(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…

Combustion differences. In an S.I. engine, a homogeneous air-fuel mixture is compressed and then ignited by a spark. The charge is homogeneous before compression; the flame starts at the plug and propagates as a premixed flame front. Its speed is governed by mixture composition, turbulence, and charge temperature, so the event is relatively fast and largely kinetically controlled. In a C.I. engine, only air is compressed; fuel is injected late in compression, near TDC. The hot compressed air autoignites the fuel after an ignition delay, and burning continues mainly by diffusion of air into fuel-rich zones. Thus C.I. combustion is heterogeneous and mixing-controlled: the effective flame speed is set by mixing rather than by a premixed chemical flame front, ignition delay is significant, and the combustion duration is longer because mixing and heat release must keep pace. This is why C.I. chambers are shaped to intensify air motion. The difference also explains why C.I. engines rely on injection timing and chamber geometry to manage the delay between injection and ignition.

Combustion-induced swirl and chamber sketches. Combustion-induced swirl is organised air motion generated by the piston’s compression stroke into a specially shaped chamber or pre-chamber, rather than by intake ducts. It increases air-fuel mixing, shortens ignition delay, and helps complete combustion. In both sketches, the swirl raises air velocity, improves atomisation and entrainment, and reduces the time needed for fuel to find air. Sketch 1, a swirl chamber or Ricardo Comet V type, shows a separate chamber connected to the main cylinder by a tangential throat. Arrows indicate compressed air entering the throat tangentially and forming a strong spiral in the chamber; the tangential throat converts piston-driven axial flow into rotational flow. Fuel is injected into this swirling air, and during expansion the arrows reverse through the throat. Sketch 2, a pre-combustion chamber, shows a small chamber with a restricted passage. Arrows show air being drawn through the passage by the pressure difference between the main chamber and the pre-chamber, producing swirl; the restricted passage creates a pressure drop that draws air through at high velocity. Fuel is injected into the pre-chamber, and hot products then pass into the main chamber.

Regenerative cycle. State 1 is turbine inlet: 30 bar, 400°C, h1=3230.9 kJ/kg, s1=6.9212 kJ/kg K. No turbine efficiency is given, so the expansion is taken isentropic. Expansion to the 0.3 MPa heater is isentropic to state 2. At 3 bar, sf≈1.672, sg≈6.992, hf≈561.4, hfg≈2163.8 kJ/kg. Hence x2=(6.9212−1.672)/(6.992−1.672)=0.9867 and h2=561.4+0.9867×2163.8≈2696.5 kJ/kg. For the low-pressure expansion to 0.1 bar, using hf≈191.8, hfg≈2392.8, sf≈0.649, sg≈8.150, x3=(6.9212−0.649)/(8.150−0.649)=0.836 and h3≈191.8+0.836×2392.8≈2192.6 kJ/kg. The extraction fraction y is the fraction of the original steam bled at 3 bar; the remaining (1-y) kg expands to the condenser. In the direct-contact heater, condensate at h4=hf(0.1 bar)=191.8 kJ/kg is mixed with extracted steam at h2 to give saturated liquid at h6=hf(3 bar)=561.4 kJ/kg. The balance y h2+(1−y)h4=h6 gives y=(561.4−191.8)/(2696.5−191.8)=0.1476. Turbine work, neglecting pump work, is W=(h1−h2)+(1−y)(h2−h3)=534.4+0.8524×503.9≈963.7 kJ/kg. Heat supplied is Q=h1−h6=3230.9−561.4=2669.5 kJ/kg. Therefore η=963.7/2669.5≈0.361, or 36.1%.

Relative humidity. The specific humidity is ω=11.5 g/kg=0.0115 kg/kg. From ω=0.622 pv/(p−pv), rearranging gives the vapour partial pressure pv=ωp/(0.622+ω)=0.0115×90/(0.622+0.0115)=1.634 kPa. Relative humidity is the ratio of actual vapour pressure to saturation vapour pressure at the dry-bulb temperature, so φ=pv/ps=1.634/4.24≈0.385, i.e. 38.5%. The dry-bulb temperature fixes ps; the total pressure fixes the dry-air partial pressure, so the result is the fraction of saturation vapour pressure actually present.

Thus the key distinction is premixed flame propagation versus diffusion-controlled burning, the cycle efficiency is 36.1%, and the air sample is about 38.5% saturated.