a passenger car or truck, the engine provide the rotating power to drive
the wheels through the transmission and driving axles. All automobile
engines, both gasoline and diesel, are classified as internal combustion
because the combustion or burning takes place inside the engine. These
systems require an air/fuel mixture that arrives in the combustion
chamber at the correct time and an engines constructed to withstand the
temperatures and pressures created by the burning of thousands of fuel
Intake Stroke. The
first stroke of the cycle is the intake stroke. As the piston moves away
from top dead center (TDC), the intake valve opens. The downward
movement of the piston increases the volume of the cylinder above it.
This reduces the pressure in the cylinder. This reduced pressure,
commonly referred to as engine vacuum, causes the atmospheric pressure
to push a mixture of air and fuel through the open intake valve. As the
piston reaches the bottom of its stroke, the reduction in pressure
stops. This causes the intake of air/fuel mixture to slow down. It does
not stop because of the weight and movement of the air/fuel mixture. It
continues to enter the cylinder until the intake valve closes. The
intake valve closes after the piston has reached bottom dead center (BDC).
This delayed closing of the valve increases the volumetric efficiency of
the cylinder by packing as much air and fuel into it as possible.
compression stroke begins as the piston starts to move from BDC. The
intake valve closes, trapping the air/fuel mixture in the cylinder. The
upward movement of the piston compresses the air/fuel mixture, thus
heating it up. At TDC, the piston and cylinder walls from a combustion
chamber in which the fuel will be burned. The volume of the cylinder
with the piston at BDC compared to the volume of the cylinder with the
piston at TDC determines the compression ratio of the engine.
Power Stroke. The power
stroke begins as the compressed fuel mixture is ignited. An electrical
spark across the electrodes of a spark plug ignites the air/fuel
mixture. The burning fuel rapidly expands, creating a very high pressure
against the top of the piston. This drives the piston down toward BDC.
The downward movement of the piston is transmitted through the
connecting rod to the crankshaft.
Exhaust Stroke. The
exhaust valve opens just before the piston reaches BDC on the power
stroke. Pressure within the cylinder causes the exhaust gas to rush past
the open valve and into the exhaust system. Movement of the piston from
BDC pushes most of the remaining exhaust gas from the cylinder. As the
piston nears TDC, the exhaust valve begins to close as the intake valve
starts to open. The exhaust stroke completes the four-stroke cycle. The
opening of the intake valve begins the cycle again. This cycle occurs in
each cylinder and is repeated over and over, as long as the engine is