This article proposes a new misfire detection index, the ΔGap slope, for a
four-cylinder engine. However, the proposed index is not limited to
four-cylinder engines. The ΔGap slope uses the tooth time measured using the
existing crankshaft position sensor; therefore, an additional sensor is not
required, which makes it economical. The ΔGap slope is defined as the difference
between the gap slopes of the same cylinder for two adjacent cycles. Various
factors that cause deviations in gap slopes between cylinders can be eliminated
in the process of determining the difference between two gap slopes. Hence, in
contrast with the existing engine roughness method, the ΔGap slope has the
advantage of not requiring compensation for deviations between the cylinders.
The conventional gap slope method must use different sets of thresholds for each
cylinder located at the same position on the sensor wheel, which results in
multiple thresholds being applied. In contrast, the ΔGap slope can use the same
set of thresholds for all the cylinders as the deviations between the cylinders
are eliminated. Although a set of thresholds is required for diagnosing the
start and end of misfire, the values are characterized by the same absolute
magnitude and opposite signs; this is another merit of the ΔGap slope. It was
found that the average misfire detection rate of the ΔGap slope is 90.2% for all
the test conditions of idle to 6,000 rpm and neutral to 100% load, and this
increased to 93% and 98% between 1,500 rpm and 4,000 rpm. However, the misfire
detection rate tends to decrease below the average value as the load approaches
neutral and the engine speed exceeds 4,000 rpm.
