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:
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
Almost all cars today use a
reciprocating internal combustion engine because this engine
Relatively efficient (compared to an external
Relatively inexpensive (compared to a gas
Relatively easy to refuel (compared to an
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
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).
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
Click on image to see animation Figure 2. Inline
- The cylinders are arranged in a line in a single
Click on image to see animation Figure 3. V -
The cylinders are arranged in two banks set at an angle to one
Click on image to see animation Figure 4. Flat -
The cylinders are arranged in two banks on opposite sides of
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
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
Crank shaft The crank shaft turns the piston's up
and down motion into circular motion just like a crank on a
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
Bad fuel mix - A bad fuel mix can occur in several
You are out of gas, so the engine is getting air but no
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
There might be an impurity in the fuel (like water in
your gas tank) that makes the fuel not burn.
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
Click on image to see animation Figure 5. The
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
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
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
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
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
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
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
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
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
windows and seats, computers,
etc.) through the vehicle's wiring.
How to Help an Engine Produce More Power
For a complete explanation of what horsepower is
and what horsepower means, check out How
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
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
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
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
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
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