One of the most practical applications of thermodynamics is developing ways to convert heat energy into mechanical energy. A device that transforms heat into mechanical energy is called an engine.
Two of the most important types are external combustion engines and internal combustion engines. What’s the difference between the two?
External combustion engines – The combustion is carried out outside of the piston cylinder. The burnt gases are used to heat up a secondary working fluid, such as air or water. This heated fluid then moves a cylinder.
Here’s an example – Newcomen’s engine, a precursor of the steam engine. Notice the boiler heated from beneath.
This was the first practical device to harness steam to produce mechanical work. Newcomen engines were used throughout Britain and Europe, principally to pump water out of mines. Hundreds were constructed through the 18th century. (Wikipedia)
Internal combustion engines – Here, the combustion of fuel is carried out in the cylinder only. The burnt gases or combustion products do the work on the piston, producing work. The combustion products leave through exhaust valves. All modern car engines have these – so let’s take a look!
4 stroke internal combustion engines
This type of engines powers your typical automobile. (This section is from STM, SpeedTech MotorSport, Performance Vehicle Specialists, New Zealand.)
Engines have 4 cylinders.
Each cylinder has a piston, attached to the crankshaft by a connecting rod.
The crankshaft transmits engine power to the clutch, gearbox and finally the wheels.
We are going to look at how the engine produces this power.
Engineers refer to the four steps of this process as suck, squeeze, bang, blow. (Yes, really.)
Suck – intake stroke. The piston descends in the cylinder and sucks a fresh air/fuel mixture into the engine.
Squeeze – compression stroke. Piston rises up the cylinder and tightly compresses the fuel/air mixture.
(The more tightly compressed the mixture is, the more power can be extracted from it when it is ignited.)
Bang – power stroke. The spark plug ignites the air/fuel mixture. The explosion is a rapid expansion of gasses – which force the piston back down the cylinder. This transfers torque to the crankshaft.
Blow – the exhaust stroke. The burnt air/fuel mixture is forced out through the exhaust valves, as the piston travels back up the cylinder.
Once all four strokes are complete the engine is back at the start and ready to do it all again. All of this happens very quickly, with a single engine cycle only taking a mere 17 milliseconds at 7000 rpm!
How a car engine works
This next infographic was created by Jacob O’Neal, graphic designer.
For the full set of his infographics, on X, Y, and Z, see Car engine animagraffs or Animagraffs.com How-a-car-engine-works.
Similar principles operate in the aerospace industry. Honeywell jet engine cores are a good example.
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields.
Clarification Statements: Examples of phenomena at the macroscopic scale could include evaporation and condensation, the conversion of kinetic energy to thermal energy,
HS-PS3-4a. Provide evidence that when two objects of different temperature are in thermal contact within a closed system, the transfer of thermal energy from higher temperature objects to lower-temperature objects results in thermal equilibrium, or a more uniform energy distribution among the objects and that temperature changes
necessary to achieve thermal equilibrium depend on the specific heat values of the two substances. Energy changes should be described both quantitatively in a single phase (Q =m·c·∆T) and conceptually either in a single phase or during a phase change.
HS-PS3-4. Plan and conduct an investigation to provide evidence that the transfer of thermal energy when two components of different temperature are combined within a closed system results in a more uniform energy distribution among the components in the system (second law of thermodynamics).
Influence of Science, Engineering and Technology on Society and the Natural World: Modern civilization depends on major technological systems. Engineers continuously modify these technological systems by applying scientific knowledge and engineering design practices to increase benefits while decreasing costs and risks. (HS-PS3-3)
Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system. (HS-PS3-3)
Energy cannot be created or destroyed—only moves between one place and another place, between objects and/or fields, or between systems. (HS-PS3-2)
7.B.2.1: The student is able to connect qualitatively the second law of thermodynamics in terms of the state function called entropy and how it (entropy) behaves in reversible and irreversible processes. [SP 7.1]
– AP Physics Course and Exam Description