Why Select Two Strokes?Written by Thomas Yoon
In our previous publication, we touched on 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, piston has to move up, and then down to complete one cycle of engine. In those 2 strokes of piston, crankshaft will have turned 1 revolution. In those 2 strokes of piston engine has also completed 4 stages of combustion cycle - air intake, compression, combustion and exhaust.
Now, compare that with a 4-stroke engine. The piston moving down will complete air intake stroke. When it moves up again, piston completes compression stroke. Next comes injection of fuel. Combustion takes place. The piston is forced to move down by pressure of gases. When piston moves up again, it drives out all exhaust gases in exhaust stroke.
The piston in a 4-stroke engine will move 4 strokes in order to complete 4 stages of combustion cycle - air intake, compression, combustion and exhaust.
Lubrication - The Silent Component of MachineryWritten 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 for oil film to be formed between moving bearing components, film must be sufficiently thick even under heavy load, high temperatures or vibrations.
Some sleeve bearings that has very heavy loads, like in crosshead bearings of diesel engines, usually have provisions for injecting pressurized oil to float shafts. This method is called hydrostatic lubrication.
However, most common method of lubrication for sleeve bearings is by hydrodynamic method. When two surfaces of a bearing and shaft move rapidly relative to one another, oil is carried along shaft to fill gap between shaft and bearing. When 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, rotating shaft does not have sufficient speed to pick up lubricant. The film separating moving surfaces is very thin - with only thickness of a molecule. This is a condition called boundary-layer lubrication. With this condition, friction losses increases, producing heat, which raises temperature of lubricant, thereby reducing its viscosity so that load-carrying capacity of film is even lower. In worst case conditions, surfaces can even seize together.