Field of the Invention
This invention relates to a vehicle fuel system
with on-board diagnostics for vapour integrity testing.
Background of the Invent ion
Vehicle fuel systems are required to control emission of
fuel vapour. This is done by collecting vapour emitted from the fuel tank in a purge
canister containing carbon to absorb the vapour. The canister is purged of collected
vapour when the engine is running by drawing air through the canister into the engine,
relying on manifold vacuum. The system is sealed except for venting to the atmosphere
via the purge canister. On-board vapour integrity testing is required so that a
warning is given if vapour loss from the sealed system exceeds predetermined levels.
Typical known vapour integrity testing systems are described
US patents 5,333,590
The latter patent describes a basic test in which the manifold
vacuum is used to pump out the fuel tank and the return of tank pressure to atmospheric
("bleedup") is monitored. If bleedup exceeds a certain threshold value R the system
is determined to have an unacceptable vapour integrity. If the bleedup is less than
R, it assumed that vapour integrity is acceptable. Low level loss of vapour integrity
cannot be reliably detected with this basic system because vapour generation from
fuel in the tank can cause pressure in the evacuated system to recover more rapidly
than air ingress due to a low level loss of vapour integrity.
In addition, the bleedup for a particular level of vapour
integrity depends on vapour volume, that is the volume of free space above the fuel
tank and in the purge canister and connecting passages. Vapour volume is itself
directly related to fuel level.
Thus, in order to improve the sensitivity of the basic bleedup test, measures must
be taken to correct for different operating conditions, particularly the fuel level
and the rate of vapour generation in the tank.
US patent 5,333,590
uses a threshold value R which is not fixed but is related to vapour volume
and fuel temperature.
US patent 5,680,849
adjusts a leakage occurrence threshold in dependence on pressure sensor
precision, pressure leakage in the canister valve, the fuel tank capacity and the
level of remaining fuel in the tank.
It is also known to improve the sensitivity of vapour integrity
testing by using a two stage test. The first stage is a bleedup test in which pressure
increase over a certain period (period_A) is measured. A second stage is carried
out in which pressure rise of the closed system from atmospheric over a second period
(period_B) is monitored. The second stage gives an indication of vapour generation
in the tank under prevailing conditions. A constant scaling factor is used to deduct
a proportion of pressure rise found during the second stage to provide a value which
more closely represents the level of bleedup due to air ingress into the tank during
the first stage of the test.
Summary of the Invention
A source of error that is not dealt with in the existing systems described above
arises from variations in temperature of the gaseous contents of the tank at the
start of bleedup, due in the main to variations in the evacuation. Evacuation results
in the temperature of the vapour contents being reduced below ambient temperature
by an amount which depends on the nature of the evacuation (fast, slow, early or
late). Without any compensation for such temperature variation, a worst case error
is may be equivalent to a hole diameter of around 0.5mm. Errors of this magnitude
are not acceptable when small leaks equivalent to a 0.5 mm diameter hole are required
to be detected.
According to the present invention a vehicle fuel system
with on-board diagnostics for vapour integrity testing comprises:
characterised in that the ECU is adapted to measure pressure in the tank at intervals
during the evacuation phase and to make a correction to the vapour integrity indication
by using an algorithm for calculating temperature variations and based on the values
and timing of the pressure measurements made during the evacuation phase, the correction
being effective to reduce errors in the vapour integrity indication due to temperature
variations in the air/vapour in the tank at the commencement of bleedup due to variations
in the evacuation phase.
- a) a fuel tank for containing fuel for delivery to an internal combustion engine;
- b) a purge canister connected to the space in the tank above the fuel;
- c) a canister vent valve (CVV) for connecting the purge canister to the atmosphere;
- d) a purge valve for connecting the purge canister to the engine; and
- e) an electronic control unit (ECU) arranged for monitoring pressure and fuel
level in the tank and other engine, vehicle and ambient conditions and for controlling
opening and closing of the valves;
- f) the CVV and the purge valve adapted to be controlled by the ECU for venting
the tank to atmosphere via the purge canister (purge valve closed, CVV open), and
for purging vapour from the canister by allowing air to be drawn through the canister
by manifold vacuum (both valves open);
- g) the ECU being arranged to carry out a periodic vapour integrity test, when
the engine is running;
- h) the vapour integrity test adpated to:
- i) evacuate the tank with the purge valve open and the CVV closed (evacuation
- ii) monitor pressure rise in the tank with both valves closed (bleedup phase);
- iii) developing an indication of vapour integrity from time and pressure values
measured during the bleedup;
The improved fuel system test contemplated by the invention
is preferably implemented using the vehicle's existing electronic engine control
unit and the fuel system pressure sensor which is used for other purposes. As a
consequence, the benefits of the invention may be obtained at very little additional
These and other features and advantages of the present
invention may be better understood by considering the following detailed description
of a preferred embodiment of the invention.
Brief description of the drawings
The invention will now be described further, by way of
example, with reference to the accompanying drawings, in which:
Description of the Preferred Embodiment
- Figure 1 is a schematic diagram of a vehicle fuel system with on-board diagnostics
for vapour integrity testing which utilises the principles of the invention;
- Figure 2 is a graph of the pressure changes which take place in a first stage
of the vapour integrity test carried out in the system shown in Figure 1;
- Figure 3 is a graph of the pressure changes which take place in a second stage
of the vapour integrity test carried out in the system shown in Figure 1; out in
the system shown in Figure 1, illustrating the effect of an early slow or late rapid
- Figure 5 is a graph of the pressure changes which take place in a first stage
of the vapour integrity test carried out in the system shown in Figure 1, illustrating
the effect of an evacuation that results in the tank pressure being held at low
pressure for a longer period.
A two stage diagnostic procedure for vapour integrity testing
is performed automatically at predetermined intervals by an electronic control unit
(ECU) 10 seen in Fig. 1. The test is aborted if prevailing conditions (fuel sloshing,
heavy acceleration etc) are such that a reliable test result cannot be expected.
The ECU 10 is connected to a fuel sender 11 for sensing
the level of fuel 12 in a fuel tank 13, an ambient temperature transducer 14, and
a fuel tank pressure transducer 15.
The ECU controls a vapour management valve (VMV) 16 and
a normally open canister vent valve (CVV) 18. The CVV controls the air flow through
a filtered passageway 19 which connects a purge canister 20 containing charcoal
for absorbing fuel vapour to an atmospheric vent 22. The VMV 16, when open, connects
the purge canister 20 to the intake manifold 17 of the vehicle engine via lines
38 and 39.
The closed fuel system seen in Fig. 1 further includes
a vacuum/pressure relief valve within a cap 25 which closes the fuel inlet passageway
26 of the fuel tank 13. A passageway 30 extends from a rollover valve 31 at the
top of the tank 13 to both the purge canister 20 and the VMV 16. A running-loss
vapour control valve 32 connects the passageway running-loss vapour control valve
32 connects the passageway 30 to the upper portion of the fuel inlet passageway
26 via a branch passageway 33.
When the vehicle engine in not running the ECU closes the
VMV 16 and opens the CVV 18 so that fuel vapour is absorbed by carbon in the purge
canister before reaching the atmosphere. Moreover, air may enter the fuel system
via the purge canister 20 if pressure in the tank falls below atmospheric due to
condensation of vapour. When the engine is running, the ECU from time to time opens
both VMV 16 and CVV 18 so that air is drawn through the purge canister by manifold
vacuum to purge fuel vapour from the canister.
The diagnostic vapour integrity testing procedure takes
place in two stages. In stage A the pressure changes in the tank 13 as measured
by the pressure sensor 15 are illustrated in Figure 2. During an evacuation phase
34 the ECU closes the CVV 18 and opens the VMV 16 so that air and vapour are pumped
out of the tank 13 and canister 20 by manifold vacuum until a desired pressure p1
is achieved. The evacuation phase is followed by a holding stage 35 of several seconds.
After the holding phase, the ECU closes both the VMV 16 and the CW 18, sealing the
system. The tank pressure as indicated by the pressure sensor 15 is monitored by
the ECU during a bleedup phase 36. At the point in time that the tank pressure recovers
to p2, the ECU starts counting out period_A, monitors the pressure p3 at the end
of period_A and calculates and saves the pressure difference dP_A = p2 - p3.
In stage B, which may take place before or after stage
A, the pressure changes in the tank 13 are as illustrated in Figure 3. After initial
venting 37 to allow the pressure to go to atmospheric, the ECU closes both the CVV
18 and the VMV 16 and starts period_B. During period_B, the pressure will normally
rise due to vapour generation, but may fall are such that vapour condenses in the
tank. At end period_B the ECU monitors the tank pressure p4 and calculates and saves
the pressure increase above atmospheric dP_B = p4 - p_atm.
The holding period is intended to allow conditions in the
tank to approach a steady state and reduce variability due to the speed of evacuation
(which is influenced by the level of manifold vacuum, in turn influenced by engine
load and throttle position). In practice, it is not feasible to have a sufficiently
long holding period to avoid errors in the pressure measurements.
Accuracy of the results from the vapour integrity test
strategy depends both on accurate measurement of those parameters for which sensors
are provided (pressure, fuel tank volume etc) and on control of test conditions
under which the test is carried out (15-85% tank volume limits, abort on high fuel
There are several factors which influence the test result
but may be impossible to measure yet occur regularly under normal driving conditions.
For example, driver input during evacuation and venting processes alters the gas
properties and result in over- or under- estimation of the perceived leak size.
The primary effect of unpredictable inputs during evacuation
is their influence on tank vapour temperature. A gas temperature sensor would enable
discrimination between the effect on pressure of gas temperature and other factors
such as vapour generation or a genuine loss of vapour integrity. A sensor, however,
would require a relatively fast response (typically 1 sec) and would add to the
system cost. It would also require its own diagnostics.
The present invention estimates corrections for the dynamic
temperature changes from the measured pressure during evacuation.
The theory behind temperature compensation and the algorithms
to enable it to be inferred from available pressure data are explained below.
Without any compensation the worst-case error is, typically,
equivalent to a hole diameter of around 0.5mm. Even a proportion of this error is
significant for lmm detection. For 0.5 mm detection this factor alone amounts to
a maximum of 100% noise and it is obviously important that this error is reduced.
To illustrate the concept of temperature error consider
a sealed tank under ideal conditions - no vapour generation or loss of vapour integrity,
and with tank and contents stabilised at the same temperature (TO). If the tank
pressure is reduced rapidly by -2 kPa (this is a typical level of pressure reduction
for the evacuation phase) then the temperature of the vapour contents will be reduced,
by around 0.7 to 1.1°C depending on the fuel vapour properties within the tank.
If the tank is then sealed the temperature will rise towards its original value
(TO), due to heat transfer between the gas and the surroundings, and the pressure
also will rise accordingly (eventually by around 0.2 to 0.35 kPa). The effect applies
whenever there is a pressure change, up or down, and influences both test stages
irrespective of the order in which they are executed.
The pressure and temperature changes involved in the test
are relatively small (e.g., +/- 2%) and so the principal of superposition is assumed
for the effects of the loss of vapour integrity and associated errors. Hence the
transient temperature error described above may be superimposed on any pressure
changes present, whether due to vapour or a genuine loss of vapour integrity. The
net effect of these errors is to cause over-estimation of the size of any loss of
vapour integrity (or to indicate a loss of vapour integrity when none is present).
It is possible to minimise the effects of thermal in-equilibrium
by setting target values for evacuation and venting processes within the strategy
and optimising the strategy for these values. However, some uncertainties, or noise,
will still exist and the errors cannot be completely eliminated by this method.
By estimating the dynamic temperature its contribution to pressure can be estimated
and the net pressure change due to other factors (loss of vapour integrity & vapour)
can be identified.
The sources of test temperature variation and alternative
ways of compensation are discussed below:
a) Primary sources of error
The test temperature(s) will be influenced by the following
For test repeatability, it is clearly desirable to have target values for all of
these. The most basic targets for evacuation would be a linear evacuation to a set
depression in a target time, followed by holding phase of fixed duration at this
depression prior to commencement of stage A. This desired or optimum evacuation
characteristic is shown in Figure 2. Ideally venting to atmosphere via the CVV (Figure
3) would also be in a controlled manner.
- i. evacuation duration
- ii. evacuation characteristics
- iii. holding time at the start of period_A
- iv. venting at the end of stage A (if stage B follows)
- v. additional conditional procedures (re-evacuation etc).
In practice, driver input influences manifold pressure
and both loss of vapour integrity and vapour generation affects the volume of gases
that must be evacuated to achieve the desired pressure. These effects make it impossible
to achieve both the target evacuation time and profile. Additional (conditional)
phases introduce further deviations from the basic strategy.
b) Principle of temperature compensation
Non-achievement of target evacuation time and/or profile
will introduce a noise equivalent to an unknown proportion of the 100% or so range
referred to above. The use of a temperature model allows optimisation for a target
strategy with temperature compensation for deviations or, alternatively, the development
of an absolute strategy using basic thermodynamics. Algorithms to assist these,
together with simplifications for the former, are described here.
C) Analytical algorithm for temperature compensation
The algorithm is based purely on the ratiometric temperature
changes resulting from a pressure history, thus avoiding the need for any absolute
reference temperature, either measured or inferred.
Over any time interval At the measured pressure P changes
by AP. The gas temperature will be driven both by this pressure change and by heat
- P is measured tank pressure;
- TO is the estimated temperature at the start of the stage;
- t_therm is the fuel tank-vapour thermal time constant; and
- &ggr;f= adiabatic index for fuel vapour.
Substituting non-dimensional factors Tr = T/TO (TO refers
to start of test)
and Tr at any time is calculated by summing &Dgr;Tr/&Dgr;t from an initial
condition Tr=1. It is assumed that digital processing will be used. In an analog
system the dTr/dt would be integrated.
Application of Tr
The bulk of the tank vapour experiences a change in pressure
and temperature due to volumetric compression caused by vapour formation together
with leak flow :
Knowing &Dgr;P, P & V by measurement and &Dgr;Tr and
Tr from above the true volumetric flow can be calculated
Analysis can then separate the contribution due to vapour
from that of leak flow without the residual error caused by the unknown temperature
d. Simplified algorithms
The above calculation may be excessively time-consuming
during evacuation in a real engine management system. Alternatively a first-order
correction based on monitoring pressure during evacuation as described below may
Figure 2 shows a vapour integrity test evacuation and stage
A bleedup an which optimum rate of evacuation 34 has been achieved followed by hold
35 at pressure p1 and bleedup 36. Pressure difference dP_A will give a correct value
for combined vapour generation and loss of vapour integrity.
The extremes of evacuation profiles compared to the optimum
34 are shown in Figures 4 and 5. In Figure 4 a late rapid evacuation 40 to the target
pressure p1 results in minimum settling time and hence has the lowest temperature
at stage A commencement. This may occur if the test takes place at an initially
low manifold depression 42 (acceleration) followed by a high manifold depression
43 (reduced throttle). Temperature recovery continues during bleedup 44 and contributes
to a more rapid rise in pressure than for the test shown in Figure 2 (for comparison
the Figure 2 test pressure variations are shown in dotted lines in Figures 4 and
5). The more rapid rise in pressure compared to Figure 2 gives a greater increase
in pressure over the period_A' than over the period_A Figure 2. The measured pressure
change dP_A' is greater than dP_A, and absent temperature compensation, this would
result in an over estimation of hole size.
Figure 5 shows another extreme case. A rapid initial but
incomplete evacuation 45 is followed by a slow evacuation 46 down to pressure p1.
This results in the maximum settling time at or near pressure p1 prior to stage
A commencement. The temperature at the start of bleedup 47 is a higher temperature
than for the optimum test of Figure 2. The measured pressure change dP_A" is less
than dP_A and hole size, without temperature compensation will be underestimated.
According to a preferred embodiment of the invention, the
evacuation profiles is characterised by integrating, or summing, the measured depression
during evacuation and dividing it by both the target depression and the target time.
The resultant value (within the range 0 to 1) is used to
generate a correction to the following stage pressure rise. The target straight-line
characteristic 34 gives a value of 0.5 and zero temperature correction. The corrections
to dP_A for other values of the summation are bi-directional around zero as shown
in the following table. The Figure 4 characteristic gives a summation value of about
0.8 and the Figure 5 characteristic gives a value of about 0.2.
Value of temp. error Indicator
Correction Applied to dP A
A similar algorithm can be applied to the effect of venting
on stage B, if appropriate. Should stage A follow stage B then the algorithm would
be adjusted accordingly to reflect the transition from a positive pressure at the
end of stage B to the target depression prior to stage A.
It is to be understood that the embodiment of the invention
described above is merely illustrative on one application of the principles of the
invention. Numerous modifications may be made to the methods and apparatus described
without departing from the scope of the invention as set forth in the following