The Four Stroke Diesel's Operation Cycle

The combustion cycle of a 4-stroke Diesel Engine is quite similar to that of a Gasoline 4-stroke engine.

You have Intake, Compression, Ignition, and Exhaust, except that in the Diesel Engine, you do not have an ignition 'source', i.e. spark plug, nor is the fuel pre-mixed with the air before the compression stroke ever begins as in a Gasoline engine. Gasoline pistons are also typically flat, diesel pistons on the other hand, almost always have a 'bowl'

Compare Gasoline Piston to Diesel Piston) -Figure 1-

where the actual combustion event occurs. The piston also comes much closer to the cylinder head, as close as .005 The Cylinder Head is also flat, as it does not have a 'combustion chamber'

Compare a Gasoline Head to a Diesel Head -Figure 2-

Intake: Air is drawn in, and the piston falls to the bottom of the cylinder, intake valve closes, and piston begins to come up.

Compression: The air is becoming compressed and is now becoming warmer, and as the piston nears the top of it's stroke, the air is near 500degrees (must be at least 500 for a successful cold start), and just as it reaches the top of it's stroke, the Diesel Fuel Injector Injects a highly metered amount of fuel which is almost immediately ignited, although quality of fuel and it's Cetane Rating (similar to Octane, which is the resistance to self ignition, but the reverse, Cetane is the fuel's ability to self ignite)

(See The Cetane Rating and why it's Important Article, for more info)

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Power Stroke: The fuel has begun igniting at the top of the piston's stroke, but because Diesel Fuel has an inherent delay of ignition (the higher the Cetane Rating the shorter this delay is) it takes a split second before it fully ignites. The fuel is igniting, and the piston is being forced down, fuel continues to burn until the piston nears the bottom of it's stroke.

Exhaust: The piston has now reached the bottom of it's stroke for the second time, and now the exhaust valve opens, and the piston forces the exhaust out. The exhaust may go straight to the exhaust system, or it may be scavenged to spool a turbocharger, which forces intake air into the cylinder at higher than athmospheric pressures, or 'boosts' the intake charge.