How Car Engines Work
Here we'll discuss the basic idea behind an engine, and then go into detail about how all
the pieces fit together and how to increase performance!
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. They are:
? Intake stroke or suck stroke
? Compression stroke or squeeze stroke
? Combustion stroke or bang stroke
? Exhaust stroke or blow stroke
The piston is connected to the crank shaft by a connecting rod. Here's what happens as the engine goes through its cycle:
1. 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.
2. Then the piston moves back up to compress this fuel/air mixture. Compression
makes the explosion more powerful.
3. 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
4. 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.
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. 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.
Almost all cars today use a reciprocating internal combustion engine because this engine
? 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
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.
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.
Figure 2. Inline - The cylinders are arranged in a line in a single bank.
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.
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.
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.
The sump surrounds the crankshaft. It contains some amount of oil, which collects in the
bottom of the sump (the oil pan).
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:
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."
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
? 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.