Octane (C8H18) is burned with 250 percent theoretical air, which enters the combustion chamber at 25°C. Assume complete combustion and a total pressure of 1.8 atm. determine (a) the air-fuel ratio and (b) the dew-point temperature of the products.

a) The air-fuel ratio is 37.83 kg air/kg fuel

b) The dew-point temperature is 47.02°C

Explanation:

a) The combustion equation is equal to:

C₈H₁₈ + 2.5a(O₂ + 3.76N₂) = xCO₂ + yH₂O + 9.4aN₂ + 1.5aO₂

The mass balance for each elements is equal to:

C: 8 = x

H: 18 = 2y, y = 9

O: 5a = 2x + y + 3a, a = 12.5

C₈H₁₈ + 31.25(O₂ + 3.76N₂) = 8CO₂ + 9H₂O + 117.5N₂ + 18.75O₂

The air fuel ratio is equal to:

Taking the molar mass of octane = 114 kg/mol

molar mass of air = 29 kg/mol

b) The dew-point temperature is:

Where Nv = moles of water vapor in products

Nprod = moles in the products

Pprod = total pressure = 1.8 atm = 182.38 kPa

The saturation temperature at 10.71 kPa is obtained from saturated-water temperature tables and interpolation:

T₁ = 45°C

P₁ = 9.5953 kPa

T₂ = 50°C

P₂ = 12.352 kPa

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