CANISTER PURGING FOR PLUG-IN HYDRID ELECTRIC VEHICLES

A method for desorbing fuel vapors from a canister of a vehicle having an internal combustion engine and a fuel tank includes detecting the temperature within the interior of the fuel tank using a temperature sensor positioned within the fuel tank. The canister is in fluid communication with the fuel tank and an intake manifold of the engine of the vehicle. A vapor bypass valve is positioned along a flow line between the fuel tank and the canister. The vapor bypass valve is opened if the temperature inside the fuel tank falls below a pre-determined value. The low pressure region created within the fuel tank due to fall in temperature is utilized to route ambient air along with the fuel-vapors contained within the canister, towards the fuel tank, through the opened vapor bypass valve.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Definitions: The following definitions are used in this disclosure.

Artificial vacuum: A vacuum created by a vacuum pump or other mechanical device.

Communication or fluid communication: A mechanism by which gas or liquid or both can travel from one point to another. Commonly, this might be effected by tubing or piping.

Natural vacuum: A vacuum created by the contraction of gas, liquid or both as a result of decrease in ambient temperature. No mechanical vacuum pump or other mechanical method is used to generate a natural vacuum. However, mechanical valves can be used to control a natural vacuum.

The following detailed description illustrates aspects of the disclosure and its implementation. This description should not be understood as defining or limiting the scope of the present disclosure, however, such definition or limitation being solely contained in the claims appended thereto. Although the best mode of carrying out the invention has been disclosed, those in the art will recognize that other embodiments for carrying out or practicing the invention are also possible.

Carbon canisters collect vapor emerging from the fuel tank to prevent vapor from escaping into the environment. The canister is generally filled with charcoal or activated carbon, to adsorb fuel vapor. The canister receives vapor inputs from the fuel tank, retains those vapors for a time, and then outputs them to the engine intake manifold. Flow lines extend from the fuel tank to the canister input port, and from the canister output port to the intake manifold. A canister purge valve lies in the line between the intake manifold and the canister, and a vapor bypass valve lies in the flow line between the canister and the fuel tank. A third line connects the canister to ambient atmosphere, through a canister valve solenoid. The activated carbon within the canister captures fuel vapor emerging from the fuel tank. When the engine starts, suction within the engine intake manifold opens the canister purge valve and polls fuel vapor accumulated within the canister into the intake manifold, where it is burned. As is well known, plug-in hybrid electric vehicles (PHEV's) minimize engine running time, which has the effect of minimizing canister purging Running the engine solely to purge the canister may waste substantial fuel. On the other hand, accumulation of fuel vapor in the canister beyond a certain level presents an environmental hazard. Diurnal temperature variation may increase pressure within the fuel tank as the day heats up, forcing fuel vapors into the canister. If the canister becomes overloaded, fuel vapor may vent to the atmosphere.

Environmental regulations are steadily tightening the standards for vehicle vapor emissions. Environmental authorities in certain regions, such as California, typically require less than about 500 mg of hydrocarbons released as vehicle evaporative emissions in a standard 3 day test. Given other sources of emissions, the standard effectively limits canister emissions to less than about 200 mg. Euro 5/6 regulations enforce a limit of about 2 grams of evaporative emissions per day. Such stringent conditions demand a highly efficient and effective evaporative emission control system, which in turn requires regular canister purging.

The present disclosure provides a more efficient and fuel-economic emission control system as well as a method for controlling fuel-vapor emissions in a plug-in hybrid electric vehicle. Accumulated vapors within the vehicle's carbon canister can be easily desorbed without the need for the engine of the vehicle to be turned on. The disclosed emission control system utilizes the low pressure region created naturally within the fuel tank of a vehicle at certain times of the day to desorb fuel vapors from the canister and route the vapors back to the fuel tank.

FIG. 1is a schematic view of an evaporative emission control system100for a vehicle, configured to facilitate purging fuel vapors from a carbon canister. As shown, the system includes a carbon canister102linked by vapor flow lines126with a fuel tank114and the vehicle's intake manifold115. O. A normally closed vapor bypass valve (VBV)122lies in the line between the canister102and the fuel tank114and canister purge valve118, also normally closed, lies in the line between the canister102and intake manifold115. A fresh air line138, controlled by normally open canister valve solenoid (CVS)137, opens to the atmosphere. A programmable control module125is coupled to CPV118, VBV122, and CVS1372effect their operation. A fuel pump110impels fuel from the tank114to intake manifold115through a fuel line, not shown.

A temperature sensor106is also positioned within fuel tank114, to measure the temperature there. Though only one temperature sensor106is shown, multiple such sensors may be disposed within the fuel tank114. An average of the temperature values detected by those sensors can be taken in some embodiments, to obtain a more precise measure of the temperature within the interior of the fuel tank114.

Evaporative emission control system100operates as follows. As noted above, CPV118and VBV122are normally closed. Thus, canister102is generally sealed off from both of the fuel tank114and the intake manifold115. As the engine starts, CPV118opens, and the suction created within intake manifold draws air through normally open CVS139, through fresh air line138and canister102, and then on through flow line126and CPV118, and into intake manifold115. As the fresh air passes through canister102, hydrocarbons accumulated in the activated carbon are desorbed and entrained by the airflow. These hydrocarbons accompany the air into the intake manifold115, and into the engine (not shown), where they are burned. This purging action can only occur, of course, when the engine is running.

As known by those in the art, an overall goal of evaporative emission control system100is to compensate for the expansion and contraction of fuel vapors, without allowing any of those vapors to escape to the atmosphere. The valves disclosed here allow the fuel system to be considered as a sealed system, and thus the behavior of the fuel vapor can be modeled by the ideal gas equation, PV=nRT. In general terms, as the temperature within fuel tank114rises, either due to an increase in ambient temperature or operation of the vehicle itself, fuel vapor134will tend to expand, while an opposite change in temperature leads to contraction.

The temperature within fuel tank114is continuously monitored by PCM125, which receives temperature signals from the temperature sensor106. By determining the volume of vapor space137(derived from the known fuel level) knowing the temperature allows calculation of pressure levels. Here, reduced temperature within fuel tank114indicates a low pressure within vapor space137. When pressure is sufficiently low that suction is created within vapor space137, VBV122is opened, this time allowing fresh air to flow through CVS139, through canister102, and into fuel tank114. There, fuel vapor134is subjected to sufficiently low pressure that it condenses back into liquid form. In this manner, canister102is purged of accumulated hydrocarbons.

Effectively, the present disclosure eliminates the need to rely on the engine to purge canister102. The evaporative emission control system100automatically purges fuel-vapors from the canister102by routing them towards the fuel tank114regularly, whenever the temperature within the fuel tank114falls.

FIG. 2is a chart illustrating the diurnal range of temperature variation inside the vehicle fuel tank. As would be expected, temperature begins rising around dawn and keeps rising until around sunset. From that high point, temperature falls steadily until the sun reappears. It will be understood that the temperature curve ofFIG. 2assumes that the automotive vehicle spends most of its time outdoors. No matter where the car is housed, however, its ambient temperature will follow some sort of diurnal cycle up and down

In this illustration, from midnight till early morning hours. The temperature may eventually fall from 50° F. to about 44° F. on a summer day. However, the range of variation may be substantially different from the illustrated temperature curve, based on the climatic conditions of a specific region. From morning hours, till the afternoon, the temperature in this illustration may eventually rise to about 58° F., and thereafter, starts falling again.

A drop in temperature is accompanied by a vacuum buildup within the fuel tank, while a rise in temperature has the opposite effect, increasing pressure within the tank. In an embodiment, the pre-determined temperature of the fuel tank, below which enough suction pressure is created within the fuel tank for drawing vapors from the canister, may be varied between about 45° F.-50° F. When the temperature sensor in the fuel tank detects that the fuel tank's temperature is below the chosen pre-determined temperature, the programmable control module opens the vapor bypass valve. Otherwise, vapor bypass is kept closed.

FIG. 3illustrates the steps involved in an exemplary method according to the present disclosure, for purging the carbon canister of a plug-in hybrid electric vehicle, using low pressure region created within the vehicle fuel tank. At step302, the method involves detecting temperature within the vehicle fuel tank. At step306, the method involves checking whether the detected temperature is below a threshold predetermined temperature value. If “yes”, then at step310, the PCM opens the vapor bypass on the flow line between the canister and the fuel tank. This allows ambient air along with fuel-vapors accumulated in the canister to be sucked into the fuel tank. If “no” at step310, the method loops back to step302and detection of the temperature of the fuel tank continues.

At step314, once the vapor bypass has been opened, the method has continuously checking of whether temperature within the fuel tank has risen again above the pre-determined value. If “yes”, then at step318, the programmable control module closes the vapor bypass. However, if “no”, then the VBV is kept open.

The method and the system of the present disclosure can be used for any vehicle, preferably for a hybrid vehicle, most preferably for a plug-in hybrid electric vehicle, including a car, an SUV, a truck, etc. The method is much more efficient and does not waste fuel and energy by requiring the vehicle's engine to be turned on for purging hydrocarbon vapors from the carbon canister of a vehicle.

Although the current invention has been described comprehensively, in considerable detail to cover many possible aspects and embodiments, those skilled in the art would recognize that other versions of the invention are also possible.