WE CLAIM:
1. An excess air ratio calculation device comprising:
an oxygen sensor, having a detection unit installed in contact with exhaust from
an internal combustion engine equipped with a fuel injection valve to detect an oxygen
concentration in the exhaust and having a temperature characteristic in which a detected
value from the detection unit varies with a temperature of the detection unit;
a temperature detection unit that estimates or detects the temperature of the
detection unit; and
an excess ratio calculation unit that calculates an excess air ratio λ of the
exhaust, based on the detected value and temperature, using data obtained by linearly
converting the detected value with respect to the excess air ratio while compensating for
the temperature characteristic,
wherein the excess ratio calculation unit includes:
a torque computation unit that calculates a torque value of the internal
combustion engine based on a crank angular velocity of the internal combustion engine;
a limit threshold setting unit that sets a conversion limit threshold for
the linearization conversion;
a storage unit that stores a duration Ti1 of fuel injection by the fuel
injection valve when the detected value or the linearly-converted data is less than or equal
to the conversion limit threshold, the torque value as a stored torque value TQ1, and the
excess air ratio λ with respect to the conversion limit threshold as a stored excess air ratio
λb;
a torque contribution setting unit that sets a torque contribution Tc; and
a substitution value computation unit that calculates a substitution value
R by using a following equation, with Ti2 as a duration for the fuel injection used when
the detected value or the linearly-converted data exceeds the conversion limit threshold
and with TQ2 as the torque value at the latest time:
R = ((Ti1 ÷ Ti2) ÷ (TQ1 ÷ TQ2)) × λb × Tc,
wherein, in a case where the detected value or the linearly-converted data
exceeds the conversion limit threshold, the substitution value R is considered as the
excess air ratio λ in the exhaust, instead of the linearly-converted data.
2. The excess air ratio calculation device according to claim 1, wherein the torque
contribution setting unit sets the value of the torque contribution Tc according to the latest
torque value TQ2, based on a lookup table that associates the torque contribution Tc with
the torque value.
3. The excess air ratio calculation device according to claim 1, wherein the storage
unit stores a respective moving averages as the fuel injection duration Ti1 and the stored
torque value TQ1.
4. The excess air ratio calculation device according to claim 1, wherein the storage
unit stores a moving average of the linearly-converted data as the stored excess air ratio
λb with respect to the conversion limit threshold.
5. The excess air ratio calculation device according to claim 1, wherein the excess
ratio calculation unit uses a predefined value as the stored excess air ratio λb with respect
to the conversion limit threshold.

6. The excess air ratio calculation device according to claim 1, wherein the limit
threshold setting unit sets the conversion limit threshold according to the temperature of
the detection unit, based on a lookup table that associates the temperature of the detection
unit with the conversion limit threshold.

7. The excess air ratio calculation device according to claims 1 to 6, wherein:
the oxygen sensor is a resistive oxygen sensor whose resistance value changes
with an oxygen concentration;
the excess ratio calculation unit has a data map that associates the temperature
and detected value of the detection unit of the resistive oxygen sensor with the excess air ratio λ in the exhaust; and
the linearly-converted data is acquired using the data map, and the linearlyconverted data is considered as the excess air ratio λ in the exhaust when the detected
value or the linearly-converted data is less than or equal to the conversion limit threshold.