suck, squeeze, bang, blow

In a four-stroke internal combustion engine, the piston makes four distinct strokes inside the cylinder, and each stroke has a job: intake, compression, combustion, and exhaust. Mechanics and engineers use the phrase "suck, squeeze, bang, blow" as a memorable way to describe this sequence. It is crude but precise, and it sticks in the mind better than formal terms do.

The first stroke, "suck," draws a fresh charge of air and fuel into the cylinder as the piston moves away from the cylinder head (intake valve open, exhaust valve closed). The second stroke, "squeeze," compresses that charge as the piston moves back toward the head (both valves now closed). At the top of the stroke, ignition happens: "bang." A spark plug ignites the compressed mixture, and the expanding gases push the piston back down with force. The fourth stroke, "blow," expels the burnt gases through the open exhaust valve as the piston rises again.

This cycle then repeats. In a running engine, multiple cylinders are staggered so that one is always on its power stroke, delivering a steady stream of pushing force to the crankshaft. The timing of the valve openings and closings, controlled by the camshaft, is critical: overlap them by even a few degrees too much or too little, and combustion efficiency suffers, fuel economy drops, and emissions rise.

The four-stroke cycle is the dominant design in automotive and small industrial engines because it is mechanically robust and thermally efficient compared to two-stroke designs. It requires more parts: two camshafts and multiple valves per cylinder in modern engines, plus more complex valve timing systems. But this complexity pays off in durability and power output over the engine's life.

The phrase comes directly from the German engineer Nikolaus Otto, whose 1876 patent described this sequence. It is shop shorthand, used by technicians diagnosing valve timing problems, tuning fuel injection, or explaining wear patterns in piston rings. When a mechanic says an engine "lost compression," understanding suck-squeeze-bang-blow helps identify whether the problem is in the intake, the compression ratio, the spark, or the exhaust scavenging.