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Valves
Spark Plug
Combustion Chamber
Piston
Connecting Rod
Cylinder
Crankshaft
Intake Stroke
 | The
Piston is going down so the
volume of the cylinder
is getting
larger
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| The Intake valve is open.
A vacuum, or pressure of less
than atmospheric is created in
the cylinder.
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| Atmospheric pressure forces
air into the cylinder.
14.7 psi
at sea level.
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| Fuel is mixed with the air,
and the cylinder is
filled with
the mixture of air and fuel.
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Compression Stroke
| The Piston is going up so the volume of the cylinder is
getting smaller.
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| Both valves are closed. Air/fuel mixture is trapped
in the cylinder.
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| Pressure is increasing.
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| Mixture is compressed into approximately 1/8 it's
original volume.
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| 150 psi built up. As the mixture is
compressed, it heats up.
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Power Stroke
| The
Piston is going down. Volume of the cylinder is increasing.
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| Both valves closed. Spark plug fires, causing
compressed air/fuel mixture to burn.
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| The flame front travels across the combustion
chamber.
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| The burning fuel creates tremendous pressure which
forces the piston down. turning the crankshaft and producing power.
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| This is not an explosion, but a smooth, controlled
burn.
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Exhaust Stroke
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The Piston is going up, so the volume of the cylinder is
getting smaller.
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| The Piston pushes the burned exhaust gasses out past the
open exhaust valve, and out through the exhaust system and into the atmosphere.
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| Both valves are open for a short period of
time at TDC. This is called "Valve Overlap"
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| The intake stroke of the new cycle follows this
exhaust stroke.
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For more info on the four stroke cycle engine, check this out.....
http://www.autoshop-online.com/auto101/eng.html
Engine terms
| Top Dead Center (TDC) - Point of uppermost travel of the piston and
crankshaft.
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| Bottom Dead Center (BDC) - Point of lowermost travel of the piston and
crankshaft.
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| Bore - Diameter of the cylinder.
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| Stroke - Distance the piston travels on a stroke, or the distance from
TDC to BDC. The stroke is twice the distance of the throw.
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| Throw - The distance from the center of the main journal of the
crankshaft to its rod journal. The throw is one half the distance of the stroke.
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Compression Ratio
- is the amount the mixture is squeezed on the
compression stroke.
-all other things being equal, the more the fuel/air
mixture is compressed, the more power the engine produces.
| As the mixture is compressed, it becomes hotter. If
it becomes hotter than the flash point of the fuel, it explodes prematurely, and engine
damage results. This is called pre-ignition, or detonation; or more commonly, knock, or
ping.
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| High NOx emissions result from the combustion
temperature being too high, and therefore the Nitrogen in the air, oxidizes, or burns.
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Compression Ratios of Different Engines:
4 : 1 Vintage cars -because of poor quality
of fuel, and hand crank starting system.
8 : 1 - 10 : 1 Modern Production Cars
-higher compression ratios with computerized
knock sensor ignition systems.
Up to 12.5 : 1 Muscle Cars from the late 1960's -as low
emissions became more important,
compression ratios were reduced, to reduce NOx.
Up to 17 : 1 Race Cars - because they use better
quality, high octane gasoline, or alcohol; and because of restrictor plates in the intake system, they are able to use higher compression ratios.
Up to 25 : 1 Diesel Engines - diesels take only air in,
on the intake stroke, and use the high
temperature of the air on compression stroke to ignite the fuel which is injected
directly
into the combustion chamber.
Engine Size ( cubic displacement)
What is it?
- The total amount of fuel /
air mixture the engine is theoretically capable of taking in
during
one cycle.
- The volume of all the
cylinders, but not the combustion chambers, put together.
- Measured in Cubic Inches
(cu. in.) in imperial measurement, or in Cubic Centimeters ( c.c.'s ) or Liters ( l ) in
metric measurement.
The amount of
power an engine produces, is a direct function of how much fuel the engine burns, and the
larger the cylinders are, the more fuel and air they can take in, and therefore burn. One
engine that is twice as big as another, does not necessarily have twice as much power
though, because it also has much more internal friction, or drag. It does, however use
twice as much fuel.
Engine Performance
To get more performance from an engine, you have to
burn more fuel. You must have air, specifically oxygen, to support the combustion. The
fuel is no problem. If that was all there was to it, we could put a huge fuel pump in the
car that would pump the tank dry in seconds. The problem is with getting enough air in to
support the combustion. Air - fuel ratio is expressed as ratio by weight, and is around 15
: 1. If we expressed it by volume ( air weighs a lot less than gas ), it would be around
10,000 parts of air to one part fuel.
How do engine parts affect performance?
Air Cleaner: A less restrictive air cleaner, or no air cleaner and a
velocity stack, or an air cleaner which takes in cooler, denser air, will increase the
amount of air entering the engine.
4 Barrel Carburetor or larger throttle body: Reduces restriction at
high speed. A carburetor, by definition, has a restriction, ( the venturi ) and therefore,
theoretically, has more power than a carburated engine.
Intake Manifold: Less restrictive intake manifolds reduce restriction
at high RPM, but can hurt low RPM power by reducing turbulence, and therefore reducing
mixing of the air and fuel, and therefore reducing vaporization of the gas, and hurting
low speed power.
Big Valve Heads: Again reduces restriction, but only at high RPM.
Free Flowing Exhaust: Headers, free flowing catalytic converters, and
large pipe, dual exhaust, increase breathing by reducing exhaust back pressure.
Camshaft: A long duration, long valve overlap, high lift camshaft, will
increase high RPM power, but will reduce power at low RPM.
R.P.M.: The higher the speed the engine runs at, the more power strokes
per minute it will have, and therefore more power. Balancing the reciprocating and
rotating assembly allows the engine to turn higher RPM safely without blowing up.
Superchargers and Turbochargers: Rather than depending on atmospheric
pressure to fill the cylinders with fuel and air, a blower forces it in under pressure.
This greatly increases power.
Nitrous Oxide: Nitrous is an oxidizer, not a fuel. It is 35% oxygen
instead of 20%, as air is. Because it is higher in O2 than air, more fuel can be burned.
Unfortunately, it can only be used for short periods of time, because the pistons would
melt if it was used longer.
Mechanical Reality
Any point in the operation of the engine can be
described as the number of degrees before, or after, Top Dead Center ( TDC ), or
Bottom Dead Center ( BDC ).
Because of the speed the engine runs at, we can't just
slam the valves open and closed at Top Dead Center and Bottom Dead Center. The valve train
parts just wouldn't last. They must be opened and closed relatively gently, over a few
degrees of crankshaft rotation.
The crankshaft describes a circle. We call it rotary motion. The piston
moves up and down, or back and forth, or in and out. We call it reciprocating
motion. When the piston and crankshaft are at BDC, and to a lesser degree, TDC; the piston
moves very little, for a relatively large amount of crankshaft travel. In fact, for about
45 degrees before and after BDC, and for about 25 degrees before and after TDC, the piston
barely move at all; so we can use that time to gradually open and close the valves.
We think of air as being nothing. Air is not nothing.
It has weight. It has mass; and therefore, it has inertia. If you get it moving, it wants
to keep moving. ( Newton's third law; a body in motion, tends to stay in motion, etc.) Air
moving into the engine at high RPM is moving at as much as 150 M.P.H. or more, so
even though the piston has stopped moving down on the intake stroke at BDC, the air,
because of it's own inertia, tends to keep filling the cylinder with fuel and air. If the
intake valve is left open longer, even after the piston is coming back up on the
compression stroke, more air and fuel will enter the cylinder, and we will get more power
at high RPM. The longer we leave the intake valve open, the more high RPM power we get.
Leaving the intake valve open longer, hurts low RPM power, though, because we don't get
the 'ram effect', we get at high RPM, and the piston starts to force mixture back out
through the still open intake valve, ruining engine vacuum, causing rough idle, killing
low RPM power, and hurting engine emissions.
In the above engine, the intake valve starts to open at 5
degrees Before TDC ( BTDC ); it is open all the way round until 45 degrees after BDC (
ABDC ), for a total duration of 230 degrees. The longer we leave the valve open for, the
higher the speed the engine is designed to run at. This engine is an extremely low speed
engine.
The compression stroke starts with the closing of the
intake valve at 45 degrees ABDC and concludes with the firing of the spark plug just
before TDC. The duration of the compression stroke is around 130 degrees. It really
doesn't take long to compress the mixture. We're not really moving anything; we're just
compressing what's already in there.
The power stroke starts with the firing of the spark
plug, and ends with the opening of the exhaust valve. It really doesn't take much time for
the fuel to burn, and push the piston down. By about 100 degrees ATDC, there is really no
point in leaving the valve shut. All the energy is gone out of the burning fuel, and the
cylinder is now so large that cylinder pressure is now very low. In the above engine, the
power stroke is about 140 degrees of crankshaft travel long.
The exhaust stroke starts with the opening of the
exhaust valve at 45 degrees BBDC, lasts through 180 degrees from BDC to TDC, and lasts
until 5 degrees ATDC; for a total duration of 230 degrees.
If you do the math, one cycle in this engine lasts
through 730 degrees of crankshaft rotation. This is because both valves are open for a
short period of time at TDC between the exhaust and intake strokes. This is called valve
overlap. The longer the valve overlap is, the rougher the idle, and the higher the speed
the engine makes its power at.
For all intents and purposes, we will call the cycle 2
revolutions of the crankshaft, or 720 degrees long. The power stroke is the only stroke
where power is actually added to the engine. All other strokes take power from the engine
to make them happen. In a single cylinder engine, the only thing which keeps the engine
running through its non-power strokes, is the inertia of the flywheel. It is 720 degrees
between power strokes in a single cylinder engine. This makes a single cylinder engine
rough running especially at idle speed; and the larger the displacement, the larger the
piston, and the more it vibrates. By taking the same displacement, splitting it up into
two cylinders, we can make it only 360 degrees between power strokes. This makes the
engine run much smoother. A four cylinder engine has 180 degrees between power strokes,
which makes it smoother still. Remember the engine in our example above? The power stroke
was 130 degrees long. An eight cylinder engine has only 90 degrees between power strokes,
so there is no time in the operation of the engine when there isn't a power stroke
happening somewhere in the engine. The greater the number of cylinders, the smoother
running the engine will be.
Of all the different cylinder arrangements, the least efficient has to be
the inline arrangement. They are relatively long, and large, heavy counterweights must be
used on the crankshaft, to off-set the weight of the pistons, and keep the engine from
vibrating.
Vee- type cylinder arrangements make for a much
shorter engine and therefore, a shorter, stiffer crankshaft. They don't need as heavy a
counterweights on the crankshaft because the weight of one bank of pistons partially
off-sets the weight of the other bank.
The flat, or horizontally opposed arrangement, is the
most efficient of all. It has a short stiff crankshaft, and the weight of one bank of
pistons totally off-sets the weight of the other bank. This engine is said to be inherently
balanced. Because of its efficiency, and light weight, the horizontally opposed
engine is used almost exclusively in light aircraft. While light weight is
important in cars, it is even more important in aircraft. Lighter weight means
more payload, longer range, and more performance. Larger aircraft, not so much
today (because of the use of gas turbine engines) as in the past, used radial
engines with up to three banks of nine cylinders, or twenty seven cylinders,
with up to 3500 cu.in. and 3000 horsepower.
...so, you think you know it all? ...take the following self test.
Click on the best answer:
How many degrees of crankshaft rotation are there between power
strokes in a single cylinder engine?
A: 360 B:
720 C:
180 D: 90
How many degrees of crankshaft rotation are there between power
strokes in a four cylinder engine?
A: 360 B:
720 C:
180 D: 90
How many degrees of crankshaft rotation are there between power
strokes in a V-eight engine?
A: 360 B:
720 C:
180 D: 90
Compression Stroke
Exhaust Stroke
Intake Stroke
Power Stroke
If an engine had a cylinder volume (between TDC and BDC) of 80
cc, and a combustion chamber volume of 10 cc, what is its' compression ratio?
A: 8 : 1
B: 9 : 1
C: 80 : 10
D: 1 : 80
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