As part of our mission to help you better
understand what goes into the proper upkeep of your vehicle, World Motors
provides this section to help explain the typical automotive systems at
work in your vehicle. It will let you in on the various systems, what they
do and how they work. It's all part of keeping you involved in the decision-making
process when it comes to your car. Auto service shouldn't be a mystery;
at World Motors, the next move is always up to you.
Please select from one of
the section on the left hand side:
Understanding
Car Engine:
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?
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).
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.
Click on image to see animation
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.
Click on image to see animation
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."
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
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 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.
