Purge fuel flow rate determination method

Details Purge fuel flow rate determination method Purge fuel flow rate determination method

2001-03-22

2002-09-10

Argenbright, Tony M. (Department: 3747)

Internal-combustion engines

Charge forming device

Including exhaust gas condition responsive means

C123S519000, C123S533000

Reexamination Certificate

active

06446618

ABSTRACT:

The present invention generally relates to the control of fuel vapour generated within an internal combustion engine installation, and in particular to a method for determining the amount of fuel vapour purged from a fuel vapour collection device of the engine installation.
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 onto the activated carbon.
The fuel vapour collection device generally has a limited capacity for storing fuel vapour and must therefore be purged to some extent of its contents in the course of vehicle operation. The accumulated fuel vapour is normally purged into the intake manifold of the engine by way of air drawn through the fuel vapour collection device, the purged fuel vapour being subsequently combusted within 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 the saturation level in the fuel vapour collection device. As the amount of purged fuel vapour is typically not able to be measured in systems not having an air/fuel ratio feedback mechanism (commonly known as open loop systems), the engine control system for such open loop systems generally cannot compensate for the increased fuelling rate to the engine. This can cause an increase in the engine torque which may result 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.
In the Applicant's U.S. Pat. No. 5,245,974 there is described a fuel vapour control system for an internal combustion engine, the details of which are incorporated herein by reference. This document discloses 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 dual fluid 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 vapour by way of drawing air through the fuel vapour collection device using the air compressor. The air compressor then supplies the air which now carries the fuel vapour 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 vapour. 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.
A proposal for dealing with this problem is described in the Applicant's International Publication No. WO 00/01663, the details of which are also incorporated herein by reference. This document describes a method of controlling the flow rate of a purge flow passing through a fuel vapour collection device by controlling the opening of a flow control valve located between the vapour collection device and the engine. The method controls the flow control valve as a function of engine operating conditions. However, the described method does not actually determine the amount of fuel vapour in the purge flow to the engine. An iterative method for providing an estimation of the fuel flow rate based on empirical data is actually used. This application also describes a method of determining the amount of fuel vapour being purged during closed loop operation of the engine. The engine is typically operated in this manner when the engine is at idle. It may also be possible to operate the engine under closed loop control when operating at stoichiometric air/fuel ratio conditions. However, at other engine loads such as at partial loads, it is necessary to operate the engine under open loop control where the fuel purge flow rate cannot be directly determined.
It would therefore be advantageous to be able to determine the actual amount of fuel within the purge flow to the engine under most, if not all engine operating conditions.
With this in mind, an object of the present invention is to provide an improved method for determining a purge fuel mass flow rate from a fuel vapour control system to an internal combustion engine, at least under most engine operating conditions.
According to the present invention, there is provided a method for determining a purge fuel mass flow rate from a fuel vapour control system to an internal combustion engine having a compressor for delivering purge gas from the fuel vapour control system to the engine, the method including:
determining the temperature rise of the purge gas passing through the compressor;
determining the specific heat ratio of the purge gas as a function of the temperature rise; and
determining the purge fuel mass flow rate as a function of the specific heat ratio of the purge gas.
Preferably, the fuel vapour control system includes an air/fuel separation means for collecting fuel vapour generated within the engine. Preferably, the compressor is arranged to deliver purge gas from the air/fuel separation means to the engine. It is however contemplated that the compressor may deliver to the engine purge gas or fuel vapour generated or present anywhere within the engine.
As the purge fuel mass flow rate is determined as a function of the temperature rise of the purge gas as it passes though the compressor, the determination of the purge fuel mass flow rate is independent of the engine operating conditions. Therefore, the purge fuel mass flow rate can be determined under most, if not all engine loads and speeds.
The specific heat ratio of the purge gas varies in dependence on the purge fuel concentration of the purge gas. Further, the specific heat ratio of purge fuel is significantly different from the specific heat ratio of air. For example, the specific heat ratio of air is about 1.4, whereas typical purge fuel species such as C
3
H
8
, C
4
H
10
, and C
5
H
14
have specific heat ratios of between 1.06 to 1.11. Generally, the higher the molecular weight of the gas, the lower the value of the specific heat ratio.
Therefore, as the concentration of purge fuel or fuel vapour within the purge gas increases, the specific heat ratio of the purge gas decreases. Hence, by monitoring the change in the adiabatic temperature rise of the gas passing through the compressor, the purge fuel mass flow rate may be determined. That is, as the compressor goes from delivering solely air to the engine to delivering both air and purge fuel, the specific heat ratio of the gas passing through the compressor will change.
Most positive displacement compressors provide approximately adiabatic compression of gas passing through the compressor. However, the compressor can never provide truly adiabatic compression under actual operating conditions due to heat losses within the compressor and in general a real compressor provides compression which is neither adiabatic nor isothermal. It can be modelled however by using the polytropic compression equation:
T
OUT
=T
IN
×PR
nl(n−l)
where
T
OUT
is the compressor discharge temperature;
T
IN
is the compressor inlet temperature;
PR is

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