--- title: "Gas temperature variation and heat flow" subject: "Internal Combustion Engines" chapter: 7 key: "semester_6/internal_combustion_engines:gas-temperature-variation-heat-flow" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/gas-temperature-variation-heat-flow site: BCT Engineering --- # Gas temperature variation and heat flow Internal Combustion Engines · Chapter 7: Engine cooling - Asked in: year not given, 4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/gas-temperature-variation-heat-flow - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-7 ## Question 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. ## 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. ## Answer #### Variation of gas temperature The gas temperature changes strongly during the cycle. ```text 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: $$ Q = \frac{T_g - T_c}{\dfrac{1}{h_g A} + \dfrac{t}{kA} + \dfrac{1}{h_c A}} $$ 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When does the gas temperature peak, and roughly how high in an SI engine? A. At the start of suction, about 300 K B. Just after TDC after ignition, about 2200-2800 K C. At the end of exhaust, about 2500 K D. At BDC, about 1000 K Answer: B. After ignition temperature rises quickly to a peak of about 2200-2800 K a little after TDC, lasting only briefly. (Key point: Peak gas temperature) 2. What is the path of heat from gas to coolant? A. Conduction in gas, radiation in the wall, convection to coolant B. Convection and radiation from gas to wall, conduction through wall, convection to coolant C. Only radiation throughout D. Only conduction throughout Answer: B. Heat goes by convection and radiation to the wall, conducts through it, then convects to the coolant. (Key point: Heat flow path) 3. Why is the wall temperature almost steady though the gas temperature fluctuates strongly? A. The wall is kept at 2000 C B. The metal thermal inertia averages the fluctuations, so heat flow is based on mean gas temperature C. The coolant boils constantly D. Radiation cancels convection Answer: B. The wall thermal inertia smooths the gas temperature swings, so the mean gas temperature is used. (Key point: Wall temperature steadiness) 4. Why is radiation heat transfer more important in CI engines? A. Because of soot in the flame B. Because gas temperature is higher than SI C. Because walls are hotter D. Because there is no convection Answer: A. Soot in diesel combustion radiates strongly, so radiation matters mainly in CI engines. 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