Suck, squeeze, bang, blow

A way to remember the cycle of events that causes a normal four-stroke internal combustion engine to function.

These are the illustrations for an OHV/OHC engine. They would be slightly different for a flathead engine.

                      +=====================+
intake valve-->     --|  +----+
spark plug-->        -|- |    +------------------
exhaust valve-->    --|  +----+
                      +=====================+

Suck
The intake valve opens, the air/fuel mixture is sucked into the cylinder.                      
                      
                      +=====================+
                      --|           +----+  piston moves down-->
                     -|-            |    +------------------
                    --|             +----+
                      +=====================+
Squeeze
Both valves are closed, the piston moves upwards and squeezes the air/fuel mixture.
                      +=====================+
                    --| . .+----+  <--piston moves up
                     -|-. .|    +----------------
                    --| . .+----+
                      +==^==================+
                         |
                      squeezed fuel
Bang
Both valves are closed, the spark plug sparks and ignites the air/fuel mixture, forcing the piston downwards.
                       
                       +=====================+
                     --| *         +----+  piston forced down-->  
                      -|-**bang!   |    +------------------
                     --| *         +----+
                       +=====================+
Blow
The exhaust valve opens, exhaust is blown out of the cylinder.
                       
                       +=====================+
                     --|.  .+----+ <--piston moves up
                      -|-. .|    +------------------
                    .. --|. +----+
                    ^  +==^==================+
                    |     |
                    exhaust

Then, the whole thing starts over, several thousand times a minute.

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Suck The intake valve opens, the air/fuel mixture is sucked into the cylinder. +=====================+ --| +----+ piston moves down--> -|- | +------------------ --| +----+ +=====================+

Squeeze Both valves are closed, the piston moves upwards and squeezes the air/fuel mixture. +=====================+ --| . .+----+ <--piston moves up -|-. .| +---------------- --| . .+----+ +==^==================+ | squeezed fuel

Bang Both valves are closed, the spark plug sparks and ignites the air/fuel mixture, forcing the piston downwards. +=====================+ --| * +----+ piston forced down--> -|-**bang! | +------------------ --| * +----+ +=====================+

Blow The exhaust valve opens, exhaust is blown out of the cylinder. +=====================+ --|. .+----+ <--piston moves up -|-. .| +------------------ .. --|. +----+ ^ +==^==================+ | | exhaust