What is an External Combustion Engine?
An external combustion engine (EC engine) is a reciprocating heat engine where a working fluid, contained internally, is heated by combustion in an external source, through the engine wall or a heat exchanger. The fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable work.
The fluid is then dumped (open cycle), or cooled, compressed and reused (closed cycle). In these types of engines, the combustion is primarily used as a heat source, and the engine can work equally well with other types of heat sources.
Understanding the External Combustion Engine
The term “engine” is typically defined as any device that transforms the chemical energy inside a combustible fuel into mechanical energy that can be used to do work. The two most common types of engines are the external combustion engine and the more famous internal combustion engine. External combustion engines were developed first. The modern steam engine was pioneered by inventors Thomas Savery, Thomas Newcomen, and James Watt.
An external combustion engine works by using an external heat source (like burning coal) to heat a working fluid (like water). As the temperature of the fluid increases, the fluid expands, and this expansion is harnessed and used to do work (like moving a vehicle).
The steam engine is the most famous example of an external combustion engine. In classic locomotive steam engines, a combustion chamber was filled with burning coal and placed beside a second chamber full of cool water.
The water absorbed the heat from the burning coal and was transformed into steam. The pressure from the steam was then used to push a piston, turn a crankshaft, and spin a wheel that moved the train forward. This type of engine is classified as an external combustion engine because the combustion of the fuel took place in a chamber outside the cylinder.
External combustion engines and internal combustion engines serve the same purpose: turning the energy inside combustible fuel into usable mechanical energy. But the designs of the engines are very different. The main difference is in the location of combustion.
In external combustion engines, fuel combustion occurs in a combustion chamber located outside of the rest of the engine. In internal combustion engines, combustion takes place inside the engine. In modern motor vehicles, fuel and air are drawn into each of the engine’s cylinders and burned. The expansion of the burning gas drives the pistons, turns the crankshaft, and produces motion in the vehicle’s wheels.
What is Combustion?
“Combustion” refers to burning fuel with an oxidizer, to supply the heat. Engines of similar (or even identical) configurations and operations may use a supply of heat from other sources such as nuclear, solar, geothermal, or exothermic reactions not involving combustion; they are not then strictly classed as external combustion engines but as external thermal engines.
Working fluid
The working fluid can be of any composition and the system may be single-phase (liquid only or gas only) or dual-phase (liquid/gas).
- Single phase: Gas is used in a Stirling engine. Single-phase liquid may sometimes be used.
- Dual phase: Dual-phase external combustion engines use a phase transition to convert temperature to usable work, for example from liquid to (generally much larger) gas. This type of engine follows variants of the Rankine cycle. Steam engines are a common example of dual-phase engines. Another example is engines that use the Organic Rankine cycle.
Examples
- CANDU reactor (a type of nuclear power plant)
- coal-fired power plant
- natural gas power plant
- steam locomotive (although there are very few operational examples left)
- Solar thermal power plant
- Stirling engine
Advantages of external combustion engines
- Almost any kind of fuel that is available can be used
- Since power is not generated due to detonation of fuel hence very less noise is generated.
- Engine emissions are also very low
- Very economical for huge power generation
Disadvantages of external combustion engines
- Not suitable for low load requirements
- Leakage of working fluid
- Lubricant consumption
- Size of engine is relatively big
- Overall working temperature is high so great care and special building materials are required
Applications of external combustion engines
- Steam engines: Locomotive, Marine
- Stirling Engines: Experimental space vehicles
- Steam Turbines: Power, Large Marine
- Closed Cycle Gas Turbine: Power, Marine