The problem gives the compression ratio of 17, so I have V1/V2=17 same as V4/V3=
ID: 1861592 • Letter: T
Question
The problem gives the compression ratio of 17, so I have V1/V2=17 same as V4/V3=17 (like in Otto cycle). Why In state 4 they are not using Vr4=Vr3(V4/V3) =.4518(17) instead they are using some relation i dont understand. Can someone explain:
1. Why they are not simply using .4518(17)?
2. What relation are they using? Please, step by step
3. If I cant use r=V1/V2=V4/V3 like in Otto cycle, can you explain why?
KNOWN: An air-standard Diesel cycle has a known compression ratio and a specified state at the beginning of compression. The maximum cycle temperature is given. FIND: Determine (a) the net work, (b) the thermal efficiency, (c) the mean effective pressure, and (d) the cutoff ratio. SCHEMATIC GIVEN DATA: ENGINEERING MODEL: See example 9.2. Begin by fixing each principal state of the cycle (Table A - 22E). T1 = 520 degree R u1 = 88.62 ur1 = 158.58 For the isentropic compression Thus, T2 = 1534.5 degree R, h2 = 378.32 T3 = 4000 degree R h3 = 1088.3 vr3 = .4518 For the isentropic expansion and Thus, T4 = 2253.7 degree R, u4 = 421.25 For the cycle, w cycle =q cycle. Thus w cycle = Q23 - Q41 = m [(h3 - h2) - (u4 - u1)] Evaluating m Thus The thermal efficiency isExplanation / Answer
Firstly, an ideal diesel cycle works on a constant pressure combustion but not a constant volume combustion. So, you can use compression ratio(r) = V1/V2 but not r=V4/V3. Thats your first mistake. You took similarity in combustion for a diesel and otto cycle but the combustion in these ideal cycles are different(constant pressure vs constant volume)
So, in the state four for V4/V3 they took V4/V2 x V2/V3 and as V4=V1, it becomes V1/V2 x V2/V3. You know V1/V2 is the comp. ratio r and V2/V3 is T2/T3 as the process involved is constant pressure in which V/T is a constant.
I feel the rest can be figured out but if you have any problem, do comment and I can write the solution for you. Bye.
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