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)
where the actual combustion event occurs.
The piston also comes much closer to the
cylinder head, as close as .005
Cylinder Head is also flat, as it does not have a 'combustion
Compare a Gasoline Head to a Diesel Head
Air is drawn in,
and the piston falls to the bottom of the cylinder, intake valve closes, and
piston begins to come up.
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)
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
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.
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
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.