Engine Problems
(Source: http://www.howstuffworks.com)

So you go out one morning and your engine will turn over but it won't start... What could be wrong? Now that you know how an engine works, you can understand the basic things that can keep an engine from running. Three fundamental things can happen: a bad fuel mix, lack of compression or lack of spark. Beyond that, thousands of minor things can create problems, but these are the "big three." Based on the simple engine we have been discussing, here is a quick rundown on how these problems affect your engine:
Bad fuel mix - A bad fuel mix can occur in several ways:

• You are out of gas, so the engine is getting air but no fuel.
• The air intake might be clogged, so there is fuel but not enough air.
• The fuel system might be supplying too much or too little fuel to the mix, meaning that combustion does not occur properly.
• There might be an impurity in the fuel (like water in your gas tank) that makes the fuel not burn.

Lack of compression - If the charge of air and fuel cannot be compressed properly, the combustion process will not work like it should. Lack of compression might occur for these reasons:

• Your piston rings are worn (allowing air/fuel to leak past the piston during compression).
• The intake or exhaust valves are not sealing properly, again allowing a leak during compression.
• There is a hole in the cylinder.
  

The most common "hole" in a cylinder occurs where the top of the cylinder (holding the valves and spark plug and also known as the cylinder head) attaches to the cylinder itself. Generally, the cylinder and the cylinder head bolt together with a thin gasket pressed between them to ensure a good seal. If the gasket breaks down, small holes develop between the cylinder and the cylinder head, and these holes cause leaks.

Lack of spark - The spark might be nonexistent or weak for a number of reasons:

• If your spark plug or the wire leading to it is worn out, the spark will be weak.
• If the wire is cut or missing, or if the system that sends a spark down the wire is not working properly, there will be no spark.
• If the spark occurs either too early or too late in the cycle (i.e. if the ignition timing is off), the fuel will not ignite at the right time, and this can cause all sorts of problems.
  

Many other things can go wrong. For example:

• If the battery is dead, you cannot turn over the engine to start it.
• If the bearings that allow the crankshaft to turn freely are worn out, the crankshaft cannot turn so the engine cannot run.
• If the valves do not open and close at the right time or at all, air cannot get in and exhaust cannot get out, so the engine cannot run.
• If someone sticks a potato up your tailpipe, exhaust cannot exit the cylinder so the engine will not run.
• If you run out of oil, the piston cannot move up and down freely in the cylinder, and the engine will seize.
  

In a properly running engine, all of these factors are within tolerance.

p.s. click here to see the structure of engine in 3D-animation. Fantabulousent!