Returnless fuel delivery system

A returnless fuel delivery control system is disclosed which regulates fuel rail pressure at the level needed for precise control of fuel mass flow to fuel injectors at both normal and elevated engine temperatures. This regulation is accomplished by precisely controlling the speed of the fuel pump motor as a function of the projected fuel demand based on engine RPM and injector pulse width. The projection is modified as a function of differential pressure error. The differential pressure error responds to a fuel temperature strategy which increases the target differential pressure as a function of fuel temperature.

TECHNICAL FIELD 
This invention relates to a fuel delivery system for fuel injected internal 
combustion engines, and more particularly to a fuel delivery system which 
eliminates the conventional pressure regulator and fuel return line and 
the attendant fuel tank vapor formation problems, by precisely controlling 
the speed of the fuel pump to achieve an optimum intake manifold/fuel rail 
differential pressure under both normal and high fuel temperature 
conditions. 
BACKGROUND ART 
A conventional fuel delivery system includes a fuel tank with a fuel pump 
located therein for supplying fuel to a plurality of fuel injectors 
located in a fuel rail. Each fuel injector is controlled by an electronic 
control unit (ECU) which is responsive to various engine operating 
conditions to provide a variable pulse width control signal to each 
injector to meet the fuel demand of the engine. A pressure regulator is 
interposed between the pump and the rail and is designed to maintain the 
fuel pressure in the rail at approximately 40 psi greater than engine 
intake manifold vacuum. 
The fuel pump runs at a constant speed and may deliver, for example, 90 
liters per hour. Under idle conditions the engine needs only about 3 
liters per hour, so in that case 87 liters per hour is returned to the 
fuel tank from the pressure regulator through return lines. There are a 
number of problems associated with return of fuel from the high 
temperature engine area to the relatively low pressure and low temperature 
which exist in the fuel tank. Because of the high temperature and pressure 
of the fuel being returned, substantial amounts of fuel vapor are 
generated and exist in the tank which must be vented to the atmosphere 
which may in turn create environmental problems. 
SUMMARY OF THE INVENTION 
In view of the above, it is an object of the present invention to provide a 
fuel delivery system which avoids the need for a pressure regulator and 
fuel tank return line, while compensating for above normal engine 
temperature by raising the fuel pressure at the fuel rail by increasing 
the speed of the fuel pump. 
It is another object of the present invention to improve performance of an 
engine provided with an injection type fuel delivery system while lowering 
evaporative emissions. 
It is another object of the present invention to provide a method of 
controlling the speed of a fuel pump in order to provide a compensating 
fuel pressure increase to offset the reduction in mass fuel flow which 
normally accompanies high engine temperature. 
It is another object of the present invention to increase the life of the 
vehicle fuel pump by reducing the electrical load on the pump motor. 
It is another object of the present invention to provide a fuel delivery 
system that compares fuel mass flow prediction or demand with the fuel 
mass flow actually supplied to the injectors in order to obtain an 
indication of fuel leakage. 
In accordance with the present invention a fuel delivery control system is 
provided which regulates fuel rail pressure at the level needed for 
precise control of fuel flow at both normal and intermediate engine 
temperatures. This regulation is accomplished without resort to the usual 
differential pressure regulator with the attendant return line to the fuel 
tank. Since fuel is not recirculated, the fuel in the tank stays at 
approximately ambient temperature or as much as 20-30 degrees cooler in 
certain areas during the summer months. Also, since hot fuel is not 
returned to the tank, vapor reduction is very significant. Moreover, by 
precisely controlling the speed of the fuel pump motor to control 
pressure, pump noise is reduced and the effects of electrical system 
voltage variation of fuel pump operation is reduced.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to the drawings, and initially to FIG. 1, a fuel delivery 
system in accordance with the present invention is shown and includes a 
fuel pump generally designated 10 located within a fuel tank 12 of a 
vehicle. The pump 10 supplies fuel through a supply line 14 to a fuel rail 
16 for distribution to a plurality of injectors 18. The speed of the pump 
10 is controlled by an engine control module 20. The module 20 supplies 
control signals which are amplified and frequency multiplied by a power 
driver 22 and supplied to the pump 10. The module 20 receives a fuel 
temperature input from a fuel temperature sensor 24 as well as an input 
from a differential pressure sensor 26. The sensor 26 responds to intake 
manifold vacuum and to the pressure in the fuel rail 16 to provide the 
differential pressure signal to the module 20. The module 20 uses this 
information to determine the fuel pump voltage needed to provide the 
engine with optimum fuel pressure and fuel flow rate. A pressure relief 
valve 28 is positioned in parallel with a check valve which is in the fuel 
supply line 14. The parallel connected relief valve prevents excessive 
pressure in the fuel rail 16 during engine-off hot soaks. Also, the relief 
valve 28 assists in smoothing engine-running transient pressure 
fluctuations. Though not shown in the drawing it will be understood by 
those skilled in the art that the module 20 also controls the pulse width 
of a fuel injector signal applied to the injectors 18 in order to control 
the amount of fuel injected into the engine cylinders in accordance with a 
control algorithm. This signal is a variable frequency, variable pulse 
width signal that controls injector valve open time. 
Referring now to FIG. 2, the module 20 generates a constant frequency pulse 
width modulated (PWM) fuel pump control signal in accordance with an 
overall control strategy which includes a Proportional-Integral-Derivative 
(PID) feedback loop generally designated 30 and a feedforward loop 
generally designated 32. The loop 30 includes a control strategy block 34 
which responds to the error output of a comparator 36 which represents the 
difference between a desired differential pressure input and the actual 
differential pressure as input from the sensor 26. Under normal 
circumstances the desired pressure may be for example 40 psid. The output 
of the block 34 represents the time history of the error input and is 
combined in a summer 38 with the output of a fuel flow prediction block 40 
to vary the duty cycle of the PWM signal to the pump 10 in a sense to 
reduce the error input to the block 34 toward zero and maintain a 
substantially constant 40 psid. Since the PID loop responds to 
differential pressure, a sudden change in manifold vacuum as would occurs 
for example if the driver requests full throttle, can produce substantial 
instability in the PID loop. The block 40 compensates for this 
instability. The block 40 utilizes engine RPM and injector pulse width 
(PW) to predict mass fuel flow. The variables are obtained by monitoring 
one of the fuel injector control lines. From these inputs defining a 
particular engine operating point, a look up table is accessed to provide 
an optimum duty cycle for the PWM control signal to the pump 10. The fuel 
flow prediction provides a very quick response to engine operating 
conditions which cannot be adequately controlled by the PID loop 30. The 
PID loop provides a fine tuning of the overall control strategy and 
compensates for pump and engine variability. The fuel flow prediction is 
also used as an indicator of gross fuel leaks in the delivery system as 
might occur during a broken supply line during an accident. The mass fuel 
flow prediction is compared with the fuel mass being supplied by the pump 
and if the prediction or demand is substantially less than the actual fuel 
mass being supplied then the pump is turned off. 
While it is desirable to eliminate the return line to the fuel tank, doing 
so prevents the fuel from being used as a coolant. At idle, where fuel 
flow is low, the fuel in the rail is heated by convection from the engine 
and if the target fuel reaches its vapor point on the distillation curve 
it will vaporize causing less fuel to be delivered through the injectors 
for a given pulse width injector control signal. A temperature strategy 
block 42 is employed to compensate for this potential mass flow reduction. 
The block 42 responds to the output of the fuel temperature sensor 24 and 
modifies the desired pressure input to the comparator 40 as a function of 
the temperature of the fuel in the rail. Thus, as the fuel temperature 
increases the error signal to the PID control strategy block 34 resulting 
in an increase in the duty cycle of the control signal to the pump 10 
which raises the fuel pressure in the rail 16 thus maintaining the mass 
flow through the injectors. The same amount of fuel is thus delivered to 
cylinders regardless of temperature change and without having to alter the 
pulse width of the fuel injector control signal. This permits the present 
invention to be added to a vehicle without modifying the injection pulse 
width control signal algorithm. The block 42 raises the input to the 
comparator 36 above 40 psi to compensate for higher than normal fuel 
temperature. Thus, the PID loop is primarily responsible for the increase 
in fuel pressure in response to temperature increases. Under low 
temperature conditions the speed of the pump 10 is primarily determined by 
the fuel flow prediction term of the block 40. 
If the fuel flow prediction table is calibrated for the default desired 
differential pressure of 40 psi it will not produce an accurate prediction 
of fuel flow when the desired pressure is raised in response to increases 
in temperature. Since there is a square root relationship between pressure 
and mass fuel flow, this inaccuracy may be alleviated if desired by 
modifying the duty cycle prediction by a multiplication factor 
corresponding to the square root of the percentage increase in the desired 
pressure above the default 40 psi. resulting from implementation of the 
temperature strategy 42. Alternatively, a plurality of lookup tables 
associated with different differential pressure values may be employed and 
selectively accessed. 
A flow chart of the duty cycle control program or algorithm as implemented 
on a microprocessor based control module such as the module 20 is shown in 
FIG. 3. The blocks in the flow chart are identified by numeral with angel 
brackets. The module 20 monitors &lt;50&gt; the differential pressure output of 
the sensor 26 and compared &lt;52&gt; the periodic readings to a target 
differential pressure &lt;48&gt; of for example 40 psid. If the pressure is less 
&lt;54&gt; than the target then the PID control strategy output &lt;56&gt; is added 
&lt;58&gt; to the FF term and the duty cycle of the fuel pump PWM control signal 
is increased &lt;60&gt; to raise the fuel pressure in the rail. On the other 
hand if the differential pressure is less than the target pressure the PID 
control strategy output &lt;62&gt; is subtracted &lt;64&gt; from the FF term and duty 
cycle of the fuel pump PWM control signal is decreased &lt;66&gt; to lower the 
fuel pressure in the rail. The gains implemented in the PID control 
strategy &lt;62&gt;, &lt;56&gt; are different because of the time lag associated with 
reducing the pressure in the rail. 
With reference to FIG. 4, the feed forward routine which provides the term 
used in the main routine at &lt;58&gt; and &lt;64&gt; of FIG. 3 is shown. As 
previously stated, an indication of the present fuel demand &lt;70&gt; of the 
engine is obtained by monitoring one of the fuel injector control signal 
to obtain the pulse width (PW) and period of the signal. If the fuel 
demand is substantially less than fuel supply &lt;72&gt; the pump is turned off 
&lt;74&gt;. Otherwise, from the period or frequency of the fuel injector control 
signal, engine RPM is directly obtained. The feed forward routine &lt;76&gt; is 
basically a look up table routine which includes interpolation where 
necessary. From two inputs, RPM and PW, a two dimensional lookup table is 
entered and provides a best estimate of the duty cycle of the pump PWM 
control signal needed to meet the fuel demand. This best estimate of duty 
cycle is preferably in terms of a specific number of computer clock 
cycles, which is proportional to pump duty cycle. 
The temperature strategy routine used to determined the target differential 
pressure used in the main routine of FIG. 3 is shown in FIG. 4 as being 
executed following the fuel demand calculation. It will be understood 
however that the routines for calculating target pressure as well as fuel 
demand may be called at the times shown in FIG. 3 namely the Target block 
and blocks &lt;58&gt;, &lt;60&gt;. The fuel temperature in the rail is read &lt;78&gt; and 
if the temperature is above a predetermined value at which fuel 
vaporization is likely to take place &lt;80&gt;, the target differential fuel 
rail pressure is increased &lt;82&gt; from a nominal 40 psi to a value that will 
cause the PID loop to increase the duty cycle of the pump to ensure the 
desired fuel mass flow through the injectors. Hysteresis &lt;84&gt; is added so 
that the temperature/pressure relationship follows a different path when 
the temperature is increasing above normal than when decreasing toward 
normal. In other words the pressure returns to the nominal 40 psi in a 
slightly different rate over the last 10 degrees on either side of the 
predetermined fuel vaporization temperature so that the pressure reaches 
40 psi at a lower temperature. This prevents chattering in the case where 
temperature begins to exceed the set point of fuel vaporization and 
prevents the system from being "fooled" by the cooling effect of for 
example an open throttle condition where the instantaneous temperature of 
the fuel might drop &lt;86&gt; the instantaneous temperature below the set 
point. 
While the best mode for carrying out the invention has been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.