Combustion Engine
The
most commonly used type of car engine today, is based on the Otto cycle,
named after its creator Nikolaus Otto. The term, four-stroke refers to
the four distinguished motions that the piston goes through during the
conversion of chemical energy into rotational energy that can be harnessed
for a practical use, in this case, to propel a car.
This
picture is used as a reference of the parts of the engine which are mentioned
on this page of the website.They
are described in greater detail on the PARTS page.
1.Intake
Stroke
The
first stroke of the cycle is described as the intake cycle, where the piston
that starts at the top of the cylinder chamber, begins to move downward.
At the same time that the piston begins it path downward, the intake valve
opens and allows air to be drawn into the cylinder chamber by the downward
moving piston. Also during this time a small quantity of gasoline is squirted
into the chamber through a fuel injector and mixes with the air. The gasoline
must be mixed with the air because liquid gasoline will not burn so it
must be vaporized by the injector and mixed with air.The
perfect air to gas ratio is 14 parts air to one part fuel.This
ratio is electronically controlled by a computer connected to the fuel
pump and injectors that supply the amount of fuel in relation to the amount
of air that the engine can draw into the cylinder.
2.
Compression Stroke
The
second stroke, also known as the compression stroke, begins with the closing
of the intake valve. As the intake valve closes a sealed chamber is created
between the piston and the top of the cylinder where the valves are located.
The piston then begins its upward path, the gasoline and air mixture is
compressed at a ratio of roughly 10:1. This ratio comes from the differences
in volume between the volume of the cylinder chamber at the top of the
pistons stroke compared to the volume of the cylinder chamber when the
piston is at the bottom of its path.The
greater that this ratio can be made the more power an engine can produce.When
cars a given 454 in3 or 5.0 liters, this is the total volume
ofall cylinders at their intake
stroke.Therefore the 454 in3
engine with 8 cylinders can hold 56.75 in3 per cylinder and therefore with
a compression ratio of 10:1 can compress this to 5.67 in3. This
compression creates a lot of pressure in the cylinder chamber.
3.
Combustion Stroke
The third stroke of the cycle, the power stroke refers to the combustion itself. Now that the cylinder chamber is full of highly compressed air and gasoline, a spark from a spark plug initiates an explosion in the chamber that causes a rapid expansion of the compressed mixture, resulting in the piston being forced downward very quickly. The expansion of gas, caused by the combustion is the single most important stage of the cycle. It is also very important that there are no leaks in the system or the pressure will be lost and result in a power loss.
4.
Exhaust Stroke
The
fourth and final stroke, is known as the exhaust stroke. Once the piston
reaches the bottom of its path after the explosion, all that remains in
the cylinder chamber is waste. Once the piston begins its movement upward
in the cylinder the exhaust valve opens and the piston forces the exhaust
out of the chamber and away from the engine. Following this exhaust removal
the intake valve opens allowing air to enter the chamber and continue the
cycle.
Pressure vs. Volume relationship
