>> How Things Work >> How Diesel Engines Work
Diesel developed the idea for the diesel engine and obtained
the German patent for it in 1892. His goal was to create an
engine with high efficiency. Gasoline engines had been invented
in 1876 and, especially at that time, were not very efficient.
Engines vs. Gasoline Engines
differences between the gasoline engine and the diesel engine
- A gasoline
engine intakes a mixture of gas and air, compresses it and
ignites the mixture with a spark. A diesel engine takes
in just air, compresses it and then injects fuel into the
compressed air. The heat of the compressed air lights the
- 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.
The higher compression ratio of the diesel engine leads
to better efficiency.
engines generally use either carburetion, in which the air
and fuel is mixed long before the air enters the cylinder,
or port fuel injection, in which the fuel is injected just
prior to the intake stroke (outside the cylinder). Diesel
engines use direct fuel injection -- the diesel fuel is
injected directly into the cylinder.
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
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.
difference between a diesel engine and a gas engine is in
the injection process. Most car engines use port injection
or a carburetor rather than direct injection. 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. A diesel compresses
only air, so the compression ratio can be much higher. The
higher the compression ratio, the more power is generated.
engines contain a glow plug of some sort (not shown in this
figure). 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:
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.
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
have ever compared diesel fuel and gasoline, you know that
they are different. They certainly smell different. Diesel
fuel is heavier and oilier. Diesel fuel 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 is so
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 is generally cheaper than gasoline.
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.
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