Why Select Two Strokes?

Written by Thomas Yoon

In our previous publication, we touched onrepparttar subject of 2-stroke diesel engines. http://www.free-engineering.com/fwezine26.htm . Today, we ask why we choose to use 2-strokes?

As you might remember, for a 2-stroke engine,repparttar 102780 piston has to move up, and then down to complete one cycle of repparttar 102781 engine. In those 2 strokes ofrepparttar 102782 piston,repparttar 102783 crankshaft will have turned 1 revolution. In those 2 strokes ofrepparttar 102784 pistonrepparttar 102785 engine has also completed 4 stages ofrepparttar 102786 combustion cycle - air intake, compression, combustion and exhaust.

Now, compare that with a 4-stroke engine. The piston moving down will completerepparttar 102787 air intake stroke. When it moves up again,repparttar 102788 piston completesrepparttar 102789 compression stroke. Next comesrepparttar 102790 injection of fuel. Combustion takes place. The piston is forced to move down byrepparttar 102791 pressure ofrepparttar 102792 gases. Whenrepparttar 102793 piston moves up again, it drives out allrepparttar 102794 exhaust gases inrepparttar 102795 exhaust stroke.

The piston in a 4-stroke engine will move 4 strokes in order to completerepparttar 102796 4 stages ofrepparttar 102797 combustion cycle - air intake, compression, combustion and exhaust.

Lubrication - The Silent Component of Machinery

Written by Thomas Yoon

The function of a lubrication medium is:

1. To form a film between moving bearing components so that metal to metal contact is prevented. 2. To reduce friction and eliminate wear 3. To protect against corrosion 4. To seal against impurities like dust, dirt, water.

In order forrepparttar oil film to be formed betweenrepparttar 102779 moving bearing components,repparttar 102780 film must be sufficiently thick even under heavy load, high temperatures or vibrations.

Some sleeve bearings that has very heavy loads, like inrepparttar 102781 crosshead bearings of diesel engines, usually have provisions for injecting pressurized oil to floatrepparttar 102782 shafts. This method is called hydrostatic lubrication.

However,repparttar 102783 most common method of lubrication for sleeve bearings is byrepparttar 102784 hydrodynamic method. Whenrepparttar 102785 two surfaces of a bearing and shaft move rapidly relative to one another,repparttar 102786 oil is carried alongrepparttar 102787 shaft to fillrepparttar 102788 gap between shaft and bearing. When repparttar 102789 moving components become completely separated by a cohesive film of lubricant, hydrodynamic lubrication occurs. Hydrodynamic lubrication prevents wear in moving parts, as there is no metallic contact between them. The bearing metals can last for a long time.

During starting time,repparttar 102790 rotating shaft does not have sufficient speed to pick uprepparttar 102791 lubricant. The film separatingrepparttar 102792 moving surfaces is very thin - with onlyrepparttar 102793 thickness of a molecule. This is a condition called boundary-layer lubrication. With this condition, friction losses increases, producing heat, which raises repparttar 102794 temperature ofrepparttar 102795 lubricant, thereby reducing its viscosity so thatrepparttar 102796 load-carrying capacity ofrepparttar 102797 film is even lower. In worst case conditions,repparttar 102798 surfaces can even seize together.

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