How Gasoline Engines Work

Power Stroke
At a precisely timed moment, the spark plug ignites this highly volatile mixture, and the heat of combustion causes expansion, which forces the piston down in the cylinder. This force is applied to the crankshaft via the connecting rod, which is in turn connected to the vehicle's wheels via the transmission.

Exhaust Stroke
When the piston reaches the bottom of the power stroke, the exhaust valve in the cylinder head opens and the piston rises on its exhaust stroke. The burned fuel/air mixture is pushed out until the piston reaches the top of the cylinder once again at which point the exhaust valve closes and the whole process repeats itself. This goes on endlessly until the fuel tank is empty, or you arrive at Blockbuster.


To make one power stroke, the crankshaft must rotate two full revolutions; the mass of the heavy steel crankshaft keeps the engine coasting around during the three other strokes the piston must complete to be ready for the next power stroke. A one-cylinder engine powerful enough to move a car would be prohibitively rough and impractical. Besides, combustion efficiency tapers off when cylinders measure much more than about four inches across, so beyond a certain point, the path to more power is more cylinders, not one bigger cylinder. By adding multiple cylinders all driving the same crankshaft, designers can get the power they're after and gain operating smoothness.


The arrangement of the cylinders (in-line, in the shape of a V or whatever) also has a direct effect on the sound and smoothness of the engine (though not necessarily the power it produces). All those reciprocating pistons create vibration, so the cylinder layout and engine-balancing strategy that tames the shakes is both art and science. Each of the popular engine configurations has its own unique attributes and characteristics. In-line four cylinder engines are compact and light, and are easy to fit into the chassis design of a typical car, but they vibrate more than other commonly used designs. V-6 engines offer an excellent combination of smoothness and reasonable size. In-line 6 engines are generally smoother still, though their length can make them harder to package in a crowded engine bay. V-8s and V-12s are the answer when more power and smoothness is required, but they eat up underhood real estate with their sprawling dimensions. Flat or opposed engines (like those from Porsche or Subaru) can be a viable option if low engine height is more important to a car's design mission than minimizing overall width.


Though the basic principles of four-stroke engine design are constant, the engineering that goes into our cars advances at an impressive pace. Entire careers are devoted to optimizing the air flow in and out of engines and fine-tuning the design of the valves, cam systems and thousands of other details. Courtesy of that hard work, current engines are radically more powerful, fuel efficient, cleaner and lighter than the engines of only a generation ago. And every year that same hypothetical soda can of gasoline seems to pack a slightly bigger wallop.