Valve overlap: Definition, Diagram, Purpose, Calculate

Valve overlap occurs due to the overlapping of the intake valve duration and exhaust valve duration. Therefore for some period both the intake and exhaust valves remain open at the same time which is known as valve overlap. Thus in this article, we are discussing the valve overlap in detail.

Contents

What is Valve overlap?

Valve overlap is the degrees of crankshaft rotation during which the intake valve and exhaust valve remain open at the same time. The valve overlap occurs a few degrees before the end of the exhaust stroke or a few degrees after the start of the suction stroke.

The below figure shows the actual valve timing diagram for a 4-stroke IC engine which indicates the opening and closing of intake and exhaust valves relative to the crankshaft angle.

The above figure shows that the intake valve opens a few degrees before the piston reaches at TDC during the exhaust stroke and the exhaust valve closes a few degrees after the piston starts to move in a downward direction during the suction stroke.

Thus during this period from the opening of the intake valve to the closing of the exhaust valve, both valve remains open at the same time which is known as Valve overlap.

Valve overlap is measured in terms of an angle turned by a crankshaft from the opening of the intake valve to the closing of the exhaust valve.

Purpose:

The valve overlap in the IC engine is used for the following purpose:-

1] It promotes scavenging:-
The piston does not move above the TDC of the cylinder, thus the piston itself cant push out the exhaust gases present in the region of clearance volume.

Due to the flow of exhaust gases, a small low-pressure region is created over a piston head. During the valve overlap, the fresh intake charge fills this low-pressure region and helps to expel the exhaust gases completely.

2] Avoid dilution of charge:-
Without valve overlap, some portion of exhaust gases remains in the cylinder. During the suction stroke, these gases mix with the intake charge and dilute the fresh mixture.

As the valve overlap helps to expel out the maximum amount of exhaust gases thus it helps to avoid the dilution of the air-fuel mixture.

3] Cooling of exhaust valve:- The valve overlap also helps to cool the exhaust valve.

What determines the amount of valve overlap?

The amount of valve overlap depends on the following factors:-

1] Speed of engine:-
The engines running at higher speeds (e.g., Sports cars) require a larger angle of valve overlap so that the engine will get enough time for scavenging at a higher speed.

While the engine running at lower rpm (e.g., street cars) can get enough time for scavenging at a lower valve overlap angle.

2] Number of valves present in cylinder head:-
As the 4-valve engine has more intake and exhaust flow than the 2-valve engine, thus the 4-valve engine requires less valve overlap than the 2-valve engine.

3] Loss of intake mixture:-
The excessive valve overlap causes loss of intake mixture through the exhaust valve. Therefore the maximum valve overlap should be optimized to avoid the loss of fresh intake mixture.

Formula:

If the crankshaft angle during the opening of the intake valve and closing of the exhaust valve is known then in this case the valve overlap is given by,

= Angle turned by a crankshaft from IVO to EVC

Valve overlap = [Angle between IVO & TDC] + [Angle between EVC & TDC]

Where,
IVO = Intake valve open
EVC = Exhaust valve closes

If the intake duration and exhaust duration (in terms of crankshaft angle) and the lobe separation angle (discussed below) are known, then the valve overlap is given by,

Valve overlap = \mathbf{\frac{\text{Intake duration + Exhaust duration}}{2}} – 2(Lobe separation angle)

How to calculate valve overlap?

It can be calculated by using the following two methods:-

1] From valve timing diagram:-

For the above valve timing diagram, the valve overlap is measured as follows,

Valve overlap = [Angle between IVO & TDC] + [Angle between EVC & TDC]

Example:- For the particular engine, the intake and exhaust valve operates as follows:-
Intake valve open (IVO):- 24° before TDC
Intake valve closes (IVC):- 50° after BDC
Exhaust valve opens (EVO):- 60° before BDC
Exhaust valve closes (EVC):- 20° after TDC

For the above information, the valve timing diagram is as follows:-

Thus the valve overlap is given by,

= [Angle between IVO & TDC] + [Angle between EVC & TDC]

= 24° + 20°

Valve overlap = 44°

2] From lobe separation angle, Intake & Exhaust duration:-

The below figure shows the position of the cam lobes of intake and exhaust valves.

The lobe separation angle [LSA] is the angle between the centerline of the intake valve lobe and the centerline of the exhaust valve lobe.

In this case, the valve overlap is given by,

Valve overlap = \frac{\text{Intake duration + Exhaust duration}}{2} – 2[LSA]

Where,
Intake duration = Angle turned by the crankshaft during the period of the intake valve open
Exhaust duration = Angle turned by the crankshaft during the period of the exhaust valve open

Example:-
Consider an engine with the following specifications
Intake Duration:- 254°, Exhaust Duration:- 260°, Cam lobe separation angle [LSA]:- 110°

Thus, in this case, the valve overlap is given by,

= \frac{\text{Intake duration + Exhaust duration}}{2} – 2[LSA]

= \frac{254^{\circ}+260^{\circ}}{2} – 2(110°)

Valve overlap = 37°

1. It promotes scavenging.
2. It helps to increase the intake of fresh charge at high engine speed which increases the high-end power of the vehicle.
3. It helps in cooling of the exhaust valves.

1. At lower speed, the engine creates less vacuum in the intake manifold, which affects the power brake system, Therefore the brake pedal feels harder.
2. The valve overlap is not used for the engine running at low rpm because of poor low-end power.
3. The excess valve overlap causes loss of fresh intake charge through the exhaust valve.

FAQs:

1. What is the reason for valve overlap to occur?

It occurs because of the early opening of the intake valve before the piston reaches TDC and the late closing of the exhaust valve after the piston reaches TDC.

2. When does valve overlap occur?

It occurs at the end of the exhaust stroke or during the initial period of the intake stroke.

3. Why is valve overlap important for an engine?

It helps to increase the high-end power output of the engine by feeding more intake charge into the engine.

4. What does valve overlap promote?

It promotes the scavenging process, which helps to increase the power output of the engine.