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Chapter 4: Q. 4.19 (page 133)

The amount of work done by each stroke of an automobile engine is controlled by the amount of fuel injected into the cylinder: the more fuel, the higher the temperature and pressure at points 3 and 4 in the cycle. But according to equation 4.10, the efficiency of the cycle depends only on the compression ratio (which is always the same for any particular engine), not on the amount of fuel consumed. Do you think this conclusion still holds when various other effects such as friction are taken into account? Would you expect a real engine to be most efficient when operating at high power or at low power? Explain.

Short Answer

Expert verified

The real engine is most efficient in low power than the high power.

Step by step solution

01

Concept Introduction

The cycle of operation is Otto cycle why because the efficiency depends only upon the compression ratio.

In practice, there are several losses such as:

1. Time loss

2. Heat loss

3. Blow down losses

4. Friction loss

5. Pumping loss

02

Explanation

Actual P-V Diagram of Otto Cycle

Maximum efficiency condition may occur in ideal cycle but because of losses in the cycle, the efficiency decreases.

So, taking in consideration all the losses, the real engine will be most efficient when operating at low power because at high power, losses will be high too andhence , the efficiency will decrease.

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Most popular questions from this chapter

Consider a household refrigerator that uses HFC-134a as the refrigerant, operating between the pressures of 1.0barand 10bars.

(a) The compression stage of the cycle begins with saturated vapor at 1 bar and ends at 10 bars. Assuming that the entropy is constant during compression, find the approximate temperature of the vapor after it is compressed. (You'll have to do an interpolation between the values given in Table 4.4.)

(b) Determine the enthalpy at each of the points 1,2,3 and 4 , and calculate the coefficient of performance. Compare to the COP of a Carnot refrigerator operating between the same extreme temperatures. Does this temperature range seem reasonable for a household refrigerator? Explain briefly.

(c) What fraction of the liquid vaporizes during the throttling step?

Recall Problem 1.34, which concerned an ideal diatomic gas taken around a rectangular cycle on a PVdiagram. Suppose now that this system is used as a heat engine, to convert the heat added into mechanical work.

(a) Evaluate the efficiency of this engine for the case V2=3V1,P2=2P1.

(b) Calculate the efficiency of an "ideal" engine operating between the same temperature extremes.

Suppose you are told to design a household air conditioner using

HFC-134a as its working substance. Over what range of pressures would you have it operate? Explain your reasoning. Calculate the COP for your design, and compare to the COP of an ideal Carnot refrigerator operating between the same extreme temperatures.

What is the maximum possible COP for a cyclic refrigerator operating between a high-temperature reservoir at 1K and a low-temperature reservoir at 0.01 K ?

Estimate the maximum possible COP of a household air conditioner. Use any reasonable values for the reservoir temperatures.
See all solutions

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