Three And Four Valves Per Cylinder: Advantages Over Two Valves Per Cylinder Explained

Very often, you’ll see a product’s promotional material boast of the fact that the two-wheeler’s engine has four valves per cylinder. Many of us often wonder whether it is really an advantage to have more than two valves per cylinder. Do more valves really help the performance of the engine or is it just a gimmick like many other so-called technologies that some manufacturers flaunt, which don’t have much substance? Well, the simple answer to the question is – more valves per cylinder do help engine performance. In this article, we will discuss how three and four valve engines are better than their two valve counterparts, their working and the respective advantages.

What’s the working principle behind a multi valve engine?

A four stroke internal combustion engine requires the air-fuel mixture inside the combustion chamber to burn well to produce a good amount of power and deliver great fuel efficiency. The valves allow the air-fuel mixture to enter the combustion chamber. Naturally, then, if the intake valves allow for better flow of air into the combustion chamber, the engine would be able to breathe better, allowing for a more efficient burning of fuel.

To explain this mathematically, picture the intake valve on a two valve engine as a single, large circle, and the intake valves on a three or four cylinder engine as two smaller circles. Now, the amount of air that these valves can let into the combustion chamber is mathematically represented by the wall area of the imaginary cylinder that is formed when the valves are pushed inside. If the travel of the valves in all cases is the same, say X, we need to do the following math.

Assuming that the radius of the large single valve is 30mm, and that of the two smaller valves is 15mm each, let’s see the difference in the air inflow

Formula for the surface area of the imaginary cylinder through which the air will flow – 2*pi*R*X, where:

pi is the ratio of a circle’s circumference to its diameter, or 22/7 or 3.14159

R is the radius of the circle, or the valve, in this case

X is the travel of the valve

Air flow area for two valve per cylinder layout: 2pi30X = 60piX

Air flow area for three or four valves per cylinder layout:  2pi15X + 2pi15X = 90piX

As you can see, by increasing the number of valves, the air intake capacity of the engine went up by 50% (60 Vs 90). So mathematically, breaking a large valve into two smaller valves of half its radius would result in 50% more airflow into the engine.

Now, of the inlet and exhaust valves, the priority is always inlet valves, as it’s always more difficult to let more air into the combustion chamber than pushing it out. Adding multiple smaller valves in place of a single large valve can be costly. Thus, in some cases, manufacturers use two valves for inlet, but only one for exhaust to save on cost. This keeps the engine design relatively simpler, and enhances the engine performance to an extent, without pushing the cost up too much. This type of valve design is called a three-valve per cylinder configuration and does a good job of balancing the performance to cost ratio.

The TVS nTorq 125’s engine, for example, features a three-valves per cylinder configuration, offering best in class power and performance, without being more expensive than its competitors.

Other advantages of multi-valve engines in addition to enhanced air flow

While enhanced engine breathing owing to better air flow is one of the biggest advantages of more valves per cylinder, it’s not the only one. Multiple valves per cylinder offer a few other advantages too, which further help enhance an engine’s performance.

Since the individual mass of multiple smaller valves is lesser than the individual mass of a larger single valve, the lighter, smaller valves move more freely and at a higher frequency, allowing the engine to run at a higher rpm. This advantages results in a better specific output for the engine. So for any given cubic capacity, a well-engineered 4-valve engine should be able to rev higher, and thus produce more power.

Being lighter, the smaller valves also put lesser pressure on the camshaft lobes which control their lift, and if well-engineered, can enhance the durability of the engine.

Unlike a two-valve engine, where there is only one intake valve, a three or four-valve setup allows manufacturers to design the engine in such a way that each intake valve opens at a slightly different time. The technique is also known as variable valve timing. This technique creates a swirling, or turbulent effect inside the combustion chamber, which allows for a more fine grained control over the mixing of air at various engine speeds and allows for better performance.

Another important performance aspect associated with the four valve geometry is the flexibility to position the spark plug on the cylinder head. Position of the spark plug on the cylinder head is a very important for optimum flame propagation. Now, with a two-valve layout, the central area of the cylinder head doesn’t have much space for a spark plug. But with the four valves setup, there is sufficient space to place the spark plug in dead centre of the cylinder head, allowing for better spark propagation and more efficient combustion. More valves also provide additional cooling to the cylinder head, which allows the engine to perform without heating up for longer durations.

The advantages of a two-valve per cylinder setup

Now, as you have seen, the advantages of three or four valve cylinder over their two-valve counterparts are many. However, it’s not that there are no downsides. A two-valve cylinder has its own set of advantages as well, and that’s exactly the reason why there still are so many mainstream two-wheelers which have two valves per cylinder. Here are a few of the advantages a two-valve configuration has, which makes them relevant even today.

A two-valve per cylinder layout is simple and cost effective. A four-valve system may be more capable, but it’s more complex and more difficult to design and produce. To get a four-valve layout right from a metallurgy and thermodynamics perspective is way more difficult than a two-valve setup. The complexity associated with a four-valve system also leads to escalation in costs. Thus, four-valve engines are considerably more expensive and are not always the most suitable option for a price-sensitive market like ours.

Finally, in some cases, the design of a two valve per cylinder engine tends to facilitate better low and mid range torque as the air flow is restricted. This, however, is negated by 4 valve engines which have variable valve timing.

In a nutshell, three or four-valve engines are generally better than their two valve counterparts and are more desirable if you don’t mind the extra cost. Two-valve units, however, are still pretty competent if price is a concern; they’re reliable, fuel efficient and do a good job of offering reliable performance for everyday use.

We hope this article dispelled any doubts you had about two, three and four-valve engines. If, however, you have any further doubts or questions, share them with us through the comments section below and we’ll have them answered at the earliest. Also, do not forget to share this informative article with your friends who you think would benefit from it.