Valve timing Daigram
- Ideal valve timing diagram for four stroke
Cycle completed in – 2rev
Total crank angle – 2×360= 720
IVO- inlet valve open
IVC- inlet valve closed
EVO- exhaust valve open
EVC- exhaust valve closed
ACTUAL VALVE TIMING DIAGRAM
The actual valve timing diagram is differ from theoretical valve timing diagram due to–
i) No process can occour instantaneously.
i. e every process can take some time to happen
ii) due to dyanmic/inertia effect of gas.
INTAKE VALVE TIMING
1) it shows the actual quantity of air sucked during suction stroke which reflex the volumetric efficiency.
2) 10° before TDC to open ensure that valbe will be fully open and fresh charge starting to flow into the cylinder as soon as possible after TDC.
3) when piston reaches BDC and start compression stroke the inertia of fresh charge cause it to continue to move into the cylinder to take advantage of this valve is clised after BDC so that maximum charge is taken in this is called ram effect.
4) however if intake valve is remain open for too long time beyond BDC compression stroke force the charge return back into tha intake manifold.
EXHAUST VALVE TIMING DIAGRAM
1) The overall effect of opening and closing of exhaust valve also reflect the volumetric efficiency.
2) it is set to open before BDC, if it is not start untill BDC the pressure in the cylinder would be considerably higher than atmospheric pressure which increase the work required for expelled the exhaust gases.
3) By closing the exhaust valve after TDC (15° in slow speed and 20° in high speed ) the inertia of exhaust gases tend to scaveng the cylinder so that maximum fresh charge remain in the clearance volume.
there may be a period that when both intake and exhaust valve remains open at the same time this is called valve overlap.
15°-30° — slow speed
55° —- fast speed
DISADVANTAGE OF VALVE OVERLAP
this overlap should not be excessive otherwise it will allow the burnt gases sucked into the intake manifold or fresh charge to escape through the exhaust valve.