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The net work done by a cyclical process is the area inside the closed path on a PV size 12{ ital "PV"} {} diagram, such as that inside path ABCDA in [link] . Note that in every imaginable cyclical process, it is absolutely necessary for heat transfer from the system to occur in order to get a net work output. In the Otto cycle, heat transfer occurs along path DA. If no heat transfer occurs, then the return path is the same, and the net work output is zero. The lower the temperature on the path AB, the less work has to be done to compress the gas. The area inside the closed path is then greater, and so the engine does more work and is thus more efficient. Similarly, the higher the temperature along path CD, the more work output there is. (See [link] .) So efficiency is related to the temperatures of the hot and cold reservoirs. In the next section, we shall see what the absolute limit to the efficiency of a heat engine is, and how it is related to temperature.

The figure shows four diagrams a, b, c, and d representing four stages of a four stroke gasoline engine. The construction of the engine has the base chamber whose cross section is in the shape of a square with flat corners, the top portion of the chamber is extended into a cylindrical section. The cylindrical section ends in the upper section with two valves, an inlet and an outlet. The cylindrical section has a movable cylinder with a piston attached to it. The piston is connected to the crank shaft in the base gas chamber. There is a spark plug on the top most part of the cylinder between the two valves. The four parts of the diagram show various stages of this four stoke engine. Part a of the diagram shows the air fuel mixture enters through the inlet valve in the upper section of the engine. The outlet valve is shown to be closed. The air and fuel is shown to exert a pressure on the piston acting downward. This force is shown to move the rotating crank shaft in clockwise direction in the gas chamber. This is the intake stroke. Part b of the diagram shows the compression stroke. Both the inlet and outlet valves are closed. The air and fuel mixture is shown to be compressed. The piston is shown to rise up as shown by a vertically pointing arrow. The piston is at the edge of the cylinder near the valves. The crankshaft in the gas chamber has shown to complete one complete cycle of rotation in the gas chamber. Part c of the diagram shows the power stroke. It has two parts, first the ignition stroke. This shows ignition of the fuel in the cylinder and pressure buildup in the region. Then in the second part the piston is shown to descend down the cylinder moving the crankshaft in the gas chamber in the clockwise direction. Part d of the figure shows the exhaust stroke. The piston expels the hot gas by moving upward and the gas is expelled through the exhaust valve.
In the four-stroke internal combustion gasoline engine, heat transfer into work takes place in the cyclical process shown here. The piston is connected to a rotating crankshaft, which both takes work out of and does work on the gas in the cylinder. (a) Air is mixed with fuel during the intake stroke. (b) During the compression stroke, the air-fuel mixture is rapidly compressed in a nearly adiabatic process, as the piston rises with the valves closed. Work is done on the gas. (c) The power stroke has two distinct parts. First, the air-fuel mixture is ignited, converting chemical potential energy into thermal energy almost instantaneously, which leads to a great increase in pressure. Then the piston descends, and the gas does work by exerting a force through a distance in a nearly adiabatic process. (d) The exhaust stroke expels the hot gas to prepare the engine for another cycle, starting again with the intake stroke.
Part a of the figure shows a graph of pressure P versus volume V for an Otto cycle. The pressure P is along the Y axis and the volume V is along the X axis. The graph shows a complete cycle A B C D.  The path begins at point A; the curve rises upward from point A to point B along the direction of the negative X axis. This is marked as an adiabatic process. Then the curve rises vertically up from point B to point C in a direction perpendicular to the X axis. Then the curve moves smoothly down to point D along the direction of the positive X axis. This is also marked as an adiabatic process. The last part of the curve drops vertically down from point D back to point A. The path A B is slightly lower than path C D. Heat Q sub h is shown to enter the system as shown by a bold arrow to the curve B C. Heat Q sub c is shown to leave the system as shown by a bold arrow near D A. The area inside A B C D is shaded, and the area of the shaded region is shown proportional to the work W. Part b of the diagram shows an internal combustion engine represented as a circle. The hot reservoir is a rectangular section at the top of the circle shown at temperature T sub h. A cold reservoir is shown as a rectangular section in the bottom part of the circle at temperature T sub c. Heat Q sub h is shown to enter the heat engine, as shown by a bold arrow. Work W is produced as output, shown to leave the system, and the remaining heat Q sub c is returned back to the cold reservoir as shown by bold arrow toward it.
PV size 12{ ital "PV"} {} diagram for a simplified Otto cycle, analogous to that employed in an internal combustion engine. Point A corresponds to the start of the compression stroke of an internal combustion engine. Paths AB and CD are adiabatic and correspond to the compression and power strokes of an internal combustion engine, respectively. Paths BC and DA are isochoric and accomplish similar results to the ignition and exhaust-intake portions, respectively, of the internal combustion engine’s cycle. Work is done on the gas along path AB, but more work is done by the gas along path CD, so that there is a net work output.
Part a of the figure shows a graph of pressure P versus volume V for an Otto cycle. The pressure P is along the Y axis and the volume V is along the X axis. The graph shows a complete cycle A B C D. The path begins at point A; the curve rises upward from point A to point B along the direction of the negative X axis. This is marked as an adiabatic process. Then the curve rises vertically up from point B to point C in a direction perpendicular to the X axis. Then the curve moves smoothly down to point D along the direction of the positive X axis. This is also marked as an adiabatic process. The last part of the curve drops vertically down from point D back to point A. The path A B is much lower than path C D, which shows that the starting temperature of path C D is higher than A B. Heat Q sub h prime enters the system as shown by a bold arrow to the curve B C. Heat Q sub c prime leaves the system, as shown by a bold arrow near D A. The area inside A B C D is shaded. Part b of the diagram shows an internal combustion engine represented as a circle. The hot reservoir is a rectangular section at the top of the circle shown at temperature T sub h prime. A cold reservoir is shown as a rectangular section in the bottom part of the circle at temperature T sub c prime. Heat Q sub h prime enters the heat engine as shown by a bold arrow, a work W prime is produced as output, shown to leave the system, and the remaining heat Q sub c prime is returned back to the cold reservoir, as shown by a bold arrow toward it.
This Otto cycle produces a greater work output than the one in [link] , because the starting temperature of path CD is higher and the starting temperature of path AB is lower. The area inside the loop is greater, corresponding to greater net work output.

Section summary

  • The two expressions of the second law of thermodynamics are: (i) Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction; and (ii) It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
  • Irreversible processes depend on path and do not return to their original state. Cyclical processes are processes that return to their original state at the end of every cycle.
  • In a cyclical process, such as a heat engine, the net work done by the system equals the net heat transfer into the system, or W = Q h Q c , where Q h is the heat transfer from the hot object (hot reservoir), and Q c is the heat transfer into the cold object (cold reservoir).
  • Efficiency can be expressed as Eff = W Q h size 12{ ital "Eff"= { {W} over {Q rSub { size 8{h} } } } } {} , the ratio of work output divided by the amount of energy input.
  • The four-stroke gasoline engine is often explained in terms of the Otto cycle, which is a repeating sequence of processes that convert heat into work.

Questions & Answers

A Body of maas m slides down an incline and reached the bottoms with a velocity v
Aditya Reply
What is mass
Aditya
mass is the amount of matter a body contains.
prakash
what is model
Chisom Reply
what is a dimension
John
When using the Conservation of Energy equation, do we substitute the energy as a negative quantity when the energies on a single object are exerting forces opposite to one another?
Jennifer Reply
Ex. On an inclined plane, gravitational potential energy, friction energy/work and spring potential energy. (Let's say that the spring is keeping the box from sliding down the slope.) How do we use this in the equation? I'm so confused
Jennifer
Oh! And if there's kinetic energy that is exerting a force opposite to the spring, what do we do?
Jennifer
Then we subtract the k.e. from force exerted from newton's 2nd law.
Prem
Subtract energy from force? They're different units
Jennifer
why is it dat when using double pan balance the known and unknown mass are the same
Victor Reply
discuss the uses of energy in the following sectors of economy security and education
amajuoyi
why the current produce during dc motot is not use for its working rather we have to supply current outside
Tanveer
is there more then 4 dimensions
Miguel Reply
hii
princy
hi
Miguel
hello I kinda need help in physics... a lot
Brown
Brown. what kind of help
Jeff
when it comes to physics stick with the basics don't overthink things
Jeff
yes
ayesha
sticking to the basics will take you farther than overwhelming yourself with more than you need to physics is simple keep it simple
Jeff
thk u Ayesha
Jeff
for real....? so I've got to know the fundamentals and use the formula to solve any problem
Brown
read Stephan hawkings a brief history of time
ayesha
it'll help you understand better than summing up formulas or ready textbooks
ayesha
physics isn't hard it's just understanding and applying the formulas if u need help ask any question
ayesha
okay...because I've got an exam next year February a Computer based exam
Brown
start with a pace a plan and stick to it
ayesha
well best of luck can't help you much there contact your teachers for tips and helpful notes
ayesha
hi
Varun
hello
Ibrahim
how can we find absolute uncertainty
ayesha Reply
it what?
Luke
in physics
ayesha
the basic formula is uncertainty in momentum multiplied buy uncertainty In position is greater than or equal to 4×pi/2. same formula for energy and time
Luke
I have this one question can you please look it up it's 9702/22/O/N/17 Question 1 B 3
ayesha
what
uma
would you like physics?
Suthar
yes
farooq
precision or absolute uncertainty is always equal to least count of that instrument
Iram
how do I unlock the MCQ and the Essay?
Ojeh Reply
what is the dimension of strain
Joy Reply
Is there a formula for time of free fall given that the body has initial velocity? In other words, formula for time that takes a downward-shot projectile to hit the ground. Thanks!
Cyclone Reply
hi
Agboro
hiii
Chandan
Hi
Sahim
hi
Jeff
hey
Priscilla
sup guys
Bile
Hy
Kulsum
What is unit of watt?
Kulsum
watt is the unit of power
Rahul
p=f.v
Rahul
watt can also be expressed as Nm/s
Rahul
what s i unit of mass
Maxamed
SI unit of mass is Kg(kilogram).
Robel
what is formula of distance
Maxamed
Formula for for the falling body with initial velocity is:v^2=v(initial)^2+2*g*h
Mateo
i can't understand
Maxamed
we can't do this calculation without knowing the height of the initial position of the particle
Chathu
sorry but no more in science
Imoreh
2 forces whose resultant is 100N, are at right angle to each other .if one of them makes an angle of 30 degree with the resultant determine it's magnitude
Victor Reply
50 N... (50 *1.732)N
Sahim
Plz cheak the ans and give reply..
Sahim
50 N...(50 *1.732)N
Ibrahim
show the working
usiomon
what is the value of f1 and f2
Syed
what is the value of force 1 and force 2.
Syed
.
muhammad
Is earth is an inertial frame?
Sahim Reply
The abacus (plural abaci or abacuses), also called a counting frame, is a calculating tool that was in use in Europe, China and Russia, centuries before the adoption of the written Hindu–Arabic numeral system
Sahim
thanks
Irungu
Most welcome
Sahim
Hey.. I've a question.
Sahim Reply
Is earth inertia frame?
Sahim
only the center
Shii
What is an abucus?
Irungu
what would be the correct interrogation "what is time?" or "how much has your watch ticked?"
prakash Reply
someone please give answer to this.
prakash
a load of 20N on a wire of cross sectional area 8×10^-7m produces an extension of 10.4m. calculate the young modules of the material of the wire is of length 5m
Ebenezer Reply
Young's modulus = stress/strain strain = extension/length (x/l) stress = force/area (F/A) stress/strain is F l/A x
El
so solve it
Ebenezer
please
Ebenezer
two bodies x and y start from rest and move with uniform acceleration of a and 4a respectively. if the bodies cover the same distance in terms of tx and ty what is the ratio of tx to ty
Oluwatola Reply
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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