Page 155 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 139

on small two-stroke engines, such as those on motorbikes and outboard motors,
use a ‘petroil’ mixture of petrol and a specially formulated biodegradable oil.
Other types of oil should not be used.
Several international standards (described in a later section) describe the
properties of an oil, but some original equipment manufacturers (OEMs) are now
specifying their own. The safest advice on what oil to use is always to follow
manufacturers’ recommendations.
If a fi lm of lubricant is used between the surfaces, the process is called boundary
lubrication ( Fig. 2.116 ). This type of lubrication is introduced between the moving
surfaces by splash feed or an oil mist. The best example is on the side of the
pistons but all sliding components use this method.
Boundary lubrication also commonly occurs on some bearings during starting
of the engine. Pressure is generated once the engine is moving as a result of the
shape and motion of the surfaces. This is known as hydrodynamic lubrication.
For example, the rotation of the crankshaft in a bearing forces lubricant into
the wedge-shaped space between the shaft and the bearing ( Fig. 2.117 ). The
clearance between the bearing and the shaft, the load on the shaft, the speed of
rotation and the viscosity of the lubricant have a marked effect on this process.

Figure 2.116 Boundary lubrication uses the oil to prevent two rough surfaces from touching

Figure 2.117 Con rod big end lubrication: 1, bearing shell; 2, load; 3, shaft; 4, lubrication
pressure; 5, oil supply