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Valve timing diagram of four-stroke engine

Short answer

Valve timing is the setting of crank angles for opening and closing inlet and exhaust valves relative to dead centres. Valves open before TDC/BDC and close after to utilise gas inertia for better filling and blowdown for scavenging. Valve overlap is the period near TDC when both valves are open simultaneously, helping clear exhaust and cool the valves.

What is meant by valve timing? Draw a typical valve timing diagram for a four-stroke petrol engine and explain why the valves open before the dead centres and close after them. What is valve overlap?

Valve timing is the setting of the crank angles at which the inlet and exhaust valves open and close with respect to the dead centres. In theory the valves would open and close exactly at TDC and BDC, but this does not work in a real engine.

Typical diagram (petrol engine, crank angles)

           TDC (end of exhaust, start of suction)
              EVC 15 | IVO 10
         . - ' ' -  -|-  - ' ' - .
      /     overlap 25 deg         \
    |       exhaust  |  suction     |
    |                |              |
      \   EVO 40     |      IVC 35 /
         ' - . _ _ _|_ _ _ . - '
           BDC (EVO before, IVC after)

Angles are in crank degrees: EVC after TDC, IVO before TDC, EVO before BDC, IVC after BDC.

EventTypical angle
Inlet valve opens (IVO)10-20 degrees before TDC
Inlet valve closes (IVC)30-50 degrees after BDC
Spark20-35 degrees before TDC
Exhaust valve opens (EVO)30-50 degrees before BDC
Exhaust valve closes (EVC)10-20 degrees after TDC

(For diesel engines the figures are similar, with injection at about 15-25 degrees before TDC.)

Reasons

  • EVO before BDC: the gas pressure is still high, so the valve is opened early to let most gas escape (blowdown) and reduce the back pressure on the piston in the exhaust stroke. The small loss in expansion work is less than the gain.
  • EVC after TDC: the moving gas column has inertia, so the cylinder is scavenged more fully.
  • IVO before TDC: so that the valve is fully open when the suction stroke starts. This gives less throttling.
  • IVC after BDC: the fresh charge keeps moving in due to its inertia (ram effect), so cylinder filling is better and the volumetric efficiency is higher.

Valve overlap

The period during which both valves are open together near TDC at the end of exhaust and start of suction (here 25 degrees). It helps scavenging of exhaust gas and cools the valves, but too much overlap lets fresh mixture escape at low speed.

✓ Answer checked by Claude Opus 5.5 (high effort)

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