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## otto cycle constant specific heat Post Review

Otto Cycle : For constant specific heat at constant pressure and constant volume.All processes are quasi-static.

## otto cycle constant specific heat Q&A Review

### What is otto cycle?

It is thermodynamic cycle which is used in internal combustion engine to produce work. It is mostly used in automobile ,petrol engine,. This is the P-V diagram of Otto cycle:- There are different type of processes in Otto cycle :- 1) ,Adiabatic compression (2-3):,- air and fuel mixture is compressed isentropically inside the cylinder. 2) ,Constant volume heat addition (3-4),:- combustion take place inside the cylinder due to external source. 3) ,Adiabatic expansion (4-5),:- high pressure and high temperature gases expand isentropically inside the cylinder. 4) ,Constant volume heat rejection (5-6),:- gases come out from the cylinder and cycle is completed. Now new mixture will come and cycle will be continued. Where r is compression ratio and y is specific heat ratio.

### Why do we call otto/diesel/dual cycle an air standard cycle?

For the purpose of thermodynamic analysis of the internal combustion engines, the following approximations are made: The engine is assumed to operate on a closed cycle with a fixed mass of air which does not undergo any chemical change. The combustion process is replaced by an equivalent energy addition process from an external source. The exhaust process is replaced by an equivalent energy rejection process to external surroundings by means of which the working fluid is restored to the initial state. The air is assumed to behave like an ideal gas with constant specific heat. These cycles are usually referred to ,as air standard cycle,.

### What is the difference between air standard cycles and stroke engines?

Air standard cycles that most closely resemble internal combustion engines (Otto and Diesel), have some key differences to real working engines, in the case of the Otto ASC: It's typically taught and modeled as a closed cycle with no gas exchange (no intake and exhaust process). The lack of gas exchange eliminates the pumping loop of real engines. The pumping loop in a naturally aspirated engine represents negative work on the P-V diagram. See here: Image from . Ideal gas is assumed, so the specific heat capacity of the working fluid is different from a real engine. The ideal gas specific heat ratio (gamma) for air is 1.4, but for an air/fuel mixture is about 1.35 There is a constant volume heat addition at the completion of the compression stroke (analogous to the combustion process). In a real engine this heat addition occurs over a much longer period of time, with 50% of the mass fraction burned occurring at 8deg ATDC at MBT (no knock limit) spark advance The Otto ASC compression and expansion strokes are isentropic (adiabatic and reversible). In a real engine these processes approach this ideal but there are heat losses to the cylinder walls, cylinder head and piston as well as working fluid losses via leakage past the piston rings (blow-by) and the valves. These losses are very small however. There is a constant volume heat rejection at the end of the expansion stroke. In a real engine the exhaust valve must open before the completion of the expansion stroke, so most of the working fluid is expelled before the expansion process is completed. Because of these differences, a real engine cannot approach the efficiency of the air standard cycle

### How do diesel engines run without spark plugs?

All the answers look good, but let me inject some basic textbook concepts. The traditional gasoline engine is an Otto Cycle engine, named after Nikolaus Otto. The diesel engine, named after Rudolf Diesel, uses the Diesel Cycle. The ,cycles, in these cases refer to a graph of pressure versus volume as one cylinder goes through its 2 or 4 strokes and is ready to repeat. In the idealized Otto Cycle, air is compressed, then heat is added quickly--at constant volume--when the spark plug detonates the mixture. The heat raises the pressure, so that work will be done as the heated air expands in the power stroke. In the idealized diesel cycle, the compressed air is hot enough to start combustion, then the fuel is added over a non-zero time during the power stroke so that the gas is heated at constant pressure during part of the power stroke. To an engineer, the idealized cycle defines each type of engine. When the air in the cylinder is hot enough to ignite the diesel fuel, then the time of fuel burning is controlled by the time during which the injector sprays a mist of fuel. The Otto cycle calls for quick burning, so the air and fuel can be pre-mixed, ready to burn ASAP when the spark plug sparks. If you can gradually absorb the idea of the idealized cycles, then you can read Wikipedia articles on different interesting engines. For example, some current Ford hybrid cars have Atkinson-cycle engines. They are like Otto cycle engines, but less air is taken in during the cycle, which raises efficiency. Wikipedia has an article called ,Heat engine, which will give you links to other articles on specific idealized cycles. Traditional Otto Cycle engines depend on keeping the air/gasoline ratio in a range where the flame will propagate after it's ignited by the spark. In diesel engines, the air/fuel ratio is not a critical parameter in the same way. Of course more recent gasoline engines use new technologies to bend the old rules and save fuel.