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Tuesday, March 18, 2014

I,C,P,E or Suck, Squish, Bang, Blow... an engine operates HOW?

A friend asked me the other day to diagnose the misfire on his little car and recommend how to fix it when we got onto a sidetracked into a discussion about how the engine actually functions and makes power.  I have answered this question so many times over the years for so many people that I figured I should just get it down in writing.  So here goes- Caution though, there be Physics ahead!
    This explanation needs to be fairly basic to accomplish the goal of assisting folk with no real physics background to get the ideas here.  I will not bore anyone with the math behind it, just the concepts and functional ideas. 
     Lets dive straight in through the air filter.  Every engine has an air filter that is there to make sure that all the dust is removed from the intake air.  It is that intake air and its 21% oxygen that is the key to the concept of an internal combustion engine.  A spark ignition engine requires a throttle valve in the air supply to control how much mixture the engine gets to burn, thus how much power it makes.
    After the air filter in spark-ignition (gasoline, CNG...) engines is a carburetor or fuel injector(s) that adds and mixes in a precise amount of fuel to match exactly the amount of air moving into the engine.  This precise mixture is referred to as 'stoichiometric' when, after combustion, there is no unburnt fuel or leftover oxygen in the exhaust.  Too much fuel is known as a 'rich' mix and will result in black soot and low power output while not enough fuel is known as a 'lean' mixture and will result in extremely hot combustion and  likely engine damage, but lots of output power.  Generally a gasoline engine will need 14.7 parts air to 1 part fuel- by mass, not volume.  That equates to around 8,300 gallons of air at sea level for every gallon of fuel burned.  Primarily, an engine is just a big air pump- following that air in through the intake and out the exhaust will expose the process that makes the power we all take for granted.
  The basics of the 4 stroke engine are (NAMED) from the start to finish of one cycle or two revolutions of the crankshaft (the output shaft) 
    1-(INTAKE STROKE) Air is pushed into the cylinder past the open intake valve by atmospheric pressure because the piston traveling down in the cylinder creates an area of lower pressure that wants to equalize with outside air pressure.  This is often referred to as the engine 'sucking' in its air.  In this case I won't argue... much. The end of the intake stroke is marked by the closing of the intake valve, and the cylinder is full of air/fuel mix.
    2-(COMPRESSION STROKE) That flammable mixture, now trapped in a closed space by the closing of the valves, is squeezed to a tiny fraction of its former volume by the piston moving back up in the cylinder which in turn increases the temperature of the air inside which will increase the speed that the fuel mix burns when the spark happens.
    3-(POWER STROKE) This is where all the work takes place.  Here, a perfectly timed spark ignites the precise mixture of fuel and air to burn very quickly which raises the temperature many hundreds of degrees almost instantly.  It is this rapid spike in heat that causes expansion of the combustion gasses that pushes the piston down violently and becomes rotary motion in the crankshaft.  The temperature and pressure drops rapidly as the volume of the cylinder increases due to the piston moving back down and extracting work from the expanding gasses.  It is for this reason that these machines are called 'heat engines'.
    4-(EXHAUST STROKE) Here the piston moves back up to the starting position.  The leftover gasses and heat are released and forcefully ejected out the exhaust valve to the atmosphere again after as much of the viable energy has been collected to do work. When all of the exhaust is purged out, the exhaust valve closes, the intake valve opens and the cycle starts over again.
    Some of my buddies call it 'suck, squish, bang, blow'. This gets the point across and stuck just as well as any other mnemonic, and is a bit more fun to say as well.  
    Each stroke takes 180 degrees of rotation or half a turn of the crankshaft.  Thus, for one full I,C,P,E cycle the crankshaft must rotate twice or 720 degrees.  Strokes 1,2 and 4 all take work out of the engine to perform, while only stroke 3 adds work. 
    It takes work to intake, compress and exhaust the gasses (pumping losses), while only the rapid expansion of the burnt gasses during the power stroke provide work output.  Because intake and exhaust strokes comprise half of the engines total running time, the easier it is for engines to move the huge amounts of air involved, the more efficient the engine can be overall.  This is the reason you see and hear the big, loud exhaust systems and oversized air filters and intakes on high performance vehicles.  Any bit of work NOT done to pump air is power usable at the tire/road interface.  Choking an engine, preventing it from breathing by restricting either the intake or the exhaust, is the fastest way an engineer can ruin the efficiency of an engine. 
    To produce a smooth powerful engine and allow this process to happen at 50 times per second (6000rpm, 4cyl) or more, many things need to happen together repeatedly, in perfect unison and with precise timing and tight tolerances.  A bit of stretch in a timing belt or chain, wear in gear-trains, or the deformation of bearings or shafts can reduce the operating efficiency of an engine drastically by changing the timing of crucial processes by only a degree or less.

      Til next time, MW :-)

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