Diesel engine
Diesel Fuel Injection
One big difference between a diesel engine and a
gas engine
is in the injection process. 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.
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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.
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More on diesel
Diesel fuel has a
higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x10
6 joules (147,000 BTU), while 1 gallon of gasoline contains 132x10
6
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 --
relys 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.
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.