Four stroke cycle engine

This section looks at major components of the four strokes and two petrol engines.

Because they operate on similar principles, many of their components is similar too. For example their engine blocks both operate under working conditions, a very high pressures and temperatures. And new designs and materials to make them work more efficiently are always being tested. Many vehicles still use cast iron but some have aluminum alloy blocks. The main game is a saving in white.

Aluminum alloys can be made as strong as cast iron but they are lighter. This is also true for cylinder heads. Combustion chambers in the cylinder head come in many different designs. To help haul in the extreme heat and pressure of combustion, a seal is made between the block and cylinder head by a head gasket.

The four stroke petrol engine has other passages cast into the head to carry oil for lubrication and coolant for cooling.

What is a stroke? It is the movement of the piston from top dead center to bottom dead center. Calling an engine four stroke means its engine cycle has four strokes. A four stroke petrol engine uses internal combustion so those four strokes must include the five key events common to all internal combustion engines; intake, compression, ignition, power, and exhaust.

Let us look at a simplified model. The intake strokes starts with the exhaust valve close, in inlet valve opening and the piston at its highest point top dead center. It starts to move down increasing the volume about the top of the piston. This makes pressure inside the cylinder lower than pressure outside. This higher outside air pressure forces air fuel mixture into the cylinder. The piston reaches bottom dead center, the inlet valve closes and the intake stroke ends. Both valves stay close as the piston leaves bottom dead center. The piston moves up, squeezing the air fuel mixture into a smaller and smaller volume which compresses it. That makes its temperature rise and that makes combustion easier. Just before the piston reaches top dead center, the next key event occurs, ignition. Burning expanding gas generates the force to push down the piston. This is the power stroke that drives the engine. The piston now moves from bottom dead center to top dead center. The exhaust valve opens and the piston pushes out the left over gases.

Let us look at the cycle operating. Intake takes in air fuel mixture. Compression squeezes the air fuel mixture into a smaller and smaller volume. Ignition, the mixture and the pressure is burned. Power, burning expanding gases pushes the piston down creating power strokes that turns the crank shaft. Exhaust, the pistons move upward forcing burned gases from the chamber. All five events occur in this order in each of the engine’s cylinders.


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