tailieunhanh - Internal Combustion Engines Fundamentals Episode 1 part 6

Tham khảo tài liệu 'internal combustion engines fundamentals episode 1 part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 172 INTERNAL COMBUSTION ENGINE FUNDAMENTALS The mixture temperature at point 1 in the cycle can be related to the inlet mixture temperature 7J with Eq. . For a working fluid with c and cp constant this equation becomes cp 7 x cp T6 1 - xr cp 7 - fa - 1 rc Pi Use of Eqs. and leads to the relation 7Ị _1 -xr 7J 1 l iyrJtpj pi y 1 -38 Extensive results for the constant-volume cycle with y can be found in Limited- and Constant- Pressure Cycles The constant-pressure cycle is a limited-pressure cycle with p3 p2. For the limited-pressure cycle the compression work remains wc me Tị - T2 The expansion work from Eq. becomes WE m c T3b - TJ p3 v3b - c3a For the combustion process Eqs. h give m 2-3aổLHV mCtẮ 3a 2 m 3a-3 ổLHV mCp 3i T3a or ổLHV m c 3a 2 Cp 3b 3a for a working fluid with c and cp constant throughout the cycle. Combining Eqs. and to and simplifying gives T4-T1 1 T3a - T2 y r36 - r3a Use of the isentropic relationships for the working fluid along 1-2 and 3h-4 with the substitutions Pi 3a b leads to the result a -1 Vf-i - 1 _ ry- _ay P - 1 a 1_ For p 1 this result becomes the constant-volume cycle efficiency Eq. . For a 1 this result gives the constant-pressure cycle efficiency as a special case. IDEAL MODELS OF ENGINE CYCLES 173 The mean effective pressure is related to Pj and p3 via imep _ Q rc Pl cv 71 7 - 1 rc - 1 imep1 Q V 1 V rc Pa xrỊ Cp tJv - 1 Arc - 1 Cycle Comparison The above expressions are most useful if values for y and ổ c 71 are chosen to match real working fluid properties. Figure 5-5 has already shown the sensitivity of rif i for the constant-volume cycle to the value of y chosen. In Sec. average values of y and yb were determined which match real working fluid properties over the compression and expansion strokes respectively. Values for a stoichiometric mixture appropriate to an SI engine