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I believe that the heat source is the spark plug because it heats up the fuel-air mixture and that the working substance is the fuel-gas mixture. Then, I know that the explosion created by the fuel forces the piston down, turning the wheels. However, I cannot identify the heat sink in an internal combustion. What does the fuel transfer its heat to to do work?

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    $\begingroup$ The exhaust.... $\endgroup$ Feb 16, 2015 at 2:00
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    $\begingroup$ " heat source is the spark plug " So, diesel engines don't have a heat source? $\endgroup$
    – DJohnM
    Feb 16, 2015 at 2:07
  • $\begingroup$ @JerrySchirmer it's not just the exhaust. You've got some form of engine cooler, usually an air or water cooled radiator. $\endgroup$
    – DanielSank
    Sep 3, 2015 at 4:37

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The spark plug merely initiates the chemical reaction of gasoline combining with oxygen. This reaction is what produces the heat. The combustion inside of the engine is typically $800-1200 ^\circ \text{C}$. The surrounding environment is the heat sink, whatever the outside temperature is. Some small engines are cooled directly by the air with fins, and most large engines have a liquid cooling system with a radiator. Either way, all of the heat produced by the engine finds it's way to the air outside either through cooling fins, a radiator, or the exhaust gases.

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  • $\begingroup$ An interesting note: radiators are not just for cooling. They are placed in Front of the engine so the air can blow through them and over the engine. This is to assure the engine has a consistent temperature all over by warming the cooler areas, saving the engine block from cracking. $\endgroup$ Jan 13, 2020 at 8:16
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The spark plug is not a significant heat source. Imagine a cube in an incline barely holding. A little push and the cube goes tumbling down. The spark plug gives the little push. The chemical energy in the fuel + oxygen system will become thermal energy. The sink is everything else. Imagine that you are driving in hell. The engine is at the same temperature as the combusting fuel and the pressure of the exploding fuel will be comparable to the ambient pressure. The exploding fuel cannot expand. The engine does not work.

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    $\begingroup$ +1, especially for "imagine you are driving in hell", reminds me of this en.wikipedia.org/wiki/Bat_Out_of_Hell $\endgroup$ Sep 3, 2015 at 0:02
  • $\begingroup$ But wouldn't there still be a pressure difference that still does work? $\endgroup$
    – Milind R
    Oct 20, 2015 at 2:43
  • $\begingroup$ Releasing chemical energy through combustion will always increase the temperature, no matter how hot the reactants are. (Ignoring autoignition, phase changes, etc) The amount of work you can get from that release of energy depends on the difference in temperature of the hot and cold sinks. That's why your engine runs better on a cold day than on a really hot one. $\endgroup$
    – user235504
    Oct 30, 2015 at 6:41
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The burning fuel/air mixture generates a lot of heat, which greatly increases the temperature and pressure inside the cylinder. The high pressure gases then push the piston down, providing expansion work that turns the crankshaft. Some of the heat from this process transfers into the water jacket of the engine, where circulating water (from the water pump) flows to the radiator and expels its heat into the ambient air (the environment). The rest of the heat exits the cylinder when the exhaust valve opens, and it travels through the exhaust manifold and exits out the tail pipe.

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The heat sink is the environment, at the temperature of the air entering the engine. When you put the heat cycle on paper, you close the cycle between exhaust and admission with a heat exchanger with the environment; the gas enters the heat excahger at the engine exhaust temperature, cools by sinking heat to the environment and exits the heat exchanger at the environment's temperature, where it cycles into the engine again. For air breathing engines at athmospheric pressure there's no need for this heat exchanger since the exchange is done by direct mixing of the exhaust gases into the air outside; it would also make internal combustion impractical, hence it is used in some external combustion engine configurations, in high pressure gas cycles such as Stirling, or in steam cycles, where you cannot mix the working fluid with air at athmospheric pressure.

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heat source can be assumed in two ways : the spark plug ( while the sink is the fuel ). in this cycle, the energy is used by the fuel to combust and produce energy. fuel ( exhaust and the surroundings become sink ) in this cycle , the energy is used to run the vehicle.

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