Actually, it is Otto, as in 4-stroke spark ignition.

nate

A 4-stroke diesel is still an Otto engine. An Otto cycle engine is one
with four strokes, intake, compression, power, and exhaust. Doesn't
care whether SI or CI.

No, it really isn't, at least not in common usage. If you want to be
ridiculously pedantic, then the "Diesel cycle" is an Otto cycle with
constant-pressure combustion under varying cylinder volume, whereas what
is commonly just called the "Otto Cycle" is a spark-ignited Otto cycle
with constant-volume combustion under varying cylinder pressure.

Neither cycle attains that ideal, so there's some overlap, but if you
look at real-world pressure-volume diagrams for Diesel and
spark-ignition engines, the difference is obvious.