Bigger Power with Two Strokes!
What is a 2-stroke diesel engine?
As the name suggests, these engines work on reciprocating
actions of pistons. One stroke means one linear motion of the
piston in one direction. When the piston moves in the opposite
direction, that is counted as another stroke.
For a 2-stroke engine, the piston has to move up, and then down
to complete one cycle of the engine. In those 2 strokes of the
piston, the crankshaft will have turned 1 revolution.
The 2 strokes of the piston is to complete 4 stages of the
combustion cycle - air intake, compression, combustion and
exhaust. As you might have known, in order for a fire (or
explosion) to occur to power the engine, we need a fuel, a heat
source, and oxygen coming together at the same time.
Remember how the crankshaft and the piston moves in a cylinder?
The reciprocating movement of the piston is converted to rotary
motion by means of a crankshaft. The piston reciprocates between
Top Dead Center and Bottom Dead Center inside the cylinder liner.
One of the most prominent feature of a 2-stroke engine is the
presence of air intake ports on the cylinder liner that will
enable fresh air to be led in for combustion.
These air inlet ports are cut into the cylinder liner somewhere
at the lower end of the latter. In a typical design, the air
inlet port are located about 35 degrees from the Bottom Dead
Center (BDC).
In many designs, the fresh air that is introduced into the
cylinder is also used to drive out the spent exhaust gases
inside it. Many designs make use of scavenging air fans like
electric blowers or turbochargers to slightly pressurize the
intake air before the latter is introduced into the cylinder.
In order to allow the exhaust gases to be expelled from the
cylinder, exhaust ports are often cut into the cylinder just
like the air intake ports. A typical design will have the
exhaust port located about 50 degrees from Bottom Dead Center.
So, let's describe our engine stages at the combustion space
between the piston top and the cylinder head.
>From Bottom Dead Center (BDC), the piston, full of fresh air,
moves up the cylinder liner until it covers up the air intake
port. The air intake process stops.
The piston moves further up. It then covers up the exhaust port
on the cylinder liner. The exhaust process stops.
The piston moves further up. The air inside the combustion space
is compressed and becomes hot. The piston has nearly reached Top
Dead Center at this point.
Highly atomized fuel is then injected into the combustion space.
The fuel burns rapidly causing an explosion inside the
combustion space. The explosion causes a tremendous rise in
pressure and the piston is pushed down towards BDC.
As the piston moves down, the exhaust ports uncover about 50
degrees from BDC. Exhaust gases are thus led out from the
cylinder. The pressure inside the cylinder drops immediately.
The piston moves further down. At 35 degrees from BDC, it
uncovers the air intake port. Fresh air is then led into the
cylinder.
The piston then reaches BDC. Because of the momentum created
from the force of the explosion, the piston reverses in
direction and moves upwards towards Top Dead Center (TDC).
The process then repeats itself.
In some designs, the exhaust gases are removed through an
exhaust valve, located at the cylinder head and very similar to
the 4-stroke engines. This type of scavenging is called uniflow
scavenging. The timing of the valve opening and closing will be
controlled by a camshaft, push rods, rocker arms or other
similar devices.
Well folks, why do we want to squeeze all the strokes into 2
cycles? That could be discussed further...
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