Suck, squeeze, bang, blow.
Not a sexual maneuver, but rather the common description for how an internal combustion engine works. The basic way all internal combustion engines work is to suck in a mixture of fuel and air, compress it, ignite it either with a spark plug or by self-igntion (in the case of a diesel engine), allow the explosion of combusting gasses to force the piston back down and then expel the exhaust gas. The vertical movement of the piston is converted into rotary motion in the crank via connecting rods. The crank then goes out to the gearbox via a flywheel and clutch, and the gearbox sends the rotary motion to the wheels, driving the vehicle forwards.

The following diagram is for reference for the technical jargon that will pop out on the rest of this page. It shows an inline- 4 engine with dual overhead cams.

Engine layouts
Below are some illustrations of the most common types of cylinder layout you'll find in engines today. Singles are typically used in motorbikes, snowblowers, chainsaws etc. V-twins are also found in motorbikes. The triple is almost unique to Triumph motorbikes where they call it the Speed Triple, or the 675. Inline-fours are the mainstay of car engines, as well as being found in some motorbikes too such as the BMW K1200S. Inline fives used to be used a lot in Audis but have found a new home in current Volvos. The V5 is something you'll find in some VWs. The V6 has the benefits of being smoother than an inline-four but without the fuel economy issues of a V8. Boxer engines are found in BMW motorbikes (twins) and Porsches and Subarus (fours and sixes). You had no idea, did you?

The difference between 4-stroke and 2-stroke engines
First, some basic concepts. Well one basic concept really - the most common types of internal combustion engine and how they work. It's worth reading this bit first otherwise the whole section on octane later in the page will seem a bit odd.The Almost every car sold today has a 4-stroke engine. So do a lot of motorbikes, lawnmowers, snowblowers and other mechanical equipment. But there are still a lot of 2-stroke engines about in smaller motorbikes, smaller lawnmowers, leaf-blowers, snowblowers and such.
The difference between the two engine types is the number of times the piston moves up and down in the cylinder for a single combustion cycle. A combustion cycle is the entire process of sucking fuel and air into the piston, igniting it and expelling the exhaust. There are two other types of engines - rotary or wankel engines, only used by Mazda in their "R" sports cars, and diesel engines. As this page gets updated, those will be dealt with accordingly.
2-stroke engines
A 2-stroke engine is different from a 4-stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to premix the oil and petrol yourself in a container, then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2-stroke oil and then the engine has a small pump which mixes the oil and petrol together for you. The simplicity of a 2-stroke engine lies in the reed valve and the design of the piston itself. The picture on the right shows a 4-stroke piston (left) and a 2-stroke piston (right). The 2-stroke piston is generally taller than the 4-stroke version, and it has two slots cut into one side of it. These slots, combined with the reed valve, are what make a 2-stroke engine work the way it does. The following animation shows a 2-stroke combustion cycle. As the piston (red) reaches the top of its stroke, the spark plug ignites the fuel-air-oil mixture. The piston begins to retreat. As it does, the slots cut into the piston on the right begin to align with the bypass port in the cylinder wall (the green oblong on the right). The receding piston pressurises the crank case which forces the reed or flapper valve (purple in this animation) to close, and at the same time forces the fuel-airoil mixture already in the crankcase out through the piston slots and into the bypass port. This effectively routes the mixture up the side of the cylinder and squirts it into the combustion chamber above the piston, forcing the
exhaust gas to expel through the green exhaust port on the left. Once the piston begins to advance again, it generates a vacuum in the crank case. The reed or flapper valve is sucked open and a fresh charge of fuel-air-oil mix is sucked into the crank case. When the piston reaches the top of its travel, the spark plug ignites the mixture and the cycle begins again.

For the same cylinder capacity, 2-stroke engines are typically more powerful than 4-stroke versions. The downside is
the pollutants in the exhaust; because oil is mixed with the petrol, every 2-stroke engine expels burned oil with the exhaust.
2-stroke oils are typically designed to burn cleaner than their 4-stroke counterparts, but nevertheless, the 2-stroke engine
can be a smoky beast. If, like me, you grew up somewhere in Europe where scooters were all the rage for teenagers, then
the mere smell of 2-stroke exhaust can bring back fond memories. The other disadvantage of 2-stroke engines is that they
are noisy compared to 4-stroke engines. Typically the noise is described as "buzzy".
4-stroke engines
4-stroke engines are typically much larger capacity than 2-stroke ones, and have a lot more complexity to them. Rather
than relying on the simple mechanical concept of reed valves, 4-stroke engines typically have valves at the top of
the combustion chamber. The simplest type has one intake and one exhaust valve. More complex engines have two of one
and one of the other, or two of each. So when you see "16v" on the badge on the back of a car, it means it's a 4-
cylinder engine with 4 valves per cylinder - two intake and two exhaust - thus 16 valves, or "16v". The valves are opened
and closed by a rotating camshaft at the top of the engine. The camshaft is driven by either gears directly from the crank,
or more commonly by a timing belt.
The following animation shows a 4-stroke combustion cycle. As the piston (red) retreats on the first stroke, the intake
valve (left green valve) is opened and the fuel-air mixture is sucked into the combustion chamber. The valve closes as
the piston bottoms out. As the piston begins to advance, it compresses the fuel-air mix. As it reaches the top of it's stroke,
the spark plug ignites the fuel-air mix and it burns. The expanding gasses force the piston back down on its second stroke.
At the bottom of this stroke, the exhaust valve (right green valve) opens, and as the piston advances for a second time,
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it forces the spent gasses out of the exhaust port. As the piston begins to retreat again, the cycle starts over, sucking a
fresh charge of fuel-air mix into the combustion chamber.

Introduction: The History
Many people claimed the invention of the internal combustion engine in the 1860's, but only one has the patent on the four stroke operating sequence. In 1867, Nikolaus August Otto, a German engineer, developed the four-stroke "Otto" cycle, which is widely used in transportation even today. Otto developed the four-stroke internal combustion engine when he was 34 years old.
The Diesel Engine came about in 1892 by another German engineer, Rudolph Diesel. The Diesel engine is designed heavier and more powerful than gasoline engines and utilizes oil as fuel. Diesel engines are a commonly used in heavy machinery, locomotives, ships, and some automobiles.
It is important to mention that the basic operating principles of these engines have been around for more than a hundred years and they are still in place. Some people get discouraged when they look under the hood and cannot recognize a thing on their automobile. Rest assured that underneath all of those wires and sensors lies an engine with the same basic operating principles of that "Otto" engine over a century old.
The Internal Combustion Engine:
Before explaining the operation of the four-stroke engine, some of the internal parts must be identified. Refer to the drawing of the basic internal combustion engine. Throughout the presentation, these parts are mentioned, so an understanding of what they do should be helpful. The Intake Valve opens at a precise time to allow the air/fuel mixture to enter the cylinder. The Exhaust Valve opens at a precise time to allow the burned gases to leave the cylinder. The Spark Plug ignites the air/fuel mixture in the cylinder, which creates an explosion. The force of the explosion is transferred to the Piston. The piston travels up and down in a Reciprocation Motion. The force from the piston is then transferred to the Crankshaft through the Piston Rod (connecting rod). The piston rod converts the reciprocating motion of the piston, to the Rotating Motion of the crankshaft. Now that the basic parts are identified, lets go through the four strokes of the internal combustion engine, which are Intake, Compression, Power, and Exhaust.



The Intake Stroke:
On the intake stroke, the intake valve has opened. The piston is moving down, and a mixture of air and vaporized fuel is being pushed by atmospheric pressure into the cylinder through the intake valve port.


The Compression Stroke:
After the piston reaches the lower limit of its travel, it begins to move upward. As this happens, the intake valve closes. The exhaust valve is also closed, so the cylinder is sealed. As the piston moves upward, the air/fuel mixture is compressed. On some small high compression engines, by the time the piston reaches the top of its travel, the mixture is compressed to as little as one-tenth its original volume. Thus, the compression of the air/fuel mixture increases the pressure in the cylinder. The compression process also creates the air/fuel mixture to increase in temperature.


The Power Stroke:
As the piston reaches the top of its travel on the compression stroke, an electric spark is produced at the spark plug. The ignition system delivers a high voltage surge of electricity to the spark plug to create the spark. The spark ignites the air/fuel mixture. The mixture burns rapidly and cylinder pressure increases to as much as (600psi). All of this pressure against the piston forces it down in the cylinder. The power impulse is transmitted down through the piston, through the piston rod (connecting rod), and to the crankshaft. The crankshaft is rotated due to the force.


The Exhaust Stroke:
As the piston reaches the bottom of its travel, the exhaust valve opens. Now, as the piston moves up on the exhaust stroke, it forces the burned gases out of the cylinder through the exhaust port. When the piston reaches the top of its travel, the exhaust valve closes, and the intake valve opens. The cycle repeats again with the intake stroke. The four strokes are continuously repeated during the operation of the engine.


The Diesel Engine:
Not much is different in the Diesel engine. In the Diesel, the fuel is not mixed with the air entering the cylinder during the intake stroke. Air alone is compressed during the compression stroke. The Diesel fuel oil is injected or sprayed into the cylinder at the end of the compression stroke. In Diesel engines, compression ratios are as high as (22.5 to 1) and provide pressures of (500psi) at the end of the compression stroke. Through the compression process, the air can be heated up (1000 degrees F). This temperature is high enough to spontaneously ignite the fuel as it is injected into the cylinder. The high pressure of the explosion forces the piston down as in the gasoline engine.

Summary:
The four strokes of the internal combustion engine are as follows (and in order): Intake, Compression, Power, and Exhaust. These four strokes require two revolutions of the crankshaft. The process continuously repeats itself during the operation of the engine. So, if a four-cylinder engine requires two complete revolutions of the crankshaft to ignite all of its cylinders, how many revolutions does an eight-cylinder engine require. Hopefully you answered two. It only takes two revolutions of the crankshaft to fire all of the cylinders of any four-stroke engine. The connecting rod converts the reciprocating motion (up and down) of the piston to the rotating motion of the crankshaft. The Diesel engine differs from the gasoline engine in that the intake stroke only pulls in air, not air and fuel. The fuel is injected into the cylinder at the end of the compression stroke. The fuel burns immediately (without the use of a spark plug) because of the high temperature of air in the cylinder.