Subtopic: Bio-fuels and their use in farm mechanization
Question Statement:
An E20 fuel blend contains $20\%$ anhydrous ethanol ($C_2H_5OH$) and $80\%$ gasoline by mass. The stoichiometric air-fuel ratio for pure ethanol is $(A/F)_{st,eth} = 9.00$, while for pure gasoline it is $(A/F)_{st,gas} = 14.70$. The stoichiometric air-fuel ratio of the E20 fuel blend on a mass basis is ________ (round off to 2 decimal places).
Answer Key:
13.56
Step-by-Step Solution & Explanation
The stoichiometric air requirement of a fuel blend is the mass-weighted sum of individual fuel air requirements:
$(A/F)_{st,blend} = w_{eth} \cdot (A/F)_{st,eth} + w_{gas} \cdot (A/F)_{st,gas}$
Given $w_{eth} = 0.20$ and $w_{gas} = 0.80$:
$(A/F)_{st,blend} = (0.20 \times 9.00) + (0.80 \times 14.70) = 1.80 + 11.76 = 13.56$