Engine Compression Theory

Four stroke cycle(gasoline engine)

Intake stroke– Piston going down - intake valve open.– Low pressure area is created in cylinder.– This stroke is where volumetric efficiency

applies - maximum at w.o.t.

Four stroke cycle

Compression stroke– Piston going up - both valves closed.– Mixture squeezed to increase

combustibility by keeping molecules tight and by raising temp.

– Needs good seal to compress and combust.

Four stroke cycle

Power stroke– Piston going down - both valves closed.– Stroke where work is created by expanding

gasses.– Power used during only first 25% of stroke.– Only 1/3 of energy is used on piston.

Four stroke cycle

Exhaust stroke– Piston going up - exhaust valve open.– Exhaust gasses exit.– Any exhaust left over dilutes incoming a/f

mixture with inert gas (exhaust) Causes poor quality and quantity of charge.

Valves

Valves serve two purposes: Sealing - during compression and

power strokes Opening up of ports - during intake and

exhaust strokes

Sealing of cylinder

Performed by tight seal of valve face against seat to seal in compression pressures

Most common loss of compression is valve not sealing properly

Typical valve failures

Valve face and or seat worn due to excessive mileage

Valve burnt due to lack of contact– Valve adjustments necessary – Foreign material on seat

Valve not closing due to valve spring failure

Valve bent due to piston contact

Opening of cylinder

Controlled directly or indirectly by camshaft

Three factors that are related to the opening of the valves are: – Size of the opening– How far the valve opens (lift) – The amount of time that it is open

(duration)

A problem with valve opening will affect either volumetric

efficiency or the quantity of the incoming fuel mixture

Factors related to SIZE of opening

Big valves and ports

Good for high rpm Will experience loss of ram effect at low

RPMs (related to momentum)

Multiple valves

Multiple valve heads will act same as big valve heads

Secondary throttle plates can be used to eliminate loss of ram effect

Lift

More lift = loss of ram (momentum) Lots good for high rpm

Head design

Wedge style still most common and not as effective in allowing air fuel mixture to flow

Hemi design very efficient Porting & polishing increases air flow by

creating less restriction and reducing turbulence in the ports

Factors related to TIME of opening

Duration - the amount of time valve is open– Long good for high rpm– Short good for low rpm– Need more time at high rpm

Typical failures affecting opening of valves

Carbon build up on intake valve stems– Caused by valve guide, seal or stem wear

(blue smoke)– Affects volumetric efficiency of cylinder

Flat cams– Intake lobe = loss of v.e.– Exhaust lobe = air/fuel mixture dilution and

backfire but no loss of v.e

Testing

Vacuum gauge testing

Provides quick test for compression problems

Should read 15-20” of manifold vacuum @ idle

Cranking vacuum will be 3-5”

Vacuum test results

Dramatically pulsing needle @ idle or while cranking usually indicates compression problems

Extremely low vacuum can indicate valve timing problems

Cranking compression test

Used for checking engine seal– Must set up conditions

W.O.T., bat. charger, disable ign., all plugs out

– Compare readings - 10% max difference– Typical range: 110 - 150 psi

Wet test is performed when cranking compression is low– Adding SMALL amount of oil to cylinder will

help seal worn rings

Leak down test

Used to identify location of sealing problem

Cylinder must be at TDC compression Problem is detected if loss is greater

than 20% - leak location needs to be identified

Identifying location of leak

Air escaping through exhaust indicates exhaust valve sealing problems

Must loosen valve adjuster

Air escaping through intake indicates intake valve sealing problems

Must loosen valve adjuster

Air escaping through valve cover indicates worn piston problems

Air escaping through radiator indicates blown into coolant jacket

Running compression test

Test cylinder seal at idle All readings will be low because there is

less time to fill cylinder Compare readings - no specs Will identify compression losses only

apparent while running– Broken valve springs – Worn valve guides

Snap compression test

Tests for problems with v.e. Allows rush of air at relatively low rpm

nearly filling cylinder which will cause increased compression pressures– If a cylinder has a restricted intake snap

pressures will be lower Compare readings - no specs

Valve clearance

Too little will cause sealing problems Too much will cause excessive noise,

wear and loss of lift

Hydraulic tappets

Zero lash is obtained with hydraulic lifters

No periodic adjustments Can fail by pumping up or collapsing

Solid tappets

Periodic adjustment necessary to maintain proper lash– Compensates for seat recession in head– Compensate for valve train component wear

Lash will be checked will feeler gauge– May have cold or hot specs

Can be adjusted by turning various adjustment screws or by replacing shims

Valve timing control

Advanced camshafts can increase low end performance but will sacrifice high rpm horsepower

Retarded camshafts will increase high end power but will lose low end torque

As timing belts or chains wear, cam timing will retard

Variable valve timing

Many variations but all are to get benefits of advanced and retarded timing

Some will vary overall timing Some will vary only one valve

Timing chains

Can use tensioner to maintain minimum slack

Most common with cam in block (OHV) designs

Timing gears

Some engines use gear to gear Most gears are helical cut Usually stronger than chain or belt

Timing belts

Commonly fail by breaking or stretching enough to allow it to jump a tooth or two

Most have service intervals of 60K -90K miles

All will have tensioners to maintain tension of belt– Tension can be maintained with spring

pressure, oil pressure or internally maintained by different devices

Timing belts

Many manufactures recommend never using a belt twice

If a belt is reused be sure it is installed in same direction of rotation

Belts must be replaced if oil or antifreeze soaked

Testing for valve timing problems

Low vacuum and low compression are typical of valve timing problems

Back lash can be measured by watching rotor for movement while turning crank by hand

Erratic timing as seen with a timing light can indicate excessive slack

Belts are often tested for tension by twisting slack side of belt - ½ twist is considered acceptable