How Car Engines
Work Have you ever opened the
hood of your car and wondered what was going on in
there? A car engine can look like a big confusing
jumble of metal, tubes and wires to the
uninitiated. You might want to know what's going
on in there simply out of curiosity. After all,
you ride in your car every day -- wouldn't it be
nice to know how it works? Or maybe you are tired
of going to the mechanic and hearing things that
are totally meaningless to you and then paying
$750 for whatever that stuff means. Or perhaps you
are buying a new car, and you hear funny words
like "3.0 liter V-6" and "dual overhead cams" and
"tuned port fuel injection." What does all of that
mean?
If you have ever
wondered about this kind of stuff, then read on!
In this edition of HowStuffWorks, we'll discuss
the basic idea behind an engine, and then go into
detail about how all the pieces fit together, what
can go wrong and how to increase
performance!
Internal
Combustion To
understand the basic idea behind how a
reciprocating internal combustion engine works, it
is helpful to have a good mental image of how
"internal combustion" works. One good example is
an old Revolutionary War cannon. You have probably
seen these in movies, where the soldiers load the
cannon with gun powder and a cannon ball and light
it. That is internal combustion, but it is hard to
imagine that having anything to do with engines.
A more
relevant example might be this: Say that you took
a big piece of plastic sewer pipe, maybe 3 inches
in diameter and 3 feet long, and you put a cap on
one end of it. Then say that you sprayed a little
WD-40 into the pipe, or put in a tiny drop of
gasoline. Then say that you stuffed a potato down
the pipe. Like this:
I am
not recommending that you do this! But
say you did... What we have here is a device
commonly known as a potato cannon. When you
introduce a spark, you can ignite the fuel. What
is interesting, and the reason we are talking
about such a device, is that a potato cannon can
launch a potato about 500 feet through the air!
The
potato cannon uses the basic principle behind any
reciprocating internal combustion engine: If you
put a tiny amount of high-energy fuel (like
gasoline) in a small, enclosed space and ignite
it, an incredible amount of energy is released in
the form of expanding gas. You can use that energy
to propel a potato 500 feet. In this case, the
energy is translated into potato motion. You can
also use it for more interesting purposes. For
example, if you can create a cycle that allows you
to set off explosions like this hundreds of times
per minute, and if you can harness that energy in
a useful way, what you have is the core of a car
engine!
Almost
all cars currently use what is called a
four-stroke combustion cycle to convert
gasoline into motion. The four-stroke approach is
also known as the Otto cycle, in honor of
Nikolaus Otto, who invented it in 1867. The four
strokes are illustrated in Figure 1. They
are:
Intake
stroke
Compression
stroke
Combustion
stroke
Exhaust
stroke
Figure 1
You
can see in the figure that a device called a
piston replaces the potato in the potato
cannon. The piston is connected to the crank
shaft by a connecting rod. As the
crankshaft revolves, it has the effect of
"resetting the cannon." Here's what happens as
the engine goes through its cycle:
The
piston starts at the top, the intake valve
opens, and the piston moves down to let the
engine take in a cylinder-full of air and
gasoline. This is the intake stroke. Only
the tiniest drop of gasoline needs to be mixed
into the air for this to work. (Part 1 of the
figure)
Then
the piston moves back up to compress this
fuel/air mixture. Compression makes the
explosion more powerful. (Part 2 of the figure)
When
the piston reaches the top of its stroke, the
spark plug emits a spark to ignite the gasoline.
The gasoline charge in the cylinder
explodes, driving the piston down. (Part
3 of the figure)
Once
the piston hits the bottom of its stroke, the
exhaust valve opens and the exhaust
leaves the cylinder to go out the tail pipe.
(Part 4 of the figure)
Now
the engine is ready for the next cycle, so it
intakes another charge of air and gas.
Notice
that the motion that comes out of an internal
combustion engine is rotational, while the
motion produced by a potato cannon is
linear (straight). In an engine the linear
motion is converted into rotational motion by the
crank shaft. The rotational motion is nice because
we plan to turn (rotate) the car's wheels with it
anyway.
Two
other things that are good to note:
There
are different kinds of internal combustion
engines. The gas turbine engine is another form
of internal combustion engine. A gas turbine
engine has interesting advantages and
disadvantages, but its main disadvantage right
now is an extremely high manufacturing cost
(which means it costs more than the piston
engine used in cars today). Click here for more
information on gas turbines.
There
is such a thing as an external combustion
engine. A steam engine in old-fashioned trains
and steam boats is the best example of an
external combustion engine. The fuel (coal,
wood, oil, whatever) in a steam engine burns
outside the engine to create steam, and the
steam creates motion inside the engine. It turns
out internal combustion is a lot more efficient
(takes less fuel per mile) than external
combustion, plus an internal combustion engine
is a lot smaller than an equivalent external
combustion engine. This explains why we don't
see any cars from Ford and GM using steam
engines.
Almost
all cars today use a reciprocating internal
combustion engine because this engine is:
Relatively
efficient (compared to an external
combustion engine)
Relatively
inexpensive (compared to a gas turbine)
Relatively
easy to refuel (compared to an electric car)
These
advantages beat any other existing technology for
moving a car around.
Now
let's look at all the parts that work together to
make this happen.
Parts
of an Engine Let's
use the same diagram you saw in the previous
article on internal combustion to identify all of
the different parts in a simple four-cycle engine
(see Figure 1 again below).
Figure 1
Here's
a quick description of each one, along with a lot
of vocabulary that will help you understand what
all the car ads are talking about.
Cylinder The
core of the engine is the cylinder. The piston
moves up and down inside the cylinder. The engine
described here has one cylinder. That is typical
of most lawn mowers, but most cars have more than
one cylinder (four, six and eight cylinders are
common). In a multi-cylinder engine the cylinders
usually are arranged in one of three ways:
inline, V or flat (also known
as horizontally opposed or boxer), as shown in the
following figures.
Click on
image to see animation Figure 2. Inline - The
cylinders are arranged in a line in a single
bank.
Click on image to
see animation Figure 3. V - The cylinders are
arranged in two banks set at an angle to one
another.
Figure 4. Flat - The cylinders are
arranged in two banks on opposite sides of the
engine.
Different
configurations have different smoothness,
manufacturing-cost and shape characteristics that
make them more suitable in some vehicles.
Spark
plug The spark plug supplies the spark that
ignites the air/fuel mixture so that combustion
can occur. The spark must happen at just the right
moment for things to work properly.
Valves The
intake and exhaust valves open at the proper time
to let in air and fuel and to let out exhaust.
Note that both valves are closed during
compression and combustion so that the combustion
chamber is sealed.
Piston A
piston is a cylindrical piece of metal that moves
up and down inside the cylinder.
Piston
rings Piston rings provide a sliding seal
between the outer edge of the piston and the inner
edge of the cylinder. The rings serve two
purposes:
They
prevent the fuel/air mixture and exhaust in the
combustion chamber from leaking into the sump
during compression and combustion.
They
keep oil in the sump from leaking into the
combustion area, where it would be burned and
lost.
Most
cars that "burn oil" and have to have a quart
added every 1,000 miles are burning it because the
engine is old and the rings no longer seal things
properly.
Combustion
chamber The combustion chamber is the area
where compression and combustion take place. As
the piston moves up and down, you can see that the
size of the combustion chamber changes. It has
some maximum volume as well as a minimum volume.
The difference between the maximum and minimum is
called the displacement and is measured in
liters or CCs (Cubic Centimeters, where 1,000
cubic centimeters equals a liter). So if you have
a 4-cylinder engine and each cylinder displaces
half a liter, then the entire engine is a "2.0
liter engine." If each cylinder displaces half a
liter and there are six cylinders arranged in a V
configuration, you have a "3.0 liter V-6."
Generally, the displacement tells you something
about how much power an engine has. A cylinder
that displaces half a liter can hold twice as much
fuel/air mixture as a cylinder that displaces a
quarter of a liter, and therefore you would expect
about twice as much power from the larger cylinder
(if everything else is equal). So a 2.0 liter
engine is roughly half as powerful as a 4.0 liter
engine. You can get more displacement either by
increasing the number of cylinders or by making
the combustion chambers of all the cylinders
bigger (or both).
Connecting
rod The connecting rod connects the piston
to the crankshaft. It can rotate at both ends so
that its angle can change as the piston moves and
the crankshaft rotates.
Crank
shaft The crank shaft turns the piston's up
and down motion into circular motion just like a
crank on a jack-in-the-box does.
Sump The
sump surrounds the crankshaft. It contains some
amount of oil, which collects in the bottom of the
sump (the oil pan).
What
Can Go Wrong So you
go out one morning and your engine will turn over
but it won't start... What could be wrong? Now
that you know how an engine works, you can
understand the basic things that can keep an
engine from running. Three fundamental things can
happen: a bad fuel mix, lack of compression or
lack of spark. Beyond that, thousands of minor
things can create problems, but these are the "big
three." Based on the simple engine we have been
discussing, here is a quick run-down on how these
problems affect your engine:
Bad
fuel mix - A bad fuel mix can occur in several
ways:
You
are out of gas, so the engine is getting air but
no fuel.
The
air intake might be clogged, so there is fuel
but not enough air.
The
fuel system might be supplying too much or too
little fuel to the mix, meaning that combustion
does not occur properly.
There
might be an impurity in the fuel (like water in
your gas tank) that makes the fuel not burn.
Lack
of compression - If the charge of air and fuel
cannot be compressed properly, the combustion
process will not work like it should. Lack of
compression might occur for these reasons:
Your
piston rings are worn (allowing air/fuel to leak
past the piston during compression).
The
intake or exhaust valves are not sealing
properly, again allowing a leak during
compression.
There
is a hole in the cylinder.
The
most common "hole" in a cylinder occurs where the
top of the cylinder (holding the valves and spark
plug and also known as the cylinder head)
attaches to the cylinder itself. Generally, the
cylinder and the cylinder head bolt together with
a thin gasket pressed between them to
ensure a good seal. If the gasket breaks down,
small holes develop between the cylinder and the
cylinder head, and these holes cause leaks.
Lack of spark
- The spark might be nonexistent or weak for a
number of reasons:
If
your spark plug or the wire leading to it is
worn out, the spark will be weak.
If
the wire is cut or missing, or if the system
that sends a spark down the wire is not working
properly, there will be no spark.
If
the spark occurs either too early or too late in
the cycle (i.e. if the ignition timing is off),
the fuel will not ignite at the right time, and
this can cause all sorts of problems.
Many
other things can go wrong. For example:
If
the battery is dead, you cannot turn over the
engine to start it.
If
the bearings that allow the crankshaft to turn
freely are worn out, the crankshaft cannot turn
so the engine cannot run.
If
the valves do not open and close at the right
time or at all, air cannot get in and exhaust
cannot get out, so the engine cannot run.
If
someone sticks a potato up your tailpipe,
exhaust cannot exit the cylinder so the engine
will not run.
If
you run out of oil, the piston cannot move up
and down freely in the cylinder, and the engine
will seize.
In a
properly running engine, all of these factors are
within tolerance.
Engine
Subsystems As you
can see in the previous descriptions under "What
Can Go Wrong," an engine has a number of systems
that help it do its job of converting fuel into
motion. Most of these subsystems can be
implemented using different technologies, and
better technologies can improve the performance of
the engine. Here's a look at all of the different
subsystems used in modern engines:
Valve
train The valve train consists of the
valves and a mechanism that opens and closes them.
The opening and closing system is called a
camshaft. The camshaft has lobes on it that
move the valves up and down, as shown in Figure
5.
Figure 5. The
camshaft
Most
modern engines have what are called overhead
cams. This means that the camshaft is located
above the valves, as you see in Figure 5. The cams
on the shaft activate the valves directly or
through a very short linkage. Older engines used a
camshaft located in the sump near the crankshaft.
Rods linked the cam below to valve
lifters above the valves. This approach has
more moving parts and also causes more lag between
the cam's activation of the valve and the valve's
subsequent motion. A timing belt or timing
chain links the crankshaft to the camshaft so that
the valves are in sync with the pistons. The
camshaft is geared to turn at one-half the rate of
the crankshaft. Many high-performance engines have
four valves per cylinder (two for intake, two for
exhaust), and this arrangement requires two
camshafts per bank of cylinders, hence the phrase
"dual overhead cams."
See
How Camshafts Work for details.
Ignition
system The
ignition system (Figure 6) produces a
high-voltage electrical charge and transmits it to
the spark plugs via ignition wires. The
charge first flows to a distributor, which
you can easily find under the hood of most cars.
The distributor has one wire going in the center
and four, six, or eight wires (depending on the
number of cylinders) coming out of it. These
ignition wires send the charge to each
spark plug. The engine is timed so that only one
cylinder receives a spark from the distributor at
a time. This approach provides maximum smoothness.
Figure
6. The ignition system
See
How Automobile Ignition Systems Work for more
details.
Cooling
system The cooling system in most cars
consists of the radiator and water pump. Water
circulates through passages around the cylinders
and then travels through the radiator to cool it
off. In a few cars (most notably Volkswagen
Beetles), as well as most motorcycles and lawn
mowers, the engine is air-cooled instead (You can
tell an air-cooled engine by the fins adorning the
outside of each cylinder to help dissipate heat.).
Air-cooling makes the engine lighter but hotter,
generally decreasing engine life and overall
performance.
Diagram of a cooling system showing
how all the plumbing is connected
See
How Car Cooling Systems Work for details.
Air
intake system Most cars are normally
aspirated, which means that air flows through
an air filter and directly into the cylinders.
High-performance engines are either
turbocharged or supercharged, which
means that air coming into the engine is first
pressurized (so that more air/fuel mixture can be
squeezed into each cylinder) to increase
performance. The amount of pressurization is
called boost. A turbocharger uses a small
turbine attached to the exhaust pipe to spin a
compressing turbine in the incoming air stream. A
supercharger is attached directly to the engine to
spin the compressor.
Starting
system The starting system consists of an
electric starter motor and a starter
solenoid. When you turn the ignition key, the
starter motor spins the engine a few revolutions
so that the combustion process can start. It takes
a powerful motor to spin a cold engine. The
starter motor must overcome:
All
of the internal friction caused by the piston
rings
The
compression pressure of any cylinder(s) that
happens to be in the compression stroke
The
energy needed to open and close valves with the
camshaft
All
of the "other" things directly attached to the
engine, like the water pump, oil pump,
alternator, etc.
Because
so much energy is needed and because a car uses a
12-volt electrical system, hundreds of amps of
electricity must flow into the starter motor. The
starter solenoid is essentially a large electronic
switch that can handle that much current. When you
turn the ignition key, it activates the solenoid
to power the motor.
Lubrication
system The lubrication system makes sure
that every moving part in the engine gets oil so
that it can move easily. The two main parts
needing oil are the pistons (so they can slide
easily in their cylinders) and any bearings that
allow things like the crankshaft and camshafts to
rotate freely. In most cars, oil is sucked out of
the oil pan by the oil pump, run through the oil
filter to remove any grit, and then squirted under
high pressure onto bearings and the cylinder
walls. The oil then trickles down into the sump,
where it is collected again and the cycle repeats.
Fuel
system The fuel system pumps gas from the
gas tank and mixes it with air so that the proper
air/fuel mixture can flow into the cylinders. Fuel
is delivered in three common ways: carburetion,
port fuel injection and direct fuel injection.
In
carburetion, a device called a carburetor
mixes gas into air as the air flows into the
engine.
In a
fuel-injected engine, the right amount of fuel
is injected individually into each cylinder
either right above the intake valve (port fuel
injection) or directly into the cylinder (direct
fuel injection).
See
How Fuel Injection Systems Work for more details.
Exhaust
system The exhaust system includes the
exhaust pipe and the muffler. Without a muffler,
what you would hear is the sound of thousands of
small explosions coming out your tailpipe. A
muffler dampens the sound. The exhaust system also
includes a catalytic converter. See How Catalytic
Converters Work for details.
Emission
control system The emission control system
in modern cars consists of a catalytic
converter, a collection of sensors and
actuators, and a computer to monitor and adjust
everything. For example, the catalytic converter
uses a catalyst and oxygen to burn off any unused
fuel and certain other chemicals in the exhaust.
An oxygen sensor in the exhaust stream makes sure
there is enough oxygen available for the catalyst
to work and adjusts things if necessary.
Electrical
system The electrical system consists of a
battery and an alternator. The
alternator is connected to the engine by a belt
and generates electricity to recharge the battery.
The battery makes 12-volt power available to
everything in the car needing electricity (the
ignition system, radio, headlights, windshield
wipers, power windows and seats, computers, etc.)
through the vehicle's wiring.
How
to Help an Engine Produce More
Power Using
all of this information, you can begin to see that
there are lots of different ways to make an engine
perform better. Car manufacturers are constantly
playing with all of the following variables to
make an engine more powerful and/or more fuel
efficient.
Increase
displacement - More displacement means more
power because you can burn more gas during each
revolution of the engine. You can increase
displacement by making the cylinders bigger or by
adding more cylinders. Twelve cylinders seems to
be the practical limit.
Increase
the compression ratio - Higher compression
ratios produce more power, up to a point. The more
you compress the air/fuel mixture, however, the
more likely it is to spontaneously burst into
flame (before the spark plug ignites it).
Higher-octane gasolines prevent this sort of early
combustion. That is why high-performance cars
generally need high-octane gasoline -- their
engines are using higher compression ratios to get
more power.
Stuff
more into each cylinder - If you can cram more
air (and therefore fuel) into a cylinder of a
given size, you can get more power from the
cylinder (in the same way that you would by
increasing the size of the cylinder).
Turbochargers and superchargers pressurize the
incoming air to effectively cram more air into a
cylinder. See How Turbochargers Work for details.
Cool
the incoming air - Compressing air raises its
temperature. However, you would like to have the
coolest air possible in the cylinder because the
hotter the air is, the less it will expand when
combustion takes place. Therefore, many
turbocharged and supercharged cars have an
intercooler. An intercooler is a special
radiator through which the compressed air passes
to cool it off before it enters the cylinder. See
How Car Cooling Systems Work for details.
Let
air come in more easily - As a piston moves
down in the intake stroke, air resistance can rob
power from the engine. Air resistance can be
lessened dramatically by putting two intake valves
in each cylinder. Some newer cars are also using
polished intake manifolds to eliminate air
resistance there. Bigger air filters can also
improve air flow.
Let
exhaust exit more easily - If air resistance
makes it hard for exhaust to exit a cylinder, it
robs the engine of power. Air resistance can be
lessened by adding a second exhaust valve to each
cylinder (a car with two intake and two exhaust
valves has four valves per cylinder, which
improves performance -- when you hear a car ad
tell you the car has four cylinders and 16 valves,
what the ad is saying is that the engine has four
valves per cylinder). If the exhaust pipe is too
small or the muffler has a lot of air resistance,
this can cause back-pressure, which has the same
effect. High-performance exhaust systems use
headers, big tail pipes and free-flowing mufflers
to eliminate back-pressure in the exhaust system.
When you hear that a car has "dual exhaust," the
goal is to improve the flow of exhaust by having
two exhaust pipes instead of one.
Make
everything lighter - Lightweight parts help
the engine perform better. Each time a piston
changes direction, it uses up energy to stop the
travel in one direction and start it in another.
The lighter the piston, the less energy it takes.
Inject
the fuel - Fuel injection allows very precise
metering of fuel to each cylinder. This improves
performance and fuel economy. See How Fuel
Injection Systems Work for details.
Q
and A Here
is a set of questions from readers:
What
is the difference between a gasoline engine and
a diesel engine? In a diesel engine, there
is no spark plug. Instead, diesel fuel is
injected into the cylinder, and the heat and
pressure of the compression stroke cause the
fuel to ignite. Diesel fuel has a higher energy
density than gasoline, so a diesel engine gets
better mileage. See How Diesel Engines Work for
more information.
What
is the difference between a two-stroke and a
four-stroke engine? Most chain saws and boat
motors use two-stroke engines. A two-stroke
engine has no moving valves, and the spark plug
fires each time the piston hits the top of its
cycle. A hole in the lower part of the cylinder
wall lets in gas and air. As the piston moves up
it is compressed, the spark plug ignites
combustion, and exhaust exits through another
hole in the cylinder. You have to mix oil into
the gas in a two-stroke engine because the holes
in the cylinder wall prevent the use of rings to
seal the combustion chamber. Generally, a
two-stroke engine produces a lot of power for
its size because there are twice as many
combustion cycles occurring per rotation.
However, a two-stroke engine uses more gasoline
and burns lots of oil, so it is far more
polluting. See How Two-stroke Engines Work for
more information.
You
mentioned steam engines in this article -- are
there any advantages to steam engines and other
external combustion engines? The main
advantage of a steam engine is that you can use
anything that burns as the fuel. For example, a
steam engine can use coal, newspaper or wood for
the fuel, while an internal combustion engine
needs pure, high-quality liquid or gaseous fuel.
See How Steam Engines Work for more information.
Are
there any other cycles besides the Otto cycle
used in car engines? The two-stroke engine
cycle is different, as is the diesel cycle
described above. The engine in the Mazda
Millennia uses a modification of the Otto cycle
called the Miller cycle. Gas turbine engines use
the Brayton cycle. Wankle rotary engines use the
Otto cycle, but they do it in a very different
way than four-stroke piston engines.
Why
have eight cylinders in an engine? Why not have
one big cylinder of the same displacement of the
eight cylinders instead? There are a couple
of reasons why a big 4.0-liter engine has eight
half-liter cylinders rather than one big 4-liter
cylinder. The main reason is smoothness. A V-8
engine is much smoother because it has eight
evenly spaced explosions instead of one big
explosion. Another reason is starting torque.
When you start a V-8 engine, you are only
driving two cylinders (1 liter) through their
compression strokes, but with one big cylinder
you would have to compress 4 liters
instead
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