Air flow measurement apparatus

An air flow measurement apparatus is proposed for measuring the air quantity aspirated by internal combustion engines, which has at least one temperature-dependent resistor disposed in the air flow, whose temperature and/or resistance is controlled in accordance with the flow rate to provide a controller output as a gauge for the quantity of the medium. The apparatus includes a flow restriction, which is preferably formed in plate-like configuration with a wedge-shaped cross section and the narraw tip facing into the air flow and is disposed in the flow downstream of the temperature-dependent resistor in such a manner that in the event of a flow reversal, such as during backfiring in the intake manifold of an internal combustion engine, the flow restriction reduces the flow velocity in the vicinity of the temperature-dependent resistor.

BACKGROUND OF THE INVENTION 
The invention relates to an air flow measurement apparatus. An air flow 
measurement apparatus in which a hot wire is used as the 
temperature-resistant resistor in the intake manifold of an internal 
combustion engine is already known. In such an apparatus, however, there 
is the danger that in backfiring (that is, when the direction of flow is 
reversed), the hot wire is destroyed by the pressure waves and high flow 
velocities in the intake manifold which then occur. 
OBJECT AND SUMMARY OF THE INVENTION 
The apparatus in accordance with the invention has the advantage over the 
prior art that in the event of backfiring in the intake manifold of an 
internal combustion engine, that is, a shock-like flow reversal across a 
measurement location, the flow velocity is reduced in the region of the 
temperature-dependent resistor and thus the destruction of this resistor 
is prevented. 
The invention will be better understood as well as further objects and 
advantages thereof become more apparent from the ensuing detailed 
description of a preferred embodiment taken in conjunction with the 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The air flow measurement apparatus of the invention is used, as an example, 
for the measurement of a quantity of air aspirated by an internal 
combustion engine. Referring now to FIG. 1, there is shown an induction 
tube 1 incorporated in the intake manifold of an internal combustion 
engine (not shown) through which air flows in the direction of the arrows 
2. The induction tube 1 includes a measurement sensor 20 having at least 
one temperature-dependent resistor 3, for example a hot wire or hot film, 
which carries an electrical current of controlled magnitude and which 
supplies the actual value signal for a controller 11. 
The controller 11 maintains the temperature of the resistor 3 at a level 
somewhat above the average air temperature. If the air flow rate 
increases, i.e., the velocity of the flowing air increases, the 
temperature of the resistor 3 tends to drop. This drop in temperature 
results in a change of resistance and hence a change in the voltage drop 
across the resistor 3 which is sensed at the input of the controller 11, 
which thereby changes its output current until the predetermined 
temperature of the resistor 3 is attained again. Thus the controller 11, 
preferably in the form of a differential amplifier, continuously attempts 
to hold the temperature of the resistor 3 at a constant temperature in 
spite of fluctuations in the air flow rate, so that its output current at 
the same time may be used in a fuel metering system to adapt the quantity 
of fuel required by the engine to the prevailing air flow rate. 
The measuring resistor 3, which is temperature-dependent, may be connected 
in series with a fixed resistor 4 in a first arm of a measuring bridge 
which has a second arm consisting of series-connected fixed resistors 5 
and 6. The two arms of the bridge are connected in parallel at junctions 9 
and 10. There is a pickup point 8 between resistors 5 and 6, and a pickup 
point 7 is located between the measuring resistor 3 and the resistor 4. 
The diagonal voltage of the bridge which appears between the pickup points 
7 and 8 is applied to the input of the controller or differential 
amplifier 11. The points 9 and 10 are connected across the outputs of the 
differential amplifier 11 and are thus supplied with operating voltage or 
current. 
The output voltage of the differential amplifier 11, designated U.sub.S, 
may be used externally, at contacts 12 and 13, for the purpose of 
providing an input signal to fuel management systems and the like. In 
particular, the signal U.sub.S may be used to control the amount of fuel 
supplied to the engine for the prevailing air flow rate in a known but 
unillustrated fuel metering system. Thus an optimum fuel-air mixture can 
be attained for maximum output with the lowest possible proportion of 
toxic exhaust components. 
The current flowing through the temperature-dependent resistor 3 heats this 
resistor 4 until the input voltage to the differential amplifier 11 equals 
the bridge diagonal voltage, zero, or some different, predetermined 
voltage. The output of the amplifier 11 is thus a controlled current 
flowing into the bridge circuit. If the air flow rate changes, the 
temperature of the resistor 3 also changes, as does its resistance, which 
causes a change in the voltage difference between the points 7 and 8, 
causing the amplifier 11 to correct the output current supplied to the 
points 9 and 10 until such time as the bridge is balanced or has reached a 
predetermined degree of imbalance. Accordingly, the output voltage 
U.sub.S, as well as the current through the resistor 3, constitute a 
measure for the prevailing air flow rate. 
In order to compensate for the influence of the temperature of the 
aspirated air on the measured result, it may be suitable to supply a 
second temperature-dependent compensating resistor 14 in the second arm of 
the bridge. The magnitude of the resistances of the resistors 5, 6 and 14 
should be so chosen that the power loss due to the current flowing through 
the second temperature-dependent resistor 14 is so low that its 
temperature is unaffected by changes of the bridge voltage but always 
corresponds substantially to the temperature of the air flowing around it. 
Downstream of the temperature-dependent resistor 3, a flow restriction 15 
is disposed in the induction tube 1 of the intake manifold of the engine. 
As shown best in FIG. 2, this flow restriction 15 is preferably formed in 
a plate-like configuration. The cross section of the flow restriction 15 
is advantageously wedge-shaped, with the narrow tip of the flow 
restriction 15 facing into the air flow. The flow restriction 15 is 
disposed in an air flow traveling in the direction of the arrows 2 in the 
lee, or flow shelter, offered by the temperature-dependent resistor 3. 
In the event of possible backfiring in the intake manifold of an engine, 
with a simultaneous flow reversal in the direction of the arrow 16, high 
flow velocities and pressure waves occur, which can cause the destruction 
of the temperature-dependent resistor 3. By disposing the flow restriction 
15 facing a flow in the direction of the arrows 2 downstream of the 
temperature-dependent resistor 3, however, a boundary layer is caused to 
be formed in the air flow at the flow restriction 15 in the event of 
backfiring, which severely reduces the flow velocity. 
In accordance with the invention, the temperature-dependent resistor 3 is 
intended to lie in the vicinity of the reduced flow velocity caused by the 
flow restriction 15 in the event of backfiring, so that destruction of the 
temperature-dependent resistor 3 is prevented. The thickness of the 
boundary layer which builds up at the flow restriction 15 is in proportion 
to .fourthroot.x, where x is the length of the flow restriction. When the 
air flow is in the direction of the arrows 2, the flow at the 
temperature-dependent resistor 3 is not affected by the wedge-like 
conformation of the flow restriction 15. 
The foregoing relates to a preferred embodiment of the invention, it being 
understood that other embodiments and variants thereof are possible within 
the spirit and scope of the invention, the latter being defined by the 
appended claims.