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Four Stroke Cycle• Intake stroke
– Piston going down - intake valve open.– Low pressure created in cylinder.– Volumetric Efficiency maximum at w.o.t.
• Compression stroke– Piston going up - both valves closed.– Mixture squeezed to increase combustibility, molecules, raises
temp.– Needs good seal to compress and combust.
• Power stroke– Piston going down - both valves closed.– Work is performed by expanding gasses.
• 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
Cylinder Sealing
• Performed by tight seal of valve face against seat to seal in compression pressures
• Most common loss of compression is valve not sealing properly
Volumetric Efficiency
• Three factors affect VE: – How far the valve opens (lift) – The amount of time that it is open (duration– Size of opening
• A problem with valve opening will affect volumetric efficiency or the quantity of incoming fuel mixture
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 most common, cost effective design
• Hemi style very efficient air flow, expensive
• Porting & polishing increases air flow, less restriction & reduced turbulence in ports
Carbon Buildup• Carbon build up
on intake valve– Worn valve
guide/seal (oil)– Fuel deposits– Affects V.E.
• Flat cams– Valve doesn’t
open fully
Vacuum gauge testing
• Provides quick test for compression problems
• Should read 15-20” of manifold vacuum @ idle
• Cranking vacuum will be 3-5”
Sticking Valve
• Irregular needle drop between 1 & 3 in Hg.
• Multiple cylinder misfire – rich or lean mixture
Cranking Compression Test
• Used for checking engine seal– Must set up conditions
• W.O.T., bat. charger, disable ignition, all plugs out
– Compare readings - 10% max difference– Typical range: 110 - 150 psi
• Wet test is performed when cranking compression is low– Add SMALL amount of oil to cylinder, helps
seal worn rings
Leak Down Test• Used to identify location of sealing problem
• Cylinder must be at TDC, comp. stroke
• Problem is detected if loss is greater than 20%
Identifying Location of Leak• Air escaping through exhaust, open valve
• Air escaping through intake, open valve
• Air escaping through valve cover, worn piston rings
• Air escaping through radiator, blown head gasket
Running Compression Test
• Test cylinder seal at idle
• Readings will be low, less time to fill cylinder
• Compare readings - no specs
• Identifies compression loss apparent while running– Broken valve springs – Worn valve guided– Burned valve
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
Improper Valve Lash
• Too little will cause sealing problems
• Too much will cause excessive noise, wear and loss of lift
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