Heat Engine

1. What is a heat engine?

Heat engine is a device which converts heat into useful mechanical energy.

2. Name the two types of heat engines.
3. A) External combustion engine B) Internal combustion engine.
4. Explain the basic principle of a heat engine.

If a liquid is heated it gets converted into gaseous form. The volume of the gas is very large when compared to its liquid form. This gas obtained is combined to a metallic cylinder fitted with a piston. The expanding gases exert a great force on the walls of the cylinder and the piston. So, the piston moves. In this way heat is used to move the piston and other objects connected to it. After this, the gas no longer exerts force on the piston. Piston falls back to its original position. The gases are sent out of the cylinder and fresh supply of gas is maintained. Thus a continuous to and fro motion of the piston takes place.

4. What is the main difference between external and internal combustion engines?

In an external combustion engine, fuel burns outside the engine.

In an internal combustion engine, fuel burns inside the engine.

5. Give an example for external combustion engine.

Steam engine.

6. Explain with a sketch the working of a steam engine.

Expansion stroke.

A typical steam engine consists of a cylinder fitted with an airtight piston. The piston is connected to a crankshaft by means of a connecting rod. A boiler is connected to the cylinder through a valve called inlet valve. Cylinder is also connected to a condenser through another valve called outlet valve. The condenser is connected to the boiler through a pump. The water in the boiler is heated to generate steam which gets stored at high pressure. The steam under high pressure gets into the cylinder through the inlet valve. The expanding steam forces the piston to move outwards. This stroke is called expansion stroke. During the caparison stroke the outlet valve remains closed. At the end of the stroke the inlet valve gets closed. As the piston moves inwards the outlet valve opens. The spent steam in the cylinder is pushed into the condenser. The cycle repeats.

7. Mention disadvantages of an external combustion engine.

• It is bulky, hence connote be used to run small machines.
• It cannot be started immediately
• Its efficiency is less
• There is risk of accident due to bursting of boiler.

8. Give two examples for an internal combustion engine.
   1. Petrol engine 2. Diesel engine.
9. Draw a sketch of a petrol engine and label its parts.

10. Describe with neat sketches the four strokes of a petrol engine.

(1) Intake stroke: The piston moves outwards away from the cylinder head. This results in low pressure inside the cylinder and the inlet valve opens. The fuel mixture from the carburettor enters the cylinder. The outlet valve remains closed.

(2) Compression stroke: Both the inlet and outlet valves are closed and the mixture is compressed by the piston moving towards head

(3) Power stroke: The compressed mixture is ignited by the spark produced by the spark plug. A lot of heat is released. The product of combustion consisting of carbon dioxide and water vapours expands suddenly. The piston is pushed outwards with the great force

4) Exhaust stroke: The exhaust valve opens. The piston moves back. The waste gases are pushed out of the cylinder through the exhaust valve

The above cycle is repeated. Piston moves to and fro quickly. The movement of the piston pushes the crankshaft and makes it rotate rapidly.

11. What are the advantages of internal combustion engine over external combustion engine?

• The efficiency of an internal combustion engine is greater than that of an external combustion engine.
• It can be started instantaneously
• Internal combustion engine can be manufactured in a range of size. They can be fitted to small machines.
• There is no risk of accident due to the storage of material at high pressure.

12. Mention the differences between diesel and petrol engines.