Malfunction detector for an oil-fuel heat exchanger

A detector for detecting the level of oil in an oil reservoir is disclosed having particular application for use in a lubricating system of a gas turbine engine in order to determine a malfunction of an oil-fuel heat exchanger. The device uses an electrically actuated level sensor within the oil reservoir connected to a first electrical circuit having an outlet junction at which is applied a voltage U.sub.1 which varies with the level of oil in the reservoir, and a second circuit having a second outlet junction at which is applied a voltage U.sub.2 which decreases as the oil level decreases, but which does not increase as the oil level increases. A comparison circuit connected to the outlet junctions of the first and second circuits compares the difference between the voltages U.sub.1 and U.sub.2 to a predetermined reference voltage U.sub.0 and, if the difference exceeds this reference voltage, provides an output signal to actuate a visual indicator.

BACKGROUND OF THE INVENTION 
The present invention relates to a malfunction detector for an oil-fuel 
heat exchanger used in the lubrication circuit of an aircraft gas turbine 
engine. 
Fire on board an aircraft represents a most critical occurrence, one which 
may easily result in catastrophe. A modem gas turbine aircraft engine 
requires the use of flammable materials, such as fuel and lubricating oil. 
It is imperative, therefore, to preclude leaks of these flammable 
materials, which may be occasioned by poor seals and degradation of 
mechanical parts in the fuel and lubricating systems in order to prevent 
such fluids from contacting high temperature parts of the gas turbine 
engine. 
Most turbojet engines are equipped with an oil-fuel heat exchanger mounted 
in the lubrication circuit. When the heat exchanger fails, fuel and oil 
from the two fluid circuits may intermix which, in most cases, will 
involve the flow of fuel from the fuel circuit into the lubricating oil 
circuit due to the higher fuel pressure within the heat exchanger. 
The intermixing of the fuel and oil circuits are manifestly undesirable. 
Dilution of the oil with the fuel substantially increases the danger of an 
internal engine fire, especially when the aircraft is on the ground after 
operation, since the labrynth seals of the aircraft engine may be 
diminished and the engine temperature is great enough to cause ignition of 
the fuel and the oil. Moreover, the fuel degrades the lubricating 
properties of the lubricating oil, which may further raise the temperature 
of the moving parts of the engine. The addition of the fuel to the 
lubricating circuit will cause the lubricating oil reservoir to overflow 
when the reservoir is at atmospheric pressure. If the reservoir is located 
within an enclosure, the pressure within the enclosure increases and the 
enclosure may be flooded. 
U.S. Pat. No. 4,163,391 describes a device to monitor the level of the 
liquid in a reservoir using a high-temperature coefficient resistor whose 
resistance changes as a function of its temperature. This device compares 
several voltages and requires the use of a calibration and computation 
chart or graph, in particular to calculate the operating time of a delay 
system. 
SUMMARY OF THE INVENTION 
A detector for detecting the level of oil in an oil reservoir is disclosed 
having particular application for use in a lubricating system of a gas 
turbine engine in order to determine a malfunction of an oil-fuel heat 
exchanger. The device uses an electrically actuated level sensor within 
the oil reservoir, such sensor applying a voltage U to an outlet terminal. 
Connected to the outlet terminal of the sensor is a first electrical 
circuit having an outlet junction at which is applied a voltage U.sub.1 
which varies with the voltage U, and a second circuit having a second 
outlet junction at which is applied a voltage U.sub.2 which decreases as 
the voltage U decreases, but which does not increase as voltage U 
increases. A comparison circuit connected to the outlet junctions of the 
first and second circuits compares the difference between the voltages 
U.sub.1 and U.sub.2 to a predetermined reference voltage U.sub.0 and, if 
the difference exceeds this reference voltage, provides an output signal 
to actuate a visual indicator. 
The failure of the fuel-oil heat exchanger will typically cause the level 
within the lubricating oil reservoir to increase. Thus, an increase 
between voltages U.sub.1 and U.sub.2 will indicate an increase in the 
level of the fluid in the reservoir. The predetermined reference voltage 
U.sub.0 can be set at a level which can be exceeded only by fuel 
intermixing with the lubricating oil, thereby indicating a failure of the 
oil-fuel heat exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic diagram of the detector according to the present 
invention which comprises an electrically actuated level sensor 1 located 
within the lubricating oil reservoir 2 of a gas turbine engine lubricating 
circuit (not otherwise shown). The level sensor 1 is electrically powered 
in known fashion by a current source 3. The level sensor 1 has an 
electrical outlet terminal 4 at which is applied a positive voltage U 
which varies in direct relationship to the level of the oil within the 
reservoir 2. 
An electrical circuit 12 is connected between the outlet terminal 4 and 
ground, and comprises a resistor R.sub.1 and a capacitor C.sub.1 connected 
in series. Circuit 12 has an outlet junction 6 at which is applied a 
voltage U.sub.1 and which is located between the resistor R.sub.1 and the 
capacitor C.sub.1. 
A second electrical circuit 13 is also connected between outlet terminal 4 
and ground in parallel with the first electrical circuit 12. Circuit 13 
comprises a resistor R.sub.2, a diode D.sub.1 and a capacitor C.sub.2 
connected in series such that the diode D.sub.1 is located between the 
resistor R.sub.2 and the capacitor C.sub.2. Circuit 13 also comprises a 
subassembly E.sub.1 which consists of a resistor R.sub.3 and a switch I 
connected between the outlet terminal 4 and the outlet junction 5. As can 
be seen, the outlet junction 5 is located between the capacitor C.sub.2 
and the diode D.sub.1. The circuit 13 applies a voltage U.sub.2 at the 
outlet junction 5. 
The outlet junctions 5 and 6 are both connected to a malfunction detector 7 
which comprises a first inverting operational amplifier A.sub.1 with a 
gain of -1 and having the outlet junction 6 connected to its negative 
input terminal such that the output of the amplifier A.sub.1 is a voltage 
-U.sub.1. A second, adding amplifier A.sub.2 has its negative input 
connected to the output of the amplifier A.sub.1, along with the outlet 
junction 5 and means for generating a reference voltage U.sub.0. As can be 
seen, both of the positive amplifier inputs are connected to ground. The 
output of the amplifier A.sub.2 is connected to a diode D.sub.2 which, in 
turn, is connected to a visual indicating device L, which may be a light 
bulb, or the like. 
In operation, the switch I is dosed when the gas turbine engine is started. 
The voltage U is applied to the capacitors C.sub.1 and C.sub.2 such that 
U=U.sub.1 =U.sub.2. When the gas turbine engine has been started and is 
running, the switch I is opened. When the oil level drops in the reservoir 
2, which will normally take place following engine start up due to the 
lubricating oil filling the lubricating circuit, the voltages U, U.sub.1 
and U.sub.2 remain equal, but all decrease from their initial values. The 
capacitor C.sub.2 is also discharging through the diode D.sub.1. 
In the event the level inside the reservoir 2 should increase, voltages U 
and U.sub.1 will also increase in the same manner. However, voltage 
U.sub.2 remains at its previous lower value because of the blocking action 
of the diode D.sub.1. An increase in the fluid level within the reservoir 
2 will usually be caused by fuel entering the lubricating oil circuit due 
to a malfunction of the oil-heat exchanger mounted in the lubrication 
system. Accordingly, the voltage U.sub.2 is the minimum voltage supplied 
by the sensor 1 and corresponds to the minimum oil level in the reservoir 
2. 
The increasing of the fluid level within the reservoir 2 will cause an 
increase in the voltages U and U.sub.1, thereby increasing the difference 
between voltages U.sub.1 and U.sub.2. This difference is measured by the 
malfunction detector 7 and, when the difference exceeds a reference value 
U.sub.0, the diode D.sub.2 will enable the output signal of the amplifier 
A.sub.2 to pass, thereby illuminating the visual indicator L. When the 
difference U.sub.1 -U.sub.2 is less than the reference voltage U.sub.0, 
the diode D.sub.2 is in a blocking condition and the visual indicator L is 
off. The reference voltage U.sub.0 takes into account the normal parameter 
changes of the lubricating oil circuit over the entire light spectrum 
which will effect the level of the oil within the reservoir. 
When the switch I is open and the capacitor C.sub.2 is discharged, such as 
before starting the gas turbine engine, the application of electrical 
power to the sensor 1 causes diode D.sub.2 to be conducting, thereby 
illuminating the visual indicator L. This allows the visual indicator L to 
be tested to ensure that it is in working order prior to engine starting. 
Upon the closing of switch I, the capacitor C.sub.2 is charged to the 
value +U and the visual indicator L is extinguished. 
The foregoing description is provided for illustrative purposes only and 
should not be construed as in any way limiting this invention, the scope 
of which is defined solely by the appended claims.