Internal Combustion Engines by John B. Heywood (Lecture Slides)
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12TOPICEngine Friction and LubricationSections: 13.1, 13.2,
13.3, 13,4, 13.8
Background and definitions
Friction losses are classified into two groups, depending on the
type of dissipation which occursOne type is friction between two
metal surfaces in relative motion, with a lubricant in between
(rubbing friction)The other type is turbulent dissipationFriction
fundamentals
Lubricated frictionSchematic of a lubricated journal and a
slider bearing
Lubricated friction
Part of the total friction work is spent in pumping fluids
through flow restrictionsThis work is dissipated in turbulent
mixing processesPressure difference required to pump these fluids
is proportional to v2Turbulent dissipation
Work per cycle for each component i of the total friction
Total friction work per cycle (and thus tfmep) for a given
engine geometry will vary with speedTotal friction
Measurement methodsMeasurement of fmep from imepDirect motoring
testsWillans lineMorse test
imepg is obtained from pdV over the compression and expansion
strokes for four-stroke engine and over whole cycle for a
two-stroke engine, and pressure and volume datapmep for four-stroke
engines is also obtained from p-V dataTotal friction mean effective
pressure is
Combined rubbing friction plus auxiliary requirements
areMeasurement of fmep from imep
Direct motoring testEngines temperatures are maintained as close
to normal operating temperatures as possibleThis can be done either
by heating the water and oil or by conducting a grab motoring test
where the engine is switched rapidly from firing to motored
operationThe power required to motor the engine equals total
friction power (including pumping power)Motoring test on a
progressively disassembled engine can be used to identify the
contribution that each major component of the engine makes to the
total friction losses
Willans lineWillans line is an approximate equivalent of the
direct motoring test for diesel engines
Morse testIndividual cylinders in a multicyhlinder engine are
cut out from firing, and the reduction in brake torque is
determined while maintaining the same engine speedRemaining
cylinders drive the cylinder cut out
Functions of lubricating systemReduce frictional resistance of
the engine to a minimum to ensure maximum mechanical
efficiencyProtect the engine against wearContribute to cooling the
piston and regions of the engine where friction work is
dissipatedRemove all injurious impurities from lubricated
regionsHold gas and oil leakage (especially in the ring region) at
an acceptable minimum level
