Diesel engines work by compressing only the air. This increases the air temperature inside the cylinder to such a high degree that atomised diesel fuel injected into the combustion chamber ignites spontaneously. ... They were originally used as a more efficient replacement for stationary steam engines.
Step 1: Intake (Down) Stroke 1: The piston moves down drawing air into the cylinder.
Step 2: Compression (Up) Stroke 1: The piston moves up compressing the newly drawn air in the cylinder. Before the piston reaches top-dead-center (TDC), diesel fuel is injected directly into the cylinder.