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DIESEL CYCLE (1892) AND ITS PROCESS

 The limitation on the compression ratio in SI Engine can be overcome by compressing air alone, instead   of fuel-air mixture, and then injecting the fuel into the cylinder in spray form when combustion is   desired.  The C.I Engine, first proposed by Rudolph Diesel in the 1890's is very similar to S.I Engine,   differing mainly in the method of initiating combustion. The S.I Engines, a mixture of air and fuel is   compressed during compression stroke, and the compression ratios are limited by the onset of   autoignition or engine knock. In C.I engines, only air is compressed during the compression stroke.   Therefore, diesel engines can operate at much higher compression ratios, typically 12 and 24. The spark   plug and carburetor (for mixing fuel and air) are replaced by a fuel injector in diesel engines. The   temperature of air after compression must be high enough so that the fuel sprayed into the hot air burns     spontaneously. The rate of burning can, to some extent, be controlled by the rate of injection of fuel. An   engine operating in this way is called a compression ignition (C.I) Engine. 




The sequence of Processes in the elementary operation of a C.I Engine,

 Process 1-2, Intake. 

The air valve is open. The Piston moves out admitting air into the cylinder at constant  pressure.

 Process 2-3 , Compression. 

 The air is then compressed by the piston to the minimum volume with all the valves closed.

 Process 3-4 , Fuel Injection and Combustion. 

 The fuel valve is open, fuel is sprayed into the hot air, and combustion takes place at constant pressure. 

 Process 4-5 , Expansion. 

 The combustion products expand, doing work on the piston which moves out to the maximum volume. 

Process 5-6 , Blow - down.

The exhaust valve opens, and the pressure drops to the initial pressure.

Process 6-1 , Exhaust.

With the exhaust valve open, the piston moves towards the cylinder cover driving away the combustion products from the cylinder at constant pressure.

The above processes constitute an engine cycle, which is completed in four strokes of the piston or two revolutions of the crank shaft.




Two reversible adiabatic, and one reversible isobar, and one reversible isochore. 

Air is compressed reversibly and adiabatically in process 1-2.Heat is then added to it from an external source reversibly at constant pressure in process 2-3. Air then expands reversibly and adiabatically in  process 3-4. Heat is rejected reversibly at constant volume in process 4-1, and the cycle repeats itself. 

For m kg of air in the cylinder , the efficiency analysis of  the cycle can be made as given below, 

Heat Supplied, 

Q1 = Q2-3 = mCp ( T3 - T2 ) 

Q2 = Q4-1 = mCv ( T4-T1 )

Efficiency N= 1- ( Q2 /Q1) = 1 - ((mCv(T4-T1)) / mCp(T3-T2)) 

                 N = 1 - ((T4 -T1) / Y( T3 -T2))

The efficiency may be expressed in terms of any two of the following three ratios

Compression ratio, rk = V1 / V2 =  v1 / v2

Expansion ratio, re = V4 / V3 = v4 /v3

Cut - off ratio, rc = V3 / V2 = v3 / v2.

It is seen that rk = re.rc

Process 3-4 

T4 / T3  = (v3 / v4) ^ (Y-1) = 1 / re ^(Y-1)

T4 = T3 ((rc^(y-1))/(rk^(y-1))

Process 2-3 

T2 / T3 = p2v2 / p3v3 = v2 / v3 = 1 / rc

Process 1-2 

T1 / T2 = (v2 / v1 )^ ( y-1) = 1 /( rk ^ (y-1))  

T1 = T2 / (rk^ (y-1))

     = T3 / (rc * rk ^ (y-1)

Substituting the values of T1, T2  and T4 in the expression of efficiency.

N diesel = 1 - (((rc^y) - 1) /(y (rk^(y-1))(rc-1) )

As rc > 1 ,  (1 / Y) ((rc^y)-1) / (rc-1)) is also greater than unity. Therefore, the efficiency of the Diesel cycle is less than that of the Otto cycle for the same compression ratio.


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