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Why can diesel engines be difficult to start in cold conditions?

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

Diesel engines rely on compression-ignition to operate, meaning they initiate combustion by injecting fuel into air that has been heated to a high temperature through compression. Because the temperature of the air within the cylinders at start-up is determined by both the engine's compression ratio and its current operating temperature, cold conditions can prevent the air from reaching the necessary "auto-ignition" temperature required for the fuel to spontaneously react and burn.1

To overcome this, diesel engines often employ supplemental heating during start-up to ensure the combustion chamber reaches the required temperature. This is frequently achieved using a glow plug, which is a heating element located in the cylinder designed to preheat the air and facilitate starting.12