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How a piston engine works

The big loud magic!

Engines may seem like magic, and if they do, that’s not a problem at all.  By the end of this TOPIC you should be able to fully explain and understand how they work, and what the components are that make them up!

We’ll cover all the basic components that comprise your engine, starting right below with the mags!

Magnetos

What it is….  A magneto is a magnet that spins around a coil of wire.  This generates electricity, and through a series of windings, steps up the voltage so that it is producing relatively little amperage (power, or “flow”), but it does produce very high voltage.  The high voltage electricity seeks the path of least resistance and is forced to jump across the gap of the spark plug to then flow through “ground” and the engine block (we’ll cover electricity much more in-depth in another TOPIC).  The spark from the spark plug inside the engine cylinder is what ignites the fuel and air and makes “combustion” or power for the engine.

Every aircraft engine has a dual ignition system (that’s a fancy way of saying two magnetos).  The purpose of having two magnetos is for redundancy as well as better combustion of the fuel-air mixture helping the engine to make more power.  You can control whether you are running off of the left or right magneto, or both, with the ignition switch.  The ignition switch simply determines if the sparks (or really electricity) made from the magnetos is forced to flow through the spark plugs and jump across the gap on the spark plug (thus igniting the fuel and air in the cylinder) or if instead the electricity made by the magneto simply bypassing the spark plug and flowing through an easier path back to “ground” through the engine block and airframe.

What’s that clicking noise?

When a mechanic turns a propeller by hand you will generally hear either one or two clicks every half rotation or so.  The clicks you hear are the IMPULSE COUPLING.  The job of the impulse coupling is to help generate spark to start the engine when it is being spun by the electric starter motor (at low RPM obviously, since the starter doesn’t have the power to spin it at high RPM).  The impulse coupling interfaces between the engine accessory gears and the magneto to “store up” energy in springs as the propeller turns at low rpm.  Once the propeller hits a predetermined point (where at least one of the pistons in the engine is right at the top of its cylinder compressing air and fuel and ready to fire it off), the impulse coupling “fires” and releases its stored energy from the springs and helps momentarily rotate the magneto a little faster to generate a stronger spark.  The click you hear is the metal smacking together as the springs release their stored energy.  The impulse coupling only does this at low rpm and disconnects by about 300-500 rpm thanks to centrifugal force from a set of fly-weights that are flung outward as the engine spins the magneto at a higher rpm after it starts.

P-lead

No, don’t follow Paul’s lead.

The P-lead is a small wire that runs from the magneto back to the ignition switch.  With the switch in the “off” position, you are actually connecting the P-leads from both magnetos directly to the frame of the aircraft.  This “grounds out” the magnetos and any electricity produced will flow through the frame of the airplane back to the battery rather than take the path with more resistance and jump across the gap of the spark plug in the cylinder.  With these P-leads grounded, there shouldn’t be any spark left to go to the spark plugs and keep the engine running.  When you select “Left” “Right” or “Both”, that is actually disconnecting the wires (P-leads) and opening the circuit.  The spark then jumps across the open gap on the spark plug because it is less resistance than the spark trying to jump the open circuit of the ignition switch and flow back through the airframe.

Four Strokes

Different strokes for different folks!  However, engines all have the same four strokes in our world (or same two strokes in the dirtbike and boating world).  The four strokes are:

  • Intake
  • Compression
  • Combustion (power stroke)
  • Exhaust

Induction System

That sucks.  No, it doesn’t, but your induction system does!  This is where air from outside is sucked in and begins its journey through our engine (it’s going to leave our engine in a very hot and dirty mess!)

  • The first part of this system is the air filter, which you can clearly see from the front of the airplane.
  • After the air filter the intake air goes to the “airbox” where it is either fed to the carburetor, or dumped overboard if you have turned on “carb heat” and instead are using the hot and unfiltered air flowing in over the exhaust manifold.
  • After the airbox, the air flows into the carburetor where it is passed through a VENTURI which accelerates the airflow, and through increasing its speed also lowers its pressure and temperature.  This lower pressure air moving through the carburetor draws in fuel from the carburetor and mixes to make a nice fuel/air charge to be burnt in the engine cylinders.
  • The fuel-air charge then makes its way through the intake “pipes” or manifold and past the intake valve (when it opens) and into the cylinder, to be compressed by the piston, and then ignited by the spark plug.
  • Once the spark plug sparks, the fuel/air charge begins to burn and create lots of heat and pressure pushing the piston back down on the “power” stroke
  • After the completion of the power stroke, the exhaust valve opens and the burnt air and fuel is pushed out of the cylinder as the piston comes back up on the exhaust stroke.
  • It then flows out through the exhaust manifold (pipes) to the muffler (makes it a little quieter and actually helps “flow” of exhaust to help the engine make more power) and then finally out the bottom of the cowling through what we call the “exhaust stack” or pipe.

Carburetor

The French part of the engine.  Now the carburetor smells like raw gasoline and not freshly baked croissants, but it is still French.

The whole idea behind the carburetor is to create an area of low-pressure air by accelerating it through a VENTURI (narrow “neck” or tube) and using that to help mix the fuel and air together.  Here’s a video on it in action:

Mixture

Finding the right mix is tough in life, and it can be in flying sometimes too.  The MIXTURE control is usually a big red knob in the cockpit next to the throttle.  The purpose of this knob is to regulate the ratio (or mixture) of fuel and air going to the engine.  We typically aim for about 15 parts air to one part fuel.  If you add more fuel for the same amount of air changing the ration we call this more “RICH” and less fuel for the same amount of air would be more “LEAN”.  This knob is connected to the carburetor via a cable and controls how much fuel the carburetor mixes with the air coming into the engine.

mixture throttle knob ground school course

More thirsty than a Saturday Football Tailgate

Expect your engine in your flight training airplane to burn between 5 and 10 gallons of fuel per hour.  The exact amounts will be found in the POH and your instructor can tell you about their personal history flying the plane and what you can expect it to burn.  The FAA requires when you are flying VFR to carry enough fuel to fly to your destination and then 30 minutes thereafter (at normal cruise speed) or for 45 minutes thereafter if you are flying at night.  I’d recommend never landing with less than an hour of fuel on board.  In some airplanes, an hour of fuel is only 2.5 gallons in each wing sloshing around (that’s not a lot!)