Effic

dynamics)

Otto

High

Fair

Poor

None

Diesel

High

Fair

Poor

None

Rankine

Low

Good

Fair

Large

Stirling

Moderate

Good

Very good

Medium

Brayton (o.c.)

Mod./high1

Very good

Good

None

Brayton (c.c.)

Moderate

Very good

Good

Medium

Development of more modern materials is expected to permit use of higher temperature ratios.

Development of more modern materials is expected to permit use of higher temperature ratios.

accomplished by separate devices, and combustion occurs in its special chamber. This allows optimization of each part.

In Otto and Diesel engines, three processes are carried out in the same engine part—the cylinder. The fourth process—heat rejection—is carried out in the environment. The multiple functions of the cylinder require design compromises with resulting lower component efficiency.

Although Rankine and Brayton engines are of the continuous combustion types, Otto and Diesel operate intermittently and are able to tolerate higher working temperatures, since the combustion phase lasts only briefly. Also, heat is generated directly in the working fluid, thereby doing away with heat transfer problems encountered in all external combustion engines.

The Stirling engine, which will be examined in greater detail toward the end of this chapter, holds considerable promise, but, although quite old in concept, it has never achieved the popularity that it appears to deserve. It is, at least theoretically, the most efficient of the mechanical heat engines. It is less polluting than the Otto and the Diesel because it burns fuel externally. In addition, since the Stirling engine involves no explosions, it runs more smoothly and with less noise than other engines. Finally, in spite of being a close-cycle device, it requires only moderate heat rejection equipment.

The Stirling cycle also finds application in some refrigeration equipment. Since it operates with helium, hydrogen, or air, it employs no freons or other ecologically damaging fluids.

D. G. Wilson (1978) summarized the characteristics of the most common mechanical heat engines, as shown in Table 3.5.

3.4 Efficiency of an Otto Engine

In an ideal Otto cycle, a fuel/air mixture is compressed adiabatically from a volume, V, to a volume, V2, during the compression phase. The gas a

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