1. VALVE TIMING DIAGRAM OF FOUR STROKE ENGINES
IKRAM SINGH HARIKA
B TECH- 6MAE-1Y A

2. VALVE TIMING
Valve timing is the regulation of points in the cycle, at which the valves are set to open and close.
Since the valves require a finite period of time for opening and closing therefore a ‘slight lead time’ is necessary for proper opening and closing of valves.
The design of the valve operating cam provides for the smooth transition from one position to another while the cam setting determines the timing of the valve.

3. INTAKE VALVE OPENING
Theoretically the intake valve should open at TDC but almost all spark ignition engines employ an intake valve opening of a few degrees before TDC on the exhaust stroke.
This is done to ensure that the valve is fully open and the fresh charge starts to flow into the cylinder as soon as the piston reaches TDC.

4. INTAKE VALVE CLOSING FOR LOW SPEED ENGINES
When the piston reaches BDC and starts to ascend on the compression stroke, the inertia of the incoming fresh charge tends to cause it to continue to move into the cylinder.
At low engine speeds the inertia of the charge is relatively low. If the intake were to remain open much beyond BDC, the up moving piston would force the charge already in the cylinder back in the intake manifold causing a significant reduction in volumetric efficiency. Hence the intake valve is closed relatively early after BDC for a slow speed engine.
In low speed engines the intake valve closes 60 ͦafter BDC.

5. INTAKE VALVE CLOSING FOR HIGH SPEED ENGINES
High speed engines bring in the charge through the inlet manifold at great speeds and the charge has great inertia.
As the piston moves up, the incoming fresh charge produces a ‘ram’ effect, which tends to pack more charge into the cylinder.
Thus in high speed engines the intake valve timing is delayed for a greater period of time after BDC in order to take advantage of this ‘ram’ effect and to induct maximum quantity of charge.

6. EXHAUST VALVE OPENING
The exhaust valve is set to open 25 ͦ before BDC in low speed engines and 55 ͦ before BDC in high speed engines.
If the exhaust valve did not open until BDC, the pressure in the cylinder would be considerably above atmospheric pressure increasing the work required to expel the exhaust gases.
Opening the exhaust valve earlier than BDC reduces the pressure which results in overall gain in output.

7. EXHAUST VALVE CLOSING
During the exhaust stroke, the piston forces the burnt gases out at a high velocity. If the closing of the exhaust valve is delayed beyond TDC, the inertia of the exhaust gases tends to scavenge the cylinder better by carrying out, a greater mass of gases left in the clearance volume, which results in increased volumetric efficiency.

8. VALVE OVERLAP
Is should be noted that it is quite possible for the intake and the exhaust valve to remain open or partially open at the same time. This is known as valve overlap.
This overlap should not be excessive enough to allow the burnt gases to be sucked back into the cylinder, or the fresh charge to escape through the exhaust valve.

9. VALVE TIMING DIAGRAM OF A LOW SPEED 4 STROKE ENGINE

10. VALVE TIMING DIAGRAM OF A HIGH SPEED 4 STROKE ENGINE

11. POSITION
THEORETICAL
LOW SPEED ENGINE
HIGH SPEED ENGINE
INLET VALVE OPENS
TDC
10 ͦ before BDC
10 ͦ before TDC
INLET VALVE CLOSES
BDC
10 ͦafter BDC
60 ͦ after BDC
INLET VALVE IS OPEN FOR
180 ͦ
200 ͦ
250 ͦ
EXHAUST VALVE OPENS AT
25 ͦ before BDC
55 ͦ before BDC
EXHAUST VALVE CLOSES AT
5 ͦ after TDC
20 ͦ after TDC
EXHAUST VALVE IS OPEN FOR
210 ͦ
255 ͦ
VALVE OVERLAP
NIL
15 ͦ
30 ͦ
SPARK
15 ͦ before TDC
30 ͦ before TDC