Straddled vehicle engine unit and straddled vehicle

A straddled vehicle engine unit mountable to a straddled vehicle,

• the straddled vehicle engine unit comprising:
• an internal combustion engine, including
• a crankshaft, and
• a crank angle signal output unit that periodically outputs a crank angle signal in accordance with rotation of the crankshaft,
• the internal combustion engine being configured to repeat at least two kinds of strokes on a cycle of every 720-degree rotation of the crankshaft;
• and
• a misfire detection device, including a crankshaft rotation speed fluctuation physical quantity acquisition unit and a misfire determination unit, wherein
• the crankshaft rotation speed fluctuation physical quantity acquisition unit is configured to acquire a physical quantity related to an amount of fluctuation in a rotation speed of the crankshaft in one kind of stroke, out of the at least two kinds of strokes, based on the crank angle signal from the crank angle signal output unit, and
• the misfire determination unit is configured to perform a misfire determination in response to each acquisition of the crankshaft rotation speed fluctuation physical quantity, whereby the misfire determination unit determines that a misfire has occurred in the internal combustion engine upon detecting all of:
• lowering of the rotation speed of the crankshaft at a first preceding timing based on a differential between a first preceding physical quantity and a second preceding physical quantity, the first preceding physical quantity being the physical quantity acquired at the first preceding timing, the second preceding physical quantity being the physical quantity acquired at a second preceding timing, the first preceding timing corresponding to the one kind of stroke before an acquisition timing at which the crankshaft rotation speed fluctuation physical quantity is acquired, the second preceding timing corresponding to the one kind of stroke before the first preceding timing,
• completion of the lowering of the rotation speed at the acquisition timing based on a differential between the crankshaft rotation speed fluctuation physical quantity and the first preceding physical quantity, completion of the lowering of the rotation speed corresponding to rising or maintaining of the rotation speed, and
• a situation where the rotation speed of the crankshaft at the second preceding timing is within a predetermined range based on a differential between a third preceding physical quantity and the second preceding physical quantity, the third preceding physical quantity being a physical quantity that is acquired at a third preceding timing corresponding to the one kind of stroke before the second preceding timing, wherein
• the predetermined range has an upper limit of a positive value, and a lower limit of a negative value.