386 Chapter 13 Energy and Power
Using Equation (13.1), we can determine the required work to go from 0 to 60 mph.
The power requirement to perform this work in 6.7 seconds is
The power expressed in horsepower is
Keeping in mind that power is needed to overcome air resistance and that there are always
additional mechanical losses in the car, it is still safe to say the claim is good.13.5 Efficiency
As we mentioned earlier, there is always some loss associated with a dynamic system. In engi-
neering, when we wish to show how well a machine or a system is functioning, we express its
efficiency. In general, the overall efficiency of a system is defined as:(13.15)
All machines and engineering systems require more input than what they put out. In the next
few sections, we will look at the efficiencies of common engineering components and systems.Power Plant Efficiency
Water is used in all steam power-generating plants to produce electricity. A simple schematic of a
power plant is shown in Figure 13.10. Fuel is burned in a boiler to generate heat, which in turn
is added to liquid water to change its phase to steam; steam passes through turbine blades, turn-
ing the blades, which in effect runs the generator connected to the turbine, creating electricity. The
low-pressure steam liquefies in a condenser and is pumped through the boiler again, completing
a cycle, as shown in Figure 13.10. The overall efficiency of a steam power plant is defined aspower plant efficiency (13.16)
energy generated
energy input from fuel
efficiency
actual output
required input
power85,530a
lbf#ft
s
b°
1hp
550
lbf#ft
s
¢155.5 hp
power
work
time
573,056 lbf#ft
6.7 s
85,530
lbf#ft
s
work 1 2
1
2
1 148 slugs2a 88
ft
s
b
2
0 573,056 lbf#ftwork 1 2
1
2
mV
2
2 1
2
mV
2
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