said pressure sensor to the output; a sealer having an input and an output, the input connected to the output of said sample and hold circuit, wherein the output provides a signal a predetermined fraction of the pressure output of said pressure transducer; a comparator having a first input, a second input and an output, the first input connected to the output of said sealer, the second input is connected to the pressure output of said pressure sensor; and a flip flop having a dock input, a data input, and an output, the data input conneded to the output of said comparator, the dodc input connected to the control input of said sample and hold circuit and providing a test signal input node, and the output for providing a fault indication signal responsive to a to be provided test signal for indicating when a pressure signal provided at the pressure output of said pressure sensor when the to be provided test signal transits to a logical low state is greater than a predetermined fraction of the pressure output of said pressure transducer provided at the output, of said sealer when the to be provided test signal is in a logical high state.

16. A sensor in accordance with daim 15 wherein said pressure sensor is an absolute pressure sensor.