Mechanical Friction and Lubrication

Internal Combustion Engines

Mechanical Friction and Lubrication

Points of contact resist motion (friction) and become hot, sometimes to the point of welding

Characteristics of lubricants Adhering to solid surfaces

Resisting being squeezed out

Low shear force

Hydraulic floating of shafts

δ ~ 2 μm

Total Engine Friction Friction:

Friction mep:

Empirical relation

A: Boundary friction in case of insufficient lubricants

Metal-to-metal contact between the piston and cylinder

B: Hydraulic shear

C: Loss from turbulent dissipation

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Total Engine Friction

Total friction: fmep ~ 10% nmep at WOT fmep ~ 100% nmep at idle

Lower percent friction loss for turbocharged engines

Most power loss due to friction ends up heating engine oils and coolants

Total friction obtained from integration of an indicator diagram and measuring brake power with a dynamo

Friction in Engine Components

Friction in engine components measured with motored engines (motoring method)

Piston assembly: ~50%

Valve train: 25%

Crankshaft bearing: 10%

Engine-drive accessories: 15%

75% at light loads

Maximum force near TDC, BDC: Oils squeezed out

Higher friction during the expansion stroke

Piston rings: 20% Oil rings distribute oils Thin compression rings

Ex. 11-1, 2

Friction in Engine Accessories

Fuel and water pumps

Old pumps were driven mechanically off the crankshaft

Modern engines have electric pumps, whose power comes

from the alternator in turn driven off the crankshaft

Cooling fans

Some fans are powered by direct mechanical linkage to the

crankshaft

Fans speed up at high speeds where cooling is not needed

due to high incoming air velocity

Most fans are electrically driven and turned on/off with a

thermal switch

Forces on Pistons

Force balance in the X direction

Force balance in the Y direction

Side thrust force

It varies with the crank angle

Major: Power and intake strokes

Minor: Exhaust and compression strokes

Variations in wear on the cylinder wall

Additional wear in the other rotation planes

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Friction Reduction

Less masses and shorter skirts in pistons to reduce friction Less masses reduce the acceleration term

Shorter skirts reduce the friction

The wrist pin sometimes offset by 1-2 mm

towards the minor thrust side

Shorter strokes to reduce friction → However, larger bores increase heat loss

Piston side thrust forces eliminated using a crosshead in large CI engines

Engine Lubrication Systems

Splash system: Crankcase used as an oil sump The rotating crankshaft distributing oil

All moving components, including valve trains and camshaft, open to the crankcase

Pressurized oil distribution system: Oil pump Oil sprayed under pressure

Dry sump systems in most aircraft engines

Electric or mechanically driven at 300-400 kPa

Dual distribution systems for most automobiles and large stationary engines

Excess wear at engine start-up (high viscosity)

Turbocharged engines better to idle before turn-off because of high speeds

Two-stroke Cycle Engines

Crankcase used as a compressor → Oil carried with the inlet air just like fuel

Fuel-to-oil ratio: 30:1 – 400:1

Oil is premixed with fuel or fed into the fuel supply line or into the inlet air flow

Deposits formed on the combustion chamber and valves with too much oil

Excess wear with too little oil

Oil contributes to HC emissions

Some engines with external superchargers use pressurized/splash lubrication systems

Lubricating Oil

Requirements: Lubrication, coolant, remover of contaminants, enhancer of ring seal, reducer of blowby, slow corrosion, stability over a large temperature, long life span, low cost

Crude oil base with additives, e.g. antifoam agent, oxidation inhibitor, antiwear agent, friction reducer

Oil rated with viscosity (SAE grade) Low viscosity good in cold weather

High viscosity good in high temp

Multigrade oils: See figure

Lubricating oil standards (API)

Synthetic oils

Oil Filters

To remove impurities from the engine oil

Impurities from the incoming air (dust) and fuel (sulfur), non-

ideal combustion (soot)

Compromise in filter pore sizes: Better filtration but

higher flow resistance with smaller pores

Impurities fill filter pores with usage → The filter

cartridge should be replaced

Oil circulation systems

Full-flow oil filtration: Large filter pore size

Bypass oil filtration: Finer filter possible

Combination: Full-flow + bypass

Shunt filtration: Full-flow filter and a bypass valve

Crankcase Explosion

Remote chance of explosion in the crankcase

Oxygen (air) and oil vapor and/or fuel vapor from blowby

Ignition source: Hot spot, flame past the piston, spark from a

broken component

Engines with crankcase volume less than 6.1 m3 or bore

less than 0.2 m are considered to be safe for crankcase

explosion

Larger engines require an explosion relief valve to

release the pressure buildup over 5-20 kPa