Altitude compensation apparatus

Altitude compensation apparatus for use with a carburetion system for an internal combustion engine, the system having a passage through which air is drawn into the engine, a throttle valve positioned in the passage and movable between an open and a closed position to control the flow of air therethrough and a fuel circuit through which fuel is delivered to the passage for mixing with air to form an air-fuel mixture combusted in the engine. The position of the throttle valve, the flow rate of air through the passage and the vacuum level in the engine are each sensed and respective electrical signals representative thereof are supplied. The quantity of fuel supplied by the fuel circuit to the passage is metered thereby to maintain the air-fuel ratio of the mixture produced at a preselected value. In response to the aforesaid signals, the density of air being drawn into the engine is calculated and a control signal is generated to control the metering of fuel. The calculated air density is a function of the altitude at which the engine is operated and the control signal has characteristics which are a function of the calculated air density.

BACKGROUND OF THE INVENTION 
This invention relates to carburetion systems for internal combustion 
engines and more particularly to a carburetion system in which adjustments 
in the delivery of fuel are made to compensate for changes in altitude. 
With the present emphasis on fuel economy and reduced engine emissions for 
automobiles, numerous schemes have been developed for controlling the 
quantities of air and fuel mixed together and supplied to an automobile 
engine for combustion. A number of factors determine the correct 
proportions of air and fuel and one of these factors is the altitude at 
which an engine is operated. As an engine is operated at higher altitudes, 
less air is drawn into the engine and the resultant air-fuel mixture tends 
to become richer unless a correction is made in the quantity of fuel 
supplied to the engine. One way of compensating for altitude changes in a 
conventional carburetor is disclosed in U.S. Pat. No. 3,872,188 issued 
Mar. 18, 1975 to Brown et al and assigned to the same assignee as the 
present application. As shown in this patent, a capsule or bellows is 
responsive to changes in atmosphere to adjust the position of a metering 
pin and bleed more or less air into a carburetor fuel system, thereby 
controlling the quantity of fuel delivered to the engine. While such an 
apparatus does improve fuel economy and reduce emissions, the mechanical 
parts are subject to failure due to wear, engine vibrations, heat, etc. 
Further, other carburetion systems for an automobile engine do not 
function in the same manner as conventional carburetors but altitude 
compensation is still necessary to obtain the above stated goals. 
SUMMARY OF THE INVENTION 
Among the several objects of the present invention may be noted the 
provision of altitude compensating apparatus for use with a carburetion 
system for an internal combustion engine; the provision of such apparatus 
which may be used both with a conventional carburetor and with other types 
of carburetion systems; the provision of such apparatus for maintaining 
the air-fuel ratio of the mixture supplied to the engine at a preselected 
value; and the provision of such apparatus which promotes fuel economy and 
reduced engine emissions and which is less susceptible to wear or failure. 
Briefly, altitude compensation apparatus of the present invention is for 
use with a carburetion system for an internal combustion engine, the 
system having a passage through which air is drawn into the engine, a 
throttle valve positioned in the passage and movable between an open and a 
closed position to control the flow of air therethrough and a fuel circuit 
through which fuel is delivered to the passage for mixing with air to form 
an air-fuel mixture combusted in the engine. The apparatus comprises 
respective means for sensing the position of the throttle valve, the flow 
rate of air through the passage and the vacuum level in the engine and for 
supplying respective electrical signals representative thereof. The 
quantity of fuel supplied by the fuel circuit to the passage is metered 
thereby to maintain the air-fuel ratio of the mixture produced at a 
preselected value. Means responsive to the aforesaid signals calculates 
the density of air being drawn into the engine and generates a control 
signal to control the metering of fuel. The air density calculated by the 
signal responsive means is a function of the altitude at which the engine 
is operated and the control signal has characteristics which are a 
function of the calculated air density. Other objects and features will be 
in part apparent and in part pointed out hereinafter.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the drawings, altitude compensation apparatus of the present 
invention is designated generally 1 and is for use with a carburetion 
system C for an internal combustion engine (not shown). As shown in FIG. 
2, the carburetion system has a passage P through which air is drawn into 
the engine, the passage having a restriction or venturi V. A throttle 
valve generally indicated T is positioned in the passage and is comprised 
of a movable plate 3 mounted on a rotatable shaft 5 for movement of the 
throttle valve between an open and a closed position to admit more or less 
air into the engine. It will be understood that the throttle valve 
structure shown in FIG. 2 is for illustrative purposes only and that other 
throttle valve designs would serve equally well in practicing the present 
invention. Fuel is delivered from a fuel source, a fuel bowl B, to air 
passage P through a fuel circuit F. The fuel and air mix to form an 
air-fuel mixture combusted in the engine. 
Apparatus 1 comprises means 7 for sensing the position of throttle valve T 
and for supplying an electrical signal representative of the position. The 
means includes a potentiometer 9 connected between a battery 11 and 
electrical ground. The resistance of the potentiometer is a function of 
the extent to which the throttle valve is open. Wiper arm 13 of the 
potentiometer is operatively connected to the throttle valve to change the 
resistance value of the potentiometer as the throttle valve moves between 
its fully open and closed positions. 
A second sensing means 15 senses the flow rate of air through passage P and 
supplies an electrical signal representative of the flow rate. Sensing 
means 15 may be any of a number of flow rate sensors whose design and 
operation are well known in the art. As shown in FIG. 2, the sensing means 
comprises a vortex-shedding flowmeter 17, the design and operation of 
which is well known in the art and is described, for example, in "Vortex 
shedding provides accurate flow", The Oil and Gas Journal, Aug. 4, 1975, 
pp. 84-88. 
A third sensing means 19 senses the vacuum level in the engine and supplies 
an electrical signal representative of this level. As shown in FIG. 2, 
means 19 is a differential pressure sensor and comprises a housing 21 in 
which a piston 23 is slidably movable and a spring 25 seated against the 
piston and one end of the housing. The housing has an opening 27 in the 
end against which spring 25 seats and the opposite end of the housing is 
vented to the atmosphere. Sensing means 19 further includes a 
potentiometer 29 connected between electrical ground and a battery 31. 
Wiper arm 33 of the potentiometer is operatively connected to piston 23 to 
change the resistance value of the potentiometer as the absolute pressure 
level (engine vacuum level) changes. 
An auxiliary air passage A communicates with fuel circuit F. The inlet to 
this auxiliary air passage is shown in FIG. 2 as being in air passage P at 
a point above Venturi V. It will be understood however, that the inlet to 
passage A may be at some other location. A portion of the air entering air 
passage P enters auxiliary passage A and modulates the pressure to which 
fuel circuit F is subjected. 
A fuel metering means M meters the quantity of fuel supplied by the fuel 
circuit to passage P to maintain the air-fuel ratio of the resultant 
mixture at a preselected value. As shown in FIG. 2, means M comprises a 
solenoid 35 and an air metering rod 37 movable by the solenoid. The 
metering rod extends into auxiliary air passage A through an opening O 
therein. 
The metering rod is movable into and out of the auxiliary air passage to 
adjust the quantity of air flowing therethrough to fuel circuit F. An 
increase or decrease in the amount of air flowing to fuel circuit F 
results in less or more fuel flowing to passage P. 
The flow of air through auxiliary air passage A is controlled on the basis 
of the density of the air being drawn into the engine. An electronic 
control unit ECU is responsive to the signals supplied by sensing means 7, 
15 and 19 to calculate the density of the air being drawn into the engine. 
The electronics control unit calculates the air density according to the 
formula 
##EQU1## 
where flow area is a function of the position of throttle valve T, g.sub.c 
is the gravitational constant, volumetric air flow is a function of the 
flow rate of air through the engine and bore area is a constant equal to 
the cross-sectional area of passage P. This formula is derived from the 
flow equation for carburetors which, in turn, is derived from Bernoulli's 
equation. The basic flow equation is set forth, for example, in 
Introduction to Fluid Mechanics, by James E. A John and William Haberman, 
Prentice Hall, Inc. 1971, p. 45, as well as in Elements of 
Internal-Combustion Engines by A. B. Rogowski, McGraw-Hill Book Company, 
1953, p. 142. The electronics control unit is, for example, a 
microprocessor, having appropriate input, output and memory circuits of 
the type well known in the art. This circuitry responds to the three input 
signals from carburetion system C to calculate air density in accordance 
with the formula and supply a control signal to the metering means to 
control the metering of fuel. As shown in FIG. 2, the control signal is 
supplied from unit ECU to solenoid 35 via electrical path 39. The control 
signal has characteristics which are a function of the calculated air 
density. Thus, for example, the control signal may be frequency variable 
with the frequency varying as a function of calculated air density or the 
control signal may be a d.c. level the amplitude of which varies as a 
function of calculated air density. 
Altitude compensation apparatus 1 of the invention is summarized in block 
diagram form in FIG. 1. Thus, a carburetion system C provides three output 
signals to an electronics control unit ECU; these signals being 
representative of throttle position or angle, air flow rate and engine 
vacuum. The electronics control unit employs these signals to calculate 
air density according to the above stated formula and produces a control 
signal which is supplied to a fuel meter M. The fuel meter is responsive 
to the control signal to supply an appropriate amount of fuel to the 
carburetion system for proper operation of the engine at a given altitude. 
The resultant air-fuel mixture produced by the carburetion system may be 
held at a specific air-fuel ratio and this helps increase fuel economy and 
reduce emissions. In addition, the parts employed in the apparatus are not 
as susceptible to wear or breakdown as mechanical systems performing a 
similar function. 
Referring to FIG. 3, a second embodiment of altitude compensation apparatus 
of the invention is indicated 1' and is for use with a carburetion system 
C' having an air passage P'; a throttle valve T'; a source of fuel B' and 
a fuel circuit F'. Throttle valve T' position is sensed by a sensing means 
7' as before and engine vacuum is sensed by a sensing means 19' also as 
before. The electrical signals developed by these sensing means are 
supplied to an electronics control unit ECU'. Concerning flow rate, the 
engine on which carburetion system C' is installed may be considered as a 
positive displacement pump or meter which ingests a quantity of air and 
fuel approximately equal to its volumetric displacement during each 
complete engine cycle. For a 2-cycle engine, a complete cycle occurs every 
engine revolution; while for a 4-cycle engine, a complete cycle occurs 
every two engine revolutions. Thus, the flow rate may be measured without 
a separate flow measuring device by measuring engine revolutions. 
If, for example, the engine on which carburetion system C', is employed is 
a 4-cycle engine, then 
##EQU2## 
Engine revolutions per minute (rpm) is measured by an rpm sensor 41. Rpm 
sensor 41 is of any type well known in the art and an electrical signal 
produced by the rpm sensor is supplied to electronics control unit ECU'. 
The volumetric efficiency proportionality constant is equal to 
##EQU3## 
A discussion of volumetric efficiency is found in Internal Combustion 
Engines by Burgess H. Jennings and Edward F. Obert, International Textbook 
Company, 1944, pp. 133-134. By making the appropriate substitutions, the 
previously set forth air density formula, by which electronic control unit 
ECU' computes air density, becomes air density 
##EQU4## 
As before, the electronics control unit computes air density in response to 
the signals supplied to it representing throttle angle, engine vacuum and 
air flow. Further, the electronics control unit produces a control signal 
whose characteristics are a function of the calculated air density and 
this signal is supplied to a fuel metering means M'. This means comprises 
a positive displacement fuel pump 43 which pumps fuel from fuel bowl B' to 
passage P' via fuel circuit F'. The outlet of fuel circuit F' is at a 
point above throttle valve T' although alternately, and as shown by the 
dashed lines, the fuel circuit outlet may be below the throttle valve. In 
operation, the amount of fuel delivered to passage P' via pump 43 is 
determined by the characteristics of the control signal and the fuel pump 
pumps an amount of fuel necessary to maintain the air-fuel ratio of the 
resultant mixture at a preselected value. 
As above noted, the electronics control unit may comprise a microprocessor 
which includes a memory. An electronics control unit of this type was 
constructed for use with a carburetion system C' as shown in FIG. 3. The 
apparatus was installed on an automobile which was then driven from lower 
to higher altitudes. At preselected altitudes, the air density was 
computed according to the abovestated formula and the quantity of fuel 
required to produce an air-fuel mixture of a specified air-fuel ratio, for 
example, for stoichiometry was calculated. The resultant values were 
tabulated and a schedule developed which was loaded into the memory of 
electronic control unit ECU'. The automobile was then driven from the 
higher back to lower altitudes and it was determined that the performance 
of the altitude compensation apparatus at the altitudes for which the 
initial data was taken was in accordance with the expected results. 
In view of the above, it will be seen that the several objects of the 
invention are achieved and other advantageous results attained. 
As various changes could be made in the above constructions without 
departing from the scope of the invention it is intended that all matter 
contained in the above description or shown in the accompanying drawings 
shall be interpreted as illustrative and not in a limiting sense.