Fuel purge control

Details Fuel purge control Fuel purge control

1999-01-07

2001-10-23

Miller, Carl S. (Department: 3747)

Internal-combustion engines

Charge forming device

Having fuel vapor recovery and storage system

C123S516000

Reexamination Certificate

active

06305360

ABSTRACT:

This invention relates to the control of the purging of fuel from the fuel vapour collection device for an internal combustion engine.
The current emission regulations in many countries require the evaporative emissions from the fuel supply system of the internal combustion engines of motor vehicles to be controlled to thereby eliminate or substantially reduce the amount of fuel released into the atmosphere by such vapours. Accordingly, it is normal practice to fit a fuel vapour collection device to the vehicle to adsorb evaporative emissions from the fuel supply system under all conditions that the vehicle experiences. This fuel vapour collection device is usually of the activated carbon type and is commonly referred to as the “carbon canister”. Such a fuel vapour collection device operates on the principle of physical adsorption of fuel vapour into the activated carbon.
The fuel vapour collection device has a limited capacity for storing fuel and must therefore be purged to some extent of its contents in the course of vehicle operation. The accumulated fuel is normally purged into the intake manifold of the engine by air drawn through the fuel vapour collection device, the purged fuel being subsequently combusted by the engine. The amount of fuel vapour being purged from the fuel vapour collection device can however vary significantly for any given purge air flow rate generally depending on saturation level in the fuel vapour collection device. As the amount of purged fuel is not normally measured in systems not having an air/fuel ratio feedback mechanism (commonly known as open loop systems), the engine control system cannot compensate for the increased fuelling rate to the engine. This can cause an increase in the engine torque which results in a higher engine speed at idle or an increase in the vehicle speed off idle. Under severe conditions, the engine operation can become unstable because the actual air fuel ratio within the engine cylinders is markedly different from the air fuel ratio mapped by the engine control system.
One proposal to deal with this problem is described in the applicant's U.S. Pat. No. 5,245,974. This document shows an internal combustion engine installation having a fuel vapour collection device for removing the fuel vapour from the evaporative emissions generated within the fuel supply system. The engine includes a fuel injection system with an air compressor supplying compressed air to the fuel injection system. The fuel vapour collection device is periodically purged of accumulated fuel by drawing air through the fuel vapour collection device using the air compressor. The air compressor then supplies the air which now carries the fuel to the fuel injection system where the air is subsequently injected into the combustion chambers of the engine resulting in combustion of the purged fuel. Although the stratification within the cylinder will remain largely unaltered by the addition of the purged fuel through the injector, this patent does not particularly address the problem of lack of knowledge of the amount of fuel being supplied from the fuel vapour collection device.
It would be advantageous to provide a system which can control the air flow rate through the fuel vapour collection device to optimise the amount of fuel that is purged from the fuel vapour collection device without jeopardising engine operation.
With this in mind, it is an object of the present invention to provide an improved method and control system for controlling the air flow rate through an fuel vapour collection device for an internal combustion engine.
According to one aspect of the present invention, there is provided a method for controlling delivery of fuel from a vapour collection device to the combustion chamber of an internal combustion engine to thereby purge fuel that has accumulated in the vapour collection device by means of a purge flow passing from the vapour collection device to the engine, the purge flow rate being varied by means of a flow control valve located between the vapour collection device and the engine, the method including determining a minimum valve signal value and a maximum valve signal value as a function of the engine load and engine speed to thereby respectively define the minimum and maximum extent of valve signal values for controlling the opening of the valve, selecting either the minimum or maximum valve signal value or an interpolation between the maximum and minimum values in dependence on the engine operating conditions to optimise the amount of fuel purged from the vapour collection device to the engine under varying engine operating conditions.
This method enables at least substantially continuous purging of the fuel vapour collection device and enables the amount of fuel purged from the fuel vapour collection device to the engine to be optimised for varying operating conditions of the engine.
The fuel vapour collection device may be in communication with an intake manifold of the engine and the method can therefore control the amount of fuel purged into the intake manifold. The pressure difference between the fuel vapour collection device and the intake manifold may be sufficient to enable air to be drawn through the fuel vapour collection device to the intake manifold. The present method may however also be used in other arrangements, for example when there is purging through an air compressor as described in U.S. Pat. No. 5,245,974 referred to above.
The method may be implemented by a variable valve for controlling the air flow rate from the fuel vapour collection device and a control means for controlling the valve as a function of the engine operating conditions. The control means may be in the form of an electronic control unit (ECU) for providing the valve with the required valve signal values for controlling the progressive opening anrd closing of the valve. A given valve signal may correspond to a given valve position.
The ECU may include at least two “look-up” maps for mapping the valve signal values for controlling the valve as a function of engine operating conditions. Each look-up map may provide valve signal values against the coordinates of fuel per cycle (FPC) and engine speed (RPM). One of the maps may be a “minimum” map which maps the valve signal values when the amount of purge air flow to the engine is required to be at a minimum level. This situation can for example arise when air purged from the vapour collection device is very rich in fuel vapour. Another map may be a “maximum” map mapping the valve signal values when the purge air flow to the engine can be maximised. This takes into account situations where the air fuel ratio of the purge air is relatively low and the engine is operating at medium to high loads.
The minimum and maximum maps may therefore respectively define the minimum and maximum range of valve signal values for controlling the opening of the valve and therefore the purge air flow rate, the opening of the valve progressively increasing with increasing valve position values. The valve signal value may be obtained from either of these maps or from an interpolation between these maps in dependence on the engine operating conditions. The interpolation amount may be provided by an adaption value. The adaption value may be provided by an arbitrary value system which assigns a proportion of each of the minimum and maximum values to create a total value for valve position determination according to given condition. This adaption value may lie within the range of 0.0 to 1.0, with the 0.0 value corresponding to the minimum map and the 1.0 value corresponding to the maximum map.
An adaption value look-up map may map the adaption values as a function of the engine coolant temperature. Upon first starting up the engine, the water temperature may be measured and an initial adaption value obtained from the adaption value map. This ensures that if the engine is starting hot, the adaption value may be relatively low to restrict the flow of purged air. Under hot starting conditions, relatively large amo

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