Thermodynamics, Power Cycles and IC Engines — Practice

Question 1

What is the maximum efficiency of an engine operating between 600 K and 300 K?

Accepted Answer

1 - 300/600 = 50 percent.

Question 2

Find the Otto cycle efficiency for r = 8 and gamma = 1.4.

Accepted Answer

1 - 8^(-0.4) = 1 - 0.435 = 56.5 percent.

Question 3

Find the ideal Brayton efficiency for pressure ratio 10 and gamma 1.4.

Accepted Answer

1 - 10^(-0.2857) = 1 - 0.518 = 48.2 percent.

Question 4

An inventor claims a heat engine takes 100 kJ at 500 K, rejects 40 kJ at 300 K and gives 60 kJ of work. Is it feasible?

Accepted Answer

The efficiency is 60 percent. The Carnot limit is 1 - 300/500 = 40 percent, so the claim violates the second law.

Question 5

A four-stroke single-cylinder engine has imep 7 bar, bore 100 mm, stroke 120 mm and runs at 1500 rpm. Find the indicated power.

Accepted Answer

IP = 7e5 x 0.12 x (pi/4)(0.1)^2 x (750/60) = about 8.25 kW, since a four-stroke engine fires 750 times a minute.

Question 6

Explain how regeneration and reheat improve a Rankine cycle and give the effect on turbine exit dryness.

Accepted Answer

Regeneration heats feed water with extracted steam, raising the mean temperature of heat addition and efficiency. Reheat returns steam to the boiler for a second expansion, raising exit dryness and slightly raising efficiency.

Question 7

A reversible refrigerator works between 270 K and 300 K. Find its COP and the COP of a heat pump between the same limits.

Accepted Answer

Refrigerator: 270/30 = 9. Heat pump: 300/30 = 10, which is one more.

Question 8

Which rating describes knock resistance of petrol and which rates ignition quality of diesel?

Accepted Answer

Octane number for petrol and cetane number for diesel.

Practice — Thermodynamics, Power Cycles and IC Engines

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