the amount of air-fuel mixture, the greater the power
produced by the engine.
Increasing volumetric efficiency increases engine
performance. Volumetric efficiency can be increasedin
the following ways:
Keep the intake mixture cool by ducting intake
air from outside the engine compartment. By
keeping the fuel cool, you can keep the intake
mixture cooler. The cooler the mixture, the
higher the volumetric efficiency. This is because
a cool mixture is denser or more tightly packed.
Modify the intake passages (fig. 2-23). Changes
to the intake passages that make it easier for the
mixture to flow through will increase the
volumetric efficiency. Other changes include
reshaping ports to smooth bends, reshaping the
back of the valve heads, or polishing the inside of
Altering the time that the valves open or how far
they open can increase volumetric efficiency.
By supercharging and turbocharging, you can
bring the volumetric efficiency figures to over
MECHANICAL EFFICIENCY is the
relationship between the actual power produced in the
engine (indicated horsepower) and the actual power
delivered at the crankshaft (brake horsepower). The
actual power is always less than the power produced
within the engine. This is due to the following:
Friction losses between the many moving parts
of the engine.
In a four-stroke-cycle engine, a considerable
amount of horsepower is used to drive the valve
From a mechanical efficiency standpoint, you can
tell what percentage of power developed in the cylinder
is actually delivered by the engine. The remaining
percentage of power is consumed by friction, and it is
computed as frictional horsepower (fhp).
THERMAL EFFICIENCY is the relationship
between actual heat energy stored within the fuel and
power produced in the engine (indicated horsepower).
The thermal efficiency figure indicates the amount of
potential energy contained in the fuel that is actually
used by the engine to produce power and what amount
of energy is actually lost through heat. A large amount
of energy from the fuel is lost through heat and not used
in an internal combustion engine. This unused heat is of
no value to the engine and must be removed from it.
Heat is dissipated in the following ways:
The cooling system removes heat from the
engine to control engine operating temperature.
A major portion of the heat produced by the
engine exits through the exhaust system.
The engine radiates a portion of the heat to the
Figure 2-23.Port design consideration.