Engine Compression Theory. Four stroke cycle (gasoline engine) u Intake stroke –Piston going down...
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Transcript of Engine Compression Theory. Four stroke cycle (gasoline engine) u Intake stroke –Piston going down...
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
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
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
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