What is the heat sink in an internal combustion engine? 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?
 A: 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.
A: 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.
A: 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.
A: 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.
A: 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.
