Diesel Engines
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The 4.5-liter V-8 Duramax improves efficiency
by 25 percent
when compared with gasoline engines,
while reducing pollutants and
emissions.
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How Diesel Engines Work
BY MARSHALL BRAIN
What is the difference between a gasoline and a diesel engine?
Diesel's story actually
begins with the invention of the gasoline engine. Nikolaus August
Otto had invented and patented the gasoline engine by 1876.
This invention used the
four-stroke combustion principle, also known as the "Otto Cycle," and
it's the basic premise for most car engines today.
In its early stage, the
gasoline engine wasn't very efficient, and other major methods of transportation
such as the steam engine fared poorly as well.
Only about 10 percent of the
fuel used in these types of engines actually moved a vehicle. The rest of the
fuel simply produced useless heat.
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Rudolf Diesel,
inventor of the diesel engine.
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In 1878, Rudolf Diesel was
attending the Polytechnic High School of Germany (the equivalent of an
engineering college) when he learned about the low efficiency of gasoline and
steam engines.
This disturbing information
inspired him to create an engine with a higher efficiency, and he
devoted much of his time to developing a "Combustion Power Engine."
By 1892 Diesel had obtained a
patent for what we now call the diesel engine.
If diesel engines are so
efficient, why don't we use them more often?
You might see the words
"diesel engine" and think of big, hefty cargo trucks spewing out
black, sooty smoke and creating a loud clattering noise.
This negative image of diesel
trucks and engines has made diesel less attractive to casual drivers in the
United States -- although diesel is great for hauling large shipments over long
distances, it hasn't been the best choice for everyday commuters.
This is starting to change,
however, as people are improving the diesel engine to make it cleaner and less
noisy.
In
theory, diesel engines and gasoline engines are quite similar. They are
both internal combustion engines designed to convert the
chemical energy available in fuel into mechanical energy.
This
mechanical energy moves pistons up and down inside cylinders.
The
pistons are connected to a crankshaft, and the up-and-down motion of the
pistons, known as linear motion, creates the rotary motion needed to turn the
wheels of a car forward.
Both
diesel engines and gasoline engines convert fuel into energy through a series
of small explosions or combustions.
The
major difference between diesel and gasoline is the way these explosions
happen.
In
a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by
sparks from spark plugs.
In
a diesel engine, however, the air is compressed first, and then the fuel is
injected. Because air heats up when it's compressed, the fuel ignites.
The
diesel engine uses a four-stroke combustion cycle just like a gasoline engine.
The four strokes are:
· Intake stroke -- The intake valve opens up, letting
in air and moving the piston down.
· Compression stroke -- The piston moves back up
and compresses the air.
· Combustion stroke -- As the piston reaches the
top, fuel is injected at just the right moment and ignited, forcing the piston
back down.
· Exhaust stroke -- The piston moves back to the top,
pushing out the exhaust created from the combustion out of the exhaust valve.
Remember
that the diesel engine has no spark plug, that it intakes air and compresses
it, and that it then injects the fuel directly into the combustion chamber
(direct injection).
It
is the heat of the compressed air that lights the fuel in a diesel engine. In
the next section, we'll examine the diesel injection process.
COMPRESSION
When working on his
calculations, Rudolf Diesel theorized that higher compression leads to higher
efficiency and more power.
This happens because when the piston
squeezes air with the cylinder, the air becomes concentrated.
Diesel fuel has a high energy content, so
the likelihood of diesel reacting with the concentrated air is greater.
Another way to think of it is when air
molecules are packed so close together, fuel has a better chance of reacting
with as many oxygen molecules as possible.
Rudolf turned out to be right -- a gasoline
engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses
at a ratio of 14:1 to as high as 25:1.
Diesel Fuel Injection
One
big difference between a diesel engine and a gas engine is in the
injection process.
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six-cylinder diesel
engine
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Most car engines use port
injection or a carburetor. A port injection system injects fuel just prior to
the intake stroke (outside the cylinder).
A carburetor mixes air and
fuel long before the air enters the cylinder.
In a car engine, therefore,
all of the fuel is loaded into the cylinder during the intake stroke and then
compressed.
The compression of the
fuel/air mixture limits the compression ratio of the engine -- if it compresses
the air too much, the fuel/air mixture spontaneously ignites and causes knocking.
Because it causes excessive
heat, knocking can damage the engine.
Diesel engines use direct
fuel injection -- the diesel fuel is injected directly into the cylinder.
The injector on a diesel
engine is its most complex component and has been the subject of a great deal
of experimentation -- in any particular engine, it may be located in a variety
of places.
The injector has to be able
to withstand the temperature and pressure inside the cylinder and still deliver
the fuel in a fine mist.
Getting the mist circulated
in the cylinder so that it is evenly distributed is also a problem, so some
diesel engines employ special induction valves, pre-combustion chambers or
other devices to swirl the air in the combustion chamber or otherwise improve
the ignition and combustion process.
Some diesel engines contain
a glow plug.
When a diesel engine is cold,
the compression process may not raise the air to a high enough temperature to
ignite the fuel.
The glow plug is an electrically
heated wire (think of the hot wires you see in a toaster) that heats the
combustion chambers and raises the air temperature when the engine is cold so
that the engine can start.
According to Cley Brotherton,
a Journeyman heavy equipment technician:
“All
functions in a modern engine are controlled by the ECM communicating with an
elaborate set of sensors measuring everything from R.P.M. to engine coolant and
oil temperatures and even engine position (i.e. T.D.C.). Glow plugs are rarely
used today on larger engines. The ECM senses ambient air temperature and
retards the timing of the engine in cold weather so the injector sprays the
fuel at a later time. The air in the cylinder is compressed more, creating more
heat, which aids in starting.”
Smaller engines and engines
that do not have such advanced computer control use glow plugs to
solve the cold-starting problem.
Of course, mechanics aren't
the only difference between diesel engines and gasoline engines.
There's also the issue of the fuel itself.
Diesel Fuel
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A sample of diesel fuel
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Petroleum
fuel starts off as crude oil that's naturally found in the Earth.
When crude oil is processed at
refineries, it can be separated into several different kinds of fuels,
including gasoline, jet fuel, kerosene and, of course, diesel.
If you have ever compared
diesel fuel and gasoline, you know that they are different. They certainly
smell different.
Diesel fuel is heavier and
oilier. It evaporates much more slowly than gasoline -- its boiling point is
actually higher than the boiling point of water.
You will often hear diesel
fuel referred to as "diesel oil" because it's so oily.
Diesel fuel evaporates more
slowly because it is heavier. It contains more carbon atoms in longer
chains than gasoline does (gasoline is typically C9H20, while diesel fuel is
typically C14H30).
It takes less refining to
create diesel fuel, which is why it used to be cheaper than gasoline.
Since 2004, however, demand
for diesel has risen for several reasons, including increased industrialization
and construction in China and the U.S.
Diesel fuel has a higher
energy density than gasoline. On average, 1 gallon (3.8 L) of diesel
fuel contains approximately 155x106 joules (147,000 BTU), while
1 gallon of gasoline contains 132x106 joules (125,000 BTU).
This, combined with the
improved efficiency of diesel engines, explains why diesel engines get better
mileage than equivalent gasoline engines.
Diesel fuel is used to power
a wide variety of vehicles and operations.
It of course fuels the diesel
trucks you see lumbering down the highway, but it also helps move boats, school
buses, city buses, trains, cranes, farming equipment and various emergency
response vehicles and power generators.
Think about how important
diesel is to the economy -- without its high efficiency, both the construction
industry and farming businesses would suffer immensely from investments in
fuels with low power and efficiency.
About 94 percent of freight
-- whether it's shipped in trucks, trains or boats -- relies on diesel.
In terms of the environment,
diesel has some pros and cons.
The pros -- diesel emits very
small amounts of carbon monoxide, hydrocarbons and carbon dioxide, emissions
that lead to global warming.
The cons -- high amounts of
nitrogen compounds and particulate matter (soot) are released from burning
diesel fuel, which lead to acid rain, smog and poor health conditions.
Diesel Improvements and Biodiesel
During the big oil crisis
in the 1970s, European car companies started advertising diesel engines for
commercial use as an alternative to gasoline.
Those who tried it out were a
bit disappointed – the engines were very loud, and they would arrive home
to find their cars covered from front to back in black soot -- the same soot
responsible for smog in big cities.
Over the past 30 to 40 years,
however, vast improvements have been made on engine performance and fuel
cleanliness.
Direct injection devices are
now controlled by advanced computers that monitor fuel combustion, increasing
efficiency and reducing emissions.
Better-refined diesel fuels
such as ultra low sulfur diesel (ULSD) will lower the amount of harmful
emissions and upgrading engines to make them compatible with cleaner fuel is
becoming a simpler process.
Other technologies such as
CRT particulate filters and catalytic converters burn soot and reduce
particulate matter, carbon monoxide and hydrocarbons by as much as 90 percent.
You may have also heard of
something called biodiesel. Is it the same as diesel?
Biodiesel is an alternative
or additive to diesel fuel that can be used in diesel engines with little to no
modifications to the engines themselves.
It's not made from petroleum
-- instead it comes from plant oils or animal fats that have been chemically
altered. (Interesting fact: Rudolf Diesel had originally considered vegetable
seed oil as fuel for his invention.)
Biodiesel can either be
combined with regular diesel or used completely by itself.
From “How Diesel Engines Work” by Marshall Brain
https://auto.howstuffworks.com/diesel.htm
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