Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

388 Chapter 13 Energy and Power

As you can see, the amount of coal that could be saved is quite large! Before you get on an

elevator next time, think about the amount of fuel — not to mention the pollution — that can

be saved if people just walk up a floor!

Internal Combustion Engine Efficiency

The thermal efficiency of a typical gasoline engine is approximately 25 to 30% and for a diesel

engine is 35 to 40%. The thermal efficiency of an internal combustion engine is defined as

(13.17)

Keep in mind that when expressing the overall efficiency of a car, one must account for the

mechanical losses as well.

Motor and Pump Efficiency

As we explained in Chapter 12, motors run many devices and equipment that make our lives

comfortable and less laborious. As an example, we identified a large number of motors in var-

ious devices at home, including motors that run the compressor of your refrigerator, garbage

disposer, exhaust fans, tape player in a VCR, vacuum cleaner, turntable of a microwave, hair

dryer, electric shaver, computer fan, and computer hard drive. When selecting motors for these

products, engineers consider the efficiency of the motor as one of the design criteria. The

efficiency of an electric motor can be simply defined as

efficiency (13.18)

power input to the device being driven by the motor

electric power input to the motor

thermal efficiency

power output

heat power input as fuel is burned

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