What specific technical parameter must air reach inside the cylinder for fuel to spontaneously ignite in a Diesel engine?

Answer

A temperature sometimes surpassing $1000^{\circ}$ Fahrenheit.

The design relies on intense compression, which causes the air temperature to skyrocket, often exceeding $1000^{\circ}$ Fahrenheit, precisely when the fuel is injected to cause ignition.

What specific technical parameter must air reach inside the cylinder for fuel to spontaneously ignite in a Diesel engine?
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