11 Engine Friction And Lubrication 4g3t1b

TOPIC

12

Engine Friction and Lubrication Sections: 13.1, 13.2, 13.3, 13,4, 13.8

Background and definitions Wtf  W p  Wrf  Wa

imep g  imep n  pmep tfmep  pmep  rfmep  amep bmep  imep g  tfmep bmep  imep n  rfmep  amep

Friction fundamentals

• Friction losses are classified into two groups, depending on the type of dissipation which occurs • One type is friction between two metal surfaces in relative motion, with a lubricant in between (rubbing friction) • The other type is turbulent dissipation

Lubricated friction

Schematic of a lubricated journal and a slider bearing

Lubricated friction f   fs   1    f L

Turbulent dissipation

• Part of the total friction work is spent in pumping fluids through flow restrictions • This work is dissipated in turbulent mixing processes • Pressure difference required to pump these fluids is proportional to ρv2

Total friction

Work per cycle for each component i of the total friction

W f ,i   Ff ,i    dx Total friction work per cycle (and thus tfmep) for a given engine geometry will vary with speed

Wtf  or tfmep   C1  C2 N  C3 N 2

Measurement methods

1.

Measurement of fmep from imep

2.

Direct motoring tests

3.

Willans line

4.

Morse test

Measurement of fmep from imep • 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 data • pmep for four-stroke engines is also obtained from p-V data • Total friction mean effective pressure is

tfmep  imep g  bmep • Combined rubbing friction plus auxiliary requirements are

rfmep  amep  imep g  bmep  pmep

Direct motoring test • Engines temperatures are maintained as close to normal operating temperatures as possible • This 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 operation • The 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 line • Willans line is an approximate equivalent of the direct motoring test for diesel engines

Morse test • Individual cylinders in a multicyhlinder engine are cut out from firing, and the reduction in brake torque is determined while maintaining the same engine speed • Remaining cylinders drive the cylinder cut out

Functions of lubricating system • Reduce frictional resistance of the engine to a minimum to ensure maximum mechanical efficiency • Protect the engine against wear • Contribute to cooling the piston and regions of the engine where friction work is dissipated • Remove all injurious impurities from lubricated regions • Hold gas and oil leakage (especially in the ring region) at an acceptable minimum level

Lubricating system

Lubricant requirements • Oxidation stability • Detergency • Wear reduction • Viscousity

Lubricant requirements

Lubricant requirements

Motoring losses vs. firing losses

Motoring losses vs. firing losses