Long stroke piston enables large power on low speed

A-frame and flywheel of large 2-stroke Sulzer diesel engine

Connecting rod inside the crankcase

Large bore cylinder liner

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...

Until next time...

Thomas Yoon
Editor, Facworld Ezine

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