-:Fuel-Air Cycle:-
The theoretical Cycle which is based actual properties of the cylinder goods called the fuel-Air Cycle ,it provides a rough idea for comparison with the actual performance.
The theoretical Cycle which is based actual properties of the cylinder goods called the fuel-Air Cycle ,it provides a rough idea for comparison with the actual performance.
-: Factors Considered for fuel -air cycle Calculations:-
The following factors are taken into consideration while making fuel -air cycle Calculations-
1- Composition of the cylinder Gases :-
The cylinder gases contains fuel air , water vapour and residual gas. The fuel air ratio changes during the operation of the engine which changes the relative amounts of CO2,water vapour etc.
2-Variable specific heat :-
Specific heat increases with temperature except for mono-atomic gases ,therefore the value of Y(Gamma) also changes with temperature.
3-Effect of Dissociation :- The disintegration of combustion products at high temperature is known as Dissociation process.
The fuel and air do not completely (above 1600 k) and this leads to the presence of CO, H2 ,H and O2 at equilibrium conditions.
4- Variation in the number of molecules:- The number of molecules present after combustion depend upon fuel-Air ratio and upon the pressure and temperature after the combustion.
Assumptions made for Fuel-Air cycle Analysis:-
The following assumptions are made for the analysis of fuel air cycle-
1- There is no chemical change in either fuel or air prior to combustion .
2- Subsequent to combustion , the charge is always in chemical equilibrium .
3- There is no heat exchange between the gases and the cylinder walls in any process , i.e. the processes are adiabatic .
4- In case of reciprocating engines , it is assumed that fluid motion can be ignored inside the cylinder .
5- The fuel is completely vaporized and perfectly mixed with the air .
6- The burning takes place instantaneously at top dead center (TDC) .
Importance / Significance of Fuel-air Cycle :-
1- The air standard cycle shows the general effect of only compression ratio on engine efficiency whereas the fuel air cycle may be calculated for various fuel air ratios , inlet pressures and temperatures .
2- A good estimate of power to be expected from the actual engine can be made from fuel air cycle analysis .
3- With the help of fuel air cycle analysis , peak pressures and exhaust temperatures can be very closely approximated which affect the engine structure and design .
4- The effect of many engine variables can be understood better by fuel air cycle analysis .
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