compression failure. A type of structural failure caused by the application of too much compressive stress. Compression failure causes the material to buckle or collapse.
compression ignition. The form of ignition used in diesel engines. Air inside the cylinder of the engine is compressed, and the compression heats the air enough that when fuel is sprayed into it, the fuel ignites and burns.
compression ignition engine. A form of reciprocating engine which uses heat produced by compressing the air in the cylinder to ignite the fuel.
A diesel engine is a compression-ignition engine. The piston, moving upward in the cylinder, compresses and heats the air. When the piston is near the top of its stroke, fuel is sprayed into the hot air, and it ignites and burns.
compression molding (plastics manufacturing process). A method of molding thermosetting resins. A mold is opened and heated, and resin is put in it. The mold is then closed, and pressure is applied. The resin flows under pressure and completely fills the cavity of the mold. The mold is kept closed and heated until the resin is completely cured.
compression ratio (reciprocating engine parameter). The ratio of the volume of a cylinder of a reciprocating engine with the piston at the bottom of its stroke to the volume of the cylinder with the piston at the top of its stroke.
compression ratio (turbine engine parameter). The ratio of the pressure of the air at the discharge of a turbine engine compressor to the pressure of the air at the inlet.

compression rib (aircraft structural member). A special rib used in a Pratt truss airplane wing to take the place of a separate compression strut. A compression rib has strong cap strips and webs designed to withstand compressive loads.
compression rings (reciprocating engine piston rings). The top rings used on reciprocating engine pistons to seal the piston in the cylinder so the upward-moving piston can compress the air in the cylinder. Compression rings also transfer heat from the piston into the cylinder wall. See illustration for scraper rings.

compression spring. A form of spring which resists a force trying to compress it, or squeeze it together. Open coil springs are often used as compression springs.
compression stress. A type of stress that tries to squeeze the ends of an object closer together. A compression stress is also called a compressive stress.
compression strut (airplane wing structural member). A heavy structural member, often in the form of a steel tube, used to hold the spars of a Pratt truss airplane wing apart. A compression strut opposes compressive loads between the spars put there by the tensile loads from the drag and anti-drag wires.
compression test (reciprocating engine maintenance test). A maintenance test performed on a reciprocating engine to determine the ability of the cylinders to hold air pressure. A cylinder will fail a compression test if an excessive amount of air leaks past the piston rings, or if any air leaks past the intake or exhaust valve.