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Gas temperature variation and heat flow

Short answer

Gas temperature rises from ~350 K to ~700 K, peaks at 2200–2800 K, then drops to ~1200 K at exhaust opening. Heat flows from gas to wall by convection and radiation, through the wall by conduction, and to coolant by convection; 25–35% of fuel energy goes to coolant. Metal thermal inertia keeps the wall temperature nearly steady.

Write short notes on the variation of gas temperature in the cylinder during the cycle and the flow of heat from the gas to the coolant.

Variation of gas temperature

The gas temperature changes strongly during the cycle.

 T (K)
 2500|        /\
     |       /  \
 1500|      /    \.
     |     /       ..
  700| ___/            ..____
  300|---------------------------> crank angle
     suction comp. comb. exp. exhaust
  • During suction and compression the charge temperature rises from about 300-350 K to 700-900 K.
  • After ignition it rises quickly to a peak of about 2200-2800 K in an SI engine (about 2000 K in CI), a little after TDC.
  • During expansion it falls to about 1200-1500 K at exhaust valve opening and then to about 700-900 K in the exhaust.
  • The peak lasts only a very short time. The mean temperature over the cycle is about 1000 K, while the metal wall stays at 400-500 K.

Heat flow

Heat flows from the hot gas to the wall by convection and radiation (radiation matters mainly in CI engines because of soot), then through the wall by conduction, and finally from the outer surface to the coolant by convection:

Q=Tg−Tc1hgA+tkA+1hcAQ = \frac{T_g - T_c}{\dfrac{1}{h_g A} + \dfrac{t}{kA} + \dfrac{1}{h_c A}}

The wall temperature varies only slightly through the cycle because the metal thermal inertia averages the gas temperature fluctuations, so heat flow is based on the mean gas temperature. The heat flux is highest at the piston crown, exhaust valve and cylinder head, and depends on gas velocity, turbulence, load and speed. About 25-35% of fuel energy goes to the coolant.

✓ Answer checked by Claude Opus 5.5 (high effort)

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