What is Heat engines
An engine is an energy conversion device which transforms one form energy into another form. While a heat engine is a device which transforms the chemical energy of fuel into thermal energy and uses this energy to produce mechanical work.
Heat engines are divided into two broad classes:
1. Internal combustion engines
2. External combustion engines
Classification of Heat Engines
Internal combustion engines:
An Internal Combustion Engine (I.C. Engine) is a type of heat engine in which the combustion of fuel takes place inside the engine cylinder itself.
The high-pressure and high-temperature gases produced from combustion directly act on the piston, producing mechanical work.
Basic Principle
- Works on the conversion of chemical energy of fuel → thermal energy (by combustion) → mechanical energy (piston movement).
- Follows the law of conservation of energy.
Applications
- Automobiles (cars, bikes, trucks, buses).
- Power generation (DG sets).
- Marine engines (ships, boats).
- Aviation (small aircraft).
- Industrial uses (pumps, compressors, agricultural machinery)
Advantages
- High power-to-weight ratio.
- Compact size compared to external combustion engines.
- Quick starting and easy operation.
Disadvantages
- Limited efficiency (30–40%).
- High pollution (emission of CO, NOx, unburnt hydrocarbons).
- Requires frequent maintenance.
External combustion engines
An External Combustion Engine (E.C. Engine) is a type of heat engine in which the combustion of fuel takes place outside the engine cylinder.
The heat generated is transferred to a working fluid (like steam, air, or gas), which then expands and produces mechanical work.
Basic Principle
- Fuel combustion outside → Heat transferred to working fluid → Expansion → Mechanical work.
- Example: In a steam engine, fuel burns in a boiler to produce steam, which then drives the piston or turbine.
Examples
- Steam engines (locomotives, old industries).
- Steam turbines (power plants).
- Stirling engines.
- Hot air engines.
Applications
- Thermal power plants.
- Marine propulsion (steamships).
- Industrial power in earlier centuries.
- Nuclear power plants (steam turbines).
Advantages
- Can use a wide variety of fuels (coal, wood, nuclear, etc.).
- Smoother running due to steady external combustion.
- Less vibration compared to I.C. engines.
Disadvantages
- Bulky and heavy.
- Slow start-up (takes time to generate steam/heat).
- Lower overall efficiency compared to I.C. engines.
- Requires more space and higher maintenance.
Comparison between Internal and External Combustion Engines
| Sr. No. | IC Engines | EC Engines |
|---|---|---|
| 1 | Working fluid takes part in combustion process. | Working fluid does not take part in combustion process. |
| 2 | Heat of combustion is transferred directly to working fluid. | Heat of combustion is transferred to a second fluid which acts as working fluid. |
| 3 | High power-to-weight ratio. | Low power-to-weight ratio. |
| 4 | Absence of auxiliary apparatus like boiler or condenser. | Auxiliary apparatus like boiler, condenser required. |
| 5 | Higher overall efficiency (up to 35–40%). | Lower overall efficiency (up to 20%). |
| 6 | Heat rejection is easier. | Separate device is required for heat rejection. |
| 7 | Self-starting. | Not self-starting, requires starting torque. |
| 8 | Require high grade fuels only. | Can use solid, liquid, or gaseous fuels. |
| 9 | Applications: Automobiles, transport vehicles, aircraft, etc. | Applications: Steam power plants, gas turbine plants, closed cycle engines. |