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How a 4-Stroke Engine Works

Four-stroke process

Many who are new to automotive repair become confused when their question involves basic engine operation. Quite often, this is due to a lack of understanding as to how engines actually work. This quick primer should give you a basic understanding of the four-stroke engine process and how the major parts interact to create motion.

A reciprocating combustion engine is the most commonly used engine type in today's automotive. Both gasoline and diesel engines used the four-stroke process. In this article, we'll focus on the gasoline version, which is the most common engine on the road today. It's called a "four-stroke" because there are four basic "strokes" for the engine to turn through to create the crank motion that moves the car:

  1. Intake Stroke
  2. Compression Stroke
  3. Power Stroke
  4. Exhaust Stroke

With two or more cylinders working together, an engine's cylinders can go through each of these strokes once ever four revolutions. A four-cylinder engine, for example, may have one cylinder in any of these four stroke phases at all times. This creates continual motion as the Power Stroke, which actually produces the energy to turn the engine, creates the motion that pushes the other cylinders through their non-powered strokes.

How An Engine Operates

Imagine an engine block with four cylinders bored into it. Inside each cylinder is a piston whose head is exactly the circumference of the cylinder's bore. The bottom of these pistons connects to a common round shaft called the crankshaft. Each connection point is slightly offset from the central shaft so that when the piston pushes down, the shaft is turned, and when it reaches the other side, the piston is pushed back up.

As the pistons move up and down, the chambers on top of them, inside the cylinder, change size. During the Intake stroke, the piston pulls down and creates a vacuum into which air and fuel are pulled. On the Compression stroke, the piston pushes back up, compressing the air:fuel mix into a tight space. The Power stroke sees the air and fuel ignited, forcing the piston back down with the expanding gases. The Exhaust stroke pushes the piston back up, pushing out the exhaust gases from the spent fuel. To facilitate this intake and out-go of air, fuel, and exhaust, small valves open and close. The ignition in a gasoline engine happens when a spark plug sparks flame inside the cylinder at the beginning of the Power stroke.

Looking further atop the cylinders, we find "heads" which cover them. These heads house the valves and spark plugs responsible for the intake and exhaust cycles. On top of those cylinder heads are camshafts which turn to create the timing of the opening and closing of the valves. Spark is controlled either manually by a turning distributor, spinning in time with the engine's crank, or by a computer, which will also control fuel delivery.

The camshafts are turned by a belt or chain that is connected to the crankshaft at the core of the engine. So as the cylinders go through their strokes, thanks to that crankshaft being turned by each Power stroke in turn, so do the camshafts that time the injection and release of gases and fuel get turned by the same strokes.

When all is working well, a four-stroke engine is a synergy of motion. The opposite end of the crankshaft, by the way, is where the engine's power is given to the gearbox (transmission) to move the vehicle.