No Oil Droplets Formation in Thermopile Flow Sensors
Used in Internal Combustion Engines
Tu XianZheng
Hot-film air mass sensors are commonly used for measuring an
air-mass flow, which include a resistor for heating
and one or two other resistors for temperature sensing. It has been
reported that when hot film air mass flow sensors are used directly in the
intake tract of the internal combustion engine or in a bypass channel to the
intake tract of the internal combustion engine, oil may deposit on the sensor
chips and, in particular, on the sensor diaphragms during operation or shortly
after the internal combustion engine has been shut off. This oil deposit may
result in undesirable effects on the measuring signal of the sensor chip, in
particular because an oil film affects the thermal conductivity of the sensor
chip surface, which results in corruption of the measuring signals or a signal
drift.
As well known, condensation can produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condensed. As result droplets of water leave on the glass or can.
As well known, condensation can produce water droplets on the outside of soda cans or glasses of cold water. When warm air hits the cold surface, it reaches its dew point and condensed. As result droplets of water leave on the glass or can.
It is the same that when a thermal air mass flow sensor is
operated at the border region of the heated measuring areas oil accumulates and
over time results in oil droplets. The air flow drives the oil droplets on the
surface up to the boundary of the heated measuring area, at which a stronger
temperature gradient appears. The strong temperature gradient exerts a force
opposite to the force exerted by the air flow. Oil droplets thus accumulate on
the boundary line, which, when they reach a certain size, may be entrained
again by the air flow to then contaminate the surface of the measuring area. In
addition to the oil droplets, other contaminants also reach the surface of the
measuring area due to this effect.
How to solve this problem? Back to the above mentioned
condensation. Condensation of water occurs when water vapor within the air
cools enough in order to change into the liquid state. A good example of
condensation often occurs in the home during winter time, when water droplets
form on the surfaces of cold windows. If open the window to let the cold air
enter the room there will no any water droplets form on the surface of the
window. So the only way to solve the problem is to reduce the operation
temperature of the sensor. This can be done using a thermopile air mass flow
sensor instead of a hot film air mass flow sensor.
It does not respond to
absolute temperature, but
generates an output voltage proportional to
a local temperature difference or temperature gradient. A thermopile air mass
flow sensor is constructed with a heater for heating and several thermocouples for
temperature sensing. The thermopiles are in series and so the output voltage
due to temperature change is summed and increased over that of a single
thermocouple.
With this advantage, POSIFA has
developed thermopile air mass flow sensors with two group thermopiles
positioned at the two side of the resistor heater and each group consisting of
40 thermopiles. Since the summed output of each group is so great that the
operation temperature of the heater can be reduced as low as no condensation to
take place. For example, the operation temperature can be set 10 to 20 degree Celsius
higher than the air temperature. In this case condensation of oil vapor in the
air is almost impossible.
