Why it all matters!!! Hydrocarbons in Action
Part 1: 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 unfamiliar person.
You might want to know what's going on simply out of
curiosity. Or perhaps you are buying a new car soon, 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?
The purpose 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:
There are different kinds of internal combustion engines. Diesel engines are one form and gas
turbine engines are another. Each has its own advantages and disadvantages.
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. 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.
Let's look at the internal combustion process in more detail in the next section.
Part 2: INTERNAL COMBUSTION
The 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!
DO NOT WRITE ON/IN THIS PACKET!!!!
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 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 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. Then the piston moves back up to
compress this fuel/air mixture. Compression makes the explosion more powerful. 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. Once the piston hits the
DO NOT WRITE ON/IN THIS PACKET!!!!
bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the
tailpipe.
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 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.
Part 3: OCTANE RATING
If you've read How Car Engines Work, you know
that almost all cars use four-stroke gasoline
engines. One of the strokes is the compression
stroke, where the engine compresses a cylinderfull of air and gas into a much smaller volume
before igniting it with a spark plug. The amount
of compression is called the compression ratio of
the engine. A typical engine might have a
compression ratio of 8-to-1.
The octane rating of gasoline tells you how much the fuel can be compressed before it
spontaneously ignites. When gas ignites by compression rather than because of the spark from
the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not
something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can
handle the least amount of compression before igniting.
The compression ratio of your engine determines the octane rating of the gas you must use in
the car. One way to increase the horsepower of an engine of a given displacement is to increase
its compression ratio. So a "high-performance engine" has a higher compression ratio and
requires higher-octane fuel. The advantage of a high compression ratio is that it gives your
engine a higher horsepower rating for a given engine weight -- that is what makes the engine
"high performance." The disadvantage is that the gasoline for your engine costs more.
Part 4: HISTORY OF OCTANE RATING
The name "octane" comes from the following fact: When you take crude oil and "crack" it in a
refinery, you end up getting hydrocarbon chains of different lengths. These different chain
lengths can then be separated from each other and blended to form different fuels. For example,
you may have heard of methane, propane and butane. All three of them are hydrocarbons.
Methane has just a single carbon atom. Propane has three carbon atoms chained together.
Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has
seven and octane has eight carbons chained together.
DO NOT WRITE ON/IN THIS PACKET!!!!
It turns out that heptane handles compression very poorly. Compress it just a little and it ignites
spontaneously. Octane handles compression very well -- you can compress it a lot and nothing
happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13percent heptane (or some other combination of fuels that has the same performance of the
87/13 combination of octane/heptane). It spontaneously ignites at a given compression level,
and can only be used in engines that do not exceed that compression ratio.
During WWI, it was discovered that you can add a chemical called tetraethyl lead (TEL) to
gasoline and significantly improve its octane rating above the octane/heptane combination.
Cheaper grades of gasoline could be made usable by adding TEL. This led to the widespread use
of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:


Lead clogs a catalytic converter and renders it inoperable within minutes.
The Earth became covered in a thin layer of lead, and lead is toxic to many living things
(including humans).
When lead was banned, gasoline got more expensive because refineries could not boost the
octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline
(known as AvGas), and octane ratings of 100 or more are commonly used in super-highperformance piston airplane engines. In the case of AvGas, 100 is the gasoline's performance
rating, not the percentage of actual octane in the gas. The addition of TEL boosts the
compression level of the gasoline -- it doesn't add more octane. Currently engineers are trying to
develop airplane engines that can use unleaded gasoline. Jet engines burn kerosene, by the way.
Part 5: ENGINE QUESTIONS AND ANSWERS
Here is a set of engine-related questions from readers and their answers:
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.
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 twostroke engine uses more gasoline and burns lots of oil, so it is far more polluting.
DO NOT WRITE ON/IN THIS PACKET!!!!
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.
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.
How are 4-cylinder and V6 engines different?
The number of cylinders that an engine contains is an important factor in the overall
performance of the engine. Each cylinder contains a piston that pumps inside of it and those
pistons connect to and turn the crankshaft. The more pistons there are pumping, the more
combustive events are taking place during any given moment. That means that more power can
be generated in less time.
4-Cylinder engines commonly come in “straight” or “inline”
configurations while 6-cylinder engines are usually configured
in the more compact “V” shape, and thus are referred to as V6
engines. V6 engines have been the engine of choice for
American automakers because they’re powerful and quiet but
still light and compact enough to fit into most car designs.
Historically, American auto consumers turned their noses up at
4-cylinder engines, believing them to be slow, weak,
unbalanced and short on acceleration. However, when
Japanese auto makers, such as Honda and Toyota, began installing highly-efficient 4-cylinder
engines in their cars in the 1980s and 90s, Americans found a new appreciation for the compact
engine. Even though Japanese models, such as the Toyota Camry, began quickly outselling
comparable American models, U.S. automakers, believing that American drivers were more
concerned with power and performance, continued to produce cars with V6 engines. Today,
with rising gas prices and greater public environmental awareness, Detroit seems to be
reevaluating the 4-cylinder engine for its fuel efficiency and lower emissions.
The inline 4-cylinder engine of
the Lotus Elise.
As for the future of the V6, in recent years the
disparity between 4-cylinder and V6 engines has
lessened considerably. In order to keep up with the
demand for high gas-mileage and lower emission
DO NOT WRITE ON/IN THIS PACKET!!!!
The turbocharged 3.8-liter V6 engine of a
Nissan GT-R.
levels, automakers have worked diligently to improve the overall performance of V6 engines.
Many current V6 models come close to matching the gas-mileage and emissions standards of the
smaller, 4-cylinder engines. So, with the performance and efficiency gaps between the two
engines lessening, the decision to buy a 4-cylinder or V6 may just come down to cost. In models
that are available with either type of engine, the 4-cylinder version can run up to $1000 cheaper
than the V6. So, regardless of what kind of performance you’re looking to get out of your car, the
4-cylinder will always be the budget buy.
DO NOT WRITE ON/IN THIS PACKET!!!!