Valve timing diagram of four stroke diesel engine

1. Valve timing diagram of four
stroke diesel Engine
Rampal Singh
B.E. (Mechanical), M.Tech (Production Engineering),
Thrice GATE Qualified

5. Valve timing diagram
• Open of inlet valve is done few degrees before TDC, If it start opening
exactly at TDC, so some will be taken by valve to stand off from valve
seat, hence less mass will enter into cylinder. To compensate this inlet
valve is opened bit earlier
• It is also necessary for better evaporation and mixing of fuel, as there
is lag between ignition and supply of fuel

6. Speed of piston
is higher than
speed of air
entering
Less mass air
will enter
This air will
expand causing
pressure lesser
than
atmospheric
pressure
Hence exhaust
valve is kept
open even few
degrees after
BDC
LOGIC OF KEEP OPENING INLET VALVE AFTER BDC

7. Exhaust valve
• First requirement to open exhaust valve after BDC so that there could
be maximum expansion and maximum work could be obtained
• Second requirement to minimize the work done by piston in expelling
hot flue gases
• Trade off between work lost by combustion gas to escape before it is
fully expanded and work required by piston to expel it