Path: i-core!caeco!iconsys!uunet!samsung!usc!apple!excelan!unix!ginger.sri.com!henry
From: henry@ginger.sri.com (Henry Pasternack)
Newsgroups: rec.motorcycles
Subject: How a motorcycle works.
Message-ID: <6250@unix.SRI.COM>
Date: 29 Nov 89 18:49:01 GMT
Sender: news@unix.SRI.COM
Reply-To: henry@ginger.sri.com (Henry Pasternack)
Organization: SRI International
Lines: 96


                How a Motorcycle Works

                          by

             Henry "Credible" Pasternack

Chapter 1: The Engine.

   The engine has many components, each of which is required for the
proper functioning of the whole.  When the crankshaft spins, the
offset lobes carrying the journals set up harmonic vibrations in the
cases and cylinders.  These vibrations cause air to be sucked out of
the combustions chambers, past the rings which act like one-way valves
in much the same manner as the reed valve in a diaphragm pump.  The
resulting vacuum causes the intake and exhaust valves to open (due to
the pressure differential) allowing mixture to be drawn into the
cylinder on the carb side, and spent combustion fumes on the exhaust
side.  The momentum of the inrushing gas pushes the pistons down, in
what is known as the "intake stroke".  The connecting rods prevent the
pistons from falling out of the cylinders and into the cases, or from
hitting the combustion chamber sides.

   After the chamber is full, the pressure differential holding open
the valves is relieved, and the valves shut.  The mixture begins to
cool slightly, producing a reverse pressure gradient, owing to the
large amount of air pumped into the crankcase.  Along with the
vibratory energy supplied by the crankshaft, the pistons begin to move
back towards the tops of the cylinders in what is known as the
"compression stroke".  As the density of the cylinder gases increases,
the air and gasoline molecules are forced together more and more.
Because they are already quite hot from spent gases which were taken
in during the intake cycle, their kinetic energy is high, and many
collisions occur.  At a critical point, the air and gas molecules
suddenly combine violently.  The "combustion" causes the mixture to
implode, and cool rapidly as the kinetic and thermal energy is
absorbed.  This makes the gas pressure drop, and the pistons are
sucked strongly towards the top of he cylinders.  The resulting
unbalanced acceleration produces a driving force which is resonant
with the vibration of the crankshaft, reinforcing its motion.

   Because of the momentum imparted to the pistons during the
implosion of the fuel/air mixture, they are now moving quite
rapidly.  As they are constrained by the connecting rods, they
rebound at the top of the compression stroke, before hitting the
cylinder head.  The wrist pins and rod bearings are made of very
hard metal, so the collision is very nearly elastic, and no energy
is lost.  Consequently, the pistons rebound, and begin traveling
downward with an equal, but opposite velocity.  The cylinder volumes
thus begin to increase, and the gas pressure drops even lower than
it was after implosion.  This is called the "power stroke" because
the motion of the pistons at this point is more powerful than it
is at any other time in the cycle.

   This extreme cylinder vacuum causes the intake valves to open,
because the intake ports are maintained at atmospheric pressure.  The
exhaust valves, however, stay closed because of a vacuum which has
developed in the headers.  The cause of this vacuum is several fold:
First, a rarefaction was caused during the intake stroke when hot
exhaust gases were sucked into the cylinders.  Second, the heat
sinking effect of the long mufflers (whose job it is to cool the
exhaust in order to most efficiently extract spent implosion products)
has caused the exhaust to cool during the intake/compression cycle.
Third, the acoustical resonance of the pipes reinforces the vacuum
pulses at certain frequencies.  This is why bikes have "powerbands".

   When the intake valves open, fresh mixture rushes into the
cylinders, immediately neutralizing the vacuum due to implosion.  This
allows the pistons to move full speed to the bottoms of the cylinders
where they again rebound off the connecting rods and begin traveling
upwards.  The gas is squeezed and pressure rises, creating a
superheated condition in the spent gases.  The pressure is relieved by
the piston rings and vented into the crankcase, where the gases are
safely relieved to the exhaust pipe by the crankcase breather and
exhaust gas recirculation system.  This is called the "exhaust
stroke".

   Most engines are of the type known as "Diesel", named after Dr.
Mercedes Diesel, who invented the internal combustion engine in
Germany at the turn of the century.  Diesels have the advantage
of being able to run on relatively crude fuel.  They last a long
time because they are simple.  There is another kind of engine,
called the "Otto" engine, which burns gasoline.  The "Otto" engine
has several extra parts, namely a camshaft, ignition system, and
spark plugs.  The camshaft has eccentric lobes which are acted
upon by the movement of the valves, causing the shaft to spin.
The spinning shaft is used for timing the ignition system which
periodically sends a burst of high voltage to the spark plugs
located in the top of each cylinder.  The spark comes at the end
of the exhaust stroke, adding additional heat to the last portions
of the spent implosion gases.  This improves exhaust scavenging,
greatly increasing the power and efficiency of the engine.

   Next chapter: The Transmission.

-Henry