The fuel and air gases in the cylinders of a 1200 cm3 car engine go from 25oC before combustion and rise to a peak temperature of 2100oC after combustion. If normal atmospheric pressure is 101kPa, calculate the peak pressure reached after combustion. Although the movement of the piston changes the volume, (i) for the sake of argument assume the volume is constant.

The fuel and air gases in the cylinders of a 1200 cm³ car engine go from 25^oC before combustion and rise to a peak temperature of 2100^oC after combustion. If normal atmospheric pressure is 101kPa, calculate the peak pressure reached after combustion. Although the movement of the piston changes the volume, (i) for the sake of argument assume the volume is constant.

 Solution

·         T₁ = 25 + 273 = 298 K, T₂ = 2100 + 273 = 2373 K, P1 = 101 KPa

·         p/T = constant

·         p₁/p₂ = T₁/T2

·         p₂ = p₁ x T₂/T₁

·         p₂ = 101 x 2373/298 = 804 kPa

 

o    (ii) To be more realistic, assume the initial volume of fuel vapour plus air was 400 cm3, now re-calculate the final pressure.

o    You now need to use the full PVT expression.

o    (p₁ x V₁)/T₁ = (p₂ x V₂)/T₂

o    p₂ = (p₁ x V₁ x T₂)/(V₂ x T₁)

o    p₂ = (101 x 400 x 2373)/(1200 x 298) = 268 kPa