Lesson 2.12 Thermodynamics -
2
Overview
This lesson deals with internal combustion engines and
the Second Law of thermodynamics. On completion of the
lesson, you should be able to describe 2-cycle and 4-cycle
engines. You should also be able to discuss the concept
of entropy and life processes appear to create order in
isolated systems.
MINI
LAB
Observe the operation of
an internal combustion engine such as a lawnmower engine.
Open up the engine to see the parts.
The Stirling Engine
One of the engines that continues to receive attention is
the hot air engine - or Stirling engine. Some believe
that it is obsolete but others believe that it has use
because it can use many different types of fuel and it is
quiet. There is controversy over who invented the engine.
British literature gives credit to Sir George Cayley (1807)
America literature gives credit to Rev. Robert Stirling.
(Presented when he was 26 years old in 1816)
A Stirling engine is a
closed-cycle, regenerative heat engine which uses an
external combustion process, heat exchangers, pistons, a
'regenerator' and a gaseous working fluid contained
within the engine to convert heat to mechanical work (motion).
Internal Combustion
Engines
Internal combustion engines compress a mixture of fuel
and air, which then burns inside the engine to create a
larger quantity of gas under pressure. The pressure of
the conbustion products provides power and momentum for
the engine to complete its cyclic process.
With the exception of
Wankel engines that use rotors, all internal combustion
engines use pistons connected via a connecting rod to a
crankshaft. A flywheel connected to the crankshaft stores
momentum and provides power for the compression and
exhaust portions of the cycle.
2-Cycle Engines
In two-cycle engines, there is one power pulse per
revolution for each power cylinder. After the mixture of
fuel and air has ignited, the piston is pushed (downwards)
towards the crankshaft by the expanding gases. This helps
to partially compresses a fresh mixture of fuel and air
that was previously drawn into the crank-case when the
piston moved up to compress the fuel and air mixture in
the cylinder. As the piston moves past an opening in the
side of the cylinder near the bottom, the fuel and air
mixture enters the cylinder to provide the charge for the
next cycle. The mixture of fuel and air is directed
slightly upwards which helps to displace some of the
remaining exhaust gases that leave through an opening in
the opposite side of the cylinder.
The flywheel carries the
piston back upwards to compress the mixture which is
ignited just as it reachesthe top of the cylinder. The
gases are compressed into a small space known as the
conbustion chamber. A spark plug ignites the mixture at
just the right time to ensure that the burning gases
provide the maximum amount of power as they burn and
expand to push the piston down again.
4-Cycle Engines
In 4-cycle engines, the mixture of fuel and air is drawn
directly into the cyclinder through a valve opening at
the top of the cylinder. There are two valves at the top
of the cylinder. One valve opens to allow air and fuel to
enter. The second valve opens to allow exhaust gases to
leave. Momentum in the flywheel carries the piston down
to draw in the fuel and air – and then back up to
compress the mixture. At the top of its stroke the spark
plug ignites the compressed mixture and this provides
power as the piston moves down again. At the bottom of
this stroke, the exhaust valve starts to open and exhaust
gases leave through the valve opening helped by the
upward movement of the piston. The exhaust valve closes
when the piston reaches the top and the inlet valve opens
to allow fuel and air to enter as the piston moves down
again.
Diesel Engines
When the mixture of fuel and air is compressed, the
temperature rises considerably and if sufficiently
compressed, the mixture will ignite by itself without the
aid of a spark plug. Simple 2-cycle diesel engines –
like those used in model airplanes – rely on the
automatic combustion of a compressed mixture of fuel and
air. Most diesel engines however use a high-precision
injector system that sprays fixed quantity of fuel into
the combustion chamber at just the right time. This
atomized fuel ignites when it comes into contact with the
hot compressed air in the combustion chamber. The burning
mixture creates pressure to drive the piston downward
during the power stroke of the cycle.
Commercial diesel engines
are usually 4-cycle engines but some manufacturers
provide commercial 2-cycle diesel engines. Many diesel
engines have glow-plugs that provide an additional source
of energy to ensure that the mixture of fuel and air
ignites properly.
Diesel engines are more
expensive than spark ignition engines. The diesel
injector system requires high-precision manufacturing
processes and can cost as much to produce as the rest of
the engine.
Diesel engines are however
slightly more efficient than ignition engines and use
fuels with more energy per gallon. They are therefore
more economical to run than gasoline powered engines.
Diesel engines can also run on a variety of fuels. It was
hoped at one time that a diesel engine that could run on
powdered coal could be successfully developed but the
problems associated with abrasive powders in the
combustion products apparently killed the dream.
High Octane Fuels
Fuels that ignite too easily may auto-ignite (as in a
diesel engine) before the piston reaches the top of its
stroke. High-octane fuels do not ignite as easily as low-octane
fuels do. If auto-ignition occurs, excess pressure is
exerted on the piston as it tries to continue moving
upwards and the piston "slaps" against the side
of the cylinder. This can damage the engine. To prevent
this, engines that have higher degrees of compression
require higher octane fuels.
The Second Law
The second law is concerned with entropy, which is a
measure of disorder. The second law states that the
entropy of the universe increases or entropy in a
closed system can never decrease.
In a closed system,
available energy can never increase, so its opposite,
entropy, can never decrease.
The flow of heat from hot
things to cold, and never vice-versa.
The consequence of the
second law is that in a closed system, you can't finish
any real physical process with as much useful energy as
you had to start with — some is always wasted. This
means that a perpetual motion machine is impossible.
The overall increase in
disorder is therefore spontaneous. The molecules in one's
body exist in great order; this only happens because the
entropy of the rest of the universe is increased to a
greater amount than the entropy of the body is decreased.
Evolution
The Second Law states that every system left to its own
devices always tends to move from order to disorder. The
energy of the system tends to be transformed into lower
levels of availability, finally reaching the state of
complete randomness and unavailability for further work.
We know that crystals and
other regular configurations (such as snowflakes) can be
formed by apparently unguided processes but the recipe
for a crystal is already present in the solution it grows
from — the crystal lattice is prescribed by the
structure of the molecules that compose it. The formation
of crystals is the straightforward result of chemical and
physical laws that do not evolve and that are, compared
to genetic programs, very simple.
The rule that things never
organize themselves is also upheld in our everyday
experience. Without someone to fix it, a broken glass
never mends. Without maintenance, a house deteriorates.
Without new software, a computer never acquires new
capabilities. Never.
Even Charles Darwin
understood this universal principle. This is basic common
sense. That's why he cautioned biologists not to call
later "evolutionary" stages "higher".
Questions
- Is the Sterling
engine an internal combustion engine?
- Why are the
compression and expansion processes in internal
combustion engines considered to be adiabatic?
- What are the two
cycles in a 2-cycle engine?
- Describe the 5
processes in a 4-cycle engine.
- How do common diesel
engines differ from engines that use gasoline?
- Why do some engines
need high-octane fuel?
- Which system has more
entropy, an unbroken raw egg or a scrambled raw
egg?
- What is the Second
law of thermodynamics?
- Living organisms
appear to create order from disorder. How does
the second law apply under such circumstances?
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