SUCK-SQUEEZE-BANG-BLOW


Whoops! With a title like that you probably think you've hit the wrong website. That you were somehow spirited away from the upstanding, community-oriented Edmunds.com and dumped into one of those lewd, adult websites full of suggestive and controversial words. You know, like CNN's website.

While the above title may sound like something out of a Kenneth Starr report, it's really just a creative way of remembering the stages of the four-stroke cycle.

Hmmm, that doesn't sound much better, does it?

OK, let's start with the basics. Every internal combustion engine has cycles, which refer to how that particular engine processes fuel and air to make horsepower. There are two-stroke engines and diesel engines that, because of their relative obscurity, I'll leave out of this discussion. For this article we'll stick to the basic, four-stroke cycle that most of today's passenger cars use. The following text, and associated diagrams, will go through each stage of the four-stroke cycle. The diagram portrays a cross-section of an engine's combustion chamber with a piston moving up-and-down and two valves (intake and exhaust) at the top. There's also a spark plug at the top of the combustion chamber that fires during the ignition (BANG) cycle.

So, without further ado, let's take a close look at what is going on inside your vehicle's engine (The following diagrams and related text come courtesy of Chilton's Easy Car Care, 4th Edition):

  1. INTAKE (SUCK) The intake stroke begins with the piston near the top of its travel. As the piston begins its descent, the exhaust valve closes fully, the intake valve opens and the volume of the combustion chamber begins to increase, creating a vacuum (thus the term SUCK for this cycle). As the piston descends, an air/fuel mixture is drawn from the fuel injectors into the cylinder through the intake manifold. The intake stroke ends with the intake valve closed just after the piston has begun its upstroke.

  2. COMPRESSION (SQUEEZE) As the piston ascends, the air/fuel mixture is forced into the small chamber machined into the cylinder head. This compresses the mixture (SQUEEZE) until it occupies 1/8th to 1/11th of the volume that it did at the time the piston began its ascent. This compression raises the temperature of the mixture and increases its pressure, increasing the force generated during the ignition cycle.

  3. IGNITION (BANG) The air/fuel mixture is ignited (BANG) by the spark plug just before the piston reaches the top of its stroke so that a large portion of the fuel will have burned by the time the piston begins ascending again. The heat produced by combustion increases the pressure in the cylinder, pushing the piston down with great force. This is the cycle that actually creates power in an engine.

  4. EXHAUST (BLOW) As the piston approaches the bottom of its stroke, the exhaust valve begins opening and the pressure in the cylinder begins to force the gasses out around the valve. The ascent of the piston then forces nearly all the rest of the unburned gasses from the cylinder (BLOW). The cycle begins again as the exhaust valve closes, the intake valve opens and the piston begins descending and bringing a fresh charge of air and fuel into the combustion chamber.

Remember that this is a basic explanation of how the internal combustion engine works. Variations from this diagram include multiple valve and/or spark plug configurations for increased efficiency and domed pistons for higher compression ratios, to name just a few. But the basic process of SUCK-SQUEEZE-BANG-BLOW is the same for all four-stroke engines.

Now, go out and impress your friends with your knowledge of the internal combustion engine ... and the creative terminology you've come up with to describe it.

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