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s@ikiran
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why are heat addition and rejection processes in an otto cycle constant volume processes?similarily why does heat addition occurs at constant pressure in a diesel cycle?
These are idealizations of what actually occurs in real engines.s@ikiran said:why are heat addition and rejection processes in an otto cycle constant volume processes?similarily why does heat addition occurs at constant pressure in a diesel cycle?
thank you bro i got your pointSteamKing said:These are idealizations of what actually occurs in real engines.
For the otto cycle engine, combustion of the fuel and the exhaust of the burned fuel occur relatively quickly, while the piston is momentarily stopped at the top and bottom of the stroke, respectively.
https://en.wikipedia.org/wiki/Otto_cycle
For the diesel engine, the engine is designed so that the injection and combustion of fuel occurs just as the piston has reached top dead center. As the piston starts to move downward in the power stroke, fuel is injected and auto-ignited by the compressed air in the cylinder. For a brief time, the additional pressure created by the gases from the burned fuel offsets the loss of pressure in the cylinder as the piston travels downward.
https://en.wikipedia.org/wiki/Diesel_engine
It's not clear what you mean here.s@ikiran said:thank you bro i got your point
now i need another question to be answered
in a carnot cycle the work done is taken by the relation (work done =heat supplied -heat removed)
in a otto cycle the work done can be found by the same relation and it is also given as (work output=workdone by the system-work done on the system)
in otto cycle the work done in compression and expasion are considered whereas in carnot cycle it is not. why what is the reason behind it?
SteamKing said:It's not clear what you mean here.
Work and heat are equivalent.
In all cycles, Carnot included, the amount of work which can be provided is based on the difference between the highest temperature obtained in the cycle and the lowest temperature obtained.
https://en.wikipedia.org/wiki/Carnot_cycle
The Carnot cycle extracts all available heat between these two temperature extremes, hence it is considered to be 100% efficient. All other cycles, like the Otto and the Diesel, are not able to extract this much available heat and turn it into work, even though they may operate between the same temperature extremes as the Carnot cycle, hence their efficiencies are less than 100%.
In the Diesel and the Otto cycles, the work done compressing the gases inside the cylinder is not available as output from the engine; this is energy which is lost, as far as useful work is concerned. Similarly, the expansion of the exhaust gases also removes energy from the system, which might otherwise be converted to work.s@ikiran said:ok i will make it clear
in carnot cycle heat supplied and heat removed during isothermal processes are considered while finding work
why isn't the work done during adiabatic compression and expansion considered?
They should be.s@ikiran said:are the net work done and work output both equal?
It's going to depend on the information given in a particular problem.s@ikiran said:i saw in a book that for an otto cycle
work done=heat supplied-heat rejected i.e.,w=(cv(t3-t2)-cv(t4-t1))(constant volume process)
net work done=work done by system during adiabatic expansion-work done on system during adiabatic compression i.e.,w=((p3v3-p4v4)/(ϒ-1)-(p2v2-p1v1)/(ϒ-1))
now are these both equal.i mean if we are asked to find work can we use any of the above equations or are there any exceptions?
Randy Beikmann said:s@ikiran, you said,
"in carnot cycle heat supplied and heat removed during isothermal processes are considered while finding work
why isn't the work done during adiabatic compression and expansion considered?"
This is not true. In every thermodynamic cycle, the work done in ALL processes must be accounted for, as well as the heat transferred. In the Carnot Cycle, work is done during all four processes, positive during expansion, and negative during compression. Heat, however, is only transferred in two: the isothermal expansion and compression processes. The net work is the sum for all four processes. Divide that by the net heat addition, and you get the Carnot efficiency 1-TL/TH.
Randy Beikmann said:s@ikiran, I do need to apologize for misspeaking in my first post. I just realized it. As I said in the second post, efficiency does NOT have to do with net heat addition, only the heat added at high temperature (as from burning a fuel). So regardless of how the cycle operates, efficiency is the net work divided by the heat transferred in: Wnet/Qh.
Again, I'm sorry if I led you off course.
thank you so much sir.this piece of information you gave me is really useful and now i understand this concept very clearly.Randy Beikmann said:"are the values obtained and the significance of both equations [heat balance and work balance] equal?"
In these ideal cycles, the net heat added (based on specific heats) must equal the net work produced. In a real engine, it's better to use measured P vs. V data, because heat is lost through the cylinder walls, piston rings leak, etc.
"in common usage efficiency means ratio of work output to useful input work right?"
Energy efficiency is defined as "net work you get out" divided by "energy you put in." So there are two main types of energy efficiency: 1) thermal efficiency, and 2) mechanical efficiency.
Thermal efficiency is "net work"/"heat added." You want to know how completely the engine converted the energy you paid for (in the fuel) to work. This is used for heat engines like we are talking about.
Mechanical efficiency is "work output"/"work input." This is used to judge how well a device passes mechanical energy along. For example, if you have a gearbox and put 500 joules of work into the input shaft, and it transmits 450 joules to the output shaft, its mechanical efficiency is 90%.
I think this addresses your question "why aren't we considering this compressor work as input work ?" In an engine, the compression work is accomplished by the engine itself, not by some other device. The kinetic energy of the rotating parts is reduced during compression - the engine slows down. During the expansion, the engine speeds up. The net output work is the expansion work minus the compression work. So only the heat from a fuel is added to the engine, and that's what efficiency is based on. Thermal efficiency.
By the way, this whole area is explained much more completely in "Physics for Gearheads," Chapters 12-15. All these terms are defined and reinforced in the discussion and in the figures. ;-)
The Otto and Diesel cycles are two types of internal combustion engine cycles that are used to convert chemical energy into mechanical energy through the process of heat addition and rejection.
The main difference between the Otto and Diesel cycles is the type of fuel used. The Otto cycle uses a spark ignition system and runs on gasoline, while the Diesel cycle uses a compression ignition system and runs on diesel fuel.
The heat addition process in the Otto and Diesel cycles is necessary to convert the chemical energy in the fuel into thermal energy, which is then converted into mechanical energy. The heat rejection process is important for removing excess heat from the engine to prevent overheating and maintain its efficiency.
The efficiency of these cycles is affected by several factors, including the compression ratio, fuel-air mixture ratio, and ignition timing. The type of fuel used and the design of the engine also play a significant role in the efficiency of these cycles.
The Otto cycle is commonly used in gasoline-powered cars, while the Diesel cycle is used in diesel-powered vehicles, such as trucks and buses. Both cycles are also used in power generation, such as in generators and power plants.