Turning heat into work is usually accomplished by two types of heat engine:
- External combustion engine
- When combustion takes place outside of the engine. Heat produced during external combustion is used for inducing useful mechanical motion in the cylinder of the engine.
- Internal combustion engine
- When combustion takes place within the engine. Chemical energy of the fuel is converted to thermal energy which is then converted to mechanical energy which moves a piston up and down inside a cylinder. In an automobile power from the piston is transmitted to a crankshaft which is ultimately transmitted to the wheels.
External Combustion Engines
- The Steam engine and Sterling engine were popular in the early days of engine inventing however have since been on a rapid decline and rarely exist today. The steam engine was ultimately used on locomotives. Burning a fuel source (usually coal) to release energy stored inside it which boils water to generate high-pressure steam. The steam then goes into a cylinder with a piston, the piston gains the energy from the steam which then pushes the wheels around. The Sterling engine was used to pumping water.
- The Steam Turbine is in a way similar to the steam engine, however, it gets its energy from a steam moving past the turbine (spinning wheel). The high pressure steam turns electricity generators at incredibly high speeds. Steam turbines can be used in cooling towers in power plants and to accelerate ships with limited space.
Open Cycle and Closed Cycle Gas Turbines both use Brayton cycle. The Open cycle gas turbines are internal combustion heat engines and are typically used as jet turbines to move a jet in the air. The Closed cycle gas turbines are external combustion heat engines and are typically used in space power generation.
Open Cycle Closed Cycle
Internal Combustion Engines
- An internal combustion heat engine can be either two or four stroke.
- Two Strokes Engine: The thermodynamics cycle is completed in two strokes (power and compression) of the piston or in one revolution of the crankshaft. Thus, one power stroke is obtained in each revolution of the crankshaft with uniform turning moment and light flywheel. Should also be noted that two stroke engines are cause more pollution due to the process not being as complete as four stroke. Two cycle engines produces high power for a relatively short period.
- Four Strokes Engine: The thermodynamics cycle is completed in four strokes (intake, compression, power, exhaust) of the piston or in two revolution of the crankshaft. Thus, one power stroke is obtained in every two revolution of the crankshaft with non-uniform turning moment and heavier flywheel. Four cycle engine produces low power for a long period. Has higher thermal efficiency.
- An internal combustion heat engine can either be Spark Ignition or Compression Ignition.
- Spark Ignition (SI) Engine: It works on Otto cycle or constant volume heat addition cycle. A gaseous mixture of fuel air introduced during the suction stroke. A carburetor and an ignition system are necessary. Modern engines have gasoline injection.
- Compression Ignition (CI) Engine: It works using the diesel cycle or constant pressure heat addition cycle. Fuel is injected directly into the combustion chamber at high pressure at the end of the compression stroke. A fuel pump and injection are necessary.
- Rotary: Wankel Engine is a rotary internal combustion engine. In which the gears rotate while it intakes the gas then compresses it and once compressed is ignited to produce work and then exhausts.
The most common engine for people is usually a car engine which is a type of gasoline engine and is also an internal combustion heat engine. Gasoline engines used for automobiles are typically spark ignition engines and use the four stroke combustion cycle (Otto cycle). The cycle of an automobile engine are as follows:
- The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.
- Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.
- When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe.
Now the engine is ready for the next cycle, so it intakes another charge of air and gas.
In an engine the linear motion of the pistons is converted into rotational motion by the crankshaft. The rotational motion makes it possible for the car’s wheels to turn and therefore advance the vehicle.
I believe had the heat engine not been invented we would not be living where we are today. It is a great source of mechanical power, however there are many environmental impacts that make it less desireable. I do believe that many companies are straying away from the heat engine to make more economic products that cause less emission issues. It will be very interesting to see what comes of major machines in the next 50 years and how the advancement of particluar processes will expand, thus will cause a decline in the use of the heat engine.