Fuel vapour storage canister assembly

Abstract: A fuel vapour storage canister assembly to prevent breakthrough evaporative emission loss and facilitate purging by concentrating the fuel vapour close to the vent to the engine. The canister (12) is divided into two compartments (26,28) which communicate through a connective means (38) that only allows vapour into the second compartment when a control valve (18), located in the connective means, opens at a threshold pressure.

Description: This invention relates to vehicle fuel system evaporation loss control in general, and specifically to a fuel vapour storage canister assembly that has increased efficiency for vapour adsorption and vapour purge. Evaporative fuel vapours generated in a vehicle fuel system are adsorbed by activated carbon (charcoal) in a canister and later purged and consumed during engine combustion. During a typical period when the vehicle is parked with the engine off, called a soak, the canister becomes only partially loaded with hydrocarbon vapor, or partially saturated with hydrocarbon vapour from the fuel tank. The partially saturated canister may experience several hours of soak before it is purged or reloaded with vapours. Partially loaded or purged canisters left overnight have shown far less available vapour adsorption capacity and a higher tendency to have hydrocarbon vapour filter out of the canister through the atmospheric vent to yield breakthrough emissions, than similar canisters tested immediately. An example of the prior art, in accordance with the preamble of Claim 1, is disclosed in US-A-4280466. Recent studies have determined that the concentrated mass of vapours initially collects at the top of the canister, but with time is dispersed throughout the canister. After soaking, a canister will purge more slowly because the vapour is no longer concentrated in a small area. A fuel vapour storage canister assembly in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. The present invention is a compartmental evaporative canister and pressure control valve assembly. The pressure control valve isolates an auxiliary compartment of the canister from a main compartment. The main compartment contains one or more chambers which communicate with each other by restrictive passages. The top chamber of the main compartment is intended for storing daily vapour generation. The purpose of the auxiliary compartment is to reduce breakthrough emissions by preserving a portion of clean carbon. Since some vapour migration can occur through the compartment openings, the auxiliary compartment is isolated from the final chamber of the main compartment by using a pressure control valve. Installing a pressure control valve to isolate the bottom compartment further reduces breakthrough emissions and preserves the working capacity of a partially saturated canister. A pressure control valve similar to that disclosed in US patent no. 4,153,025 is currently used on some vehicles to reduce tank vapour generation. By installing the pressure control valve on the canister before the last compartment it can reduce both tank vapour generation and canister breakthrough emissions. Previous uses of compartmental canisters were to disperse evenly the fuel vapours to the lower chambers in order to utilize fully the whole canister or to separate the vapours from the fuel tank from those from the carburetor bowl as is US patent no. 4,203,401; US patent no. 4,308,840; and US patent no. 4,496,379. Although current evaporative systems incorporate a pressure control valve, it is used to separate the fuel vapours into individual canisters, to regulate fuel vapours from the tank to the canister, and to regulate the vapour back to the carburetor. The adverse effects of vapour migration and redistribution will become much more significant in larger canisters which may be used to store multiple diurnal emissions. Diurnal emissions are the loss of vapours from the tank resulting from the daily cyclic variations in tank temperature while the vehicle is at rest. Even though a large canister is employed for controlling multiple diurnal emissions and/or refueling emissions, only a small portion of the large canister will be utilized most of the time. Therefore, vapour migration and redistribution during soak can make it harder to purge the canister and may increase breakthrough emissions. Accordingly, the present invention has four preferred objects. 1) It reduces vapour migration throughout the canister. 2) It improves the purge rate. 3) It improves vapour adsorption on subsequent soaks. And, 4) it significantly reduces the chance of breakthrough emissions. The present invention will now be described, by way of example, with reference to the remainder of the specification, and the sole figure of the accompanying drawing which is a schematic view of a fuel vapour recovery system having a compartmental emission canister and a control valve between the two compartments of the canister, in accordance with the invention. In Figure 1 the preferred embodiment of the system comprises a fuel tank 10, and a canister 12, connected to the air induction system of a vehicle engine by conduits 14, 16, and 20. When the pressure of the air-fuel vapour mixture formed in fuel tank 10 exceeds the threshold pressure of a pressure control valve 18 the mixture is vented to canister 12 through conduit 14, where the fuel vapour component is stored in a manner more fully described below. When the vehicle is operating, engine vacuum from the air induction system opens the pressure control valve 18, allowing air flow through canister 12 to desorb the stored fuel vapours and send them back to the engine intake. The canister 12 has an exterior housing 24 of moulded plastic, which encloses an interior volume, charged with activated charcoal granules, or the like, which are capable of adsorbing the fuel portion of an air-fuel vapour mixture that is fed through canister 12. The interior volume is partitioned horizontally into a main compartment 26 and an auxiliary compartment 28. The main compartment 26 is substantially larger than the auxiliary compartment 28 and has at least two chambers 26a and 26b. Fuel vapour vented through the conduit 14 enters the canister 12 through an inlet/outlet aperture 30 and into the first chamber 26a. A partition 32 divides the two chambers 26a, 26b of the main compartment 26 and has a passage 34 that allows vapour to pass between the two chambers. The passage 34 restricts the migration of the fuel vapour to the last chamber 26b of the main compartment 26, thereby providing a more efficient desorption of the canister 12 during purge by keeping the fuel vapour concentrated near the inlet/outlet aperture 30 in the first chamber 26a. A partition 36 separates the last chamber 26b of the main compartment 26 and the auxiliary compartment 28. A connective means 38 joins the last chamber 26b and the auxiliary compartment 28. The pressure control valve 18 is positioned in the connective means 38 and opens when the pressure in the main compartment 26 reaches a threshold level during the soaking period. A diaphragm 40 in the pressure control valve 18 is biased by spring 42 to close the connective means 38 between the main compartment 26 and auxiliary compartment 28, and thereby obstruct vapour migration between the two compartments. During the soak, the pressure in the fuel tank 10 and main compartment 26 will reach a threshold to cause the diaphragm 40 to compress spring 42 and allow flow from the main compartment 26 to the auxiliary compartment 28. The diaphragm 40 will close the connective means 38 once the pressure has been relieved. Because vapour cannot migrate from main compartment 26 to auxiliary compartment 28 when diaphragm 40 closes connective means 38, the auxiliary compartment 28 remains substantially clean of fuel vapour, and therefore essentially eliminates breakthrough emissions through an atmospheric vent 22, which is located at the bottom of auxiliary chamber 28 through the exterior housing 24. The pressure control valve 18 also responds to vacuum from the manifold at port 44. During engine operation when the port 44 is subjected to the vacuum conditions below throttle blade 46, the pressure differential across the diaphragm 40 will be sufficient to overcome the bias of the spring 42 and open. At the same time, vacuum applied to inlet/outlet aperture 30 induces air flow through the atmospheric vent 22. The air will flow successively through the auxiliary compartment 28, the pressure control valve 18 in the connective means 38, the chambers 26b, 26a of the main compartment 26, and out through the inlet/outlet aperture 30 in the first chamber 26a. A purge solenoid 50, normally closed when the engine is not running, opens to return the vapour to the intake of the engine by means of conduit 16. The purge solenoid 50 does not form part of the invention as such, but would generally be present.

Claims: A fuel vapour storage canister assembly for an engine fuel system, comprising a main compartment (26) for vapour storage; an auxiliary compartment (28) for vapour storage; an inlet/outlet aperture (30) to the main compartment; an atmospheric vent (22) opening to the auxiliary compartment; and a connective means (38) between the main compartment and the auxiliary compartment; characterised by a pressure control valve (18), comprising a diaphragm (40) and spring (42), in the connective means between the main compartment and the auxiliary compartment, wherein the pressure control valve is biased to prevent vapours from flowing from the main compartment into the auxiliary compartment, and is responsive to pressure in the main compartment to permit the diaphragm to unseat to allow the connective means to be open when the pressure in the main compartment reaches a selected level, whereby substantially all vapour is collected in the main compartment. A fuel vapour storage canister assembly as claimed in Claim 1, comprising a generally elongated, adsorbent filled housing (24) having the main compartment (26) and the auxiliary compartment (28) therein. A fuel vapour storage canister assembly as claimed in Claim 2, wherein the housing (24) has a volume partitioned either vertically or horizontally into a plurality of separate chambers (26a,26b) that communicate with each other in succession to compose the main compartment (26); wherein the volume of the auxiliary compartment (28) is smaller than the volume of the main compartment; wherein the inlet/outlet aperture (30) opens to the first chamber of the main compartment; and wherein the connective means (38) is between the last chamber of the main compartment and the auxiliary compartment. A fuel vapour storage canister assembly as claimed in Claim 1, wherein the main compartment (26) is adsorbent filled and comprises multiple separate chambers (26a,26b) that communicate with each other in succession to compose the main compartment; wherein the auxiliary compartment (28) is adsorbent filled and has a volume which is smaller than the volume of the main compartment; wherein the inlet/outlet aperture (30) opens to the first chamber of the main compartment; and wherein the connective means (38) is between the last chamber of the main compartment and the auxiliary compartment. A fuel vapour storage canister assembly as claimed in any one of Claims 1 to 4, wherein the pressure control valve (18) comprises a vacuum orifice, in the connective means (38) between the main compartment (26) and the auxiliary compartment (28); a vacuum line (20) being connectable to the induction system of the engine and joining the pressure control valve at the vacuum orifice; thereby improving purging capabilities during which engine vacuum acts upon the spring (42) in the pressure control valve to unseat the diaphragm (40) which opens the pressure control valve and draws in air through the atmospheric vent (22) which successively flows into the auxiliary compartment (28), through the pressure control valve, and into the main compartment (26) to retrieve the hydrocarbon vapours and return them to the engine. A fuel vapour storage canister assembly as claimed in any one of Claims 1 to 5, wherein the inlet/outlet aperture (30) is adapted for connection to a fuel tank (10) and a source of engine vacuum.

Inventors: REDDY S RAGHUMA, REDDY, S. RAGHUMA
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1992
Main classification: F02M25
IPC codes: F02M25