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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