Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
where the value of Am is the molecular air-fuel ratio, and k is the nitrogen-to-oxygen molecular
ratio; this is conventionally taken as 79/21 or 3.76, as seen previously in Sec. 1.5.5. The
relationship between Am and the actual air-to-fuel ratio, AFR, is then given by:
Am(M0 +kMN )
AFR = J±-2* M (4.3.4)
Mc + nMH
where MQ » etc., are the molecular weights of the constituent gases to be found numerically
in Table 2.1.1. The value of Am can De determined only by balancing the equation for the
carbon, hydrogen, oxygen and nitrogen present in the combustion products:
carbon balance 1 = xi + X2 (4.3.5)
hydrogen balance n = 2x3 + 2x5 (4.3.6)
oxygen balance 2Am = xj + 2x2 + x 3 +2x4 (4.3.7)
nitrogen balance 2Amk = 2x6 (4.3.8)
4.3.2.1 Stoichiometry and equivalence ratio
In the ideal case of stoichiometry, and hence ignoring dissociation effects, the values of
xj, X4 and X5 in Eq. 4.3.3 are zero, in which case the solutions of Eqs. 4.3.5 to 4.3.8 are found
as:
Xi = 1 Xo = — and Am = 1 + —
z i 2 4
This reveals the stoichiometric air-fuel ratio, AFRS, as:
1 + i)K +kM N2)
AFRS = •> =£ (4.3.9)
Mc + nMH
To confirm this with the previous solution in Eq. 1.5.17, for iso-octane:
[ 1 + — )(31.99 + 3.76 x 28.01)
AFRS = ^ 4_J = 15.02
12.01 + 2.25 x 1.008
Using the properties of the fuels in Table 3.1, observe that the equivalent values for AFRS
for super-unleaded gasoline is 14.2, for dodecane is 14.92, and for diesel fuel is 14.42.
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