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 the ports.

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 100 percent.

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 train.

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 atmosphere.

Figure 2-23. - Port design consideration.