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How does air compression heat the air enough to ignite diesel fuel?

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diesel engine and precombustion chamber — diesel engine Diesel engine equipped with a precombustion chamber.

Diesel engines, often called compression-ignition engines, rely on the heat generated by compressing air to initiate combustion rather than using an electric spark.

In a diesel engine, the compression ratio—typically between 14:1 and 22:1—is used to compress the air charge within the cylinder. This rapid compression increases the air temperature significantly, often exceeding 526 °C (979 °F). When fuel is injected into this environment, the air temperature is higher than the fuel's "auto-ignition" temperature, causing the fuel to spontaneously react with the oxygen in the air and burn.1

Because the engine relies on this heat of compression, the temperature of the air inside the cylinder is determined by both the engine's compression ratio and its current operating temperature. At start-up, when the engine is cold, supplemental heating may be required to ensure the air reaches the necessary temperature for ignition.1