||At the heart
of any car is its engine, affectionately refered
to as the powerplant, motor, or prime mover
depending on which circles one frequents. when
someone asks what you have under the hood, they
are refering to the engine.
The engine's primary
purpose is to convert fuel and oxygen into
motion. It does this through the combustion
process which is simply a controlled explosion of
the gasoline and air mixture. The combustion
process is comprised of four primary functions or
cycles. Here's how each of the four cycles
Stroke: We'll start with the piston at its
highest point, referred to as top-dead-center.
From this point, the piston is pulled down by the
rotating crankshaft through the connection rod.
It is during this cycle that the intake valve
(see intake system) is open allowing the
air-fuel mixture into the combustion chamber.
When the piston reaches the bottom of its travel,
the intake valve closes.
compression stroke: As the crankshaft
continues to rotate, it begins to push the piston
up, again through the connecting rod. This upward
movement of the piston compresses the air-fuel
mixture, making it more dense. This compression
continues until the piston again reaches the top
of its travel.
stroke: At the point at which the piston
reaches the top of its travel, a pulse of high
voltage (see ignition system) is sent to the spark
plug, causing a spark to jump across the gap
between the electrodes. This spark ignites the
air-fuel mixture forcing the piston downward,
thus turning the crankshaft. The timing of the
combustion is such that the explosion of the
air-fuel mixture lasts for approximately the same
amount of time that it takes for the piston to
reach the bottom of its travel.
Stroke: Once the air-fuel mixture has burned,
the byproducts of the combustion must be removed
from the combustion chamber. This is handled
during the exhaust stroke. As the rotating
crankshaft again pushes the piston upward after
the power stroke, the exhaust valve opens,
allowing the piston to push the exhaust gasses
out of the combustion chamber so that a fresh
air-fuel mixture can be drawn in on the next
intake stroke. The exhaust valve remains open
until the piston again reaches top-dead-center at
which point it closes. The engine is now ready
for the next intake stroke.
This cycle is repeated
over and over again as the engine runs. Since most cars
have 4, 6, or 8 cylinders, each cylinder will go through
these same 4 cycles. Normally, each cylinder is timed
such that there is equal crankshaft rotation between the
the power strokes of each cylinder, allowing a smoother
operating engine. One could imagine how rough an engine
would operate if all of its cylinders were on the power
stroke at the same time.
The rotating crankshaft
of the engine transfers the linear motion of all of an
engine's pistons into a circular motion, but that motion
must be transfered from the engine to the wheels of the
car. This is dealt with in the drive
Drivetrain, Electronic Controls, Emission Controls, Tire Formula, Gear Math, Ignition System,
System, Suspension, Torque and Horsepower, Transmission