How four stroke engines work

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To understand the basic idea behind how a reciprocating four stroke internal combustion engine works, it is helpful to have a good mental image of how "internal combustion" works. One good example is a cannon. The type you have probably seen standing on the battlements of medieval castles, where the soldiers load the cannon with gun powder and a cannon ball. That is internal combustion. It is hard to imagine that having anything to do with engines.

Expansion gun (65299 bytes)

I don't recommend doing this at home kids, but you will have just made a cannon. When you introduce a spark, you ignite the fuel. The reason we are talking about such a device, is that this cannon uses the basic principle behind any reciprocating internal combustion engine. If you put a tiny amount of high-energy fuel (like petrol) in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. You can use that energy to propel an orange 500 feet. You can also use it for more interesting purposes. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and then if you can harness that energy in a useful way, what you have is the core of a four stroke or two 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 illustrated below. They are:

The intake stroke.
The compression stroke.
The power stroke
The exhaust stroke.

Before we start our explanation of what is going on in an engine, I will define some of the terminology. You can see below that the piston replaces the orange in the cannon. The piston is connected to the crank shaft by a connecting rod. As the crankshaft revolves, it has the effect of "resetting the cannon."

A. Camshaft
B. Tappet
C. Valve spring
D. Spark plug or glow plug
E. Exhaust port
F. Piston
G. Crankcase
H. Crankshaft
I. Big end
J. Conrod, or connecting rod, or just rod
K. Liner or cylinder
L. Gudeon pin or wrist pin
M. Combustion chamber
N. Inlet port
O. Valve

Now for how it all works.

	Step 1. The induction stroke. The piston starts at the top, the intake valve opens (on the left), and the piston moves down to let the engine take in a cylinder full of air and fuel (shown in light blue). This is the intake stroke. Only the tiniest drop of fuel needs to be mixed into the air for this to work. The correct ratio of petrol is 17:1 by mass.
	Step 2. The compression stroke. Then the piston moves back up and compresses this fuel/air mixture. Compressing the mixture makes the explosion more powerful.
	Step 3. The power stroke. When the piston reaches the top of its stroke, the spark/glow plug ignites the fuel. The charge in the cylinder explodes, driving the piston down.
	Step 4. The exhaust stroke. Once the piston hits the bottom of its stroke, the exhaust valve opens (on the right) and the exhaust leaves the cylinder to go out the silencer (not shown). Now the engine is ready for the next cycle, so it intakes another charge of air and gas. And we are back to step 1.

Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear (straight). In an engine the linear motion is converted into rotational motion by the crank shaft, and conrod.

You will also notice that the camshaft is in charge of correctly opening and closing the valves. The camshaft is driven by the crankshaft, but it is geared to turn at half the RPM of the crankshaft. You will be able to see in the above diagrams that if the crankshaft has done one revolution then the camshaft has only done half a revolution. The camshaft is specially shaped to open and close the valves at the correct time. Not all engines have the camshaft at the top of the engine. Some engines have it at the bottom and connect its movement to the tappets with a push rod, most model engines use this method rather than an overhead camshaft.

Now we will go back to our arrangement diagram of an engine and explain what some of the bits are:

Cylinder
The core of the engine is the cylinder. The piston moves up and down inside the cylinder. The engine described here has one cylinder. That is typical of most model engines, but some have more than one cylinder (four, six and eight cylinders are available)

Spark/Glow plug
The plug supplies the heat that ignites the air/fuel mixture so that combustion can occur.

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.

Piston
A piston is a cylindrical piece of metal that moves up and down inside the cylinder. The fit and finish between it and the cylinder is very sensitive, the better the fit the better the engine. Two methods exist of ensuring this fit is good

Piston rings are a hard metal ring that sits around the top edge of the piston, they are made of ring steel and keep themselves pushed outward into the liner. They effectively provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. Generally speaking, when cold, ringed engines have less compression and are therefore a bit easier to start.
ABC is the more modern approach. The piston is ever so slightly barrel shaped and made of Aluminium, and the liner is tapered with the narrow end at the top. The liner is made from a Chromed Brass, hence ABC. The idea is that when cold the piston and liner fit is very stiff, however at the optimum running temperature, the coefficients of expansion of the various metals are such that the fit between the two is perfect. Because of the lower component count (i.e. no rings) they are much more reliable at higher RPM's, in fact they are able to safely reach RPM's that ringed engines could only dream of, 40,000 plus. They are also harder to seize because the hotter the engine gets the further the liner gets away from the piston. The only draw back is because the piston and liner are tight when cold they can be hard to turnover and thus start.

Combustion chamber
The combustion chamber is the area where compression and combustion take place. As the piston moves up and down, you can see that 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 CCs (Cubic Centimetres, where 1,000 cubic centimetres equals a litre) or Cubic Inches (where 1cc = .06 cubic inches). Generally, the displacement tells you something about how much power an engine has. A cylinder that displaces 2cc can hold twice as much fuel/air mixture as a cylinder that displaces 1cc, and therefore you would expect about twice as much power from the larger cylinder (if everything else is equal).

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. The connection to the crankshaft is called the big end, and the connection to the piston is called the little end. The inner part of the little end is a steel pin that goes through the piston, this pin is called the gudeon pin.

Crank shaft
The crank shaft turns the piston's up and down motion into circular motion.

Using all of this information, you can begin to see that there are lots of different ways to make an engine perform better. Manufacturers are constantly playing with all of the following variables to make an engine more powerful and/or more fuel efficient.

Increase displacement -- More displacement means more power because you can burn more fuel during each revolution of the engine. It isn't practical to alter the displacement on model engines but it is fairly easy to buy a bigger engine.

Increase the compression ratio -- Higher compression ratios produce more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst into flame (before the spark/glow plug ignites it). Better quality fuel can prevent this sort of early combustion, but it is a fine balance

More air/fuel into each cylinder -- If you can cram more air (and therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would by increasing the size of the cylinder). Turbochargers and super chargers pressurize the incoming air to effectively cram more air into a cylinder. Although model engines are yet to be seen with turbochargers, some do come with superchargers.

Cool the incoming air -- Compressing air raises its temperature. However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand when combustion takes place. Therefore, many turbo charged and super charged cars have an intercooler. An intercooler is a special radiator through which the compressed air passes to cool it off before it enters the cylinder.

Let air come in more easily -- As a piston moves down in the intake stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves in each cylinder. Some newer engines are using polished intake manifolds to eliminate air resistance there.

Let exhaust exit more easily -- If air resistance makes it hard for exhaust to exit a cylinder, it robs the engine of power. Ultimate power is developed with no exhaust, but this tends to upset the neighbours, so you must strike a compromise between power and noise.

Make everything lighter -- Lightweight parts help the engine perform better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another. The lighter the piston, the less energy it takes.

Inject the fuel -- Fuel injection allows very precise metering of fuel to each cylinder. This improves performance and fuel economy.