How Diesel Two-Stroke Engines Work
The article How Diesel Engines Work describes the four-stroke diesel engines commonly found in cars and trucks. The article How Two-stroke Engines Work describes the small two-stroke engines found in things like chain saws, mopeds and jet skis. It turns out that diesel engine technology is often combined with a two-stroke cycle in the huge diesel engines found in locomotives, large ships and generating facilities.
Now, we will explore diesel two-stroke technology and learn about the huge engines that use it!
Understanding the Cycle
If you read
How
Two-stroke Engines Work, you learned that one big
difference between two-stroke and four-stroke
engines is the amount of power the engine can produce. The
spark plug fires twice as often in a two-stroke engine -- once
per every revolution of the crankshaft, versus once for every
two revolutions in a four-stroke engine. This means that a
two-stroke engine has the potential to produce twice as
much power as a four-stroke engine of the same size.
The two-stroke engine article also explains that the gasoline engine cycle, where gas and air are mixed and compressed together, is not really a perfect match for the two-stroke approach. The problem is that some unburned fuel leaks out each time the cylinder is recharged with the air-fuel mixture. (See How Two-stroke Engines Work for details.)
It turns out that the diesel approach, which compresses only air and then injects the fuel directly into the compressed air, is a much better match with the two-stroke cycle. Many manufacturers of large diesel engines therefore use this approach to create high-power engines.
The figure below shows the layout of a typical two-stroke diesel engine:
At the top of the cylinder are typically two or four exhaust valves that all open at the same time. There is also the diesel fuel injector (shown above in yellow). The piston is elongated, as in a gasoline two-stroke engine, so that it can act as the intake valve. At the bottom of the piston's travel, the piston uncovers the ports for air intake. The intake air is pressurized by a turbocharger or a supercharger (light blue). The crankcase is sealed and contains oil as in a four-stroke engine.
The two-stroke diesel cycle goes like this:
From this description, you can see the big difference between a diesel two-stroke engine and a gasoline two-stroke engine: In the diesel version, only air fills the cylinder, rather than gas and air mixed together. This means that a diesel two-stroke engine suffers from none of the environmental problems that plague a gasoline two-stroke engine. On the other hand, a diesel two-stroke engine must have a turbocharger or a supercharger, and this means that you will never find a diesel two-stroke on a chain saw -- it would simply be too expensive.
General Motors EMD Engines
The General
Motors EMD engine line is typical of the two-stroke diesel
breed. These engines were introduced in the 1930s and power a
large number of the diesel
locomotives found in the United States. There have been
three successive series in the EMD line: the 567 series, the
645 series, and the 710 series. The numbers refer to the
number of cubic inches per cylinder, with a typical engine
having 16 cylinders (for a total displacement on the order of
10,000 cubic inches!). When you consider that a 5-liter
(305-cubic-inch) engine is considered to be very large in an
automobile, you can see that one of these EMD engines is
massive!
Here are some of the specifications for the EMD 645E3 engine:
A typical horsepower rating for one of these engines is 4,300 hp!
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