Lesson 2/Learning Event 2
Learning Event 2:
DESCRIBE THE PRINCIPLES AND OPERATION OF COMPRESSION IGNITION
The power developed by a naturally aspirated internal combustion engine is dependent upon the density of
the air inducted into the engine. In a naturally aspirated engine, the air is drawn into the cylinder on the
intake stroke with a force acting upon the gas equal to the pressure difference between the atmosphere and
the cylinder pressure. The cylinder pressure approaches that of a perfect vacuum, which is 0 pressure.
Atmospheric pressure is about 15 PSI. Therefore, the pressure forcing the air into the cylinder is about 15
The highest density that can be attained is atmospheric air density; but this is not usually attained because of
certain losses in internal combustion engines: leakage past the piston, pressure drops across restrictions in the
manifold, leakage in the valves, and also the heating of the air as it enters the manifold (which tends to
expand the gas and decrease its density).
All of these losses make it hard for the engine to breathe. This causes slow response when the accelerator is
depressed and reduces torque output before maximum power output. As engine speed is increased, the
"breathing" of the engine becomes more difficult, because each cylinder has a shorter time to fill itself with
Supercharging is assistance given to the air intake process by use of supplementary fans, blowers, and
compressors. Supercharging greatly increases the power output of an engine with very little increase in
weight. A supercharger accomplishes this by increasing the overall efficiency of the engine.
Overall efficiency of an internal combustion engine is affected by many factors. One factor is "volumetric
efficiency." Volumetric efficiency can be defined as the ratio of the weight of the air actually drawn into the
engine over a definite period of time to the weight of the air that would be drawn in without the losses
present in any engine, such as leakage, friction losses, heat-transfer losses, and so forth.