HOW DIESEL ENGINES WORK
The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed, hot air to ignite the fuel rather than using a spark plug.
In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) (about 200 psi) in the petrol engine. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber, with spark the diesel get burned & get started to get load & cycle get repeated on each movement of pistons.

HOW DIESEL ENGINES WORK
The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed, hot air to ignite the fuel rather than using a spark plug.
In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) (about 200 psi) in the petrol engine. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber, with spark the diesel get burned & get started to get load & cycle get repeated on each movement of pistons.