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Pistons traveling up and down inside an engine are only half of the internal
combustion story. While the pistons are responsible for the grunt work of
spinning the crankshaft, it is the intake and exhaust valves that allow each
one of the cycles crucial to the operation of a four-stroke engine to occur.
The intake valve opens on the intake stroke. The piston draws air and fuel
into the cylinder as it travels downward. The intake valve then closes. The
piston moves up to compress the mixture on the compression stroke. The spark
plug ignites the mixture at the very top of piston travel. As the confined
fuel-air mixture burns, it pushes the piston back down for the power stroke.
The exhaust valve then opens. The piston pushes the spent gas out of the
cylinder for the exhaust stroke. In this way the internal combustion creates
power using the Otto cycle, so named for German engineer and inventor of the
internal combustion engine Nicolaus Otto.
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Valve timing is of great importance for the Otto-cycle engine to run smoothly
and produce power to turn the drive wheels. The intake and exhaust valves in
the cylinder head open and close in time by way of one or more camshafts. The
camshaft is also aptly referred to in some circles as a bumpstick. On the
camshaft are lobes, or circular ramps. The lobes are responsible for opening
and closing the valves at precisely the right moments in the never ending
Otto-cycle. These lobes mechanically bump the valves open and allow them to
close. Springs hold the valves closed in the cylinder head when not getting
bumped around by the camshaft lobes. The camshaft spins at exactly half the
revolutions of the engine crankshaft. Gears connected to the camshaft and
crankshaft are joined together by the timing chain or belt. If all is well,
perfect valve timing is the result.
Key To Combustion
Timing chains were much more common than timing belts. While this balance has
shifted, manufacturers still use timing chains in modern engines. An engine
with a worn and slacked timing chain will run poorly as correct valve and
engine timing cannot be achieved. An engine with a broken timing chain or belt
will not run at all. Replacing the timing chain is a major job as it involves
traveling down to the core of the engine itself. A service manual is the first
and most important tool on the list for timing chain removal and replacement.
Timing chain replacement complexity depends on engine configuration and number
of camshafts. That said, engineers build relatively foolproof methods of
lining up the gears and chains into the engine design itself. The following is
an example of the simplest of timing chains on a single camshaft pushrod
four-cylinder engine.
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