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4 STROKE ENGINES

Cylinders

Valves

Crank Shaft

Almost all cars, and some snowmobile models, currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are shown in the animation below. They are:

* Intake stroke

* Compression stroke

* Combustion stroke

* Exhaust stroke

engine description



Here's what happens as the engine goes through its 4 cycles:

1. The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.

2. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.

3. When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down.

4. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.

Now the engine is ready for the next cycle, so it intakes another charge of air and gas.

Notice that the motion that comes out of these engines is rotational. In an engine the linear motion of the pistons is converted into rotational motion by the crank shaft. The rotational motion is nice because we plan to turn (spin) the snowmobiles track with it anyway.

Now let's look at some of the elements that work together to make this happen.

Cylinders

The core of the engine is the cylinder, with the piston moving up and down inside the cylinder. The engine described above has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: in line, V or flat (also known as horizontally opposed or boxer).

Displacement

The combustion chamber is the area where compression and combustion take place. As the piston moves up and down the size of the combustion chamber changes. It has some maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement and is measured in liters or CCs (Cubic Centimeters, where 1,000 cubic centimeters equals a liter). Here are some examples:

* A chain saw might have a 40 cc engine.

* A motorcycle might have a 500 cc or a 750 cc engine.

* A sports car might have a 5.0 liter (5,000 cc) engine.

Most normal car engines fall somewhere between 1.5 liter (1,500 cc) and 4.0 liters (4,000 cc)

Generally, the displacement tells you something about how much power an engine can produce. A cylinder that displaces half a liter can hold twice as much fuel/air mixture as a cylinder that displaces a quarter of a liter, and therefore you would expect about twice as much power from the larger cylinder (if everything else is equal). So a 2.0 liter engine is roughly half as powerful as a 4.0 liter engine.

You can get more displacement in an engine either by increasing the number of cylinders or by making the combustion chambers of all the cylinders bigger (or both).

Spark plug

The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly.

Valves

The intake and exhaust valves open at the proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

The opening and closing system is called a camshaft. The camshaft has lobes on it that move the valves up and down.

Most modern engines have what are called overhead cams. This means that the camshaft is located above the valves. The cams on the shaft activate the valves directly or through a very short linkage. Rods linked the cam below to valve lifters above the valves. This approach has more moving parts and also causes more lag between the cam's activation of the valve and the valve's subsequent motion. A timing belt or timing chain links the crankshaft to the camshaft so that the valves are in sync with the pistons. Many high-performance engines have four valves per cylinder (two for intake, two for exhaust), and this arrangement requires two camshafts per bank of cylinders, hence the phrase "dual overhead cams."

Piston

A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings

Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

* They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.

* They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.

Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly.

Connecting rod

The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates.

Crank shaft

The crank shaft turns the pistons up and down motion into circular motion just like a crank on a jack-in-the-box does.

Cooling System

The cooling system in most cars consists of the radiator and water pump. Water circulates through passages around the cylinders and then travels through the radiator to cool it off. In a few cars as well as some motorcycles, the engine is air-cooled instead (You can tell an air-cooled engine by the fins adorning the outside of each cylinder to help dissipate heat.). Air-cooling makes the engine lighter but hotter, generally decreasing engine life and overall performance

Lubrication System

The lubrication system makes sure that every moving part in the engine gets oil so that it can move easily. The two main parts needing oil are the pistons (so they can slide easily in their cylinders) and any bearings that allow things like the crankshaft and camshafts to rotate freely. In most cars, oil is sucked out of the oil pan by the oil pump, run through the oil filter to remove any grit, and then squirted under high pressure onto bearings and the cylinder walls. The oil then trickles down into the sump, where it is collected again and the cycle repeats.

For more info on 4 stroke engines see How Stuff Works


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