Heat Engine Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. A heat engine is a device which transforms the _________ of a fuel into thermal energy.

1. Chemical energy
2. Electrical energy
3. Solar Energy
4. Mechanical energy

Ans. The correct answer is a. Chemical energy

Explanation: Chemical energy is generated in a fuel by means of a chemical reaction. A heat engine transforms this chemical energy into thermal energy.

Ques. Which of the following engines works on a rotary mechanism?

1. Stirling engine
2. Steam turbine
3. Steam engine
4. Diesel engine

Ans. The correct answer is b. Steam turbine

Explanation: The reciprocating mechanism is used by the steam engine, Stirling engine, and diesel engine, whereas the rotary mechanism is used by the steam engine.

Ques. The earliest internal combustion engine is credited to

1. Brayton
2. Frenchman Lenior
3. Atkinson
4. Christian Huygens

Ans. The correct answer is d. Christian Huygens

Explanation: Christian Huygens invented the first internal combustion Huygens gun power engine in 1680.

Ques. What are the principal uses of gas engines?

1. Electrical power and aircraft
2. Missiles and aircraft
3. Marine and locomotive
4. Industrial and electrical power

Ans. The correct answer is d. Industrial and electrical power

Explanation: The primary applications of a gas engine are industrial and electrical power. Diesel engines are mostly used in marine, locomotive, and electrical power applications. Missiles are the primary use for rocket engines.

Ques. Theoretically, the efficiency of the ideal Otto cycle is equal to

1. Dual cycle
2. Stirling cycle
3. Brayton cycle
4. Diesel cycle

Ans. The correct answer is c. Brayton cycle

Explanation: Because the Brayton cycle is an improved Otto cycle, the theoretical efficiency of the ideal Otto cycle is the same as that of the Brayton cycle.

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Short Answers Questions [2 Marks Questions]

Ques. What is a heat engine?

Ans. A heat engine is a type of engine that converts the chemical energy of the fuel into thermal energy, which is then utilized to perform practical work.

Ques. What is the principle of a heat engine?

Ans. The workings principle of a heat engine is limited by the first and second laws of thermodynamics. By burning fuel outside and away from the main engine, an external combustion engine generates force and motion. Steam engines are one of the best examples of external combustion engines.

Ques. What is the purpose of a temperature difference in a heat engine?

Ans. Increasing the temperature of the fluid inside the cylinder initially improves the cycle's efficiency, as does decreasing the temperature at the opposite end of the process. In other words, an efficient heat engine runs at the greatest feasible temperature difference.

Ques. What makes a heat engine impossible?

Ans. A perfect heat engine is difficult to achieve since heat engines constantly generate waste heat. Heat, to be specific, is a low-grade kind of energy. This energy can be lost by a variety of processes, including conduction, radiation, and convection.

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Long Answers Questions [3 Marks Questions]

Ques. What is the maximum efficiency of a heat engine operating between 100 ℃ and 25 ℃?

Ans. Given

• T₁ = 25 ℃ = 25 + 273 = 298 K
• T₂ = 100 ℃ = 100 + 273 = 373 K

The maximum efficiency of a heat engine is given by

ƞ = (T₂ - T₁ / T₂) x 100%

⇒ ƞ = (373 - 298 / 373) x 100%

⇒ ƞ = 20 %

Ques. What is the difference between a heat pump and a heat engine?

Ans. The difference between a heat pump and a heat engine is

Ques. What is an Ideal Heat Engine?

Ans. A heat engine is a mechanism that receives heat from a hot body (reservoir), turns some of the heat energy into work, and then transfers the remaining heat to a cold body (sink).

• The work can be electrical, mechanical, etc.
• An ideal heat engine is one that can transform all of the heat energy into work. Hence by conservation of energy, no heat is released to sink.
• The efficiency of an engine is defined as the ratio of the quantity of work performed by the engine to the amount of heat consumed by the engine. i.e.

Efficiency, ƞ = Amount of work done by the engine / Amount of heat taken as input

• Clearly, the efficiency of an ideal heat engine is 1 or 100%.

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Very Long Answers Questions [5 Marks Questions]

Ques. Describe the types of heat engines with examples.

Ans. In general, heat engines are classified into two types:

• Internal Combustion Engines: The most common type of heat engine is the internal combustion engine, which is found in boats, ships, automobiles, airplanes, and trains. They are so named because the fuel is ignited to perform work inside the engine. The equivalent combination of air and petrol is eventually discharged as exhaust. Inside the system, fuel is burned in this way. Pistons are most commonly seen in internal combustion heat engines. Inside the cylinders of heat engines, pistons move up and down. The stroke refers to a single movement of a piston in the upper and downward direction inside the cylinder.

Some examples of internal combustion engines are Gas turbines, Piston engines, and Jet engines.

• External Combustion Engines: External heat engines are simply steam engines that differ from internal heat engines in that the heat source is separate from the working gas. Because the combustion occurs outside the engine, these heat engines are commonly referred to as external combustion engines. A heat engine has a functioning fluid that is heated by combustion in an external source through a heat exchanger. Fuel is burnt outside the engine in these heat engines, or, to put it another way, outside the combustion chamber. For example, external combustion would include utilizing a flame to turn water into steam and then using the steam to operate a turbine. Internal combustion happens when gasoline ignites inside a piston, produces work, and is expelled, as in an automobile engine.

Some examples of external combustion engines are Coal-fired power plants, Nuclear reactors, and Natural gas power plants.

Ques. What are the advantages of an Internal Combustion Engine over an External Combustion Engine?

Ans. The following are the main advantages of an internal combustion engine over an external combustion engine:

• Internal combustion engines are smaller and lighter. An external combustion engine, on the other hand, is large and heavy. Because of its compact size and low weight, a petrol engine can power even small vehicles such as automobiles and scooters.
• An internal combustion engine is quite safe to use. An external combustion engine, on the other hand, is not particularly safe to operate due to the presence of highly pressurized steam in the boiler.
• Internal combustion engines may be started immediately. An external combustion engine, on the other hand, takes a considerable time to start.
• The efficiency of an internal combustion engine is greater than that of an external combustion engine. The efficiency of a steam engine is only approximately 20%, but that of an internal combustion engine might be up to 40%.

Ques. What are the major components of a heat engine?

Ans. The major components of a heat engine are

• Source (High Temperature): It is the most important part of the heat engine. The heat engine is commonly referred to as a hot body since its principal function is to convert heat into mechanical work. As a result, the device is given a source to gather heat. It is always maintained with an infinite thermal capacity, allowing it to stay stable even after producing significant quantities of heat.
• Working Object: This functioning material will gather the quantity of heat delivered to the hot source, which may vary. However, the role is to gather from the hot source and transmit it to the sink.
• Sink of Heat (At Lower Temperature): This is completely opposed to the first portion, which is the source. It has a limited thermal capacity and absorbs heat from the working material. It also helps in the preservation of mechanical work acquired from the working substance. It cannot, however, change by outside temperatures.

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