Diesel Cycle - Ideal Cycle for Compression-ignition Engines

Isentropic Compression (1-2) Click to View Movie (25.0 kB)		The only difference between ideal Otto cycle and ideal Diesel cycle is the heat addition process. Instead of constant volume heat addition process in SI engine, heat is added to the air in the Diesel engine at constant pressure. The four processes are: 1-2 Isentropic compression 2-3 Constant pressure heat addition 3-4 Isentropic expansion 4-1 Constant volume heat rejection
Constant Pressure Heat Addition (2-3) Click to View Movie (45 kB) Isentropic Expansion (3-4) Click to View Movie (29 kB) Constant Volume Heat Rejection (4-1) Click to View Movie (35 kB)		Noting that the ideal Diesel cycle is executed in a closed system and the working fluid is air according to the air-standard assumption. Also, changes in kinetic and potential energies are negligible. No heat transfer is involved in the two isentropic processes. The energy balances for these two processes are:       -w12 = u2 - u1       -w34 = u4 - u3 w12 is negative since work is needed to compress the air in the cylinder and w34 is positive since air does work to the surroundings during its expansion. In the constant pressure heat addition process, air is expanded to keep the pressure as constant during the heat addition. The expansion work equals       w23 = P2(v3 - v2) The energy balances for this process is:       q23 = u3 - u2 + w23 = h3 - h2 In the constant volume heat rejection process, no work interaction is involved since no volume change occurs. The energy balances for this process is:           q41 = u1 - u4 q23 is positive since heat is added to the air and q41 is negative since heat is rejected to the surroundings during this process. For the whole cycle, the energy balance can be determined by adding the energy balance of its four processes. That is,       q23 + q41 - w12 - w34 = 0
		The thermal efficiency of an ideal Otto cycle is       ηth,Diesel = wnet/qin According to the analysis above, the net work output is       wnet = w34 + w12 = q23 + q41       qin = q23       ηth, Diesel = 1+ q41/q23 Under the cold air-standard assumption, the thermal efficiency of an ideal Diesel cycle is
		In order to simplify the above equation, the cutoff ratio rc is defined as rc = v3/v2 Process 1-2 and process 3-4 are isentropic. Thus,        The thermal efficiency relation reduces to