Misfire determination apparatus for internal combustion engine

A misfire determination for a multi-cylinder internal combustion engine, the misfire determination apparatus comprising:

• a crank position sensor that detects a rotation position of a crankshaft;
• and a processor programmed to:
• obtain, using an output from the crank position sensor, a crank angle velocity on the basis of a time required for the crankshaft to rotate through a predetermined rotation angle width from a first crank angle to a second crank angle during an expansion stroke of each cylinder of the multi-cylinder internal combustion engine;
• obtain an angular velocity variation amount, which is a difference in the crank angle velocity between two cylinders that reach the expansion stroke consecutively;
• obtain a rotation variation mount which is a difference between a first angular velocity variation amount and a second angular velocity variation amount, the first angular velocity variation amount being obtained during the expansion stroke of an arbitrary cylinder among respective cylinders, and the second angular velocity variation amount being obtained in relation to a cylinder that has reached the expansion stroke in a Nth revolution of the crankshaft before the arbitrary cylinder reaches the expansion stroke, the N being a positive integer;
• when the rotation variation amount exceeds a predetermined threshold:
• determine that a misfire has not occurred in the multi-cylinder internal combustion engine when a predetermined condition is established, the predetermined condition being established when (i) a magnitude of the second angular velocity variation amount is greater than a magnitude of the first angular velocity variation amount, and (ii) the second angular velocity variation amount has a sign by which a value of the rotation variation amount relating to the arbitrary cylinder is oriented in a direction in which the rotation variation amount exceeds the predetermined threshold;
• and determine that a misfire has occurred in the multi-cylinder internal combustion engine when the predetermined condition is not established, such that the misfire determination apparatus does not erroneously determine a misfire due to an effect of a temporary increase in an engine rotation velocity.