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.