How Gasoline Engines Work

How Gasoline Engines Work

The miracles of internal combustion

Gasoline is incredibly powerful stuff. Just a single soda can of it packs enough energy to blast a typical car close to three miles. Not in one huge explosion, of course (that would definitely spill your latte), but in a series of about 13,000 small explosions contained in your engine. Still plenty impressive, albeit somewhat less dramatic.

More than a century of development has gone into making the modern-day automotive engine a remarkably effective device for converting fuel and air into useful, reliable forward motion. It does this through the miracle of internal combustion; as the name implies, the engine burns fuel in controlled, measured amounts inside of a combustion chamber. (Actually many of them.) This burning fuel mixture makes heat, which causes a powerful expanding force. The engine uses this expanding force to drive the vehicle's wheels.

There are a number of ways to accomplish this, but the most widely used design is the four-stroke reciprocating-piston engine. The basic power unit of a piston engine is the cylinder. It's a precisely made metal tube that's open at the bottom and sealed at the top by a component called the cylinder head. The cylinder head has an intake valve and an exhaust valve built into it, both of which can be opened on cue by a camshaft. A sparkplug is also screwed into the cylinder head, with its business end pointing into the cylinder below. A close-fitting piston slides up and down in the cylinder, and is connected to the crankshaft with the aptly named connecting rod. So as the crankshaft spins, the piston travels up and down (reciprocates) in the cylinder. Each of these motions is referred to as a stroke, and there are four strokes in a four-stroke engine-no surprise there. Metal seals around the sides of the piston (called piston rings) keep the pressure above the piston where it belongs, instead of letting it sneak out between the cylinder and piston and go places it shouldn't. A thin film of oil on all the moving parts keeps them from grinding themselves to bits, while a liquid cooling system carries away the substantial heat that is created during operation.

Intake Stroke

Let's take a look at each of the four strokes (sometimes called cycles) of a modern engine. Picture the piston in its uppermost position, almost touching the cylinder head. The small, trapped volume of air between the piston and head is caught in an area called the combustion chamber. If we rotate the crankshaft in ultra slow motion from this point, the piston descends, which increases the space above it. Pressure in the cylinder drops as a result. The intake valve in the cylinder head opens automatically at this point to allow a fresh mixture of air and a controlled amount of fuel to flow from the higher outside atmospheric pressure into the low-pressure area inside the cylinder. The piston reaches the lowest point of its stroke as the intake valve closes, leaving the cylinder full of a mixture of air and fuel. One of the four strokes-the intake stroke-is now complete.

Compression Stroke

As we continue to rotate the crankshaft the same direction, the piston begins to rise in the cylinder. As it does, the pressure in the cylinder increases dramatically until the point at the top of this "compression" stroke where pressure is about ten times more than it was at the bottom of the stroke only a moment before.

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.

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