The I.C. Piston Engine Explained

Title: The Piston Engine Explained

Author: Kevin Schappell

The engine is the heart of your car, but instead of pumping
blood, the engine pumps oil, air and fuel. The engines main function
is to convert air and fuel into rotary motion so it can drive
the wheels of the car. How does it do that ??.... Well let's
start with a cutaway of the engine and see all the major parts
then we will get into the actual mechanics. Pistons: Most common
engines have 4, 6, or 8 pistons, which move up and down in the
cylinders. On the upper side of the piston is what is called the
combustion chamber where the fuel and air mix before ignited. On
the other side is the crankcase, which is full of oil. Pistons
have rings which serve to keep the oil out of the combustion
chamber and the fuel and air out of the oil. Crankshaft: The
crankshaft is connected to the pistons via a connecting rod. As
the piston moves up and down in the cylinder it rotates the
crankshaft and converts the straight line motion into rotary
motion. Valve train: The valve train consists of valves, rocker
arms, pushrods, lifters, and the camshaft. (shown in above
picture in blue, yellow, and green) The valve train's only job
is that of a traffic cop. It lets air and fuel in and out of the
engine at the proper time. The timing is controlled by the
camshaft, which is synchronized to the crankshaft by a chain or
belt. Now that we have a general overview of the parts involved
let's talk about what happens. Most automotive engine today are
4-stroke (or 4-cycle) engines, meaning they have four distinct
events which make up the cycle. ·Intake stroke: The camshaft
opens the intake valve and the piston moves down the cylinder.
This creates vacuum and sucks in air and fuel into the
combustion chamber above the piston. ·Compression stroke: As the
piston starts moving back up the cylinder the intake valve
closes and seals off the combustion chamber. The causes the air
and fuel to compress. ·Power stroke: As the fuel is compressed
and the piston nears the top of the cylinder the spark plug
fires and ignites the fuel and air. This explosion pushes the
piston back down the cylinder and drives the crankshaft.
·Exhaust stroke: After the piston reaches the bottom of the
cylinder, the exhaust valve opens and the gasses left over from
the fuel and air are sent out to the exhaust system. Put these
four events together in the above order and you have a complete
cycle. Are you asleep yet? That's enough theory, let's talk
about the real world and problems you might encounter with the
above mentioned parts. Pistons: Remember I talked about the
rings, which seal the combustion chamber from the crankcase. The
rings over time tend to wear out. When they wear they allow the
fuel and air to enter into the oil and dilute it. This dilution
reduces the oils ability to lubricate your engine and can cause
premature wear. Also if the rings wear down they can allow oil
from the crankcase to enter the combustion chambers. This will
result in oil being burned and exiting your tailpipe as
grayish/white smoke. If your car spews grayish white smoke and
it does not go stop in the first few minutes after start-up you
might have warn rings. If the smoke goes away after start-up
look to the valve train section. Crankshaft: The crankshaft
rides on bearings, which can wear down over time. The bearings
support the crankshaft and also the rods, which connect the
pistons to the crankshaft. A loud medium pitched knocking noise
in the engine points to warn bearings most of the time. This is
usually a costly repair and involves removing the crankshaft and
either machining the surface where the bearings ride, or
replacing the entire crankshaft. To prevent this type of
problem, use a high quality oil, change your oil at suggested
intervals (3 months or 3000 miles is a safe number) and always
maintain your oil level between oil changes. Valve train:
Remember the oil smoke problem mentioned above in the piston
sections. If your car only smokes grayish/white smoke at
start-up you may have leaking valve seals. Valve seals keep oil
from above the valve from leaking into the combustion chamber.
When they wear, they can allow oil to seep into the combustion
chamber and collect there until your start the engine again. You
generally do not get oil leaking past the valve seals while the
engine is running since the seals expand with the heat of the
engine and plug the leak. Another common problem is the timing
chain or belt will slip or even break causing the cam shaft to
stop rotating. Remember the camshaft tells the valves when to
open and if it stops spinning then the valves stop opening and
closing. No valve moving, no engine running :-) A term you will
here when talking about timing chains and belts is "interference
engine". When an engine is an "interference engine" the pistons
and valves are so close together that if the valves were to stop
moving (broken belt or chain) and the crankshaft kept spinning
they would crash into the piston. (that's the interference) This
crash tends to do bad things to an engine, breaking valve,
bending pushrods, and even cracking pistons. This is why most
manufacturers recommend changing the timing chain or belt every
60,000 miles. Timing belts dry out, stretch and deteriorate over
time so even if you do not have 60,000 miles on the car think
about changing the belt after it's 6 years old. Preventive
Maintenance: ·Change your oil regularly. ·Give your engine a
chance to warm up before driving if possible. Let the oil get
into all parts of the engine before driving. This is even more
critical in cooler temperatures when the oil is cold and
sluggish. ·Change your timing belt or chain at your
manufacturer's recommended interval. ·void "snake oil" additives
advertised on late night TV. Regular oil changes and good
maintenance habits will keep your engine running it's best. ·If
you have a turbo charged engine, give the engine a minute or two
cool down before turning it off. This cool down period allows
oil to circulate and cool down the bearings. If you shut off the
engine immediately after hard driving, the oil can gum up around
the hot bearings and create problems down the road. What to
discuss with your mechanic: ·If you have to replace your engine,
discuss the benefits of buying used versus new. If you plan on
keeping your car for some time, a new engine might be the best
bet. Sometimes new engines are not much more expensive than
rebuilt ones, and offer the best solution. ·When trying to
diagnose engine noises, be as descriptive as possible. Take note
to when the noise occurs; at what throttle position, and when
the noise started occurring. Sometimes changing the weight of
oil being used can cause a new noise to crop up. Make sure you
mechanic knows if you changed oil brands or weight recently. 


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