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Published on Sep 30, 2010


Hw a four stroke engine works animation showing the four strokes involved in an internal combustion engine. Also shows the four individual strokes involved - induction, compression, power and exhaust, more easily remembered as suck squeeze bang and blow.

The four stroke engine is undoubtedly the one we use most during our everyday lives. It is used in most automobiles as it has high efficiency and high power. The diesel engine is used for more robust tasks such as transporting goods and people as it has more pulling power and higher torque.

As the name implies it works on four cycles -- induction, compression, power and exhaust.
The induction cycle is where the fuel and air mixture is sucked into the cylinders by the motion of the piston. The mechanics of the engine has opened the inlet port at the top of the cylinder, this allows the fuel air mixture in, the port has been opened by the camshaft. The piston is an airtight seal inside the cylinder, so when it moves downwards it creates a partial vacuum inside the cylinder which sucks the fuel and air mixture in. The fuel/air mixture is usually at a ratio of between 15:1 or 18:1, ie. 15 parts of air to one part of fuel for a typical 4 stroke gasoline engine.
During all four cycles there is a complex movement of all of the components of the engine, these are best explained by the video below so i will not get into them here as they are easier to understand when seen.

The compression cycle is when the piston is on its way back up inside the cylinder. The inlet port has now been closed by the camshaft, as a result the cylinder is now completely airtight inside. The upwards motion of the piston compresses the fuel and air mixture to a fraction of its original size. This also heats up the mixture and makes it easier to ignite -- when any gas is compressed it increases in temperature based on the gas laws. This happens because the individual molecules of the fuel air mixture are confined to a smaller space, as a result they have less room to move around in, this means they hit each other more often and their kinetic energy of motion is converted into heat energy.

The power cycle is the only point at which the engine creates energy. Now that the gas is compressed and heated at the top of the cylinder the spark plug emits a spark. This has been timed perfectly through the mechanisms of the engine to occur just after the piston has reached the highest point in its motion and is about to begin going down in the cylinder again, this point is called TDC (top dead centre). It is a similar idea as pushing a child on a swing -- you don't push them as the are coming towards you, you wait until they are just about to begin travelling away from you again, this means you can transmit a lot of energy to the child on the swing for the least amount of work.
When the spark plug ignites the fuel air mixture it rapidly increases in volume, again based on the gas laws -- when any gas is heated up its volume increases in proportion to its temperature. This results in the piston being moved downwards in the cylinder with great force. In the case of a single cylinder engine this is the only time the engine will produce power, therefore the engine needs to produce enough power in this one cycle for its momentum to power it through the other 3 cycles before it will produce power again.
The way an engine can keep the energy of the power stroke is by utilising a flywheel -- a flywheel is a large mass of metal that stores the energy over several cycles instead of allowing the engine to use all the power in one go, the flywheel also stabilises the whole engine by counterbalancing all of the parts of the engine.

The exhaust cycle is when the piston is now coming back up inside the cylinder. The camshaft is now opening the exhaust port. As the piston moves upwards it forces the exhaust gas out of the exhaust port and eventually out of the exhaust itself. When the piston reaches the very top of the cylinder the exhaust port will have closed and the inlet port will begin to open, allowing to engine to start the induction cycle again.

Hopefully the video above will clear up any issues and by the end of it you should understand the basic concepts and workings of the four stroke engine

engine animation, car engine, engines


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