Heat Engines: Types, Operation and Internal Combustion

Thermal Machines: Converting Energy into Mechanical Work

Thermal machines, also known as heat engines, are machines that transform thermal energy (heat) into mechanical energy.

Types of Heat Engines

Heat engines are classified into two main groups according to where combustion takes place:

• External combustion engines: the fuel burns outside the machine. The heat is transferred to a working fluid, usually water, which boils and turns into steam.
• Internal combustion engines: the fuel burns inside the machine. The combustion gases are the working fluid that directly produces mechanical work.

External Combustion Engines

These engines can be:

• Alternative (reciprocating): for example, the steam engine.
• Rotary: for example, the steam turbine.

Internal Combustion Engines

These engines can be:

• Alternative (reciprocating): spark-ignition engines (Otto cycle) and compression-ignition engines (Diesel cycle).
• Rotary: open-cycle gas turbines and the Wankel engine.

How Alternative Engines Work

Alternative engines incorporate a piston or plunger that moves back and forth (alternating motion). A rod-crank mechanism converts this linear motion into rotational motion.

How Rotary Engines Work

Rotary engines directly produce rotational motion.

How Steam Turbines Work

The steam boiler sends steam through a conduit to the turbine. The turbine has a fixed casing that contains a set of fixed blades joined to the casing, and a set of rotating blades mounted on a rotor, located on its periphery. The steam strikes the rotating blades under pressure and makes them turn.

Impulse Turbines and Reaction Turbines

• Impulse turbines: the steam is directed by fixed nozzles onto the rotating blades, which change its direction and make the rotor spin.
• Reaction turbines: the rotating blades are also inclined, so that the steam expands as it passes through them and pushes the blades by reaction, causing rotation. The process repeats successively through the stages of the turbine.

Internal Combustion Engines: Operation

These engines work by burning a fuel-air mixture inside the machine itself, without needing an intermediate element such as a boiler.

Common elements with the steam engine include the cylinders, the pistons and the rod-crank mechanism. Unlike the steam engine, they do not need a boiler: the fuel burns violently inside the cylinder, producing the pressure that drives the piston.

Types of Explosion Engines

• Spark-ignition engines (Otto cycle): the mixture is ignited by a spark plug.
• Compression-ignition engines (Diesel cycle): the mixture ignites by the heat generated during compression.

Both types can operate on a four-stroke cycle (intake, compression, power and exhaust).