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.
want to know what's going on simply out of curiosity. Or perhaps
you are buying a new car, and you hear things like "3.0
liter V-6" and "dual overhead cams" and "tuned
port fuel injection." What does all of that mean?
have ever wondered about this kind of stuff, then read on
-- In this article, 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.
of a gasoline car engine is to convert gasoline into motion
so that your car can move. Currently the easiest way to create
motion from gasoline is to burn the gasoline inside an engine.
Therefore, a car engine is an internal combustion engine --
combustion takes place internally. Two things to note:
are different kinds of internal combustion engines. Diesel
engines are one form and gas turbine engines are another.
See also the articles on HEMI engines, rotary engines and
two-stroke engines. Each has its own advantages and disadvantages.
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. 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.
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!
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:
called a piston replaces the potato in the potato cannon.
The piston is connected to the crankshaft 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
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.
the piston moves back up to compress this fuel/air mixture.
Compression makes the explosion more powerful.
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.
the piston hits the bottom of its stroke, the exhaust valve
opens and the exhaust leaves the cylinder to go out the
engine is ready for the next cycle, so it intakes another
charge of air and gas.
that comes out of an internal combustion engine is rotational,
while the motion produced by a potato cannon is linear (straight
line). In an engine the linear motion of the pistons is converted
into rotational motion by the crankshaft. The rotational motion
is nice because we plan to turn (rotate) the car's wheels
with it anyway.
look at all the parts that work together to make this happen,
starting with the cylinders.
core of the engine is the cylinder, with the piston moving
up and down inside the cylinder. The engine described above
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).
configurations have different advantages and disadvantages
in terms of smoothness, manufacturing cost and shape characteristics.
These advantages and disadvantages make them more suitable
for certain vehicles.
look at some key engine parts in more detail.
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
: 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.
: A piston is a cylindrical piece of metal that moves up
and down inside the cylinder.
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:
prevent the fuel/air mixture and exhaust in the combustion
chamber from leaking into the sump during compression
keep oil in the sump from leaking into the combustion
area, where it would be burned and lost.
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.
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.
: The crankshaft turns the piston's up and down motion into
circular motion just like a crank on a jack-in-the-box does.
: The sump surrounds the crankshaft. It contains some amount
of oil, which collects in the bottom of the sump (the oil
Engines Don't Work
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 rundown on how these
problems affect your engine:
fuel mix - A bad fuel mix can occur in several ways:
are out of gas, so the engine is getting air but no fuel.
air intake might be clogged, so there is fuel but not enough
fuel system might be supplying too much or too little fuel
to the mix, meaning that combustion does not occur properly.
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
piston rings are worn (allowing air/fuel to leak past the
piston during compression).
intake or exhaust valves are not sealing properly, again
allowing a leak during compression.
is a hole in the cylinder.
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.
of spark - The spark might be nonexistent or weak for
a number of reasons:
your spark plug or the wire leading to it is worn out, the
spark will be weak.
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.
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
things can go wrong. For example:
the battery is dead, you cannot turn over the engine to
the bearings that allow the crankshaft to turn freely are
worn out, the crankshaft cannot turn so the engine cannot
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.
someone sticks a potato up your tailpipe, exhaust cannot
exit the cylinder so the engine will not run.
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.
can see, an engine has a number of systems that help it do
its job of converting fuel into motion.
Valve Train and Ignition Systems
subsystems can be implemented using different technologies,
and better technologies can improve the performance of the
engine. Let's look at all of the different subsystems used
in modern engines, beginning with the valve train.
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.
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.
system 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.
Cooling, Air-intake and Starting Systems
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. See How Car Cooling Systems
Work for details.
you know how and why your engine stays cool. But why is air
circulation so important? 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.
your engine's performance is great, but what exactly happens
when you turn the key to start it? 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:
of the internal friction caused by the piston rings
compression pressure of any cylinder(s) that happens to
be in the compression stroke
energy needed to open and close valves with the camshaft
of the "other" things directly attached to the
engine, like the water pump, oil pump, alternator, etc.
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, Fuel, Exhaust and Electrical Systems
comes to day-to-day car maintenance, your first concern is
probably the amount of gas in your car. How does the gas that
you put in power the cylinders? The engine's 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.
carburetion, a device called a carburetor mixes gas into
air as the air flows into the engine.
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).
plays an important part. 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.
you know about some of the stuff that you put in your car,
let's look at some of the stuff that comes out of it. 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.
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.
gas, what else powers your car? 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.
More Engine Power
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
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.
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 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.
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.
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.
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.
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.
the fuel - Fuel injection allows very precise metering
of fuel to each cylinder. This improves performance and fuel
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