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.
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.
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 center 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 duration.
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.
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