Lesson 2/Learning Event 2
FIGURE 47. COMPRESSION STROKE.
The crankshaft next starts the piston up on the compression stroke. The two-stroke-cycle diesel engine
compresses or squeezes the air, just as the four-stroke-cycle diesel engine does, to very high pressure. To
compress the air to very high pressure, the engine must have a high compression ratio. Compression ratio
means how much LESS space there is above the piston when it is all the way up than when it is all the way
down. If there is eight times more space above the piston when it is all the way down, then the compression
ratio is eight to one or 8:1. This means that when the piston moves all the way up, it will have squeezed the
air into a space that is eight times smaller. Many gasoline engines use the 8:1 compression ratio. Most
compression-ignition engines have from 15 to 22:1 compression ratios. The more air is compressed, the hotter
it gets.
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