(a) (i) How does the mixture combustion in the combustion chamber of a C.I. engine differ from that of an S.I. engine? (ii) What…

(a) CI versus SI combustion, and combustion-induced swirl

(i) In an SI engine, a homogeneous air–fuel mixture is compressed and ignited by a spark. A premixed flame front spreads from the plug, and its speed depends on mixture strength, turbulence and temperature. In a CI engine only air is compressed; fuel is injected near the end of compression and autoignites after an ignition delay in the hot air. The charge is heterogeneous: part of the fuel burns quickly in the premixed phase, but most burns by diffusion, controlled by how fast air mixes with fuel. CI combustion is therefore mixing-controlled, with no flame propagating through a uniform charge, and the chamber must create strong air motion.

(ii) Combustion-induced swirl is turbulence generated by the combustion itself. Part of the fuel burns in a small auxiliary chamber, its pressure rises, and the burning gases are forced at high speed through a narrow passage into the main chamber, where they mix with the remaining air and fuel. This shortens the delay and completes combustion.

Two designs use this method:

• Pre-combustion chamber: a small chamber of about 20–30% of the clearance volume sits above the piston and connects to the main chamber by one or more narrow passages. Fuel is injected into it, partially burns, and the pressure rise blows the rich burning mixture into the main chamber, where it burns with the remaining air.
• Air-cell (energy-cell, Lanova type): a small cell lies opposite the injector, joined to the main chamber by a narrow throat. During compression, air enters the cell; after injection, burning in the cell raises its pressure and drives a violent jet of burning gas back into the main chamber.

Both give good mixing and a smooth, rapid burn, at the cost of heat loss through the passage, a higher compression ratio for starting, and poorer fuel economy than direct injection. The sketches below show both designs.

(b) Single-heater regenerative cycle

State 1: 30 bar, 400°C, h₁ = 3230.9 kJ/kg, s₁ = 6.9212 kJ/kg K. No turbine efficiency is given, so the expansion is isentropic.

Bleed at 0.3 MPa: s = s₁ gives x₂ = (6.9212 − 1.672)/(6.992 − 1.672) = 0.987 and h₂ = 561.4 + 0.987 × 2163.8 ≈ 2697 kJ/kg.

Exhaust at 0.1 bar: x₃ = (6.9212 − 0.649)/(8.150 − 0.649) = 0.836 and h₃ = 191.8 + 0.836 × 2392.8 ≈ 2192.5 kJ/kg.

In the direct-contact heater, y h₂ + (1 − y)h_f(0.1 bar) = h_f(0.3 MPa) = 561.4, so

y = (561.4 − 191.8)/(2697 − 191.8) = 0.1475 kg of steam bled per kg.

Neglecting pump work, the turbine work is w = (h₁ − h₂) + (1 − y)(h₂ − h₃) = 534 + 0.8525 × 504.5 = 964 kJ/kg. The heat supplied is q = h₁ − h_f(0.3 MPa) = 3230.9 − 561.4 = 2669.5 kJ/kg.

Cycle efficiency η = 964/2669.5 = 36.1%.

(c) Relative humidity

ω = 0.0115 kg/kg, and ω = 0.622p_v/(p − p_v) gives

p_v = ωp/(0.622 + ω) = 0.0115 × 90/0.6335 = 1.634 kPa.

φ = p_v/p_s = 1.634/4.24 = 0.385, that is, about 38.5%.