Car engines

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An internal combustion engine burns a mixture of fuel and air.  The most common type is a four-stroke, gasoline engine.  A piston slides in and out of a cylinder.  Two or more valves allow the fuel and the air to enter the cylinder and the gases that form when the fuel and air burn to leave the cylinder.  As the piston slides back and forth inside the cylinder, the volume that the gases can occupy changes drastically.

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  • The process of converting heat into work begins when the piston is pulled out of the cylinder, expanding the enclosed space and allowing fuel and air to flow into that space through a valve.  This motion is called the intake stroke or induction stroke
  • Next, the fuel and the air mixture are squeezed together by pushing the piston into the cylinder.  This is called the compression stroke.  At the end of the compression stroke, with the fuel and the air mixture squeezed as tightly as possible, the spark plug at the sealed end of the cylinder fires and ignites the mixture. 
  • The hot burning fuel has an enormous pressure and it pushes the piston out of the cylinder.  This power stroke is what provides power to the engine and the attached machinery. 
  • Finally, the burned gas is squeezed out of the cylinder through another valve in the exhaust stroke

The power stroke is the only time when the gas does work on the engine.

These four strokes repeat over and over again.  Most internal combustion engines have at least four cylinders and pistons.  There is always at least one cylinder going through the power stroke and it can carry the other cylinders through the non-power strokes.  The maximum efficiency of such an engine is emax =(Tignition - Tair)/Tignition where Tignition is the temperature of the fuel-air mixture after ignition.  To maximize the fuel efficiency, you have to create the hottest possible fuel air mixture after ignition.  The highest efficiency that has been achieved is approximately 50% of emax.

Link: The Otto Engine

The Diesel engine is another type of internal combustion engine.  A gasoline engine takes in a mixture of gas and air, compresses it and ignites the mixture with a spark.  The fuel and air mixture limits the compression ratio and therefore the temperature of the fuel-air mixture.  If the air is compressed to much then the fuel spontaneously ignites and the engine "knocks".  A diesel engine takes in just air, compresses it and then injects fuel into the compressed air.  The compression ratio and therefore the temperature can be much higher.  The diesel engine uses less refined fuel which ignites easier.  It does not need a spark plug, the heat of the compressed air lights the fuel spontaneously.  A higher temperature makes for a more efficient engine.

  • The induction stroke takes air in at constant atmospheric pressure at temperature.

  • The compression stroke compresses the air to about 1/20 of its original volume. the air heats up.

  • Atomized fuel is injected and ignites.  It expands and cools during the power stroke.

  • The exhaust stroke vents the cooled gases.

Link:  The Diesel Engine

Link:  Gasoline and Diesel Fuel

 

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