Knowing what the valve overlap is, the next question is: what is the function of valve overlap?
The purpose of valve overlap is to allow a scavenging system to take place and cool the exhaust valve.
There was a time when I noticed that my diesel generator exhaust valve was often damaged; when I troubleshooted the engine, I discovered that the valve adjustment was poor, so there was no valve overlap.
I readjusted the valve, and the engine was running fine.
Click here to read another article on the scavenging system.
Keep reading below for more details.
What Are The 2 Purpose Of Valve Overlap In An Engine
Valve overlap is a household name in internal combustion engines. It plays a crucial role for the engine to keep running. Below, I will mention 2 main purposes of valve overlap.
1.To cool the exhaust valve: During the exhaust stroke, the exhaust valve suffers heat due to the exhaust gas going out of the engine.
Therefore, if you compare the intake and exhaust valves, you will notice that the exhaust valve looked burnt more than the intake valve.
So, during the valve overlap, fresh air comes to the engine to cool the exhaust valve so that it will not be damaged due to too much heat from the exhaust heat.
The intake and exhaust valves stay slightly open during the valve overlap to allow for the entry of freshly charged air into the engine.
2. The second purpose of valve overlap is to ensure that the scavenging system takes place in the engine.
During the valve overlap, the intake valve allows air to enter the combustion chamber to push out the remaining leftover spent exhaust gases found in the combustion chamber.
The exhaust valve opens during the exhaust stroke, giving the engine’s exhaust gases a way to escape.
However, some leftover exhaust gases remain in the combustion chamber; for the remaining leftover exhaust gases to exit the combustion chamber, fresh air from the intake valve is needed to clean up the combustion chamber.
The whole process is called the scavenging system. The scavenging system is possible because of the valve overlap, which ensures that the intake and exhaust valves remain slightly open simultaneously at the end of the exhaust stroke.
These are the 2 main purposes of valve overlap.
Let us consider some frequently asked questions regarding the valve overlap.
At What Stroke Does Valve Overlap Occur
After the exhaust stroke, valve overlap happens.
As soon as the exhaust stroke ends before the intake stroke begins, valve overlap takes place to clear the remaining exhaust gases found in the combustion chamber and prepare for the next combustion cycle.
In 4 stroke engine, there are four sequences:
i. The intake stroke
ii. The combustion stroke
iii. The power stroke
iv. The exhaust stroke.
After the last stroke, the exhaust stroke, the valve overlap takes place; during the exhaust stroke, the valve opens to expel the waste or the spent gas from the combustion chamber.
It is the last stroke in every internal combustion engine, whether a two or 4-stroke engine.
What Stroke Does Valve Overlap Help
The valve overlap helps the exhaust stroke. How? As mentioned above, the valve overlap happens at the end of the exhaust stroke. Therefore, the exhaust gas is on its way down to the exhaust tailpipe.
An intake valve opening provides a pull and siphon so that a fresh mixture of air and fuel can come into the combustion chamber.
Therefore, valve overlap helps the exhaust stroke to finish its job.
However, let us look at this matter from another angle too. The valve overlap also helps the intake stroke.
It helps the intake stroke by mobbing the combustion chamber so that there will no longer be exhaust gas in the combustion chamber before the next cycle begins, which is the intake stroke.
Although primarily, the valve overlap helps the exhaust stroke, from the above analysis, it also helps the intake stroke.
What Happens During The Valve Overlap Process
During the valve overlap, both the intake and exhaust valves are slightly open. That means practically the cylinder’s pushrod cannot be rotated by your fingers.
Valve overlap angle usually refers to the period during which the engine intake valve and exhaust valve are open simultaneously.
On an ordinary engine, the opening and closing time of the intake and exhaust valves are fixed, and the valve overlap angle is also fixed. It is the optimal valve timing obtained based on experiments.
Since the intake valve opens early and the exhaust valve closes late, both valves will open simultaneously. The crankshaft angle at which the two valves open simultaneously is called the valve overlap angle. During this period, will the combustible mixture and exhaust gas flow randomly? No, this is because a. The intake and exhaust flows have flow direction and inertia, and the overlap time is very short, so there will be clarity.
What Causes Valve Overlap
Valve overlap is caused by valve timing. The quality of engine performance, efficiency, power, torque, and operating economy depends on many factors, including valve timing, that is, the opening and closing of the intake and exhaust valves.
The camshaft drives the valves of a typical four-stroke internal combustion engine.
The cam profile determines the torque, time of opening (i.e., phase width), and valve stroke.
When the intake and exhaust valves are open, for instance, narrow valve timing phases with late opening and early closing of the valves without overlapping phases are suitable for idle operation.
Put another way, widen the phases by opening the valves early and longer. In addition, for improved cylinder drainage, the overlap phase often widens with increasing revolution counts.
What Is The Valve Overlap At Tdc
It is a process that takes place between the exhaust stroke and the intake stroke. For a brief period, the intake and outlet valves are simultaneously opened.
The valve overlap can result in some advantages.
On the one hand, better filling is achieved:
Expelling the exhaust gases creates a negative pressure, allowing fresh gases to flow better when the inlet valve is opened.
However, there is a way to cut emissions: internal exhaust gas recirculation is produced when certain exhaust gases return to the combustion chamber. That may lessen the amount of nitrogen oxides released.
In addition, the exhaust valves are cooled :
That happens through a kind of flushing process that takes place by opening all the valves.
What Happens If The Valve Overlap Is Too Much High
When the valve overlap is too much, it will cause the loss of the intake mixture of air/fuel through the exhaust valve. Therefore, the degree of the valve overlap is important to prevent the wastage of air/ fuel mixture.
Fuel efficiency suffers when the valve overlap is too much because the engine needs more fuel to produce the power.
Fuel efficiency can be defined as the amount an automobile will transform fuel energy into kinetic energy for motion.
Put differently, fuel economy indicates the distance your vehicle can cover on a given gas tank.
Therefore, fuel efficiency is an integral part of the car working system.
Better fuel-efficient cars typically use less fuel to do the same activity.
Therefore, fuel waste is reduced. Choosing a fuel-efficient vehicle can bring many benefits: fuel savings, reducing carbon footprint, reducing our dependence on oil, etc.
How Does Valve Overlap Affect Engine Performance
Valve overlap affects the engine efficiency and power. Optimizing valve timing is a crucial aspect of engine performance and efficiency.
It involves adjusting the timing of the intake and exhaust valves to maximize power production and fuel efficiency.
Engineers can ensure the engine operates at its peak performance in different RPM ranges by optimizing valve timing.
That can result in improved torque, horsepower, and overall engine efficiency.
Validity timing is optimized through careful analysis and adjustment of the camshaft and valvetrain components.
It is a complex process that requires a deep understanding of engine dynamics and presentation characteristics.
What Is The Advantage Of Having A Large Overlap Period
In this subheading, let’s discuss the advantages of having a large overlap period.
Having a long overlap period allows sufficient time for scavenging to take place. That is commonly found in racing cars or performance cars.
Large overlap periods also prevent mixture dilution. If the valve overlap valve is longer, it can allow mixture dilution, which means insufficient time for scavenging.
Although the long period of valve overlap has disadvantages, it also has a lot of advantages, as discussed above.
In summary, it prevents mixture dilution and allows sufficient time for scavenging.
Conclusion
Valve overlap has two major functions: to cool the exhaust valve and to ensure that scavenging takes place in the combustion chamber.
It would help if you also remembered the advantages of valve overlap and the disadvantages of valve overlap.
Valve overlap also affects engine performance and is vital to fuel efficiency. During the valve overlap, the intake and exhaust valves simultaneously open to clean up the combustion chamber for the next cycle.
Valve overlap is caused by valve timing.
Click here to read another article on engine performance cars.