Heat engines and engineering details

Turning heat into work is usually accomplished by two types of heat engine:

  1. External combustion engine
    1. 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.
  2. Internal combustion engine
    1. 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.

types-of-heat-engines

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.

stirling_pv stirling_ts

  • 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.

rankine_pvrankine_ts

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.

fig5braytonopencycle                             fig5braytonclosedcycleopwn

Open Cycle                                                                   Closed Cycle

brayton_pvbrayton_ts

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.

otto-cycle

  • 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.

diesel-pvdiesel_ts

  • 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.

wankel-engine

Automobile engine

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:

  1. 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.
  2. Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.
  3. 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.

Personal Viewpoint

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.

References:

https://entegila.files.wordpress.com/2012/06/thermodynamics-an-engineering-approach-5th-edition-gengel-boles.pdf

http://gradestack.com/gate-exam/mechanical-engineering/thermal-sciences/i-c-engines/

http://www.hkdivedi.com/2016/11/otto-cycle-efficiency-derivation.html

http://www.mpoweruk.com/heat_engines.htm

http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node28.html

http://www.explainthatstuff.com/steam-turbines.html

Efficiency of a Heat Engine

There are many different ways to find the efficiency of a heat engine.  One of the main ways is to show it in the form of a PV diagram.  The diagram gives a visual interpretation of the work done over the system.  The diagram can then be compared to a Carnot cycle diagram, which is a heat engine with the greatest possible efficiency.  When the heat engine process is cyclic, the diagram will be a closed loop.  The area inside the loop will then be a representation of the amount of work done during a cycle.  Basically, the PV diagram provides the framework for the analysis of any heat engine which uses gas as a working substance.

heaengcyc

Figure 1: This is a simple PV diagram of a heat engine. It shows the work that is done by the cycle.

 

main-qimg-24cb9b83022fdcd5b46bfeb1881d1e67

Figure 2: This image shows the PV diagram of the Carnot cycle, so it can be compared to a heat engine.

 

Another way to express the efficiency of a heat engine is through an equation and the energy reservoir model.  A heat engine takes energy from a hot source and uses most of it to do work, but some of the energy must be exhausted to a cold reservoir.  An example of this would be a car engine.  The hot reservoir would be the burning fuel, and the cold reservoir would be the environment where the combustion products are exhausted.  Using the equation, the efficiency will be limited by that of the Carnot cycle because that is the most efficient heat engine.  This limitation can be known as the thermal bottleneck.

heaeng2

Figure 3: This diagram shows where all the energy flows to and the equation that can be used to find the efficiency of a heat engine.

A third way to determine the efficiency of a heat engine is to apply the considerations of endoreversible thermodynamics.  This would mean that the cycle would be equivalent to the Carnot cycle, but the two processes of heat transfer are not reversible.  This model can do a much better way of determining the efficiency of a real-world heat engine.

\eta =1-{\sqrt {\frac {T_{c}}{T_{h}}}}

Figure 4: This is the equation that would be used for the endoreversible heat engine.

 

Power source T2 (⁰C) T1 (⁰C) Carnot efficiency Chambadal-Novikov efficiency Observed efficiency
West Thurrock (UK) coal fired power plant 25 565 64.1% 40% 36%
CANDU (Canada) nuclear power plant 25 300 48% 28% 30%
Larderello (Italy) geothermal power plant 80 250 32.3% 17.5% 16%

Figure 5: This chart shows the efficiencies of three different engines using the Carnot method, the endoreversible method (Chambadal-Novikov) and the observed efficiency.

Personal Viewpoints: I believe the heat engine is a great engine.  It does have some environmental drawbacks, and is not completely 100% efficient, but no engine really is.  It still does a great amount of work and seems to be impactful.  For example, heat engines work great in automobiles.

Resources:

https://qph.ec.quoracdn.net/main-qimg-24cb9b83022fdcd5b46bfeb1881d1e67?convert_to_webp=true

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heaeng.html

https://en.wikipedia.org/wiki/Heat_engine#Efficiency

http://large.stanford.edu/courses/2010/ph240/askarov2/

 

Environmental Impacts of Heat Engines

Environmental impacts of heat engines:

The study of thermodynamics was initially inspired by trying to get as much energy out of heat engines as possible.[2] To this day, various fuels are used, like gasoline, coal, and uranium. All of these heat engines still operate under the limits imposed by the second law of thermodynamics. This means that various fuels are used to heat a gas and a large cold reservoir is needed in order to get rid of waste heat. Often, the waste heat goes into the atmosphere or a large body of water (the ocean, a lake, or a river).

1. Air pollution

  • The emission of the greenhouse gas (CO2), is believed to be a key contributor to global warming and climate change
  • Increasing CO2 levels in the atmosphere are related to increasing acidification of the oceans.

2. Sulphur Oxides (SOx)

  • Unless removed prior to combustion, the Sulphur present in the fossil fuel will be emiited as Sulphur Oxides. This occurs when burned. When Sulphur Oxides are released into the atmosphere, they may for Sulphuric acid, which damages plants and buildings through acid rain production.

3. Nitrogen Oxides

  • Nitrogen Oxides are produced from the Oxygen and Nitrogen gases present in high temperature coal combustion. Nitrogen oxides contribute to the greenhouse effect, formation of acid rain, ground level ozone production and photochemical smog.
  1. Thermal pollution
  • When warm water is used to cool a power plant, and the water is then released into bodies of water, it will decrease the dissolved oxygen content. When the oxygen content is decreased, this will harm species which are dependent on oxygen, whom exist in this ecosystem.
  1. Particulate matter
  • The fine particle matter which is emitted when fossil fuels are burned are harmful to human health. When the particles are breathed in, they may damage cardio-respiratory health, which may trigger lung cancer.
  • Fly ash is the large particulate matter left over after coal is burned. The fly ash contains a large quantity of silicon and calcium oxide. Additionally, it contains heavy metals.

Personal Viewpoints:

In all, I think these are fine pieces of machinery. Although, heat engines reside on the environmental harmful side, benefits may outweigh the costs at this point in time.

References:

http://www.pollutionissues.com/Te-Un/Thermal-Pollution.html

Hinrichs, Roger A., and Kleinbach, Merlin. (2001). Energy: Its Use and the Environment, 3rd edition. Monterey, CA: Brooks/Cole Publishing Company.

Ristinen, Robert A., and Kraushaar, Jack J. (1998). Energy and the Environment. New York: John Wiley & Sons.

http://www.pollutionpollution.com/2012/07/thermal-pollution.html

Heat Engine

 

INTRODUCTION

A heat engine is a device used to produce motive power from heat.  Heat, thermal energy, and chemical energy are converted into mechanical energy to do work and then are exhausted in a way that cannot be used to do work.

Most heat engines in Thermodynamics are modeled using a standard engineering model, the Otto Cycle, shown in Figure 1. In a full cycle, there are three steps that happen:

  1. Heat is added (QH)
  2. Some of the energy from heat input is used to perform work (W)
  3. The remaining heat is removed at a relatively cold temperature (QC) 1

 Figure 1.

An important measure of a heat engine is its efficiency, e: how much input energy ends up doing work.  This efficiency is calculated as a fraction of e = work done/input heat = W/QH which will be discussed later in this blog.

THERMODYNAMIC PRINCIPLES

The Pressure – Volume (PV) diagram is a primary visualization tool for studying heat engine.  Due to engines involving gas to function, the ideal gas law relates the PV diagram to the temperature.  With an internal energy depending on the temperature, a PV diagram determines the change in internal energy of the gas so the amount of heat added can be evaluated from the first law of thermodynamics.

2

Figure 2.  A visualization for a closed, cyclic heat engine process.  The area inside the loop is a representation for the amount of work done during a cycle.

Thermodynamics is defined as the study of relationships between heat and work. Containing properties of the first, second and third law of thermodynamics, heat engines incorporate multiple thermodynamic concepts.

First Law of Thermodynamics

The first law of thermodynamics states that energy is always conserved, it cannot be created or destroyed.

Second Law of Thermodynamics

The second law of thermodynamics states: “in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state” commonly referring to entropy, the measure of disorder.  This law helps to show that heat will flow from regions of a higher temperature to regions of lower temperatures.

Third Law of Thermodynamics

The entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero, or conversely, the temperature of any pure substance in thermodynamic equilibrium approaches zero when the entropy approaches zero.  This implies that a perpetual motion machine is possible, because the efficiency will always be less than 1.

Applications of the Laws of Thermodynamics

The first and second law of thermodynamics constrains the operation of a heat engine.  The first law applies the conservation of energy to the system while the second law sets limits on the possible efficiencies of the machine by determining the direction of energy flow.  Like mentioned above, the third law implies that the perpetual motion machine is possible, because the efficiency will always be less than one, as proven in lecture.

Personal Viewpoints

After thoroughly researching the heat engine, one can presume it is a great development even though it has environmental drawbacks by not being efficient.  The work an engine is able to do helps us function in today’s society and is a great use for daily transportation.  Without heat engines, we would not be as efficient as we are today.

REFERENCES

  1. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heaeng.html
  2. http://physics.bu.edu/~duffy/py105/Heatengines.html
  3. http://www.pollutionissues.com/Te-Un/Thermal-Pollution.html
  4. https://en.wikipedia.org/wiki/Heat_engine#/media/File:Heat_engine.png
  5. http://www.explainthatstuff.com/engines.html
  6. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html#c2
  7. https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookEner1.html