A spark ignition engine has a compression ratio of 8 and the volume before compression is 0.9 m³/kg. Net heat interaction per cycle is 1575 kJ/kg. What is the mean effective pressure?

Concept:

Compression ratio = \(r = \frac{{volume\;before\;compression\;}}{{volume\;after\;compression}}\)

Work done = Mean effective pressure × swept volume

swept volume = (v1 – v2)

Calculation:

Given Compression ratio (r) = 8, v₁ = 0.9 m³/kg, Net heat interaction for cycle = 1575 kJ/kg

For any cycle, the net heat interaction = Net work interaction

\(r = \frac{{{v_1}}}{{{v_2}}}\)

⇒  \({v_2} = \frac{{0.9}}{8} = 0.1125\frac{{{m^3}}}{{kg}}\)

Work done = Mean effective pressure × swept volume (v₁ – v₂)

∴ 1575 × 10³ = p_m × (0.9 – 0.1125)

p_m = 20 bar