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Friday, March 21, 2014

Hey Mr? Whats efficiency mean?

    Heya there, Machine Whisperer back again, and pondering the idea of efficiency.  I often get asked to define efficiency and to help other folks understand what it means and why it matters. I hope this can clarify a bit more for those of you who have asked or just wondered about it.  
    We hear about fuel efficiency in cars, high efficiency washers and high efficiency LED lighting in the home- how does it all tie together?  Simply put, efficiency is the amount of energy you get out of a system compared to the amount of energy that had to be put in.  When we use energy in whatever form, some of it gets lost in conversion.
This loss and waste happens inherently as a matter of course and has come to be known as the second law of thermodynamics.  The 2nd law states that the entropy of an isolated system never decreases, because isolated systems evolve toward thermodynamic equilibrium- the state of maximum entropy.  HUH???
   In lay terms, the second law states there will always be waste and that some of the energy used as input for a system will be turned into unusable heat (entropy) that cannot be used for work or anything else and simply adds to heating the universe in a miniscule way.  It takes energy to do work: an engine converts chemical potential energy in the fuel into mechanical kinetic energy that we can use to do work and save our achin' backs.
    Any and every mechanical, electrical, or chemical device wastes energy.  What matters in regards to efficiency is how much of that energy gets wasted, and how much can be preserved to do the work we need done.   Input Energy= Useable Output Energy + Waste.  Obviously, the less waste, the better.
    Efficiency is often expressed as a percentage of input energy used to do the work that needs to be done.  But more often, efficiency is stated as a categorically comparable number indicating how much an appliance or vehicle will cost to operate.  A ratio of useable output energy to total input energy is the technical definition of efficiency.  If you put in 10 units of energy, and you can use 5 units for work on the other end, the system is 50% efficient.
     The counterpart to efficiency is waste- what does not get used for productive purposes gets thrown away as heat.  Heat is always a waste product of any process. Period. Always.  Nothing can be accomplished in this universe without wasting energy as heat.  
    Lets take an applied look at one example- a vehicle that burns gasoline to move down the road and haul people and stuff around.  This is a very simplistic and generalized example, not a detailed breakdown- so yes, its missing some in-depth explanation.  Lets use a sample average vehicle on the road today with a naturally aspirated gasoline engine and an automatic transmission.
    When the spark plug ignites the fuel and air inside the cylinders of the engine, some of the energy released by the expanding combustion gasses goes into pushing the piston down to turn the crankshaft, thus providing torque that accelerates of the car.
     However, in that combustion process, the metal of the engine is absorbing some of the heat out of combustion, and much of the remaining heat goes straight out the exhaust pipe.  As much as 60% of the energy in the fuel powering an engine goes directly out the radiator and exhaust pipe as waste heat, or gets used to pump air in and out of the engine.
      Yep, thats right- over half of the energy contained in the gasoline you bought is wasted as heat without ever doing any work that benefits us.  Yikes!!
      A few percent of the remaining useful energy is used to spin fans and pumps to move that waste heat out of the engine and circulate oil through the engine.  Another few percent goes into flexing the tires, heating them and the road.  A few percent is wasted in the automatic transmission as fluid shear in the torque converter, and a couple percent is used by the alternator to make electricity from the spinning engine. An air conditioning compressor can consume several percent of a small engines output.  Depending on speed (and other conditions), bad aerodynamic factors can consume up to  10% of the useable work an engine puts out.
     So all in all- about 20% of the fuel we burn in a gasoline engine gets used to push the car down the road while the remaining 80% gets wasted as heat and thrown away.  Thats it- 20% used for transportation, 80% waste. How does that feel?
    In practical terms, out of $4 per gallon of gasoline, about $3.20 gets thrown away heating the air and the environment around us, the other 80 cents worth of fuel is what we use to move the car.
    By comparison, diesel powered vehicles typically use well over 30% of the energy in the fuel to move a load down the road.  The higher baseline efficiency is the reason that most ocean-going ships, big trucks and construction equipment (and most of the rest of the world outside the US for that matter) are powered by torquey and fuel conserving diesel engines.  In some post soon, we will go into more detail on the differences between diesel and gasoline fueled engines...
     Til next time, MW out-  back to other stuff.  :-)

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